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page.title=Data Backup
@jd:body
<div id="qv-wrapper">
<div id="qv">
<h2>Quickview</h2>
<ul>
<li>Back up the user's data to the cloud in case the user loses it</li>
<li>If the user upgrades to a new Android-powered device, your app can restore the user's
data onto the new device</li>
<li>Easily back up SharedPreferences and private files with BackupAgentHelper</li>
<li>Requires API Level 8</li>
</ul>
<h2>In this document</h2>
<ol>
<li><a href="#Basics">The Basics</a></li>
<li><a href="#BackupManifest">Declaring the Backup Agent in Your Manifest</a></li>
<li><a href="#BackupKey">Registering for Android Backup Service</a></li>
<li><a href="#BackupAgent">Extending BackupAgent</a>
<ol>
<li><a href="#RequiredMethods">Required Methods</a></li>
<li><a href="#PerformingBackup">Performing backup</a></li>
<li><a href="#PerformingRestore">Performing restore</a></li>
</ol>
</li>
<li><a href="#BackupAgentHelper">Extending BackupAgentHelper</a>
<ol>
<li><a href="#SharedPreferences">Backing up SharedPreferences</a></li>
<li><a href="#Files">Backing up Private Files</a></li>
</ol>
</li>
<li><a href="#RestoreVersion">Checking the Restore Data Version</a></li>
<li><a href="#RequestingBackup">Requesting Backup</a></li>
<li><a href="#RequestingRestore">Requesting Restore</a></li>
<li><a href="#Testing">Testing Your Backup Agent</a></li>
</ol>
<h2>Key classes</h2>
<ol>
<li>{@link android.app.backup.BackupManager}</li>
<li>{@link android.app.backup.BackupAgent}</li>
<li>{@link android.app.backup.BackupAgentHelper}</li>
</ol>
<h2>See also</h2>
<ol>
<li><a href="{@docRoot}guide/developing/tools/bmgr.html">{@code bmgr} tool</a></li>
</ol>
</div>
</div>
<p>Android's {@link android.app.backup backup} service allows you to copy your persistent
application data to remote "cloud" storage, in order to provide a restore point for the
application data and settings. If a user performs a factory reset or converts to a new
Android-powered device, the system automatically restores your backup data when the application
is re-installed. This way, your users don't need to reproduce their previous data or
application settings. This process is completely transparent to the user and does not affect the
functionality or user experience in your application.</p>
<p>During a backup operation (which your application can request), Android's Backup Manager ({@link
android.app.backup.BackupManager}) queries your application for backup data, then hands it to
a backup transport, which then delivers the data to the cloud storage. During a
restore operation, the Backup Manager retrieves the backup data from the backup transport and
returns it to your application so your application can restore the data to the device. It's
possible for your application to request a restore, but that shouldn't be necessary&mdash;Android
automatically performs a restore operation when your application is installed and there exists
backup data associated with the user. The primary scenario in which backup data is restored is when
a user resets their device or upgrades to a new device and their previously installed
applications are re-installed.</p>
<p class="note"><strong>Note:</strong> The backup service is <em>not</em> designed for
synchronizing application data with other clients or saving data that you'd like to access during
the normal application lifecycle. You cannot read or write backup data on demand and cannot access
it in any way other than through the APIs provided by the Backup Manager.</p>
<p>The backup transport is the client-side component of Android's backup framework, which is
customizable by
the device manufacturer and service provider. The backup transport may differ from device to device
and which backup transport is available on any given device is transparent to your application. The
Backup Manager APIs isolate your application from the actual backup transport available on a given
device&mdash;your application communicates with the Backup Manager through a fixed set of APIs,
regardless of the underlying transport.</p>
<p>Data backup is <em>not</em> guaranteed to be available on all Android-powered
devices. However, your application is not adversely affected in the event
that a device does not provide a backup transport. If you believe that users will benefit from data
backup in your application, then you can implement it as described in this document, test it, then
publish your application without any concern about which devices actually perform backup. When your
application runs on a device that does not provide a backup transport, your application operates
normally, but will not receive callbacks from the Backup Manager to backup data.</p>
<p>Although you cannot know what the current transport is, you are always assured that your
backup data cannot be read by other applications on the device. Only the Backup Manager and backup
transport have access to the data you provide during a backup operation.</p>
<p class="caution"><strong>Caution:</strong> Because the cloud storage and transport service can
differ from device to device, Android makes no guarantees about the security of your data while
using backup. You should always be cautious about using backup to store sensitive data, such as
usernames and passwords.</p>
<h2 id="Basics">The Basics</h2>
<p>To backup your application data, you need to implement a backup agent. Your backup
agent is called by the Backup Manager to provide the data you want to back up. It is also called
to restore your backup data when the application is re-installed. The Backup Manager handles all
your data transactions with the cloud storage (using the backup transport) and your backup agent
handles all your data transactions on the device.</p>
<p>To implement a backup agent, you must:</p>
<ol>
<li>Declare your backup agent in your manifest file with the <a
href="{@docRoot}guide/topics/manifest/application-element.html#agent">{@code
android:backupAgent}</a> attribute.</li>
<li>Register your application with a backup service. Google offers <a
href="http://code.google.com/android/backup/index.html">Android Backup Service</a> as a backup
service for most Android-powered devices, which requires that you register your application in
order for it to work. Any other backup services available might also require you to register
in order to store your data on their servers.</li>
<li>Define a backup agent by either:</p>
<ol type="a">
<li><a href="#BackupAgent">Extending BackupAgent</a>
<p>The {@link android.app.backup.BackupAgent} class provides the central interface with
which your application communicates with the Backup Manager. If you extend this class
directly, you must override {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} and {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()} to handle the backup and restore operations for your data.</p>
<p><em>Or</em></p>
<li><a href="#BackupAgentHelper">Extending BackupAgentHelper</a>
<p>The {@link android.app.backup.BackupAgentHelper} class provides a convenient
wrapper around the {@link android.app.backup.BackupAgent} class, which minimizes the amount of code
you need to write. In your {@link android.app.backup.BackupAgentHelper}, you must use one or more
"helper" objects, which automatically backup and restore certain types of data, so that you do not
need to implement {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} and {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}.</p>
<p>Android currently provides backup helpers that will backup and restore complete files
from {@link android.content.SharedPreferences} and <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal storage</a>.</p>
</li>
</ol>
</li>
</ol>
<h2 id="BackupManifest">Declaring the Backup Agent in Your Manifest</h2>
<p>This is the easiest step, so once you've decided on the class name for your backup agent, declare
it in your manifest with the <a
href="{@docRoot}guide/topics/manifest/application-element.html#agent">{@code
android:backupAgent}</a> attribute in the <a
href="{@docRoot}guide/topics/manifest/application-element.html">{@code
&lt;application&gt;}</a> tag.</p>
<p>For example:</p>
<pre>
&lt;manifest ... &gt;
...
&lt;application android:label="MyApplication"
<b>android:backupAgent="MyBackupAgent"</b>&gt;
&lt;activity ... &gt;
...
&lt;/activity&gt;
&lt;/application&gt;
&lt;/manifest&gt;
</pre>
<p>Another attribute you might want to use is <a
href="{@docRoot}guide/topics/manifest/application-element.html#restoreany">{@code
android:restoreAnyVersion}</a>. This attribute takes a boolean value to indicate whether you
want to restore the application data regardless of the current application version compared to the
version that produced the backup data. (The default value is "{@code false}".) See <a
href="#RestoreVersion">Checking the Restore Data Version</a> for more information.</p>
<p class="note"><strong>Note:</strong> The backup service and the APIs you must use are
available only on devices running API Level 8 (Android 2.2) or greater, so you should also
set your <a
href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#min">{@code android:minSdkVersion}</a>
attribute to "8". However, if you implement proper <a
href="{@docRoot}resources/articles/backward-compatibility.html">backward compatibility</a> in
your application, you can support this feature for devices running API Level 8 or greater, while
remaining compatible with older devices.</p>
<h2 id="BackupKey">Registering for Android Backup Service</h2>
<p>Google provides a backup transport with <a
href="http://code.google.com/android/backup/index.html">Android Backup Service</a> for most
Android-powered devices running Android 2.2 or greater.</p>
<p>In order for you application to perform backup using Android Backup Service, you must
register your application with the service to receive a Backup Service Key, then
declare the Backup Service Key in your Android manifest.</p>
<p>To get your Backup Service Key, <a
href="http://code.google.com/android/backup/signup.html">register for Android Backup Service</a>.
When you register, you will be provided a Backup Service Key and the appropriate {@code
&lt;meta-data&gt;} XML code for your Android manifest file, which you must include as a child of the
{@code &lt;application&gt;} element. For example:</p>
<pre>
&lt;application android:label="MyApplication"
android:backupAgent="MyBackupAgent"&gt;
...
&lt;meta-data android:name="com.google.android.backup.api_key"
android:value="AEdPqrEAAAAIDaYEVgU6DJnyJdBmU7KLH3kszDXLv_4DIsEIyQ" /&gt;
&lt;/application&gt;
</pre>
<p>The <code>android:name</code> must be <code>"com.google.android.backup.api_key"</code> and
the <code>android:value</code> must be the Backup Service Key received from the Android Backup
Service registration.</p>
<p>If you have multiple applications, you must register each one, using the respective package
name.</p>
<p class="note"><strong>Note:</strong> The backup transport provided by Android Backup Service is
not guaranteed to be available
on all Android-powered devices that support backup. Some devices might support backup
using a different transport, some devices might not support backup at all, and there is no way for
your application to know what transport is used on the device. However, if you implement backup for
your application, you should always include a Backup Service Key for Android Backup Service so
your application can perform backup when the device uses the Android Backup Service transport. If
the device does not use Android Backup Service, then the {@code &lt;meta-data&gt;} element with the
Backup Service Key is ignored.</p>
<h2 id="BackupAgent">Extending BackupAgent</h2>
<p>Most applications shouldn't need to extend the {@link android.app.backup.BackupAgent} class
directly, but should instead <a href="#BackupAgentHelper">extend BackupAgentHelper</a> to take
advantage of the built-in helper classes that automatically backup and restore your files. However,
you might want to extend {@link android.app.backup.BackupAgent} directly if you need to:</p>
<ul>
<li>Version your data format. For instance, if you anticipate the need to revise the
format in which you write your application data, you can build a backup agent to cross-check your
application version during a restore operation and perform any necessary compatibility work if the
version on the device is different than that of the backup data. For more information, see <a
href="#RestoreVersion">Checking the Restore Data Version</a>.</li>
<li>Instead of backing up an entire file, you can specify the portions of data the should be
backed up and how each portion is then restored to the device. (This can also help you manage
different versions, because you read and write your data as unique entities, rather than
complete files.)</li>
<li>Back up data in a database. If you have an SQLite database that you want to restore when
the user re-installs your application, you need to build a custom {@link
android.app.backup.BackupAgent} that reads the appropriate data during a backup operation, then
create your table and insert the data during a restore operation.</li>
</ul>
<p>If you don't need to perform any of the tasks above and want to back up complete files from
{@link android.content.SharedPreferences} or <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal storage</a>, you
should skip to <a href="#BackupAgentHelper">Extending BackupAgentHelper</a>.</p>
<h3 id="RequiredMethods">Required Methods</h3>
<p>When you create a backup agent by extending {@link android.app.backup.BackupAgent}, you
must implement the following callback methods:</p>
<dl>
<dt>{@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()}</dt>
<dd>The Backup Manager calls this method after you <a href="#RequestingBackup">request a
backup</a>. In this method, you read your application data from the device and pass the data you
want to back up to the Backup Manager, as described below in <a href="#PerformingBackup">Performing
backup</a>.</dd>
<dt>{@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}</dt>
<dd>The Backup Manager calls this method during a restore operation (you can <a
href="#RequestingRestore">request a restore</a>, but the system automatically performs restore when
the user re-installs your application). When it calls this method, the Backup Manager delivers your
backup data, which you then restore to the device, as described below in <a
href="#PerformingRestore">Performing restore</a>.</dd>
</dl>
<h3 id="PerformingBackup">Performing backup</h3>
<p>When it's time to back up your application data, the Backup Manager calls your {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method. This is where you must provide your application data to the Backup Manager so
it can be saved to cloud storage.</p>
<p>Only the Backup Manager can call your backup agent's {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method. Each time that your application data changes and you want to perform a backup,
you must request a backup operation by calling {@link
android.app.backup.BackupManager#dataChanged()} (see <a href="#RequestingBackup">Requesting
Backup</a> for more information). A backup request does not result in an immediate call to your
{@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method. Instead, the Backup Manager waits for an appropriate time, then performs
backup for all applications that have requested a backup since the last backup was performed.</p>
<p class="note"><strong>Tip:</strong> While developing your application, you can initiate an
immediate backup operation from the Backup Manager with the <a
href="{@docRoot}guide/developing/tools/bmgr.html">{@code bmgr} tool</a>.</p>
<p>When the Backup Manager calls your {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method, it passes three parameters:</p>
<dl>
<dt>{@code oldState}</dt>
<dd>An open, read-only {@link android.os.ParcelFileDescriptor} pointing to the last backup
state provided by your application. This is not the backup data from cloud storage, but a
local representation of the data that was backed up the last time {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} was called (as defined by {@code newState}, below, or from {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}&mdash;more about this in the next section). Because {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} does not allow you to read existing backup data in
the cloud storage, you can use this local representation to determine whether your data has changed
since the last backup.</dd>
<dt>{@code data}</dt>
<dd>A {@link android.app.backup.BackupDataOutput} object, which you use to deliver your backup
data to the Backup Manager.</dd>
<dt>{@code newState}</dt>
<dd>An open, read/write {@link android.os.ParcelFileDescriptor} pointing to a file in which
you must write a representation of the data that you delivered to {@code data} (a representation
can be as simple as the last-modified timestamp for your file). This object is
returned as {@code oldState} the next time the Backup Manager calls your {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method. If you do not write your backup data to {@code newState}, then {@code oldState}
will point to an empty file next time Backup Manager calls {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()}.</dd>
</dl>
<p>Using these parameters, you should implement your {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method to do the following:</p>
<ol>
<li>Check whether your data has changed since the last backup by comparing {@code oldState} to
your current data. How you read data in {@code oldState} depends on how you originally wrote it to
{@code newState} (see step 3). The easiest way to record the state of a file is with its
last-modified timestamp. For example, here's how you can read and compare a timestamp from {@code
oldState}:
<pre>
// Get the oldState input stream
FileInputStream instream = new FileInputStream(oldState.getFileDescriptor());
DataInputStream in = new DataInputStream(instream);
try {
// Get the last modified timestamp from the state file and data file
long stateModified = in.readLong();
long fileModified = mDataFile.lastModified();
if (stateModified != fileModified) {
// The file has been modified, so do a backup
// Or the time on the device changed, so be safe and do a backup
} else {
// Don't back up because the file hasn't changed
return;
}
} catch (IOException e) {
// Unable to read state file... be safe and do a backup
}
</pre>
<p>If nothing has changed and you don't need to back up, skip to step 3.</p>
</li>
<li>If your data has changed, compared to {@code oldState}, write the current data to
{@code data} to back it up to the cloud storage.
<p>You must write each chunk of data as an "entity" in the {@link
android.app.backup.BackupDataOutput}. An entity is a flattened binary data
record that is identified by a unique key string. Thus, the data set that you back up is
conceptually a set of key-value pairs.</p>
<p>To add an entity to your backup data set, you must:</p>
<ol>
<li>Call {@link android.app.backup.BackupDataOutput#writeEntityHeader(String,int)
writeEntityHeader()}, passing a unique string key for the data you're about to write and the data
size.</li>
<li>Call {@link android.app.backup.BackupDataOutput#writeEntityData(byte[],int)
writeEntityData()}, passing a byte buffer that contains your data and the number of bytes to write
from the buffer (which should match the size passed to {@link
android.app.backup.BackupDataOutput#writeEntityHeader(String,int) writeEntityHeader()}).</li>
</ol>
<p>For example, the following code flattens some data into a byte stream and writes it into a
single entity:</p>
<pre>
// Create buffer stream and data output stream for our data
ByteArrayOutputStream bufStream = new ByteArrayOutputStream();
DataOutputStream outWriter = new DataOutputStream(bufStream);
// Write structured data
outWriter.writeUTF(mPlayerName);
outWriter.writeInt(mPlayerScore);
// Send the data to the Backup Manager via the BackupDataOutput
byte[] buffer = bufStream.toByteArray();
int len = buffer.length;
data.writeEntityHeader(TOPSCORE_BACKUP_KEY, len);
data.writeEntityData(buffer, len);
</pre>
<p>Perform this for each piece of data that you want to back up. How you divide your data into
entities is up to you (and you might use just one entity).</p>
</li>
<li>Whether or not you perform a backup (in step 2), write a representation of the current data to
the {@code newState} {@link android.os.ParcelFileDescriptor}. The Backup Manager retains this object
locally as a representation of the data that is currently backed up. It passes this back to you as
{@code oldState} the next time it calls {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} so you can determine whether another backup is necessary (as handled in step 1). If you
do not write the current data state to this file, then
{@code oldState} will be empty during the next callback.
<p>The following example saves a representation of the current data into {@code newState} using
the file's last-modified timestamp:</p>
<pre>
FileOutputStream outstream = new FileOutputStream(newState.getFileDescriptor());
DataOutputStream out = new DataOutputStream(outstream);
long modified = mDataFile.lastModified();
out.writeLong(modified);
</pre>
</li>
</ol>
<p class="caution"><strong>Caution:</strong> If your application data is saved to a file, make sure
that you use synchronized statements while accessing the file so that your backup agent does not
read the file while an Activity in your application is also writing the file.</p>
<h3 id="PerformingRestore">Performing restore</h3>
<p>When it's time to restore your application data, the Backup Manager calls your backup
agent's {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()} method. When it calls this method, the Backup Manager delivers your backup data so
you can restore it onto the device.</p>
<p>Only the Backup Manager can call {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}, which happens automatically when the system installs your application and
finds existing backup data. However, you can request a restore operation for
your application by calling {@link
android.app.backup.BackupManager#requestRestore(RestoreObserver) requestRestore()} (see <a
href="#RequestingRestore">Requesting restore</a> for more information).</p>
<p class="note"><strong>Note:</strong> While developing your application, you can also request a
restore operation with the <a href="{@docRoot}guide/developing/tools/bmgr.html">{@code bmgr}
tool</a>.</p>
<p>When the Backup Manager calls your {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()} method, it passes three parameters:</p>
<dl>
<dt>{@code data}</dt>
<dd>A {@link android.app.backup.BackupDataInput}, which allows you to read your backup
data.</dd>
<dt>{@code appVersionCode}</dt>
<dd>An integer representing the value of your application's <a
href="{@docRoot}guide/topics/manifest/manifest-element.html#vcode">{@code android:versionCode}</a>
manifest attribute, as it was when this data was backed up. You can use this to cross-check the
current application version and determine if the data format is compatible. For more
information about using this to handle different versions of restore data, see the section
below about <a href="#RestoreVersion">Checking the Restore Data Version</a>.</dd>
<dt>{@code newState}</dt>
<dd>An open, read/write {@link android.os.ParcelFileDescriptor} pointing to a file in which
you must write the final backup state that was provided with {@code data}. This object is
returned as {@code oldState} the next time {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} is called. Recall that you must also write the same {@code newState} object in the
{@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} callback&mdash;also doing it here ensures that the {@code oldState} object given to
{@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} is valid even the first time {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} is called after the device is restored.</dd>
</dl>
<p>In your implementation of {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}, you should call {@link android.app.backup.BackupDataInput#readNextHeader()} on the
{@code data} to iterate
through all entities in the data set. For each entity found, do the following:</p>
<ol>
<li>Get the entity key with {@link android.app.backup.BackupDataInput#getKey()}.</li>
<li>Compare the entity key to a list of known key values that you should have declared as static
final strings inside your {@link android.app.backup.BackupAgent} class. When the key matches one of
your known key strings, enter into a statement to extract the entity data and save it to the device:
<ol>
<li>Get the entity data size with {@link
android.app.backup.BackupDataInput#getDataSize()} and create a byte array of that size.</li>
<li>Call {@link android.app.backup.BackupDataInput#readEntityData(byte[],int,int)
readEntityData()} and pass it the byte array, which is where the data will go, and specify the
start offset and the size to read.</li>
<li>Your byte array is now full and you can read the data and write it to the device
however you like.</li>
</ol>
</li>
<li>After you read and write your data back to the device, write the state of your data to the
{@code newState} parameter the same as you do during {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()}.
</ol>
<p>For example, here's how you can restore the data backed up by the example in the previous
section:</p>
<pre>
&#64;Override
public void onRestore(BackupDataInput data, int appVersionCode,
ParcelFileDescriptor newState) throws IOException {
// There should be only one entity, but the safest
// way to consume it is using a while loop
while (data.readNextHeader()) {
String key = data.getKey();
int dataSize = data.getDataSize();
// If the key is ours (for saving top score). Note this key was used when
// we wrote the backup entity header
if (TOPSCORE_BACKUP_KEY.equals(key)) {
// Create an input stream for the BackupDataInput
byte[] dataBuf = new byte[dataSize];
data.readEntityData(dataBuf, 0, dataSize);
ByteArrayInputStream baStream = new ByteArrayInputStream(dataBuf);
DataInputStream in = new DataInputStream(baStream);
// Read the player name and score from the backup data
mPlayerName = in.readUTF();
mPlayerScore = in.readInt();
// Record the score on the device (to a file or something)
recordScore(mPlayerName, mPlayerScore);
} else {
// We don't know this entity key. Skip it. (Shouldn't happen.)
data.skipEntityData();
}
}
// Finally, write to the state blob (newState) that describes the restored data
FileOutputStream outstream = new FileOutputStream(newState.getFileDescriptor());
DataOutputStream out = new DataOutputStream(outstream);
out.writeUTF(mPlayerName);
out.writeInt(mPlayerScore);
}
</pre>
<p>In this example, the {@code appVersionCode} parameter passed to {@link
android.app.backup.BackupAgent#onRestore onRestore()} is not used. However, you might want to use
it if you've chosen to perform backup when the user's version of the application has actually moved
backward (for example, the user went from version 1.5 of your app to 1.0). For more information, see
the section about <a href="#RestoreVersion">Checking the Restore Data Version</a>.</p>
<div class="special">
<p>For an example implementation of {@link android.app.backup.BackupAgent}, see the <a
href="{@docRoot}resources/samples/BackupRestore/src/com/example/android/backuprestore/ExampleAgent.html">{@code
ExampleAgent}</a> class in the <a
href="{@docRoot}resources/samples/BackupRestore/index.html">Backup and Restore</a> sample
application.</p>
</div>
<h2 id="BackupAgentHelper">Extending BackupAgentHelper</h2>
<p>You should build your backup agent using {@link android.app.backup.BackupAgentHelper} if you want
to back up complete files (from either {@link android.content.SharedPreferences} or <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal storage</a>).
Building your backup agent with {@link android.app.backup.BackupAgentHelper} requires far less
code than extending {@link android.app.backup.BackupAgent}, because you don't have to implement
{@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} and {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}.</p>
<p>Your implementation of {@link android.app.backup.BackupAgentHelper} must
use one or more backup helpers. A backup helper is a specialized
component that {@link android.app.backup.BackupAgentHelper} summons to perform backup and
restore operations for a particular type of data. The Android framework currently provides two
different helpers:</p>
<ul>
<li>{@link android.app.backup.SharedPreferencesBackupHelper} to backup {@link
android.content.SharedPreferences} files.</li>
<li>{@link android.app.backup.FileBackupHelper} to backup files from <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal storage</a>.</li>
</ul>
<p>You can include multiple helpers in your {@link android.app.backup.BackupAgentHelper}, but only
one helper is needed for each data type. That is, if you have multiple {@link
android.content.SharedPreferences} files, then you need only one {@link
android.app.backup.SharedPreferencesBackupHelper}.</p>
<p>For each helper you want to add to your {@link android.app.backup.BackupAgentHelper}, you must do
the following during your {@link android.app.backup.BackupAgent#onCreate()} method:</p>
<ol>
<li>Instantiate in instance of the desired helper class. In the class constructor, you must
specify the appropriate file(s) you want to backup.</li>
<li>Call {@link android.app.backup.BackupAgentHelper#addHelper(String,BackupHelper) addHelper()}
to add the helper to your {@link android.app.backup.BackupAgentHelper}.</li>
</ol>
<p>The following sections describe how to create a backup agent using each of the available
helpers.</p>
<h3 id="SharedPreferences">Backing up SharedPreferences</h3>
<p>When you instantiate a {@link android.app.backup.SharedPreferencesBackupHelper}, you must
include the name of one or more {@link android.content.SharedPreferences} files.</p>
<p>For example, to back up a {@link android.content.SharedPreferences} file named
"user_preferences", a complete backup agent using {@link android.app.backup.BackupAgentHelper} looks
like this:</p>
<pre>
public class MyPrefsBackupAgent extends BackupAgentHelper {
// The name of the SharedPreferences file
static final String PREFS = "user_preferences";
// A key to uniquely identify the set of backup data
static final String PREFS_BACKUP_KEY = "prefs";
// Allocate a helper and add it to the backup agent
&#64;Override
public void onCreate() {
SharedPreferencesBackupHelper helper = new SharedPreferencesBackupHelper(this, PREFS);
addHelper(PREFS_BACKUP_KEY, helper);
}
}
</pre>
<p>That's it! That's your entire backup agent. The {@link
android.app.backup.SharedPreferencesBackupHelper} includes all the code
needed to backup and restore a {@link android.content.SharedPreferences} file.</p>
<p>When the Backup Manager calls {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} and {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()}, {@link android.app.backup.BackupAgentHelper} calls your backup helpers to perform
backup and restore for your specified files.</p>
<p class="note"><strong>Note:</strong> {@link android.content.SharedPreferences} are threadsafe, so
you can safely read and write the shared preferences file from your backup agent and
other activities.</p>
<h3 id="Files">Backing up other files</h3>
<p>When you instantiate a {@link android.app.backup.FileBackupHelper}, you must include the name of
one or more files that are saved to your application's <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal storage</a>
(as specified by {@link android.content.ContextWrapper#getFilesDir()}, which is the same
location where {@link android.content.Context#openFileOutput(String,int) openFileOutput()} writes
files).</p>
<p>For example, to backup two files named "scores" and "stats," a backup agent using {@link
android.app.backup.BackupAgentHelper} looks like this:</p>
<pre>
public class MyFileBackupAgent extends BackupAgentHelper {
// The name of the SharedPreferences file
static final String TOP_SCORES = "scores";
static final String PLAYER_STATS = "stats";
// A key to uniquely identify the set of backup data
static final String FILES_BACKUP_KEY = "myfiles";
// Allocate a helper and add it to the backup agent
void onCreate() {
FileBackupHelper helper = new FileBackupHelper(this, TOP_SCORES, PLAYER_STATS);
addHelper(FILES_BACKUP_KEY, helper);
}
}
</pre>
<p>The {@link android.app.backup.FileBackupHelper} includes all the code necessary to backup and
restore files that are saved to your application's <a
href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal storage</a>..</p>
<p>However, reading and writing to files on internal storage is <strong>not threadsafe</strong>. To
ensure that your backup agent does not read or write your files at the same time as your activities,
you must use synchronized statements each time you perform a read or write. For example,
in any Activity where you read and write the file, you need an object to use as the intrinsic
lock for the synchronized statements:</p>
<div class="sidebox-wrapper">
<div class="sidebox">
<p><strong>Interesting Fact:</strong></p>
<p>A zero-length array is lighter-weight than a normal Object, so it's great for an
intrinsic lock.</p>
</div>
</div>
<pre>
// Object for intrinsic lock
static final Object[] sDataLock = new Object[0];
</pre>
<p>Then create a synchronized statement with this lock each time you read or write the files. For
example, here's a synchronized statement for writing the latest score in a game to a file:</p>
<pre>
try {
synchronized (MyActivity.sDataLock) {
File dataFile = new File({@link android.content.Context#getFilesDir()}, TOP_SCORES);
RandomAccessFile raFile = new RandomAccessFile(dataFile, "rw");
raFile.writeInt(score);
}
} catch (IOException e) {
Log.e(TAG, "Unable to write to file");
}
</pre>
<p>You should synchronize your read statements with the same lock.</p>
<p>Then, in your {@link android.app.backup.BackupAgentHelper}, you must override {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} and {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()} to synchronize the backup and restore operations with the same
intrinsic lock. For example, the {@code MyFileBackupAgent} example from above needs the following
methods:</p>
<pre>
&#64;Override
public void onBackup(ParcelFileDescriptor oldState, BackupDataOutput data,
ParcelFileDescriptor newState) throws IOException {
// Hold the lock while the FileBackupHelper performs backup
synchronized (MyActivity.sDataLock) {
super.onBackup(oldState, data, newState);
}
}
&#64;Override
public void onRestore(BackupDataInput data, int appVersionCode,
ParcelFileDescriptor newState) throws IOException {
// Hold the lock while the FileBackupHelper restores the file
synchronized (MyActivity.sDataLock) {
super.onRestore(data, appVersionCode, newState);
}
}
</pre>
<p>That's it. All you need to do is add your {@link android.app.backup.FileBackupHelper} in the
{@link android.app.backup.BackupAgent#onCreate()} method and override {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} and {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
onRestore()} to synchronize read and write operations.</p>
<div class="special">
<p>For an example implementation of {@link
android.app.backup.BackupAgentHelper} with {@link android.app.backup.FileBackupHelper}, see the
{@code FileHelperExampleAgent} class in the <a
href="{@docRoot}resources/samples/BackupRestore/index.html">Backup and Restore</a> sample
application.</p>
</div>
<h2 id="RestoreVersion">Checking the Restore Data Version</h2>
<p>When the Backup Manager saves your data to cloud storage, it automatically includes the version
of your application, as defined by your manifest file's <a
href="{@docRoot}guide/topics/manifest/manifest-element.html#vcode">{@code android:versionCode}</a>
attribute. Before the Backup Manager calls your backup agent to restore your data, it
looks at the <a
href="{@docRoot}guide/topics/manifest/manifest-element.html#vcode">{@code
android:versionCode}</a> of the installed application and compares it to the value
recorded in the restore data set. If the version recorded in the restore data set is
<em>newer</em> than the application version on the device, then the user has downgraded their
application. In this case, the Backup Manager will abort the restore operation for your application
and not call your {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}
method, because the restore set is considered meaningless to an older version.</p>
<p>You can override this behavior with the <a
href="{@docRoot}guide/topics/manifest/application-element.html#restoreany">{@code
android:restoreAnyVersion}</a> attribute. This attribute is either "{@code true}" or "{@code
false}" to indicate whether you want to restore the application regardless of the restore set
version. The default value is "{@code false}". If you define this to be "{@code true}" then the
Backup Manager will ignore the <a
href="{@docRoot}guide/topics/manifest/manifest-element.html#vcode">{@code android:versionCode}</a>
and call your {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}
method in all cases. In doing so, you can manually check for the version difference in your {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}
method and take any steps necessary to make the data compatible if the versions conflict.</p>
<p>To help you handle different versions during a restore operation, the {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}
method passes you the version code included with the restore data set as the {@code appVersionCode}
parameter. You can then query the current application's version code with the {@link
android.content.pm.PackageInfo#versionCode PackageInfo.versionCode} field. For example:</p>
<pre>
PackageInfo info;
try {
String name = {@link android.content.ContextWrapper#getPackageName() getPackageName}();
info = {@link android.content.ContextWrapper#getPackageManager
getPackageManager}().{@link android.content.pm.PackageManager#getPackageInfo(String,int)
getPackageInfo}(name,0);
} catch (NameNotFoundException nnfe) {
info = null;
}
int version;
if (info != null) {
version = info.versionCode;
}
</pre>
<p>Then simply compare the {@code version} acquired from {@link android.content.pm.PackageInfo}
to the {@code appVersionCode} passed into {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}.
</p>
<p class="caution"><strong>Caution:</strong> Be certain you understand the consequences of setting
<a href="{@docRoot}guide/topics/manifest/application-element.html#restoreany">{@code
android:restoreAnyVersion}</a> to "{@code true}" for your application. If each version of your
application that supports backup does not properly account for variations in your data format during
{@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()},
then the data on the device could be saved in a format incompatible with the version currently
installed on the device.</p>
<h2 id="RequestingBackup">Requesting Backup</h2>
<p>You can request a backup operation at any time by calling {@link
android.app.backup.BackupManager#dataChanged()}. This method notifies the Backup Manager that you'd
like to backup your data using your backup agent. The Backup Manager then calls your backup
agent's {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()} method at an opportune time in the future. Typically, you should
request a backup each time your data changes (such as when the user changes an application
preference that you'd like to back up). If you call {@link
android.app.backup.BackupManager#dataChanged()} several times consecutively, before the Backup
Manager requests a backup from your agent, your agent still receives just one call to {@link
android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
onBackup()}.</p>
<p class="note"><strong>Note:</strong> While developing your application, you can request a
backup and initiate an immediate backup operation with the <a
href="{@docRoot}guide/developing/tools/bmgr.html">{@code bmgr}
tool</a>.</p>
<h2 id="RequestingRestore">Requesting Restore</h2>
<p>During the normal life of your application, you shouldn't need to request a restore operation.
They system automatically checks for backup data and performs a restore when your application is
installed. However, you can manually request a restore operation by calling {@link
android.app.backup.BackupManager#requestRestore(RestoreObserver) requestRestore()}, if necessary. In
which case, the Backup Manager calls your {@link
android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}
implementation, passing the data from the current set of backup data.</p>
<p class="note"><strong>Note:</strong> While developing your application, you can request a
restore operation with the <a href="{@docRoot}guide/developing/tools/bmgr.html">{@code bmgr}
tool</a>.</p>
<h2 id="Testing">Testing Your Backup Agent</h2>
<p>Once you've implemented your backup agent, you can test the backup and restore functionality
with the following procedure, using <a
href="{@docRoot}guide/developing/tools/bmgr.html">{@code bmgr}</a>.</p>
<ol>
<li>Install your application on a suitable Android system image
<ul>
<li>If using the emulator, create and use an AVD with Android 2.2 (API Level 8).</li>
<li>If using a device, the device must be running Android 2.2 or greater and have Android
Market built in.</li>
</ul>
</li>
<li>Ensure that backup is enabled
<ul>
<li>If using the emulator, you can enable backup with the following command from your SDK
{@code tools/} path:
<pre class="no-pretty-print">adb shell bmgr enable true</pre>
</li>
<li>If using a device, open the system <b>Settings</b>, select <b>Privacy</b>, then enable
<b>Back up my data</b> and <b>Automatic restore</b>.
</ul>
</li>
<li>Open your application and initialize some data
<p>If you've properly implemented backup in your application, then it should request a
backup each time the data changes. For example, each time the user changes some data, your app
should call {@link android.app.backup.BackupManager#dataChanged()}, which adds a backup request to
the Backup Manager queue. For testing purposes, you can also make a request with the following
{@code bmgr} command:</p>
<pre class="no-pretty-print">adb shell bmgr backup <em>your.package.name</em></pre>
</li>
<li>Initiate a backup operation:
<pre class="no-pretty-print">adb shell bmgr run</pre>
<p>This forces the Backup Manager to perform all backup requests that are in its
queue.</p>
<li>Uninstall your application:
<pre class="no-pretty-print">adb uninstall <em>your.package.name</em></pre>
</li>
<li>Re-install your application.</li>
</ol>
<p>If your backup agent is successful, all the data you initialized in step 4 is restored.</p>

457
base/docs/html/guide/topics/data/data-storage.jd Normal file
View File

@ -0,0 +1,457 @@
page.title=Data Storage
@jd:body
<div id="qv-wrapper">
<div id="qv">
<h2>Storage quickview</h2>
<ul>
<li>Use Shared Preferences for primitive data</li>
<li>Use internal device storage for private data</li>
<li>Use external storage for large data sets that are not private</li>
<li>Use SQLite databases for structured storage</li>
</ul>
<h2>In this document</h2>
<ol>
<li><a href="#pref">Using Shared Preferences</a></li>
<li><a href="#filesInternal">Using the Internal Storage</a></li>
<li><a href="#filesExternal">Using the External Storage</a></li>
<li><a href="#db">Using Databases</a></li>
<li><a href="#netw">Using a Network Connection</a></li>
</ol>
<h2>See also</h2>
<ol>
<li><a href="#pref">Content Providers and Content Resolvers</a></li>
</ol>
</div>
</div>
<p>Android provides several options for you to save persistent application data. The solution you
choose depends on your specific needs, such as whether the data should be private to your
application or accessible to other applications (and the user) and how much space your data
requires.
</p>
<p>Your data storage options are the following:</p>
<dl>
<dt><a href="#pref">Shared Preferences</a></dt>
<dd>Store private primitive data in key-value pairs.</dd>
<dt><a href="#filesInternal">Internal Storage</a></dt>
<dd>Store private data on the device memory.</dd>
<dt><a href="#filesExternal">External Storage</a></dt>
<dd>Store public data on the shared external storage.</dd>
<dt><a href="#db">SQLite Databases</a></dt>
<dd>Store structured data in a private database.</dd>
<dt><a href="#netw">Network Connection</a></dt>
<dd>Store data on the web with your own network server.</dd>
</dl>
<p>Android provides a way for you to expose even your private data to other applications
&mdash; with a <a href="{@docRoot}guide/topics/providers/content-providers.html">content
provider</a>. A content provider is an optional component that exposes read/write access to
your application data, subject to whatever restrictions you want to impose. For more information
about using content providers, see the
<a href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a>
documentation.
</p>
<h2 id="pref">Using Shared Preferences</h2>
<p>The {@link android.content.SharedPreferences} class provides a general framework that allows you
to save and retrieve persistent key-value pairs of primitive data types. You can use {@link
android.content.SharedPreferences} to save any primitive data: booleans, floats, ints, longs, and
strings. This data will persist across user sessions (even if your application is killed).</p>
<div class="sidebox-wrapper">
<div class="sidebox">
<h3>User Preferences</h3>
<p>Shared preferences are not strictly for saving "user preferences," such as what ringtone a
user has chosen. If you're interested in creating user preferences for your application, see {@link
android.preference.PreferenceActivity}, which provides an Activity framework for you to create
user preferences, which will be automatically persisted (using shared preferences).</p>
</div>
</div>
<p>To get a {@link android.content.SharedPreferences} object for your application, use one of
two methods:</p>
<ul>
<li>{@link android.content.Context#getSharedPreferences(String,int)
getSharedPreferences()} - Use this if you need multiple preferences files identified by name,
which you specify with the first parameter.</li>
<li>{@link android.app.Activity#getPreferences(int) getPreferences()} - Use this if you need
only one preferences file for your Activity. Because this will be the only preferences file
for your Activity, you don't supply a name.</li>
</ul>
<p>To write values:</p>
<ol>
<li>Call {@link android.content.SharedPreferences#edit()} to get a {@link
android.content.SharedPreferences.Editor}.</li>
<li>Add values with methods such as {@link
android.content.SharedPreferences.Editor#putBoolean(String,boolean) putBoolean()} and {@link
android.content.SharedPreferences.Editor#putString(String,String) putString()}.</li>
<li>Commit the new values with {@link android.content.SharedPreferences.Editor#commit()}</li>
</ol>
<p>To read values, use {@link android.content.SharedPreferences} methods such as {@link
android.content.SharedPreferences#getBoolean(String,boolean) getBoolean()} and {@link
android.content.SharedPreferences#getString(String,String) getString()}.</p>
<p>
Here is an example that saves a preference for silent keypress mode in a
calculator:
</p>
<pre>
public class Calc extends Activity {
public static final String PREFS_NAME = "MyPrefsFile";
&#64;Override
protected void onCreate(Bundle state){
super.onCreate(state);
. . .
// Restore preferences
SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
boolean silent = settings.getBoolean("silentMode", false);
setSilent(silent);
}
&#64;Override
protected void onStop(){
super.onStop();
// We need an Editor object to make preference changes.
// All objects are from android.context.Context
SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
SharedPreferences.Editor editor = settings.edit();
editor.putBoolean("silentMode", mSilentMode);
// Commit the edits!
editor.commit();
}
}
</pre>
<a name="files"></a>
<h2 id="filesInternal">Using the Internal Storage</h2>
<p>You can save files directly on the device's internal storage. By default, files saved
to the internal storage are private to your application and other applications cannot access
them (nor can the user). When the user uninstalls your application, these files are removed.</p>
<p>To create and write a private file to the internal storage:</p>
<ol>
<li>Call {@link android.content.Context#openFileOutput(String,int) openFileOutput()} with the
name of the file and the operating mode. This returns a {@link java.io.FileOutputStream}.</li>
<li>Write to the file with {@link java.io.FileOutputStream#write(byte[]) write()}.</li>
<li>Close the stream with {@link java.io.FileOutputStream#close()}.</li>
</ol>
<p>For example:</p>
<pre>
String FILENAME = "hello_file";
String string = "hello world!";
FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_PRIVATE);
fos.write(string.getBytes());
fos.close();
</pre>
<p>{@link android.content.Context#MODE_PRIVATE} will create the file (or replace a file of
the same name) and make it private to your application. Other modes available are: {@link
android.content.Context#MODE_APPEND}, {@link
android.content.Context#MODE_WORLD_READABLE}, and {@link
android.content.Context#MODE_WORLD_WRITEABLE}.</p>
<p>To read a file from internal storage:</p>
<ol>
<li>Call {@link android.content.Context#openFileInput openFileInput()} and pass it the
name of the file to read. This returns a {@link java.io.FileInputStream}.</li>
<li>Read bytes from the file with {@link java.io.FileInputStream#read(byte[],int,int)
read()}.</li>
<li>Then close the stream with {@link java.io.FileInputStream#close()}.</li>
</ol>
<p class="note"><strong>Tip:</strong> If you want to save a static file in your application at
compile time, save the file in your project <code>res/raw/</code> directory. You can open it with
{@link android.content.res.Resources#openRawResource(int) openRawResource()}, passing the {@code
R.raw.<em>&lt;filename&gt;</em>} resource ID. This method returns an {@link java.io.InputStream}
that you can use to read the file (but you cannot write to the original file).
</p>
<h3 id="InternalCache">Saving cache files</h3>
<p>If you'd like to cache some data, rather than store it persistently, you should use {@link
android.content.Context#getCacheDir()} to open a {@link
java.io.File} that represents the internal directory where your application should save
temporary cache files.</p>
<p>When the device is
low on internal storage space, Android may delete these cache files to recover space. However, you
should not rely on the system to clean up these files for you. You should always maintain the cache
files yourself and stay within a reasonable limit of space consumed, such as 1MB. When the user
uninstalls your application, these files are removed.</p>
<h3 id="InternalMethods">Other useful methods</h3>
<dl>
<dt>{@link android.content.Context#getFilesDir()}</dt>
<dd>Gets the absolute path to the filesystem directory where your internal files are saved.</dd>
<dt>{@link android.content.Context#getDir(String,int) getDir()}</dt>
<dd>Creates (or opens an existing) directory within your internal storage space.</dd>
<dt>{@link android.content.Context#deleteFile(String) deleteFile()}</dt>
<dd>Deletes a file saved on the internal storage.</dd>
<dt>{@link android.content.Context#fileList()}</dt>
<dd>Returns an array of files currently saved by your application.</dd>
</dl>
<h2 id="filesExternal">Using the External Storage</h2>
<p>Every Android-compatible device supports a shared "external storage" that you can use to
save files. This can be a removable storage media (such as an SD card) or an internal
(non-removable) storage. Files saved to the external storage are world-readable and can
be modified by the user when they enable USB mass storage to transfer files on a computer.</p>
<p class="caution"><strong>Caution:</strong> External files can disappear if the user mounts the
external storage on a computer or removes the media, and there's no security enforced upon files you
save to the external storage. All applications can read and write files placed on the external
storage and the user can remove them.</p>
<h3>Checking media availability</h3>
<p>Before you do any work with the external storage, you should always call {@link
android.os.Environment#getExternalStorageState()} to check whether the media is available. The
media might be mounted to a computer, missing, read-only, or in some other state. For example,
here's how you can check the availability:</p>
<pre>
boolean mExternalStorageAvailable = false;
boolean mExternalStorageWriteable = false;
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
// We can read and write the media
mExternalStorageAvailable = mExternalStorageWriteable = true;
} else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
// We can only read the media
mExternalStorageAvailable = true;
mExternalStorageWriteable = false;
} else {
// Something else is wrong. It may be one of many other states, but all we need
// to know is we can neither read nor write
mExternalStorageAvailable = mExternalStorageWriteable = false;
}
</pre>
<p>This example checks whether the external storage is available to read and write. The
{@link android.os.Environment#getExternalStorageState()} method returns other states that you
might want to check, such as whether the media is being shared (connected to a computer), is missing
entirely, has been removed badly, etc. You can use these to notify the user with more information
when your application needs to access the media.</p>
<h3>Accessing files on external storage</h3>
<p>If you're using API Level 8 or greater, use {@link
android.content.Context#getExternalFilesDir(String) getExternalFilesDir()} to open a {@link
java.io.File} that represents the external storage directory where you should save your
files. This method takes a <code>type</code> parameter that specifies the type of subdirectory you
want, such as {@link android.os.Environment#DIRECTORY_MUSIC} and
{@link android.os.Environment#DIRECTORY_RINGTONES} (pass <code>null</code> to receive
the root of your application's file directory). This method will create the
appropriate directory if necessary. By specifying the type of directory, you
ensure that the Android's media scanner will properly categorize your files in the system (for
example, ringtones are identified as ringtones and not music). If the user uninstalls your
application, this directory and all its contents will be deleted.</p>
<p>If you're using API Level 7 or lower, use {@link
android.os.Environment#getExternalStorageDirectory()}, to open a {@link
java.io.File} representing the root of the external storage. You should then write your data in the
following directory:</p>
<pre class="no-pretty-print classic">
/Android/data/<em>&lt;package_name&gt;</em>/files/
</pre>
<p>The {@code <em>&lt;package_name&gt;</em>} is your Java-style package name, such as "{@code
com.example.android.app}". If the user's device is running API Level 8 or greater and they
uninstall your application, this directory and all its contents will be deleted.</p>
<div class="sidebox-wrapper" style="margin-top:3em">
<div class="sidebox">
<h4>Hiding your files from the Media Scanner</h4>
<p>Include an empty file named {@code .nomedia} in your external files directory (note the dot
prefix in the filename). This will prevent Android's media scanner from reading your media
files and including them in apps like Gallery or Music.</p>
</div>
</div>
<h3>Saving files that should be shared</h3>
<p>If you want to save files that are not specific to your application and that should <em>not</em>
be deleted when your application is uninstalled, save them to one of the public directories on the
external storage. These directories lay at the root of the external storage, such as {@code
Music/}, {@code Pictures/}, {@code Ringtones/}, and others.</p>
<p>In API Level 8 or greater, use {@link
android.os.Environment#getExternalStoragePublicDirectory(String)
getExternalStoragePublicDirectory()}, passing it the type of public directory you want, such as
{@link android.os.Environment#DIRECTORY_MUSIC}, {@link android.os.Environment#DIRECTORY_PICTURES},
{@link android.os.Environment#DIRECTORY_RINGTONES}, or others. This method will create the
appropriate directory if necessary.</p>
<p>If you're using API Level 7 or lower, use {@link
android.os.Environment#getExternalStorageDirectory()} to open a {@link java.io.File} that represents
the root of the external storage, then save your shared files in one of the following
directories:</p>
<ul class="nolist"></li>
<li><code>Music/</code> - Media scanner classifies all media found here as user music.</li>
<li><code>Podcasts/</code> - Media scanner classifies all media found here as a podcast.</li>
<li><code>Ringtones/ </code> - Media scanner classifies all media found here as a ringtone.</li>
<li><code>Alarms/</code> - Media scanner classifies all media found here as an alarm sound.</li>
<li><code>Notifications/</code> - Media scanner classifies all media found here as a notification
sound.</li>
<li><code>Pictures/</code> - All photos (excluding those taken with the camera).</li>
<li><code>Movies/</code> - All movies (excluding those taken with the camcorder).</li>
<li><code>Download/</code> - Miscellaneous downloads.</li>
</ul>
<h3 id="ExternalCache">Saving cache files</h3>
<p>If you're using API Level 8 or greater, use {@link
android.content.Context#getExternalCacheDir()} to open a {@link java.io.File} that represents the
external storage directory where you should save cache files. If the user uninstalls your
application, these files will be automatically deleted. However, during the life of your
application, you should manage these cache files and remove those that aren't needed in order to
preserve file space.</p>
<p>If you're using API Level 7 or lower, use {@link
android.os.Environment#getExternalStorageDirectory()} to open a {@link java.io.File} that represents
the root of the external storage, then write your cache data in the following directory:</p>
<pre class="no-pretty-print classic">
/Android/data/<em>&lt;package_name&gt;</em>/cache/
</pre>
<p>The {@code <em>&lt;package_name&gt;</em>} is your Java-style package name, such as "{@code
com.example.android.app}".</p>
<h2 id="db">Using Databases</h2>
<p>Android provides full support for <a href="http://www.sqlite.org/">SQLite</a> databases.
Any databases you create will be accessible by name to any
class in the application, but not outside the application.</p>
<p>The recommended method to create a new SQLite database is to create a subclass of {@link
android.database.sqlite.SQLiteOpenHelper} and override the {@link
android.database.sqlite.SQLiteOpenHelper#onCreate(SQLiteDatabase) onCreate()} method, in which you
can execute a SQLite command to create tables in the database. For example:</p>
<pre>
public class DictionaryOpenHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION = 2;
private static final String DICTIONARY_TABLE_NAME = "dictionary";
private static final String DICTIONARY_TABLE_CREATE =
"CREATE TABLE " + DICTIONARY_TABLE_NAME + " (" +
KEY_WORD + " TEXT, " +
KEY_DEFINITION + " TEXT);";
DictionaryOpenHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
&#64;Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DICTIONARY_TABLE_CREATE);
}
}
</pre>
<p>You can then get an instance of your {@link android.database.sqlite.SQLiteOpenHelper}
implementation using the constructor you've defined. To write to and read from the database, call
{@link android.database.sqlite.SQLiteOpenHelper#getWritableDatabase()} and {@link
android.database.sqlite.SQLiteOpenHelper#getReadableDatabase()}, respectively. These both return a
{@link android.database.sqlite.SQLiteDatabase} object that represents the database and
provides methods for SQLite operations.</p>
<div class="sidebox-wrapper">
<div class="sidebox">
<p>Android does not impose any limitations beyond the standard SQLite concepts. We do recommend
including an autoincrement value key field that can be used as a unique ID to
quickly find a record. This is not required for private data, but if you
implement a <a href="{@docRoot}guide/topics/providers/content-providers.html">content provider</a>,
you must include a unique ID using the {@link android.provider.BaseColumns#_ID BaseColumns._ID}
constant.
</p>
</div>
</div>
<p>You can execute SQLite queries using the {@link android.database.sqlite.SQLiteDatabase}
{@link
android.database.sqlite.SQLiteDatabase#query(boolean,String,String[],String,String[],String,String,String,String)
query()} methods, which accept various query parameters, such as the table to query,
the projection, selection, columns, grouping, and others. For complex queries, such as
those that require column aliases, you should use
{@link android.database.sqlite.SQLiteQueryBuilder}, which provides
several convienent methods for building queries.</p>
<p>Every SQLite query will return a {@link android.database.Cursor} that points to all the rows
found by the query. The {@link android.database.Cursor} is always the mechanism with which
you can navigate results from a database query and read rows and columns.</p>
<p>For sample apps that demonstrate how to use SQLite databases in Android, see the
<a href="{@docRoot}resources/samples/NotePad/index.html">Note Pad</a> and
<a href="{@docRoot}resources/samples/SearchableDictionary/index.html">Searchable Dictionary</a>
applications.</p>
<h3 id="dbDebugging">Database debugging</h3>
<p>The Android SDK includes a {@code sqlite3} database tool that allows you to browse
table contents, run SQL commands, and perform other useful functions on SQLite
databases. See <a href="{@docRoot}guide/developing/tools/adb.html#sqlite">Examining sqlite3
databases from a remote shell</a> to learn how to run this tool.
</p>
<h2 id="netw">Using a Network Connection</h2>
<!-- TODO MAKE THIS USEFUL!! -->
<p>You can use the network (when it's available) to store and retrieve data on your own web-based
services. To do network operations, use classes in the following packages:</p>
<ul class="no-style">
<li><code>{@link java.net java.net.*}</code></li>
<li><code>{@link android.net android.net.*}</code></li>
</ul>

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