前言

众所周知，对于binder通信来说，native层通信的基础架构是BpBinder/BBinder，Java层的基础通信架构是BinderProxy/Binder，这两者是如何统一起来的呢？

正文

在binder的跨进程传递数据实现中，数据是包裹在Parcel中的。具体的传递方式，体现在Parcel类的两个方法writeStrongBinder()和readStrongBinder()中，前者的功能是写入Binder对象，后者的功能是读取Binder对象。同样，Parcel在java层和native层各有一套。不过，查看代码可知，java层的Parcel只是native层的封装，两者通过jni建立关联。

    public final void writeStrongBinder(IBinder val){nativeWriteStrongBinder(mNativePtr, val);}public final IBinder readStrongBinder(){return nativeReadStrongBinder(mNativePtr);}private static native String nativeReadStrongBinder(long nativePtr);private static native void nativeWriteStrongBinder(long natievPtr, IBinder);

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jni层代码如下

  static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr, jobject object){Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr);if (parcel != NULL){const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object));if (err != NO_ERROR){signalExceptionForError(env, clazz, err);}}}static jobject android_os_Parcel_readStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr){Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr);if (parcel != NULL){return javaObjectForIBinder(env, parcel->readStrongBinder());}return NULL;}

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重点来了，在jni层代码中发现了两个名字起的有点意思的函数ibinderForJavaObject()和javaObjectForIBinder()，正是这两个函数，将java层和native层统一了，代码如下

    sp<IBinder> ibinderForJavaObject(JNIEnv* env, jobject obj){if (obj == NULL) return NULL;//Instance of Binder?if (env->IsInstanceOf(obj, gBinderOffsets.mClass)){JavaBBinderHolder* jbh = (JavaBBinderHolder*)env->GetLongField(obj, gBinderOffsets.mObject);return jbh->get(env, obj);}//Instance of BinderProxy?if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)){return getBPNativeData(env, obj)->mObject;}return NULL;}jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val){if (val == NULL) return NULL;if (val->checkSubclass(&gBinderOffsets)){//It's a JavaBBinder created by ibinderForJavaObjectjobject object = static_cast<JavaBBinder*>(val.get())->object();return object;}BinderProxyNativeData* nativeData = new BinderProxyNativeData();nativeData->mOrgue = new DeathRecipientList;nativeData->mObject = val;jobject object = env->CallStaticObjectMethod(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mGetInstance, (jlong)nativeData, (jlong)val.get());...BinderProxyNativeData* actualNativeData = getBPNativeData(env, object);if (actualNativeData == nativeData){//Created a new Proxy...} else {delete nativeData;}return object;}

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ibinderForJavaObject()将java binder对象转换成native层binder对象，javaObjectForIBinder()将native binder对象还原成java层binder对象。

总结

android在创建java层Binder对象和BinderProxy对象时，会同时在native层创建对应的JavaBBinderHolder和BinderProxyNativeData。前者包含一个JavaBBinder类型的成员，它继承自BBinder;后者包含BpBinder对象。这样就把java层和native层的连接建立起来了。

转自：binder是如何在java层和native层实现统一的-腾讯云开发者社区-腾讯云