IBinder
public
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 aRemoteExceptionexception 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 aDeathRecipientwith the IBinder, which will be called when its containing process goes away.
See also:
Summary
Nested classes | |
|---|---|
interface |
IBinder.DeathRecipient
Interface for receiving a callback when the process hosting an IBinder has gone away. |
Constants | |
|---|---|
int |
DUMP_TRANSACTION
IBinder protocol transaction code: dump internal state. |
int |
FIRST_CALL_TRANSACTION
The first transaction code available for user commands. |
int |
FLAG_ONEWAY
Flag to |
int |
INTERFACE_TRANSACTION
IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor. |
int |
LAST_CALL_TRANSACTION
The last transaction code available for user commands. |
int |
LIKE_TRANSACTION
IBinder protocol transaction code: tell an app asynchronously that the caller likes it. |
int |
PING_TRANSACTION
IBinder protocol transaction code: pingBinder(). |
int |
TWEET_TRANSACTION
IBinder protocol transaction code: send a tweet to the target object. |
Public methods | |
|---|---|
abstract
void
|
dump(FileDescriptor fd, String[] args)
Print the object's state into the given stream. |
abstract
void
|
dumpAsync(FileDescriptor fd, String[] args)
Like |
abstract
String
|
getInterfaceDescriptor()
Get the canonical name of the interface supported by this binder. |
static
int
|
getSuggestedMaxIpcSizeBytes()
Limit that should be placed on IPC sizes, in bytes, to keep them safely under the transaction buffer limit. |
abstract
boolean
|
isBinderAlive()
Check to see if the process that the binder is in is still alive. |
abstract
void
|
linkToDeath(IBinder.DeathRecipient recipient, int flags)
Register the recipient for a notification if this binder goes away. |
abstract
boolean
|
pingBinder()
Check to see if the object still exists. |
abstract
IInterface
|
queryLocalInterface(String descriptor)
Attempt to retrieve a local implementation of an interface for this Binder object. |
abstract
boolean
|
transact(int code, Parcel data, Parcel reply, int flags)
Perform a generic operation with the object. |
abstract
boolean
|
unlinkToDeath(IBinder.DeathRecipient recipient, int flags)
Remove a previously registered death notification. |
Constants
DUMP_TRANSACTION
public static final int DUMP_TRANSACTION
IBinder protocol transaction code: dump internal state.
Constant Value: 1598311760 (0x5f444d50)
FIRST_CALL_TRANSACTION
public static final int FIRST_CALL_TRANSACTION
The first transaction code available for user commands.
Constant Value: 1 (0x00000001)
FLAG_ONEWAY
public static final int FLAG_ONEWAY
Flag to transact(int, Parcel, Parcel, int): 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.
Constant Value: 1 (0x00000001)
INTERFACE_TRANSACTION
public static final int INTERFACE_TRANSACTION
IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor.
Constant Value: 1598968902 (0x5f4e5446)
LAST_CALL_TRANSACTION
public static final int LAST_CALL_TRANSACTION
The last transaction code available for user commands.
Constant Value: 16777215 (0x00ffffff)
LIKE_TRANSACTION
public 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.
Constant Value: 1598835019 (0x5f4c494b)
PING_TRANSACTION
public static final int PING_TRANSACTION
IBinder protocol transaction code: pingBinder().
Constant Value: 1599098439 (0x5f504e47)
TWEET_TRANSACTION
public 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!".
Constant Value: 1599362900 (0x5f545754)
Public methods
dump
public abstract void dump (FileDescriptor fd, String[] args)
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 |
String: additional arguments to the dump request.
This value may be null. |
| Throws | |
|---|---|
RemoteException |
|
dumpAsync
public abstract void dumpAsync (FileDescriptor fd, String[] args)
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 |
String: additional arguments to the dump request.
This value may be null. |
| Throws | |
|---|---|
RemoteException |
|
getInterfaceDescriptor
public abstract String getInterfaceDescriptor ()
Get the canonical name of the interface supported by this binder.
| Returns | |
|---|---|
String |
This value may be null. |
| Throws | |
|---|---|
RemoteException |
|
getSuggestedMaxIpcSizeBytes
public static int getSuggestedMaxIpcSizeBytes ()
Limit that should be placed on IPC sizes, in bytes, to keep them safely under the transaction buffer limit.
| Returns | |
|---|---|
int |
|
isBinderAlive
public abstract boolean isBinderAlive ()
Check to see if the process that the binder is in is still alive.
| Returns | |
|---|---|
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
public abstract void linkToDeath (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 DeathRecipient's
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. |
flags |
int |
| Throws | |
|---|---|
RemoteException |
if the target IBinder's process has already died. |
See also:
pingBinder
public abstract boolean pingBinder ()
Check to see if the object still exists.
| Returns | |
|---|---|
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
public abstract 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.
| Parameters | |
|---|---|
descriptor |
String: This value cannot be null. |
| Returns | |
|---|---|
IInterface |
|
transact
public abstract boolean transact (int code,
Parcel data,
Parcel reply,
int flags)
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. |
| Returns | |
|---|---|
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. |
| Throws | |
|---|---|
RemoteException |
|
unlinkToDeath
public abstract boolean unlinkToDeath (IBinder.DeathRecipient recipient, int flags)
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. |
flags |
int |
| Returns | |
|---|---|
boolean |
true if the recipient is successfully
unlinked, assuring you that its
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. |
| 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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Last updated 2024-04-11 UTC.