IBinder  |  API reference  |  Android Developers (original) (raw)

interface IBinder

Known Direct Subclasses

Base interface for a remotable object, the core part of a lightweight remote procedure call mechanism designed for high performance when performing in-process and cross-process calls. This interface describes the abstract protocol for interacting with a remotable object. Do not implement this interface directly, instead extend from [Binder](/reference/kotlin/android/os/Binder).

The key IBinder API is [transact()](#transact%28kotlin.Int,%20android.os.Parcel,%20android.os.Parcel,%20kotlin.Int%29) matched by [Binder.onTransact()](/reference/kotlin/android/os/Binder#onTransact%28kotlin.Int,%20android.os.Parcel,%20android.os.Parcel,%20kotlin.Int%29). These methods allow you to send a call to an IBinder object and receive a call coming in to a Binder object, respectively. This transaction API is synchronous, such that a call to [transact()](#transact%28kotlin.Int,%20android.os.Parcel,%20android.os.Parcel,%20kotlin.Int%29) does not return until the target has returned from [Binder.onTransact()](/reference/kotlin/android/os/Binder#onTransact%28kotlin.Int,%20android.os.Parcel,%20android.os.Parcel,%20kotlin.Int%29); this is the expected behavior when calling an object that exists in the local process, and the underlying inter-process communication (IPC) mechanism ensures that these same semantics apply when going across processes.

The data sent through transact() is a [Parcel](/reference/kotlin/android/os/Parcel), a generic buffer of data that also maintains some meta-data about its contents. The meta data is used to manage IBinder object references in the buffer, so that those references can be maintained as the buffer moves across processes. This mechanism ensures that when an IBinder is written into a Parcel and sent to another process, if that other process sends a reference to that same IBinder back to the original process, then the original process will receive the same IBinder object back. These semantics allow IBinder/Binder objects to be used as a unique identity (to serve as a token or for other purposes) that can be managed across processes.

The system maintains a pool of transaction threads in each process that it runs in. These threads are used to dispatch all IPCs coming in from other processes. For example, when an IPC is made from process A to process B, the calling thread in A blocks in transact() as it sends the transaction to process B. The next available pool thread in B receives the incoming transaction, calls Binder.onTransact() on the target object, and replies with the result Parcel. Upon receiving its result, the thread in process A returns to allow its execution to continue. In effect, other processes appear to use as additional threads that you did not create executing in your own process.

The Binder system also supports recursion across processes. For example if process A performs a transaction to process B, and process B while handling that transaction calls transact() on an IBinder that is implemented in A, then the thread in A that is currently waiting for the original transaction to finish will take care of calling Binder.onTransact() on the object being called by B. This ensures that the recursion semantics when calling remote binder object are the same as when calling local objects.

When working with remote objects, you often want to find out when they are no longer valid. There are three ways this can be determined:

• The [transact()](#transact%28kotlin.Int,%20android.os.Parcel,%20android.os.Parcel,%20kotlin.Int%29) method will throw a [RemoteException](/reference/kotlin/android/os/RemoteException) exception if you try to call it on an IBinder whose process no longer exists.
• The [pingBinder()](#pingBinder%28%29) method can be called, and will return false if the remote process no longer exists.
• The [linkToDeath()](#linkToDeath%28android.os.IBinder.DeathRecipient,%20kotlin.Int%29) method can be used to register a [DeathRecipient](/reference/kotlin/android/os/IBinder.DeathRecipient) with the IBinder, which will be called when its containing process goes away.

Summary

Constants

DUMP_TRANSACTION

static val DUMP_TRANSACTION: Int

IBinder protocol transaction code: dump internal state.

Value: 1598311760

FIRST_CALL_TRANSACTION

static val FIRST_CALL_TRANSACTION: Int

The first transaction code available for user commands.

Value: 1

FLAG_ONEWAY

static val FLAG_ONEWAY: Int

Flag to [transact](#transact%28kotlin.Int,%20android.os.Parcel,%20android.os.Parcel,%20kotlin.Int%29): this is a one-way call, meaning that the caller returns immediately, without waiting for a result from the callee. Applies only if the caller and callee are in different processes.

The system provides special ordering semantics for multiple oneway calls being made to the same IBinder object: these calls will be dispatched in the other process one at a time, with the same order as the original calls. These are still dispatched by the IPC thread pool, so may execute on different threads, but the next one will not be dispatched until the previous one completes. This ordering is not guaranteed for calls on different IBinder objects or when mixing oneway and non-oneway calls on the same IBinder object.

Value: 1

INTERFACE_TRANSACTION

static val INTERFACE_TRANSACTION: Int

IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor.

Value: 1598968902

LAST_CALL_TRANSACTION

static val LAST_CALL_TRANSACTION: Int

The last transaction code available for user commands.

Value: 16777215

LIKE_TRANSACTION

static val LIKE_TRANSACTION: Int

IBinder protocol transaction code: tell an app asynchronously that the caller likes it. The app is responsible for incrementing and maintaining its own like counter, and may display this value to the user to indicate the quality of the app. This is an optional command that applications do not need to handle, so the default implementation is to do nothing.

There is no response returned and nothing about the system will be functionally affected by it, but it will improve the app's self-esteem.

Value: 1598835019

PING_TRANSACTION

static val PING_TRANSACTION: Int

IBinder protocol transaction code: pingBinder().

Value: 1599098439

TWEET_TRANSACTION

static val TWEET_TRANSACTION: Int

IBinder protocol transaction code: send a tweet to the target object. The data in the parcel is intended to be delivered to a shared messaging service associated with the object; it can be anything, as long as it is not more than 130 UTF-8 characters to conservatively fit within common messaging services. As part of [Build.VERSION_CODES.HONEYCOMB_MR2](/reference/kotlin/android/os/Build.VERSION%5FCODES#HONEYCOMB%5FMR2:kotlin.Int), all Binder objects are expected to support this protocol for fully integrated tweeting across the platform. To support older code, the default implementation logs the tweet to the main log as a simple emulation of broadcasting it publicly over the Internet.

Also, upon completing the dispatch, the object must make a cup of tea, return it to the caller, and exclaim "jolly good message old boy!".

Value: 1599362900

Public methods

addFrozenStateChangeCallback

open fun addFrozenStateChangeCallback(
    executor: Executor,
    callback: IBinder.FrozenStateChangeCallback
): Unit

This method provides a callback mechanism to notify about process frozen/unfrozen events. Upon registration and any subsequent state changes, the callback is invoked with the latest process frozen state.

If the listener process (the one using this API) is itself frozen, state change events might be combined into a single one with the latest frozen state. This single event would then be delivered when the listener process becomes unfrozen. Similarly, if an event happens before the previous event is consumed, they might be combined. This means the callback might not be called for every single state change, so don't rely on this API to count how many times the state has changed.

The callback is automatically removed when all references to the binder proxy are dropped.

You will only receive state change notifications for remote binders, as local binders by definition can't be frozen without you being frozen too.

dump

abstract fun dump(
    fd: FileDescriptor,
    args: Array<String!>?
): Unit

Print the object's state into the given stream.

dumpAsync

abstract fun dumpAsync(
    fd: FileDescriptor,
    args: Array<String!>?
): Unit

Like [dump(java.io.FileDescriptor,java.lang.String[])](#dump%28java.io.FileDescriptor,%20kotlin.Array%29) but always executes asynchronously. If the object is local, a new thread is created to perform the dump.

getInterfaceDescriptor

abstract fun getInterfaceDescriptor(): String?

Get the canonical name of the interface supported by this binder.

getSuggestedMaxIpcSizeBytes

open static fun getSuggestedMaxIpcSizeBytes(): Int

Limit that should be placed on IPC sizes, in bytes, to keep them safely under the transaction buffer limit.

isBinderAlive

abstract fun isBinderAlive(): Boolean

Check to see if the process that the binder is in is still alive.

linkToDeath

abstract fun linkToDeath(
    recipient: IBinder.DeathRecipient,
    flags: Int
): Unit

Register the recipient for a notification if this binder goes away. If this binder object unexpectedly goes away (typically because its hosting process has been killed), then the given [DeathRecipient](/reference/kotlin/android/os/IBinder.DeathRecipient)'s android.os.IBinder.DeathRecipient#binderDied method will be called.

This will automatically be unlinked when all references to the linked binder proxy are dropped.

You will only receive death notifications for remote binders, as local binders by definition can't die without you dying as well.

pingBinder

abstract fun pingBinder(): Boolean

Check to see if the object still exists.

queryLocalInterface

abstract fun queryLocalInterface(descriptor: String): IInterface?

Attempt to retrieve a local implementation of an interface for this Binder object. If null is returned, you will need to instantiate a proxy class to marshall calls through the transact() method.

transact

abstract fun transact(
    code: Int,
    data: Parcel,
    reply: Parcel?,
    flags: Int
): Boolean

Perform a generic operation with the object.

unlinkToDeath

abstract fun unlinkToDeath(
    recipient: IBinder.DeathRecipient,
    flags: Int
): Boolean

Remove a previously registered death notification. The recipient will no longer be called if this object dies.