Added in API level 1

IBinder

interface IBinder
android.os.IBinder

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.

The key IBinder API is transact() matched by Binder.onTransact(). 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() does not return until the target has returned from Binder.onTransact(); 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, 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() method will throw a RemoteException exception if you try to call it on an IBinder whose process no longer exists.
  • The pingBinder() method can be called, and will return false if the remote process no longer exists.
  • The linkToDeath() method can be used to register a DeathRecipient with the IBinder, which will be called when its containing process goes away.

Summary

Nested classes
abstract

Interface for receiving a callback when the process hosting an IBinder has gone away.

Constants
static Int

IBinder protocol transaction code: dump internal state.

static Int

The first transaction code available for user commands.

static Int

Flag to transact: this is a one-way call, meaning that the caller returns immediately, without waiting for a result from the callee.

static Int

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

static Int

The last transaction code available for user commands.

static Int

IBinder protocol transaction code: tell an app asynchronously that the caller likes it.

static Int

IBinder protocol transaction code: pingBinder().

static Int

IBinder protocol transaction code: send a tweet to the target object.

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

Print the object's state into the given stream.

abstract Unit

Like dump(java.io.FileDescriptor,java.lang.String[]) but always executes asynchronously.

abstract String?

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

open static Int

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

abstract Boolean

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

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

Register the recipient for a notification if this binder goes away.

abstract Boolean

Check to see if the object still exists.

abstract IInterface?

Attempt to retrieve a local implementation of an interface for this Binder object.

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

Perform a generic operation with the object.

abstract Boolean

Remove a previously registered death notification.

Constants

DUMP_TRANSACTION

Added in API level 1
static val DUMP_TRANSACTION: Int

IBinder protocol transaction code: dump internal state.

Value: 1598311760

FIRST_CALL_TRANSACTION

Added in API level 1
static val FIRST_CALL_TRANSACTION: Int

The first transaction code available for user commands.

Value: 1

FLAG_ONEWAY

Added in API level 1
static val FLAG_ONEWAY: Int

Flag to transact: 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

Added in API level 1
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

Added in API level 1
static val LAST_CALL_TRANSACTION: Int

The last transaction code available for user commands.

Value: 16777215

LIKE_TRANSACTION

Added in API level 15
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

Added in API level 1
static val PING_TRANSACTION: Int

IBinder protocol transaction code: pingBinder().

Value: 1599098439

TWEET_TRANSACTION

Added in API level 13
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, 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

dump

Added in API level 3
abstract fun dump(
    fd: FileDescriptor,
    args: Array<String!>?
): Unit

Print the object's state into the given stream.

Parameters
fd FileDescriptor: The raw file descriptor that the dump is being sent to. This value cannot be null.
args Array<String!>?: additional arguments to the dump request. This value may be null.

dumpAsync

Added in API level 13
abstract fun dumpAsync(
    fd: FileDescriptor,
    args: Array<String!>?
): Unit

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

Parameters
fd FileDescriptor: The raw file descriptor that the dump is being sent to. This value cannot be null.
args Array<String!>?: additional arguments to the dump request. This value may be null.

getInterfaceDescriptor

Added in API level 1
abstract fun getInterfaceDescriptor(): String?

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

Return
String? This value may be null.

getSuggestedMaxIpcSizeBytes

Added in API level 30
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

Added in API level 1
abstract fun isBinderAlive(): Boolean

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

Return
Boolean false if the process is not alive. Note that if it returns true, the process may have died while the call is returning.

linkToDeath

Added in API level 1
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'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.

Parameters
recipient IBinder.DeathRecipient: This value cannot be null.
Exceptions
android.os.RemoteException if the target IBinder's process has already died.

See Also

pingBinder

Added in API level 1
abstract fun pingBinder(): Boolean

Check to see if the object still exists.

Return
Boolean Returns false if the hosting process is gone, otherwise the result (always by default true) returned by the pingBinder() implementation on the other side.

queryLocalInterface

Added in API level 1
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.

Parameters
descriptor String: This value cannot be null.

transact

Added in API level 1
abstract fun transact(
    code: Int,
    data: Parcel,
    reply: Parcel?,
    flags: Int
): Boolean

Perform a generic operation with the object.

Parameters
code Int: The action to perform. This should be a number between FIRST_CALL_TRANSACTION and LAST_CALL_TRANSACTION.
data Parcel: Marshalled data to send to the target. Must not be null. If you are not sending any data, you must create an empty Parcel that is given here.
reply Parcel?: Marshalled data to be received from the target. May be null if you are not interested in the return value.
flags Int: Additional operation flags. Either 0 for a normal RPC, or FLAG_ONEWAY for a one-way RPC.
Return
Boolean Returns the result from Binder#onTransact. A successful call generally returns true; false generally means the transaction code was not understood. For a oneway call to a different process false should never be returned. If a oneway call is made to code in the same process (usually to a C++ or Rust implementation), then there are no oneway semantics, and false can still be returned.

unlinkToDeath

Added in API level 1
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.

Parameters
recipient IBinder.DeathRecipient: This value cannot be null.
Return
Boolean true if the recipient is successfully unlinked, assuring you that its android.os.IBinder.DeathRecipient#binderDied method will not be called; false if the target IBinder has already died, meaning the method has been (or soon will be) called.
Exceptions
java.util.NoSuchElementException if the given recipient has not been registered with the IBinder, and the IBinder is still alive. Note that if the recipient was never registered, but the IBinder has already died, then this exception will not be thrown, and you will receive a false return value instead.