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Android Reference

IBinder


android.os

Interface IBinder

  • All Known Implementing Classes:
    ActivityManagerNative, ApplicationThreadNative, Binder, BulkCursorNative, ContentProviderNative, CursorToBulkCursorAdaptor, IMountService.Stub, IMountServiceListener.Stub, IMountShutdownObserver.Stub, IObbActionListener.Stub, ServiceManagerNative

    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 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 IBinder.DeathRecipient with the IBinder, which will be called when its containing process goes away.
    See Also:
    Binder

    • Field Detail

      • FIRST_CALL_TRANSACTION

        static final int FIRST_CALL_TRANSACTION
        The first transaction code available for user commands.
        See Also:
        Constant Field Values

      • LAST_CALL_TRANSACTION

        static final int LAST_CALL_TRANSACTION
        The last transaction code available for user commands.
        See Also:
        Constant Field Values

      • PING_TRANSACTION

        static final int PING_TRANSACTION
        IBinder protocol transaction code: pingBinder().
        See Also:
        Constant Field Values

      • DUMP_TRANSACTION

        static final int DUMP_TRANSACTION
        IBinder protocol transaction code: dump internal state.
        See Also:
        Constant Field Values

      • INTERFACE_TRANSACTION

        static final int INTERFACE_TRANSACTION
        IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor.
        See Also:
        Constant Field Values

      • TWEET_TRANSACTION

        static final 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 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!".

        See Also:
        Constant Field Values

      • LIKE_TRANSACTION

        static final 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.

        See Also:
        Constant Field Values

    • Method Detail

      • pingBinder

        boolean pingBinder()
        Check to see if the object still exists.
        Returns:
        Returns false if the hosting process is gone, otherwise the result (always by default true) returned by the pingBinder() implementation on the other side.

      • isBinderAlive

        boolean isBinderAlive()
        Check to see if the process that the binder is in is still alive.
        Returns:
        false if the process is not alive. Note that if it returns true, the process may have died while the call is returning.

      • queryLocalInterface

        IInterface queryLocalInterface(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.

      • dump

        void dump(FileDescriptor fd,
        String[] args)
          throws RemoteException
        Print the object's state into the given stream.
        Parameters:
        fd - The raw file descriptor that the dump is being sent to.
        args - additional arguments to the dump request.
        Throws:
        RemoteException

      • transact

        boolean transact(int code,
               Parcel data,
               Parcel reply,
               int flags)
                 throws RemoteException
        Perform a generic operation with the object.
        Parameters:
        code - The action to perform. This should be a number between FIRST_CALL_TRANSACTION and LAST_CALL_TRANSACTION.
        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.
        reply - Marshalled data to be received from the target. May be null if you are not interested in the return value.
        flags - Additional operation flags. Either 0 for a normal RPC, or FLAG_ONEWAY for a one-way RPC.
        Throws:
        RemoteException

      • unlinkToDeath

        boolean unlinkToDeath(IBinder.DeathRecipient recipient,
                    int flags)
        Remove a previously registered death notification. The recipient will no longer be called if this object dies.
        Returns:
        Returns true if the recipient is successfully unlinked, assuring you that its DeathRecipient.binderDied() method will not be called. Returns false if the target IBinder has already died, meaning the method has been (or soon will be) called.
        Throws:
        Throws - 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.


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