認識Android的BinderProxy和Binder類別

16lz 2021-01-23

By 高煥堂2009.7.31

台灣Android技術服務中心主持人

“Android軟硬整合高階技術”課程主講人

*** 其他相關文章

Java層的Activity透過BinderProxy來與遠距的(Remote)服務進行溝通。例如myBinder繼承Binder：

從Java層而觀之，myActivity可以經由bindService()而建立它與myBinder之間的連結。然而，這個連結是透過C++層的機制而達成的。如果你只想要開發Java層級的應用程式的話，並不需要閱讀本文。如果你想學習Android框架的原理，或修改Android框架，或撰寫Android的核心服務，進行軟硬整合，就必須熟悉幕後的重要機制。如下圖所示：

上圖之目的：myActivity想呼叫BinderProxy的IBinder::transact()函數，進而呼叫到myBinder的IBinder::onTransact()函數。就像一條橋，兩端分別是myActivity和myBinder。

為了要達成上述的目的，需要建立一些物件(如同橋墩)，例如BpBinder物件，以及JavaBBinder物件；如此才能達成目的。茲詳細說明如下：

** 當myService::onCreate()函數誕生myBinder物件時，就呼叫到Binder()函數，它將myBinder物件的mObject欄位ID存入Native公用變數gBinderOffsets.mObject裡。接著，init()將JavaBBinderHolder物件的參考存入myBinder物件的mObject欄位裡。** 此時，若已知myBinder物件，藉由ibinderForJavaObject()可取得該物件相對應的JavaBBinderHolder物件。

** 例如，當myActivitty呼叫bindService()函數，就藉由ibinderForJavaObject()可取得該物件相對應的JavaBBinderHolder物件，然後從JavaBBinderHolder物件取得關於myBinder的資訊，進而誕生JavaBBinder物件來與myBinder物件相互輝映。當已知JavaBBinder物件，可藉由javaObjectForIBinder 可取得該物件相對應的myBinder物件。

** 建立了右邊的橋墩(即JavaBBinder物件)之後，也必須建立左邊的橋墩。Android的Binder System可以幫忙建立左邊的橋墩：BpBinder物件。

** 已知BpBinder物件，藉由javaObjectForIBinder ()可取得該物件相對應的BinderProxy物件，或誕生新的BinderProxy物件。並且，將BindProxy物件的mObject欄位ID存入Native公用變數gBinderProxyOffsets.mObject裡。還將BpBinder物件的參考存入BinderProxy物件的mObject欄位裡。

** 有了BinderProxy物件，myActivity裡就能呼叫BinderProxy的IBinder::transact()函數，進而呼叫到myBinder的IBinder::onTransact()函數。也可藉由ibinderForJavaObject()可取得該物件相對應的BpBinder物件。

**** 以下是參考資料 ****

**** 是從Android框架原始程式碼裡面摘錄出來的，

提供給您參考。

---- BinderProxy類別是定義於Android 框架的/java/Binder.java檔案裡

final class BinderProxy implements IBinder {

public native boolean pingBinder();

public native boolean isBinderAlive();

public IInterface queryLocalInterface(String descriptor) {

return null;

}

public native String getInterfaceDescriptor() throws RemoteException;

public native boolean transact(int code, Parcel data, Parcel reply,

int flags) throws RemoteException;

public native void linkToDeath(DeathRecipient recipient, int flags)

throws RemoteException;

public native boolean unlinkToDeath(DeathRecipient recipient, int flags);

BinderProxy() {

mSelf = new WeakReference(this);

}

@Override

protected void finalize() throws Throwable {

try {

destroy();

} finally {

super.finalize();

}

}

private native final void destroy();

private static final void sendDeathNotice(DeathRecipient recipient) {

if (Config.LOGV) Log.v("JavaBinder", "sendDeathNotice to " + recipient);

try {

recipient.binderDied();

}

catch (RuntimeException exc) {

Log.w("BinderNative", "Uncaught exception from death notification",

exc);

}

}

final private WeakReference mSelf;

private int mObject;

}

其中，最重要的Native函數是：

public native boolean transact(int code, Parcel data, Parcel reply, int flags)

throws RemoteException;

此Native函數實作於：

---- Native函數定義於Android 框架的/jni/android_util_binder.cpp檔案裡

static struct binderproxy_offsets_t

{

// Class state.

jclass mClass;

jmethodID mConstructor;

jmethodID mSendDeathNotice;

// Object state.

jfieldID mObject;

jfieldID mSelf;

} gBinderProxyOffsets;

在這gBinderProxyOffsets.mObject裡，放的是BpBinder。

static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,

jint code, jobject dataObj,

jobject replyObj, jint flags)

{

if (dataObj == NULL) {

jniThrowException(env, "java/lang/NullPointerException", NULL);

return JNI_FALSE;

}

Parcel* data = parcelForJavaObject(env, dataObj);

if (data == NULL) {

return JNI_FALSE;

}

Parcel* reply = parcelForJavaObject(env, replyObj);

if (reply == NULL && replyObj != NULL) {

return JNI_FALSE;

}

IBinder* target = (IBinder*)

env->GetIntField(obj, gBinderProxyOffsets.mObject);

if (target == NULL) {

jniThrowException(env, "java/lang/IllegalStateException", "Binder has been finalized!");

return JNI_FALSE;

}

LOGV("Java code calling transact on %p in Java object %p with code %d/n",

target, obj, code);

//printf("Transact from Java code to %p sending: ", target); data->print();

status_t err = target->transact(code, *data, reply, flags);

//if (reply) printf("Transact from Java code to %p received: ", target); reply->print();

if (err == NO_ERROR) {

return JNI_TRUE;

} else if (err == UNKNOWN_TRANSACTION) {

return JNI_FALSE;

}

signalExceptionForError(env, obj, err);

return JNI_FALSE;

}

還含有：

sp<IBinder> ibinderForJavaObject(JNIEnv* env, jobject obj)

{

if (obj == NULL) return NULL;

if (env->IsInstanceOf(obj, gBinderOffsets.mClass)) {

JavaBBinderHolder* jbh = (JavaBBinderHolder*)

env->GetIntField(obj, gBinderOffsets.mObject);

return jbh != NULL ? jbh->get(env) : NULL;

}

if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) {

return (IBinder*)

env->GetIntField(obj, gBinderProxyOffsets.mObject);

}

LOGW("ibinderForJavaObject: %p is not a Binder object", obj);

return NULL;

}

還含有：

static void android_os_Binder_init(JNIEnv* env, jobject clazz)

{

JavaBBinderHolder* jbh = new JavaBBinderHolder(env, clazz);

if (jbh == NULL) {

jniThrowException(env, "java/lang/OutOfMemoryError", NULL);

return;

}

LOGV("Java Binder %p: acquiring first ref on holder %p", clazz, jbh);

jbh->incStrong(clazz);

env->SetIntField(clazz, gBinderOffsets.mObject, (int)jbh);

}

static int int_register_android_os_Binder(JNIEnv* env)

{

jclass clazz;

clazz = env->FindClass(kBinderPathName);

LOG_FATAL_IF(clazz == NULL, "Unable to find class android.os.Binder");

gBinderOffsets.mClass = (jclass) env->NewGlobalRef(clazz);

gBinderOffsets.mExecTransact

= env->GetMethodID(clazz, "execTransact", "(IIII)Z");

assert(gBinderOffsets.mExecTransact);

gBinderOffsets.mObject

= env->GetFieldID(clazz, "mObject", "I");

assert(gBinderOffsets.mObject);

return AndroidRuntime::registerNativeMethods(

env, kBinderPathName,

gBinderMethods, NELEM(gBinderMethods));

}

還含有：

jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val)

{

if (val == NULL) return NULL;

if (val->checkSubclass(&gBinderOffsets)) {

// One of our own!

jobject object = static_cast<JavaBBinder*>(val.get())->object();

//printf("objectForBinder %p: it's our own %p!/n", val.get(), object);

return object;

}

// For the rest of the function we will hold this lock, to serialize

// looking/creation of Java proxies for native Binder proxies.

AutoMutex _l(mProxyLock);

// Someone else's... do we know about it?

jobject object = (jobject)val->findObject(&gBinderProxyOffsets);

if (object != NULL) {

jobject res = env->CallObjectMethod(object, gWeakReferenceOffsets.mGet);

if (res != NULL) {

LOGV("objectForBinder %p: found existing %p!/n", val.get(), res);

return res;

}

LOGV("Proxy object %p of IBinder %p no longer in working set!!!", object, val.get());

android_atomic_dec(&gNumProxyRefs);

val->detachObject(&gBinderProxyOffsets);

env->DeleteGlobalRef(object);

}

object = env->NewObject(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mConstructor);

if (object != NULL) {

LOGV("objectForBinder %p: created new %p!/n", val.get(), object);

// The proxy holds a reference to the native object.

env->SetIntField(object, gBinderProxyOffsets.mObject, (int)val.get());

val->incStrong(object);

// The native object needs to hold a weak reference back to the

// proxy, so we can retrieve the same proxy if it is still active.

jobject refObject = env->NewGlobalRef(

env->GetObjectField(object, gBinderProxyOffsets.mSelf));

val->attachObject(&gBinderProxyOffsets, refObject,

jnienv_to_javavm(env), proxy_cleanup);

// Note that a new object reference has been created.

android_atomic_inc(&gNumProxyRefs);

incRefsCreated(env);

}

return object;

}

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