page.title=Application Fundamentals @jd:body

In this document

  1. Application Components
    1. Activating components: intents
    2. Shutting down components
    3. The manifest file
    4. Intent filters
  2. Activities and Tasks
    1. Affinities and new tasks
    2. Launch modes
    3. Clearing the stack
    4. Starting tasks
  3. Processes and Threads
    1. Processes
    2. Threads
    3. Remote procedure calls
    4. Thread-safe methods
  4. Component Lifecycles
    1. Activity lifecycle
    2. Service lifecycle
    3. Broadcast receiver lifecycle
    4. Processes and lifecycles

Key classes

  1. {@link android.app.Activity}
  2. {@link android.app.Service}
  3. {@link android.content.BroadcastReceiver}
  4. {@link android.content.ContentProvider}
  5. {@link android.content.Intent}

Android applications are written in the Java programming language. The compiled Java code — along with any data and resource files required by the application — is bundled by the aapt tool into an Android package, an archive file marked by an {@code .apk} suffix. This file is the vehicle for distributing the application and installing it on mobile devices; it's the file users download to their devices. All the code in a single {@code .apk} file is considered to be one application.

In many ways, each Android application lives in its own world:

It's possible to arrange for two applications to share the same user ID, in which case they will be able to see each other's files. To conserve system resources, applications with the same ID can also arrange to run in the same Linux process, sharing the same VM.

Application Components

A central feature of Android is that one application can make use of elements of other applications (provided those applications permit it). For example, if your application needs to display a scrolling list of images and another application has developed a suitable scroller and made it available to others, you can call upon that scroller to do the work, rather than develop your own. Your application doesn't incorporate the code of the other application or link to it. Rather, it simply starts up that piece of the other application when the need arises.

For this to work, the system must be able to start an application process when any part of it is needed, and instantiate the Java objects for that part. Therefore, unlike applications on most other systems, Android applications don't have a single entry point for everything in the application (no {@code main()} function, for example). Rather, they have essential components that the system can instantiate and run as needed. There are four types of components:

Activities
An activity presents a visual user interface for one focused endeavor the user can undertake. For example, an activity might present a list of menu items users can choose from or it might display photographs along with their captions. A text messaging application might have one activity that shows a list of contacts to send messages to, a second activity to write the message to the chosen contact, and other activities to review old messages or change settings. Though they work together to form a cohesive user interface, each activity is independent of the others. Each one is implemented as a subclass of the {@link android.app.Activity} base class.

An application might consist of just one activity or, like the text messaging application just mentioned, it may contain several. What the activities are, and how many there are depends, of course, on the application and its design. Typically, one of the activities is marked as the first one that should be presented to the user when the application is launched. Moving from one activity to another is accomplished by having the current activity start the next one.

Each activity is given a default window to draw in. Typically, the window fills the screen, but it might be smaller than the screen and float on top of other windows. An activity can also make use of additional windows — for example, a pop-up dialog that calls for a user response in the midst of the activity, or a window that presents users with vital information when they select a particular item on-screen.

The visual content of the window is provided by a hierarchy of views — objects derived from the base {@link android.view.View} class. Each view controls a particular rectangular space within the window. Parent views contain and organize the layout of their children. Leaf views (those at the bottom of the hierarchy) draw in the rectangles they control and respond to user actions directed at that space. Thus, views are where the activity's interaction with the user takes place. For example, a view might display a small image and initiate an action when the user taps that image. Android has a number of ready-made views that you can use — including buttons, text fields, scroll bars, menu items, check boxes, and more.

A view hierarchy is placed within an activity's window by the {@link android.app.Activity#setContentView Activity.setContentView()} method. The content view is the View object at the root of the hierarchy. (See the separate User Interface document for more information on views and the hierarchy.)

Services
A service doesn't have a visual user interface, but rather runs in the background for an indefinite period of time. For example, a service might play background music as the user attends to other matters, or it might fetch data over the network or calculate something and provide the result to activities that need it. Each service extends the {@link android.app.Service} base class.

A prime example is a media player playing songs from a play list. The player application would probably have one or more activities that allow the user to choose songs and start playing them. However, the music playback itself would not be handled by an activity because users will expect the music to keep playing even after they leave the player and begin something different. To keep the music going, the media player activity could start a service to run in the background. The system would then keep the music playback service running even after the activity that started it leaves the screen.

It's possible to connect to (bind to) an ongoing service (and start the service if it's not already running). While connected, you can communicate with the service through an interface that the service exposes. For the music service, this interface might allow users to pause, rewind, stop, and restart the playback.

Like activities and the other components, services run in the main thread of the application process. So that they won't block other components or the user interface, they often spawn another thread for time-consuming tasks (like music playback). See Processes and Threads, later.

Broadcast receivers
A broadcast receiver is a component that does nothing but receive and react to broadcast announcements. Many broadcasts originate in system code — for example, announcements that the timezone has changed, that the battery is low, that a picture has been taken, or that the user changed a language preference. Applications can also initiate broadcasts — for example, to let other applications know that some data has been downloaded to the device and is available for them to use.

An application can have any number of broadcast receivers to respond to any announcements it considers important. All receivers extend the {@link android.content.BroadcastReceiver} base class.

Broadcast receivers do not display a user interface. However, they may start an activity in response to the information they receive, or they may use the {@link android.app.NotificationManager} to alert the user. Notifications can get the user's attention in various ways — flashing the backlight, vibrating the device, playing a sound, and so on. They typically place a persistent icon in the status bar, which users can open to get the message.

Content providers
A content provider makes a specific set of the application's data available to other applications. The data can be stored in the file system, in an SQLite database, or in any other manner that makes sense. The content provider extends the {@link android.content.ContentProvider} base class to implement a standard set of methods that enable other applications to retrieve and store data of the type it controls. However, applications do not call these methods directly. Rather they use a {@link android.content.ContentResolver} object and call its methods instead. A ContentResolver can talk to any content provider; it cooperates with the provider to manage any interprocess communication that's involved.

See the separate Content Providers document for more information on using content providers.

Whenever there's a request that should be handled by a particular component, Android makes sure that the application process of the component is running, starting it if necessary, and that an appropriate instance of the component is available, creating the instance if necessary.

Activating components: intents

Content providers are activated when they're targeted by a request from a ContentResolver. The other three components — activities, services, and broadcast receivers — are activated by asynchronous messages called intents. An intent is an {@link android.content.Intent} object that holds the content of the message. For activities and services, it names the action being requested and specifies the URI of the data to act on, among other things. For example, it might convey a request for an activity to present an image to the user or let the user edit some text. For broadcast receivers, the Intent object names the action being announced. For example, it might announce to interested parties that the camera button has been pressed.

There are separate methods for activating each type of component:

For more on intent messages, see the separate article, Intents and Intent Filters.

Shutting down components

A content provider is active only while it's responding to a request from a ContentResolver. And a broadcast receiver is active only while it's responding to a broadcast message. So there's no need to explicitly shut down these components.

Activities, on the other hand, provide the user interface. They're in a long-running conversation with the user and may remain active, even when idle, as long as the conversation continues. Similarly, services may also remain running for a long time. So Android has methods to shut down activities and services in an orderly way:

Components might also be shut down by the system when they are no longer being used or when Android must reclaim memory for more active components. A later section, Component Lifecycles, discusses this possibility and its ramifications in more detail.

The manifest file

Before Android can start an application component, it must learn that the component exists. Therefore, applications declare their components in a manifest file that's bundled into the Android package, the {@code .apk} file that also holds the application's code, files, and resources.

The manifest is a structured XML file and is always named AndroidManifest.xml for all applications. It does a number of things in addition to declaring the application's components, such as naming any libraries the application needs to be linked against (besides the default Android library) and identifying any permissions the application expects to be granted.

But the principal task of the manifest is to inform Android about the application's components. For example, an activity might be declared as follows: 

<?xml version="1.0" encoding="utf-8"?>
<manifest . . . >
    <application . . . >
        <activity android:name="com.example.project.FreneticActivity"
                  android:icon="@drawable/small_pic.png"
                  android:label="@string/freneticLabel" 
                  . . .  >
        </activity>
        . . .
    </application>
</manifest>

The {@code name} attribute of the <activity> element names the {@link android.app.Activity} subclass that implements the activity. The {@code icon} and {@code label} attributes point to resource files containing an icon and label that can be displayed to users to represent the activity.

The other components are declared in a similar way — <service> elements for services, <receiver> elements for broadcast receivers, and <provider> elements for content providers. Activities, services, and content providers that are not declared in the manifest are not visible to the system and are consequently never run. However, broadcast receivers can either be declared in the manifest, or they can be created dynamically in code (as {@link android.content.BroadcastReceiver} objects) and registered with the system by calling {@link android.content.Context#registerReceiver Context.registerReceiver()}.

For more on how to structure a manifest file for your application, see The AndroidManifest.xml File.

Intent filters

An Intent object can explicitly name a target component. If it does, Android finds that component (based on the declarations in the manifest file) and activates it. But if a target is not explicitly named, Android must locate the best component to respond to the intent. It does so by comparing the Intent object to the intent filters of potential targets. A component's intent filters inform Android of the kinds of intents the component is able to handle. Like other essential information about the component, they're declared in the manifest file. Here's an extension of the previous example that adds two intent filters to the activity: 

<?xml version="1.0" encoding="utf-8"?>
<manifest . . . >
    <application . . . >
        <activity android:name="com.example.project.FreneticActivity"
                  android:icon="@drawable/small_pic.png"
                  android:label="@string/freneticLabel" 
                  . . .  >
            <intent-filter . . . >
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
            <intent-filter . . . >
                <action android:name="com.example.project.BOUNCE" />
                <data android:mimeType="image/jpeg" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
        . . .
    </application>
</manifest>

The first filter in the example — the combination of the action "{@code android.intent.action.MAIN}" and the category "{@code android.intent.category.LAUNCHER}" — is a common one. It marks the activity as one that should be represented in the application launcher, the screen listing applications users can launch on the device. In other words, the activity is the entry point for the application, the initial one users would see when they choose the application in the launcher.

The second filter declares an action that the activity can perform on a particular type of data.

A component can have any number of intent filters, each one declaring a different set of capabilities. If it doesn't have any filters, it can be activated only by intents that explicitly name the component as the target.

For a broadcast receiver that's created and registered in code, the intent filter is instantiated directly as an {@link android.content.IntentFilter} object. All other filters are set up in the manifest.

For more on intent filters, see a separate document, Intents and Intent Filters.

Activities and Tasks

As noted earlier, one activity can start another, including one defined in a different application. Suppose, for example, that you'd like to let users display a street map of some location. There's already an activity that can do that, so all your activity needs to do is put together an Intent object with the required information and pass it to {@code startActivity()}. The map viewer will display the map. When the user hits the BACK key, your activity will reappear on screen.

To the user, it will seem as if the map viewer is part of the same application as your activity, even though it's defined in another application and runs in that application's process. Android maintains this user experience by keeping both activities in the same task. Simply put, a task is what the user experiences as an "application." It's a group of related activities, arranged in a stack. The root activity in the stack is the one that began the task — typically, it's an activity the user selected in the application launcher. The activity at the top of the stack is one that's currently running — the one that is the focus for user actions. When one activity starts another, the new activity is pushed on the stack; it becomes the running activity. The previous activity remains in the stack. When the user presses the BACK key, the current activity is popped from the stack, and the previous one resumes as the running activity.

The stack contains objects, so if a task has more than one instance of the same Activity subclass open — multiple map viewers, for example — the stack has a separate entry for each instance. Activities in the stack are never rearranged, only pushed and popped.

A task is a stack of activities, not a class or an element in the manifest file. So there's no way to set values for a task independently of its activities. Values for the task as a whole are set in the root activity. For example, the next section will talk about the "affinity of a task"; that value is read from the affinity set for the task's root activity.

All the activities in a task move together as a unit. The entire task (the entire activity stack) can be brought to the foreground or sent to the background. Suppose, for instance, that the current task has four activities in its stack — three under the current activity. The user presses the HOME key, goes to the application launcher, and selects a new application (actually, a new task). The current task goes into the background and the root activity for the new task is displayed. Then, after a short period, the user goes back to the home screen and again selects the previous application (the previous task). That task, with all four activities in the stack, comes forward. When the user presses the BACK key, the screen does not display the activity the user just left (the root activity of the previous task). Rather, the activity on the top of the stack is removed and the previous activity in the same task is displayed.

The behavior just described is the default behavior for activities and tasks. But there are ways to modify almost all aspects of it. The association of activities with tasks, and the behavior of an activity within a task, is controlled by the interaction between flags set in the Intent object that started the activity and attributes set in the activity's <activity> element in the manifest. Both requester and respondent have a say in what happens.

In this regard, the principal Intent flags are:

{@code FLAG_ACTIVITY_NEW_TASK}
{@code FLAG_ACTIVITY_CLEAR_TOP}
{@code FLAG_ACTIVITY_RESET_TASK_IF_NEEDED}
{@code FLAG_ACTIVITY_SINGLE_TOP}

The principal {@code <activity>} attributes are:

{@code taskAffinity}
{@code launchMode}
{@code allowTaskReparenting}
{@code clearTaskOnLaunch}
{@code alwaysRetainTaskState}
{@code finishOnTaskLaunch}

The following sections describe what some of these flags and attributes do, how they interact, and what considerations should govern their use.

Affinities and new tasks

By default, all the activities in an application have an affinity for each other — that is, there's a preference for them all to belong to the same task. However, an individual affinity can be set for each activity with the {@code taskAffinity} attribute of the {@code <activity>} element. Activities defined in different applications can share an affinity, or activities defined in the same application can be assigned different affinities. The affinity comes into play in two circumstances: When the Intent object that launches an activity contains the {@code FLAG_ACTIVITY_NEW_TASK} flag, and when an activity has its {@code allowTaskReparenting} attribute set to "{@code true}".

The {@link android.content.Intent#FLAG_ACTIVITY_NEW_TASK} flag
As described earlier, a new activity is, by default, launched into the task of the activity that called {@code startActivity()}. It's pushed onto the same stack as the caller. However, if the Intent object passed to {@code startActivity()} contains the {@code FLAG_ACTIVITY_NEW_TASK} flag, the system looks for a different task to house the new activity. Often, as the name of the flag implies, it's a new task. However, it doesn't have to be. If there's already an existing task with the same affinity as the new activity, the activity is launched into that task. If not, it begins a new task.
The allowTaskReparenting attribute
If an activity has its {@code allowTaskReparenting} attribute set to "{@code true}", it can move from the task it starts in to the task it has an affinity for when that task comes to the fore. For example, suppose that an activity that reports weather conditions in selected cities is defined as part of a travel application. It has the same affinity as other activities in the same application (the default affinity) and it allows reparenting. One of your activities starts the weather reporter, so it initially belongs to the same task as your activity. However, when the travel application next comes forward, the weather reporter will be reassigned to and displayed with that task.

If an {@code .apk} file contains more than one "application" from the user's point of view, you will probably want to assign different affinities to the activities associated with each of them.

Launch modes

There are four different launch modes that can be assigned to an {@code <activity>} element's launchMode attribute:

"{@code standard}" (the default mode)
"{@code singleTop}"
"{@code singleTask}"
"{@code singleInstance}"

The modes differ from each other on these four points:

When an existing activity is asked to handle a new intent, the Intent object is passed to the activity in an {@link android.app.Activity#onNewIntent onNewIntent()} call. (The intent object that originally started the activity can be retrieved by calling {@link android.app.Activity#getIntent getIntent()}.)

Note that when a new instance of an Activity is created to handle a new intent, the user can always press the BACK key to return to the previous state (to the previous activity). But when an existing instance of an Activity handles a new intent, the user cannot press the BACK key to return to what that instance was doing before the new intent arrived.

For more on launch modes, see the description of the <activity> element.

Clearing the stack

If the user leaves a task for a long time, the system clears the task of all activities except the root activity. When the user returns to the task again, it's as the user left it, except that only the initial activity is present. The idea is that, after a time, users will likely have abandoned what they were doing before and are returning to the task to begin something new.

That's the default. There are some activity attributes that can be used to control this behavior and modify it:

The alwaysRetainTaskState attribute
If this attribute is set to "{@code true}" in the root activity of a task, the default behavior just described does not happen. The task retains all activities in its stack even after a long period.
The clearTaskOnLaunch attribute
If this attribute is set to "{@code true}" in the root activity of a task, the stack is cleared down to the root activity whenever the user leaves the task and returns to it. In other words, it's the polar opposite of {@code alwaysRetainTaskState}. The user always returns to the task in its initial state, even after a momentary absence.
The finishOnTaskLaunch attribute
This attribute is like {@code clearTaskOnLaunch}, but it operates on a single activity, not an entire task. And it can cause any activity to go away, including the root activity. When it's set to "{@code true}", the activity remains part of the task only for the current session. If the user leaves and then returns to the task, it no longer is present.

There's another way to force activities to be removed from the stack. If an Intent object includes the {@link android.content.Intent#FLAG_ACTIVITY_CLEAR_TOP FLAG_ACTIVITY_CLEAR_TOP} flag, and the target task already has an instance of the type of activity that should handle the intent in its stack, all activities above that instance are cleared away so that it stands at the top of the stack and can respond to the intent. If the launch mode of the designated activity is "{@code standard}", it too will be removed from the stack, and a new instance will be launched to handle the incoming intent. That's because a new instance is always created for a new intent when the launch mode is "{@code standard}".

{@code FLAG_ACTIVITY_CLEAR_TOP} is most often used in conjunction with {@code FLAG_ACTIVITY_NEW_TASK}. When used together, these flags are a way of locating an existing activity in another task and putting it in a position where it can respond to the intent.

Starting tasks

An activity is set up as the entry point for a task by giving it an intent filter with "{@code android.intent.action.MAIN}" as the specified action and "{@code android.intent.category.LAUNCHER}" as the specified category. (There's an example of this type of filter in the earlier Intent Filters section.) A filter of this kind causes an icon and label for the activity to be displayed in the application launcher, giving users a way both to launch the task and to return to it at any time after it has been launched.

This second ability is important: Users must be able to leave a task and then come back to it later. For this reason, the two launch modes that mark activities as always initiating a task, "{@code singleTask}" and "{@code singleInstance}", should be used only when the activity has a {@code MAIN} and {@code LAUNCHER} filter. Imagine, for example, what could happen if the filter is missing: An intent launches a "{@code singleTask}" activity, initiating a new task, and the user spends some time working in that task. The user then presses the HOME key. The task is now ordered behind and obscured by the home screen. And, because it is not represented in the application launcher, the user has no way to return to it.

A similar difficulty attends the {@code FLAG_ACTIVITY_NEW_TASK} flag. If this flag causes an activity to begin a new task and the user presses the HOME key to leave it, there must be some way for the user to navigate back to it again. Some entities (such as the notification manager) always start activities in an external task, never as part of their own, so they always put {@code FLAG_ACTIVITY_NEW_TASK} in the intents they pass to {@code startActivity()}. If you have an activity that can be invoked by an external entity that might use this flag, take care that the user has a independent way to get back to the task that's started.

For those cases where you don't want the user to be able to return to an activity, set the {@code <activity>} element's {@code finishOnTaskLaunch} to "{@code true}". See Clearing the stack, earlier.

Processes and Threads

When the first of an application's components needs to be run, Android starts a Linux process for it with a single thread of execution. By default, all components of the application run in that process and thread.

However, you can arrange for components to run in other processes, and you can spawn additional threads for any process.

Processes

The process where a component runs is controlled by the manifest file. The component elements — {@code <activity>}, {@code <service>}, {@code <receiver>}, and {@code <provider>} — each have a {@code process} attribute that can specify a process where that component should run. These attributes can be set so that each component runs in its own process, or so that some components share a process while others do not. They can also be set so that components of different applications run in the same process — provided that the applications share the same Linux user ID and are signed by the same authorities. The {@code <application>} element also has a {@code process} attribute, for setting a default value that applies to all components.

All components are instantiated in the main thread of the specified process, and system calls to the component are dispatched from that thread. Separate threads are not created for each instance. Consequently, methods that respond to those calls — methods like {@link android.view.View#onKeyDown View.onKeyDown()} that report user actions and the lifecycle notifications discussed later in the Component Lifecycles section — always run in the main thread of the process. This means that no component should perform long or blocking operations (such as networking operations or computation loops) when called by the system, since this will block any other components also in the process. You can spawn separate threads for long operations, as discussed under Threads, next.

Android may decide to shut down a process at some point, when memory is low and required by other processes that are more immediately serving the user. Application components running in the process are consequently destroyed. A process is restarted for those components when there's again work for them to do.

When deciding which processes to terminate, Android weighs their relative importance to the user. For example, it more readily shuts down a process with activities that are no longer visible on screen than a process with visible activities. The decision whether to terminate a process, therefore, depends on the state of the components running in that process. Those states are the subject of a later section, Component Lifecycles.

Threads

Even though you may confine your application to a single process, there will likely be times when you will need to spawn a thread to do some background work. Since the user interface must always be quick to respond to user actions, the thread that hosts an activity should not also host time-consuming operations like network downloads. Anything that may not be completed quickly should be assigned to a different thread.

Threads are created in code using standard Java {@link java.lang.Thread} objects. Android provides a number of convenience classes for managing threads — {@link android.os.Looper} for running a message loop within a thread, {@link android.os.Handler} for processing messages, and {@link android.os.HandlerThread} for setting up a thread with a message loop.

Remote procedure calls

Android has a lightweight mechanism for remote procedure calls (RPCs) — where a method is called locally, but executed remotely (in another process), with any result returned back to the caller. This entails decomposing the method call and all its attendant data to a level the operating system can understand, transmitting it from the local process and address space to the remote process and address space, and reassembling and reenacting the call there. Return values have to be transmitted in the opposite direction. Android provides all the code to do that work, so that you can concentrate on defining and implementing the RPC interface itself.

An RPC interface can include only methods. By default, all methods are executed synchronously (the local method blocks until the remote method finishes), even if there is no return value.

In brief, the mechanism works as follows: You'd begin by declaring the RPC interface you want to implement using a simple IDL (interface definition language). From that declaration, the aidl tool generates a Java interface definition that must be made available to both the local and the remote process. It contains two inner class, as shown in the following diagram:

RPC mechanism.

The inner classes have all the code needed to administer remote procedure calls for the interface you declared with the IDL. Both inner classes implement the {@link android.os.IBinder} interface. One of them is used locally and internally by the system; the code you write can ignore it. The other, called Stub, extends the {@link android.os.Binder} class. In addition to internal code for effectuating the IPC calls, it contains declarations for the methods in the RPC interface you declared. You would subclass Stub to implement those methods, as indicated in the diagram.

Typically, the remote process would be managed by a service (because a service can inform the system about the process and its connections to other processes). It would have both the interface file generated by the {@code aidl} tool and the Stub subclass implementing the RPC methods. Clients of the service would have only the interface file generated by the {@code aidl} tool.

Here's how a connection between a service and its clients is set up:

This brief description omits some details of the RPC mechanism. For more information, see Designing a Remote Interface Using AIDL and the {@link android.os.IBinder IBinder} class description.

Thread-safe methods

In a few contexts, the methods you implement may be called from more than one thread, and therefore must be written to be thread-safe.

This is primarily true for methods that can be called remotely — as in the RPC mechanism discussed in the previous section. When a call on a method implemented in an IBinder object originates in the same process as the IBinder, the method is executed in the caller's thread. However, when the call originates in another process, the method is executed in a thread chosen from a pool of threads that Android maintains in the same process as the IBinder; it's not executed in the main thread of the process. For example, whereas a service's {@code onBind()} method would be called from the main thread of the service's process, methods implemented in the object that {@code onBind()} returns (for example, a Stub subclass that implements RPC methods) would be called from threads in the pool. Since services can have more than one client, more than one pool thread can engage the same IBinder method at the same time. IBinder methods must, therefore, be implemented to be thread-safe.

Similarly, a content provider can receive data requests that originate in other processes. Although the ContentResolver and ContentProvider classes hide the details of how the interprocess communication is managed, ContentProvider methods that respond to those requests — the methods {@link android.content.ContentProvider#query query()}, {@link android.content.ContentProvider#insert insert()}, {@link android.content.ContentProvider#delete delete()}, {@link android.content.ContentProvider#update update()}, and {@link android.content.ContentProvider#getType getType()} — are called from a pool of threads in the content provider's process, not the main thread of the process. Since these methods may be called from any number of threads at the same time, they too must be implemented to be thread-safe.

Component Lifecycles

Application components have a lifecycle — a beginning when Android instantiates them to respond to intents through to an end when the instances are destroyed. In between, they may sometimes be active or inactive,or, in the case of activities, visible to the user or invisible. This section discusses the lifecycles of activities, services, and broadcast receivers — including the states that they can be in during their lifetimes, the methods that notify you of transitions between states, and the effect of those states on the possibility that the process hosting them might be terminated and the instances destroyed.

Activity lifecycle

An activity has essentially three states:

If an activity is paused or stopped, the system can drop it from memory either by asking it to finish (calling its {@link android.app.Activity#finish finish()} method), or simply killing its process. When it is displayed again to the user, it must be completely restarted and restored to its previous state.

As an activity transitions from state to state, it is notified of the change by calls to the following protected methods:

{@code void onCreate(Bundle savedInstanceState)}
{@code void onStart()}
{@code void onRestart()}
{@code void onResume()}
{@code void onPause()}
{@code void onStop()}
{@code void onDestroy()}

All of these methods are hooks that you can override to do appropriate work when the state changes. All activities must implement {@link android.app.Activity#onCreate onCreate()} to do the initial setup when the object is first instantiated. Many will also implement {@link android.app.Activity#onPause onPause()} to commit data changes and otherwise prepare to stop interacting with the user.

Calling into the superclass

An implementation of any activity lifecycle method should always first call the superclass version. For example: 

protected void onPause() {
    super.onPause();
    . . .
}

Taken together, these seven methods define the entire lifecycle of an activity. There are three nested loops that you can monitor by implementing them:

The following diagram illustrates these loops and the paths an activity may take between states. The colored ovals are major states the activity can be in. The square rectangles represent the callback methods you can implement to perform operations when the activity transitions between states.

State diagram for an Android activity lifecycle.

The following table describes each of these methods in more detail and locates it within the activity's overall lifecycle:

Method Description Killable? Next
{@link android.app.Activity#onCreate onCreate()} Called when the activity is first created. This is where you should do all of your normal static set up — create views, bind data to lists, and so on. This method is passed a Bundle object containing the activity's previous state, if that state was captured (see Saving Activity State, later).

Always followed by {@code onStart()}.

 No {@code onStart()}
     {@link android.app.Activity#onRestart onRestart()} Called after the activity has been stopped, just prior to it being started again.

Always followed by {@code onStart()}

 No {@code onStart()}
{@link android.app.Activity#onStart onStart()} Called just before the activity becomes visible to the user.

Followed by {@code onResume()} if the activity comes to the foreground, or {@code onStop()} if it becomes hidden.

 No {@code onResume()}
or
{@code onStop()}
     {@link android.app.Activity#onResume onResume()} Called just before the activity starts interacting with the user. At this point the activity is at the top of the activity stack, with user input going to it.

Always followed by {@code onPause()}.

 No {@code onPause()}
{@link android.app.Activity#onPause onPause()} Called when the system is about to start resuming another activity. This method is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, and so on. It should do whatever it does very quickly, because the next activity will not be resumed until it returns.

Followed either by {@code onResume()} if the activity returns back to the front, or by {@code onStop()} if it becomes invisible to the user.

 Yes {@code onResume()}
or
{@code onStop()}
{@link android.app.Activity#onStop onStop()} Called when the activity is no longer visible to the user. This may happen because it is being destroyed, or because another activity (either an existing one or a new one) has been resumed and is covering it.

Followed either by {@code onRestart()} if the activity is coming back to interact with the user, or by {@code onDestroy()} if this activity is going away.

 Yes {@code onRestart()}
or
{@code onDestroy()}
{@link android.app.Activity#onDestroy onDestroy()} Called before the activity is destroyed. This is the final call that the activity will receive. It could be called either because the activity is finishing (someone called {@link android.app.Activity#finish finish()} on it), or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the {@link android.app.Activity#isFinishing isFinishing()} method. Yes nothing

Note the Killable column in the table above. It indicates whether or not the system can kill the process hosting the activity at any time after the method returns, without executing another line of the activity's code. Three methods ({@code onPause()}, {@code onStop()}, and {@code onDestroy()}) are marked "Yes." Because {@code onPause()} is the first of the three, it's the only one that's guaranteed to be called before the process is killed — {@code onStop()} and {@code onDestroy()} may not be. Therefore, you should use {@code onPause()} to write any persistent data (such as user edits) to storage.

Methods that are marked "No" in the Killable column protect the process hosting the activity from being killed from the moment they are called. Thus an activity is in a killable state, for example, from the time {@code onPause()} returns to the time {@code onResume()} is called. It will not again be killable until {@code onPause()} again returns.

As noted in a later section, Processes and lifecycle, an activity that's not technically "killable" by this definition might still be killed by the system — but that would happen only in extreme and dire circumstances when there is no other recourse.

Saving activity state

When the system, rather than the user, shuts down an activity to conserve memory, the user may expect to return to the activity and find it in its previous state.

To capture that state before the activity is killed, you can implement an {@link android.app.Activity#onSaveInstanceState onSaveInstanceState()} method for the activity. Android calls this method before making the activity vulnerable to being destroyed — that is, before {@code onPause()} is called. It passes the method a {@link android.os.Bundle} object where you can record the dynamic state of the activity as name-value pairs. When the activity is again started, the Bundle is passed both to {@code onCreate()} and to a method that's called after {@code onStart()}, {@link android.app.Activity#onRestoreInstanceState onRestoreInstanceState()}, so that either or both of them can recreate the captured state.

Unlike {@code onPause()} and the other methods discussed earlier, {@code onSaveInstanceState()} and {@code onRestoreInstanceState()} are not lifecycle methods. They are not always called. For example, Android calls {@code onSaveInstanceState()} before the activity becomes vulnerable to being destroyed by the system, but does not bother calling it when the instance is actually being destroyed by a user action (such as pressing the BACK key). In that case, the user won't expect to return to the activity, so there's no reason to save its state.

Because {@code onSaveInstanceState()} is not always called, you should use it only to record the transient state of the activity, not to store persistent data. Use {@code onPause()} for that purpose instead.

Coordinating activities

When one activity starts another, they both experience lifecycle transitions. One pauses and may stop, while the other starts up. On occasion, you may need to coordinate these activities, one with the other.

The order of lifecycle callbacks is well defined, particularly when the two activities are in the same process:

  1. The current activity's {@code onPause()} method is called.
  2. Next, the starting activity's {@code onCreate()}, {@code onStart()}, and {@code onResume()} methods are called in sequence.
  3. Then, if the starting activity is no longer visible on screen, its {@code onStop()} method is called.

Service lifecycle

A service can be used in two ways:

The two modes are not entirely separate. You can bind to a service that was started with {@code startService()}. For example, a background music service could be started by calling {@code startService()} with an Intent object that identifies the music to play. Only later, possibly when the user wants to exercise some control over the player or get information about the current song, would an activity establish a connection to the service by calling {@code bindService()}. In cases like this, {@code stopService()} will not actually stop the service until the last binding is closed.

Like an activity, a service has lifecycle methods that you can implement to monitor changes in its state. But they are fewer than the activity methods — only three — and they are public, not protected:

{@code void onCreate()}
{@code void onStart(Intent intent)}
{@code void onDestroy()}

By implementing these methods, you can monitor two nested loops of the service's lifecycle:

The {@code onCreate()} and {@code onDestroy()} methods are called for all services, whether they're started by {@link android.content.Context#startService Context.startService()} or {@link android.content.Context#bindService Context.bindService()}. However, {@code onStart()} is called only for services started by {@code startService()}.

If a service permits others to bind to it, there are additional callback methods for it to implement:

{@code IBinder onBind(Intent intent)}
{@code boolean onUnbind(Intent intent)}
{@code void onRebind(Intent intent)}

The {@link android.app.Service#onBind onBind()} callback is passed the Intent object that was passed to {@code bindService} and {@link android.app.Service#onUnbind onUnbind()} is handed the intent that was passed to {@code unbindService()}. If the service permits the binding, {@code onBind()} returns the communications channel that clients use to interact with the service. The {@code onUnbind()} method can ask for {@link android.app.Service#onRebind onRebind()} to be called if a new client connects to the service.

The following diagram illustrates the callback methods for a service. Although, it separates services that are created via {@code startService} from those created by {@code bindService()}, keep in mind that any service, no matter how it's started, can potentially allow clients to bind to it, so any service may receive {@code onBind()} and {@code onUnbind()} calls.

State diagram for Service callbacks.

Broadcast receiver lifecycle

A broadcast receiver has single callback method:

{@code void onReceive(Context curContext, Intent broadcastMsg)}

When a broadcast message arrives for the receiver, Android calls its {@link android.content.BroadcastReceiver#onReceive onReceive()} method and passes it the Intent object containing the message. The broadcast receiver is considered to be active only while it is executing this method. When {@code onReceive()} returns, it is inactive.

A process with an active broadcast receiver is protected from being killed. But a process with only inactive components can be killed by the system at any time, when the memory it consumes is needed by other processes.

This presents a problem when the response to a broadcast message is time consuming and, therefore, something that should be done in a separate thread, away from the main thread where other components of the user interface run. If {@code onReceive()} spawns the thread and then returns, the entire process, including the new thread, is judged to be inactive (unless other application components are active in the process), putting it in jeopardy of being killed. The solution to this problem is for {@code onReceive()} to start a service and let the service do the job, so the system knows that there is still active work being done in the process.

The next section has more on the vulnerability of processes to being killed.

Processes and lifecycles

The Android system tries to maintain an application process for as long as possible, but eventually it will need to remove old processes when memory runs low. To determine which processes to keep and which to kill, Android places each process into an "importance hierarchy" based on the components running in it and the state of those components. Processes with the lowest importance are eliminated first, then those with the next lowest, and so on. There are five levels in the hierarchy. The following list presents them in order of importance:

  1. A foreground process is one that is required for what the user is currently doing. A process is considered to be in the foreground if any of the following conditions hold:

    Only a few foreground processes will exist at any given time. They are killed only as a last resort — if memory is so low that they cannot all continue to run. Generally, at that point, the device has reached a memory paging state, so killing some foreground processes is required to keep the user interface responsive.

  2. A visible process is one that doesn't have any foreground components, but still can affect what the user sees on screen. A process is considered to be visible if either of the following conditions holds:

    A visible process is considered extremely important and will not be killed unless doing so is required to keep all foreground processes running.

  3. A service process is one that is running a service that has been started with the {@link android.content.Context#startService startService()} method and that does not fall into either of the two higher categories. Although service processes are not directly tied to anything the user sees, they are generally doing things that the user cares about (such as playing an mp3 in the background or downloading data on the network), so the system keeps them running unless there's not enough memory to retain them along with all foreground and visible processes.

  4. A background process is one holding an activity that's not currently visible to the user (the Activity object's {@link android.app.Activity#onStop onStop()} method has been called). These processes have no direct impact on the user experience, and can be killed at any time to reclaim memory for a foreground, visible, or service process. Usually there are many background processes running, so they are kept in an LRU (least recently used) list to ensure that the process with the activity that was most recently seen by the user is the last to be killed. If an activity implements its lifecycle methods correctly, and captures its current state, killing its process will not have a deleterious effect on the user experience.

  5. An empty process is one that doesn't hold any active application components. The only reason to keep such a process around is as a cache to improve startup time the next time a component needs to run in it. The system often kills these processes in order to balance overall system resources between process caches and the underlying kernel caches.

Android ranks a process at the highest level it can, based upon the importance of the components currently active in the process. For example, if a process hosts a service and a visible activity, the process will be ranked as a visible process, not a service process.

In addition, a process's ranking may be increased because other processes are dependent on it. A process that is serving another process can never be ranked lower than the process it is serving. For example, if a content provider in process A is serving a client in process B, or if a service in process A is bound to a component in process B, process A will always be considered at least as important as process B.

Because a process running a service is ranked higher than one with background activities, an activity that initiates a long-running operation might do well to start a service for that operation, rather than simply spawn a thread — particularly if the operation will likely outlast the activity. Examples of this are playing music in the background and uploading a picture taken by the camera to a web site. Using a service guarantees that the operation will have at least "service process" priority, regardless of what happens to the activity. As noted in the Broadcast receiver lifecycle section earlier, this is the same reason that broadcast receivers should employ services rather than simply put time-consuming operations in a thread.