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IBinder.java API Doc Android 5.1 API 11628 Thu Mar 12 22:22:10 GMT 2015 android.os

IBinder

public interface 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 {@link Binder}.

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

see: Binder

Fields Summary
int
FIRST_CALL_TRANSACTION
The first transaction code available for user commands.
int
LAST_CALL_TRANSACTION
The last transaction code available for user commands.
int
PING_TRANSACTION
IBinder protocol transaction code: pingBinder().
int
DUMP_TRANSACTION
IBinder protocol transaction code: dump internal state.
int
INTERFACE_TRANSACTION
IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor.
int
TWEET_TRANSACTION
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 {@link 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!".
int
LIKE_TRANSACTION
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.
int
SYSPROPS_TRANSACTION
int
FLAG_ONEWAY
Flag to {@link #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.
Constructors Summary
Methods Summary
public void dump(java.io.FileDescriptor fd, java.lang.String[] args)
Print the object's state into the given stream.
param
fd The raw file descriptor that the dump is being sent to.
param
args additional arguments to the dump request.
public void dumpAsync(java.io.FileDescriptor fd, java.lang.String[] args)
Like {@link #dump(FileDescriptor, String[])} but always executes asynchronously. If the object is local, a new thread is created to perform the dump.
param
fd The raw file descriptor that the dump is being sent to.
param
args additional arguments to the dump request.
public java.lang.String getInterfaceDescriptor()
Get the canonical name of the interface supported by this binder.
public boolean isBinderAlive()
Check to see if the process that the binder is in is still alive.
return
false if the process is not alive. Note that if it returns true, the process may have died while the call is returning.
public void linkToDeath(android.os.IBinder$DeathRecipient recipient, int flags)
Register the recipient for a notification if this binder goes away. If this binder object unexpectedly goes away (typically because its hosting process has been killed), then the given {@link DeathRecipient}'s {@link DeathRecipient#binderDied DeathRecipient.binderDied()} method will be called.
You will only receive death notifications for remote binders, as local binders by definition can't die without you dying as well.
throws
RemoteException if the target IBinder's process has already died.
see
#unlinkToDeath
public boolean pingBinder()
Check to see if the object still exists.
return
Returns false if the hosting process is gone, otherwise the result (always by default true) returned by the pingBinder() implementation on the other side.
public IInterface queryLocalInterface(java.lang.String descriptor)
Attempt to retrieve a local implementation of an interface for this Binder object. If null is returned, you will need to instantiate a proxy class to marshall calls through the transact() method.
public boolean transact(int code, Parcel data, Parcel reply, int flags)
Perform a generic operation with the object.
param
code The action to perform. This should be a number between {@link #FIRST_CALL_TRANSACTION} and {@link #LAST_CALL_TRANSACTION}.
param
data Marshalled data to send to the target. Must not be null. If you are not sending any data, you must create an empty Parcel that is given here.
param
reply Marshalled data to be received from the target. May be null if you are not interested in the return value.
param
flags Additional operation flags. Either 0 for a normal RPC, or {@link #FLAG_ONEWAY} for a one-way RPC.
public boolean unlinkToDeath(android.os.IBinder$DeathRecipient recipient, int flags)
Remove a previously registered death notification. The recipient will no longer be called if this object dies.
return
{@code true} if the recipient is successfully unlinked, assuring you that its {@link DeathRecipient#binderDied DeathRecipient.binderDied()} method will not be called; {@code false} if the target IBinder has already died, meaning the method has been (or soon will be) called.
throws
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.