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);

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;
}

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 ibinderForJavaObject
            jobject 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;
    }

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。

binder是如何在java层和native层实现统一的

前言

正文

总结