Android Binder——APP->framework(mRemote的前世今生)

版权说明：本文为开开向前冲原创文章，转载请注明出处；
注：限于作者水平有限，文中有不对的地方还请指教
本文参考一个朋友兼同事ShadowN1ght的文章客户端到驱动通信流程；用一个简单的案例阐述了完整的Binder 通信过程；

Binder之于Android，犹如电话之于人类，都是用于传递信息；

写这篇文章前我酝酿了很久，不知道该何从下笔，文章写了又删，删了又写，因为始终感觉写的有点杂，一不注意就丢失了主线；现在通过跟踪一个完整的Binder调用来说明Binder IPC的过程——PowerManger调用isScreenOn();

Android Binder——APP->framework(mRemote的前世今生)_第1张图片

Binder框架.png

这里先PO出Binder通信架构图镇楼；

方案设计：Binder通信涉及APP层，framework层，Kernel层，虽然涉及的东西比较杂，但是都是代码实现的，既然如此，我们都可以通过增加调试Log信息来跟踪这个流程，本文的思路就是如此；

为了更好的理解Binder 通信过程，你最好有一套完整的Android 源码，再配上一个代码搜索神器OpenGrok，如果你没有android源码，不想自己搭建OpenGrok服务器，这里推荐一个公开的Android Code In OpenGrok，只不过由于网速问题你可能得多花点时间等待；

1. APP——> framework

import com.example.bindservice.ProcessInfo;

public class MainActivity extends AppCompatActivity {
    String TAG = "Bindertest MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        findViewById(R.id.mybtn).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ProcessInfo processInfo = new ProcessInfo();
                processInfo.nativeSelfCall();
                PowerManager powerManager = (PowerManager) getSystemService(Context.POWER_SERVICE);
                Log.e(TAG,"App begin nativeCall");
                boolean bool = powerManager.isScreenOn();
                Log.e(TAG,"App end nativeCall");
                Log.e(TAG,"" + bool);
            }
        });
    }
}

Binder过程很简单，就是调用了powerManager.isScreenOn();//Binder Call 代码;
processInfo.nativeSelfCall()是自己添加的JNI，目的是向Binder Kernel中传递cmd，然后在Binder Kernel中根据cmd获取到应用进程ID，过滤Log；这里暂时不说；

/frameworks/base/core/java/android/os/PowerManager.java

    @Deprecated
    public boolean isScreenOn() {
        return isInteractive();
    }

    public boolean isInteractive() {
        try {
            return mService.isInteractive();
        } catch (RemoteException e) {
            return false;
        }
    }

PowerManger.java中的isScreenOn最终会调用 mService.isInteractive()；这里的mService是什么呢？？？

mService

    final IPowerManager mService;
    
    /**
     * {@hide}
     */
    public PowerManager(Context context, IPowerManager service, Handler handler) {
        mContext = context;
        mService = service;//mService初始化是在PowerManager构造方法中
        mHandler = handler;
    }

mService 是IPowerManager 对象；这里根据名字我们就知道IPowerManager是通过AIDL生成的代码，可以在Android Studio中查找；找到：
out/target/common/obj/JAVA_LIBRARIES/framework_intermediates/src/core/java/android/os/IPowerManager.java

------> IPowerManager.java——>Proxy
      @Override
      public boolean isInteractive() throws android.os.RemoteException {
            android.os.Parcel _data = android.os.Parcel.obtain();
            android.os.Parcel _reply = android.os.Parcel.obtain();
            boolean _result;
            try {
                _data.writeInterfaceToken(DESCRIPTOR);
                mRemote.transact(Stub.TRANSACTION_isInteractive, _data, _reply, 0);//核心核心
                _reply.readException();
                _result = (0 != _reply.readInt());
            } finally {
                _reply.recycle();
                _data.recycle();
            }
            return _result;
        }

_data用于包装客户端数据，_reply用于从服务端获取数据，mRemote是android.os.IBinder对象，DESCRIPTOR标识了IPowerManager ，DESCRIPTOR = "android.os.IPowerManager"；TRANSACTION_isInteractive 是方法号，TRANSACTION_isInteractive = (android.os.IBinder.FIRST_CALL_TRANSACTION + 11)；_result即我们应用层得到的值，这里是根据reply获取的值来赋值的；

这里我重点看看mRemote；

------> IPowerManager.java——>Proxy
        ......
        private static class Proxy implements android.os.IPowerManager {
            private android.os.IBinder mRemote;

            Proxy(android.os.IBinder remote) {
                mRemote = remote;//赋值
            }

            @Override
            public android.os.IBinder asBinder() {
                return mRemote;
            }
            ......
        }

这里mRemote是在Proxy的构造方法中被调用，那Proxy是在什么地方调用呢？

------>IPowerManager.java——>Stub
        /**
         * Cast an IBinder object into an android.os.IPowerManager interface,
         * generating a proxy if needed.
         */
        public static android.os.IPowerManager asInterface(android.os.IBinder obj) {
            if ((obj == null)) {
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof android.os.IPowerManager))) {
                return ((android.os.IPowerManager) iin);
            }
            return new android.os.IPowerManager.Stub.Proxy(obj);//调用Proxy构造函数
        }

这里IPowerManger.Stub asInterface在什么地方调用呢？asInterface返回的对象是IPowerManager对象；在Android Studio 查看该方法在那些地方被调用，你会发现有很对，但是我相信你会特别在ContextImpl的；
/frameworks/base/core/java/android/app/ContextImpl.java

------> ContextImpl.java
        registerService(POWER_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    IBinder b = ServiceManager.getService(POWER_SERVICE);
                    IPowerManager service = IPowerManager.Stub.asInterface(b);
                    //调用IPowerManger.stub.asInterface，传入的IBinder对象参数是从 
                    //ServiceManager.getService(POWER_SERVICE)获取的
                    if (service == null) {
                        Log.wtf(TAG, "Failed to get power manager service.");
                    }
                    return new PowerManager(ctx.getOuterContext(),
                            service, ctx.mMainThread.getHandler());//返回PowerManager对象，
                    //这里的service就是PowerManger类中的mService，
//我们在应用层调用的  (PowerManager) getSystemService(Context.POWER_SERVICE)
//得到的powerManger对象就是这里返回的new PowerManager，这部分代码跟着逻辑就能看到；
             }});

看到registerService方法，我相信很多人都很熟悉，这里的service就是PowerManger类中的mService，
应用层调用的 (PowerManager) getSystemService(Context.POWER_SERVICE)得到的powerManger对象
就是这里返回的new PowerManager，顺着Activity的getSystemService方法的逻辑看就会清楚；
PowerManager中的mService就是这里的service，通过 IPowerManager.Stub.asInterface(b)获得；

IBinder b = ServiceManager.getService(POWER_SERVICE);
IPowerManager service = IPowerManager.Stub.asInterface(b);
这里真正开始接触IBiner，这里做个标记，等会可能会回头再来看，

这里调用 IPowerManager.Stub.asInterface(b)的IBinder b参数是从ServiceManager.getService(POWER_SERVICE)获取的，那这个IBinder b值是什么呢？？？（我这里先透露一下，IBinder b是一个BinderProxy对象）

/frameworks/base/core/java/android/os/ServiceManager.java

------> ServiceManager.java
/**
     * Returns a reference to a service with the given name.
     * 
     * @param name the name of the service to get
     * @return a reference to the service, or null if the service doesn't exist
     */
    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return getIServiceManager().getService(name);//核心核心
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }

这里先从缓存中获取IBinder，缓存中没有则调用getIServiceManager().getService(name)获取；

------> ServiceManager.java
    private static IServiceManager getIServiceManager() {
        if (sServiceManager != null) {
            return sServiceManager;
        }
        // Find the service manager
        sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());//核心核心
        return sServiceManager;
    }

/frameworks/base/core/java/android/os/ServiceManagerNative.java
ServiceManagerNative.asInterface(BinderInternal.getContextObject())的参数为BinderInternal.getContextObject();

------> ServiceManagerNative.java
  static public IServiceManager asInterface(IBinder obj)
    {
        if (obj == null) {
            return null;
        }
        IServiceManager in =
            (IServiceManager)obj.queryLocalInterface(descriptor);//如果没有真正的跨进程通信，则从这里返回
        //Binder 类有实现，BinderProxy没有实现queryLocalInterface方法；
        if (in != null) {
            return in;//判断是否是真的跨进程，比如应用内部实现service，就没真正跨进程，从这里返回；
        }
        return new ServiceManagerProxy(obj);//核心核心核心，跨进程
    }

使用OpenGrok搜索"implements IBinder"，会发现Binder.java 文件中class Binder和
class BinderProxy两个类实现了IBinder；此处我先不确定queryLocalInterface是否是调用的这里；我们先确定
static public IServiceManager asInterface(IBinder obj)的参数IBinder obj；obj是ServiceManager.java中调用ServiceManagerNative.asInterface(BinderInternal.getContextObject())传递过来的，所以
obj = BinderInternal.getContextObject()；我们看看ServiceManagerProxy的构造方法，会发现ServiceManagerProxy中
public ServiceManagerProxy(IBinder remote) {
mRemote = remote;
}，即BinderInternal.getContextObject()的返回值将赋值给mRemote

/frameworks/base/core/java/com/android/internal/os/BinderInternal.java

------> BinderInternal.java
public static final native IBinder getContextObject();

这里使用JNI来获取IBinder对象；根据Android JNI命名规则，我们知道getContextObject方法在android_util_Binder.cpp中实现；

------> android_util_Binder.cpp
static const JNINativeMethod gBinderInternalMethods[] = {
     /* name, signature, funcPtr */
    { "getContextObject", "()Landroid/os/IBinder;", (void*)android_os_BinderInternal_getContextObject },
    { "joinThreadPool", "()V", (void*)android_os_BinderInternal_joinThreadPool },
    { "disableBackgroundScheduling", "(Z)V", (void*)android_os_BinderInternal_disableBackgroundScheduling },
    { "handleGc", "()V", (void*)android_os_BinderInternal_handleGc }
};

static jobject android_os_BinderInternal_getContextObject(JNIEnv* env, jobject clazz)
{
    sp b = ProcessState::self()->getContextObject(NULL);//ProcessState采用单列，整个应用只有一个实例
    return javaObjectForIBinder(env, b);//将native binder转换为Java Binder对象，这里返回BinderProxy对象，下面会说明；
}

这里通过 ProcessState创建native IBinder对象；再调用javaObjectForIBinder将native Binder对象转换成Java层的Binder对象；

------> ProcessState.cpp
sp ProcessState::getContextObject(const sp& /*caller*/)//caller 上面传递的值为null
{
    return getStrongProxyForHandle(0);//参数为0
}

sp ProcessState::getStrongProxyForHandle(int32_t handle)
{
    sp result;

    AutoMutex _l(mLock);

    handle_entry* e = lookupHandleLocked(handle);//此时handle=0

    if (e != NULL) {
        // We need to create a new BpBinder if there isn't currently one, OR we
        // are unable to acquire a weak reference on this current one.  See comment
        // in getWeakProxyForHandle() for more info about this.
        IBinder* b = e->binder;
        if (b == NULL || !e->refs->attemptIncWeak(this)) {
            if (handle == 0) {
                // Special case for context manager...
                // The context manager is the only object for which we create
                // a BpBinder proxy without already holding a reference.
                // Perform a dummy transaction to ensure the context manager
                // is registered before we create the first local reference
                // to it (which will occur when creating the BpBinder).
                // If a local reference is created for the BpBinder when the
                // context manager is not present, the driver will fail to
                // provide a reference to the context manager, but the
                // driver API does not return status.
                //
                // Note that this is not race-free if the context manager
                // dies while this code runs.
                //
                // TODO: add a driver API to wait for context manager, or
                // stop special casing handle 0 for context manager and add
                // a driver API to get a handle to the context manager with
                // proper reference counting.

                Parcel data;
                status_t status = IPCThreadState::self()->transact(
                        0, IBinder::PING_TRANSACTION, data, NULL, 0);
                if (status == DEAD_OBJECT)
                   return NULL;
            }

            b = new BpBinder(handle); //创建BpBinder，即b = new BpBinder(0);
            e->binder = b;
            if (b) e->refs = b->getWeakRefs();
            result = b;//用new BpBinder(0)给result赋值，最后返回result
        } else {
            // This little bit of nastyness is to allow us to add a primary
            // reference to the remote proxy when this team doesn't have one
            // but another team is sending the handle to us.
            result.force_set(b);
            e->refs->decWeak(this);
        }
    }

    return result;//返回new BpBinder(0)
}

sp b = ProcessState::self()->getContextObject(NULL)返回的是new BpBinder(0);返回到android_os_BinderInternal_getContextObject方法中，接下来会调用javaObjectForIBinder方法；

------> javaObjectForIBinder
jobject javaObjectForIBinder(JNIEnv* env, const sp& val)
{
    if (val == NULL) return NULL;

    if (val->checkSubclass(&gBinderOffsets)) {
        // One of our own!
        jobject object = static_cast(val.get())->object();
        LOGDEATH("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);//gBinderProxyOffsets很重要，
//gBinderProxyOffets在int_register_android_os_BinderProxy中初始化，指向Java层的BinderProxy(核心核心核心)，
//int_register_android_os_BinderProxy在register_android_os_Binder中调用，register_android_os_Binder
//在开机过程中AndroidRuntime.startReg方法中被调用；
    if (object != NULL) {
        jobject res = jniGetReferent(env, object);
        if (res != NULL) {
            ALOGV("objectForBinder %p: found existing %p!\n", val.get(), res);
            return res;
        }
        LOGDEATH("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);
    //gBinderProxyOffsets.mClass指向BinderProxy class，gBinderProxyOffsets.mConstructor指向BinderProxy构造方法；
    if (object != NULL) {
        LOGDEATH("objectForBinder %p: created new proxy %p !\n", val.get(), object);
        // The proxy holds a reference to the native object.
        env->SetLongField(object, gBinderProxyOffsets.mObject, (jlong)val.get());//val是BpBinder，
        //这里利用JNI 调用java将读到的BpBinder 对象val存入BinderProxy的mObject变量中
        val->incStrong((void*)javaObjectForIBinder);
        // 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);
        // Also remember the death recipients registered on this proxy
        sp drl = new DeathRecipientList;
        drl->incStrong((void*)javaObjectForIBinder);
        env->SetLongField(object, gBinderProxyOffsets.mOrgue, reinterpret_cast(drl.get()));
        // Note that a new object reference has been created.
        android_atomic_inc(&gNumProxyRefs);
        incRefsCreated(env);
    }
    return object;//返回Java BinderProxy对象
}

所以前面BinderInternal.java 中 getContextObject()方法会返回一个BinderProxy对象；并将获取到的BpBinder对象存入Java层BinderProxy类的mObject变量中；

是不是有点蒙圈了，休息休息休息休息一下下，你还记得我们的这个BinderProxy返回到什么地方吗？哈哈，反正我是记得；因为我都记下来了，哈哈；BinderProxy是BinderInternal.getContextObject()返回的，即这个BinderProxy将作为ServiceManagerNative.java 中static public IServiceManager asInterface(IBinder obj)方法的参数IBinder obj；如果Binder通信确实跨进程ServiceManagerNative.java的asInterface方法将返回
new ServiceManagerProxy(BinderProxy binderProxy)

到这里我想使用goto 语句了，调到我想去的地方，还记得我们是从什么时候开始分析这个IBinder对象的吗？

------> ContextImpl.java
        registerService(POWER_SERVICE, new ServiceFetcher() {
                public Object createService(ContextImpl ctx) {
                    IBinder b = ServiceManager.getService(POWER_SERVICE);
                    IPowerManager service = IPowerManager.Stub.asInterface(b);
                    //调用IPowerManger.stub.asInterface，传入的IBinder对象参数是从 
                    //ServiceManager.getService(POWER_SERVICE)获取的
                    if (service == null) {
                        Log.wtf(TAG, "Failed to get power manager service.");
                    }
                    return new PowerManager(ctx.getOuterContext(),
                            service, ctx.mMainThread.getHandler());//返回PowerManager对象，
                    //这里的service就是PowerManger类中的mService，
//我们在应用层调用的  (PowerManager) getSystemService(Context.POWER_SERVICE)
//得到的powerManger对象就是这里返回的new PowerManager，这部分代码跟着逻辑就能看到；
             }});

我们重新回到ContextImpl.java 中开始分析， IBinder b = ServiceManager.getService(POWER_SERVICE);其实就是ServiceManagerProxy的getService方法，ServiceManagerProxy类中mRemote的值就是BinderProxy，所以 IBinder b = ServiceManager.getService(POWER_SERVICE)最终会调用ServiceManagerProxy的方法；

------> ServiceMangerNative.java ——>ServiceManagerProxy
    public IBinder getService(String name) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IServiceManager.descriptor);
        data.writeString(name);
        mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0);
        IBinder binder = reply.readStrongBinder();//这里返回IBinder对象
        reply.recycle();
        data.recycle();
        return binder;
    }

IPowerManager service = IPowerManager.Stub.asInterface(b)，这里asInterface方法的参数b就是ServiceManagerProxy类中getService返回的IBinder，这里是通过reply.readStrongBinder()；reply是一个Parcel；

/frameworks/base/core/java/android/os/Parcel.java

------> Parcel.java
 public final IBinder readStrongBinder() {
        return nativeReadStrongBinder(mNativePtr);//JNI方法
    }

我们根据Android JNI命名规则，可以到android_os_Parcel.cpp中查看nativeReadStrongBinder方法：

------> android_os_Parcel.cpp
static jobject android_os_Parcel_readStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr)
{
    Parcel* parcel = reinterpret_cast(nativePtr);
    if (parcel != NULL) {
        return javaObjectForIBinder(env, parcel->readStrongBinder());//这个方法前面有说过，
                                            //根据parcel->readStrongBinder()的值返回，
                                            //提前透露一下，parcel->readStrongBinder()返回的值是BpBinder对象
    }
    return NULL;
}

parcel->readStrongBinder()

------> Parcel.cpp
sp Parcel::readStrongBinder() const
{
    sp val;
    unflatten_binder(ProcessState::self(), *this, &val);//核心，解析Binder
    return val;
}
------> Parcel.cpp
status_t unflatten_binder(const sp& proc,
    const Parcel& in, sp* out)
{
    const flat_binder_object* flat = in.readObject(false);

    if (flat) {
        switch (flat->type) {
            case BINDER_TYPE_BINDER://Binder实体
                *out = reinterpret_cast(flat->cookie);
                return finish_unflatten_binder(NULL, *flat, in);
            case BINDER_TYPE_HANDLE://Binder 引用，我们这里是通过ServiceManager.getService获取的服务代理，即Binder引用
                *out = proc->getStrongProxyForHandle(flat->handle);//核心核心，是不是很熟悉呢？？？
                //该方法返回一个new BpBinder(flat->handle)，这里的new BpBinder将会传递给readStringBinder方法的&val然后直接返回；
                return finish_unflatten_binder(
                    static_cast(out->get()), *flat, in);//类型转换
        }
    }
    return BAD_TYPE;
}

Parcel.cpp中的readStrongBinder方法将返回一个new BpBinder()对象，接着继续返回给android_os_Parcel.cpp的android_os_Parcel_readStrongBinder，在android_os_Parcel_readStrongBinder中调用javaObjectForIBinder方法将这个BpBinder对象转换为Java 层的BinderProxy对象返回，javaObjectForIBinder还会调用SetLongField将获取到的BpBinder对象保存到java层BinderProxy类的mObject变量中，所以呢所以呢？？？ServiceManagerProxy的getService方法将返回一个BinderProxy对象

------> ServiceMangerNative.java ——>ServiceManagerProxy
    public IBinder getService(String name) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IServiceManager.descriptor);
        data.writeString(name);
        mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0);
        IBinder binder = reply.readStrongBinder();//这里返回IBinder对象
        reply.recycle();
        data.recycle();
        return binder;//经过一系列转换后，这里返回的binder对象 其实是一个BinderProxy对象
    }

再回到ContextImpl中，你将会明白所有参数：
IBinder b = ServiceManager.getService(POWER_SERVICE) //IBinder b其实就是一个BinderProxy对象，是不是和前面透露的保持一致，其实我都怕透露出错，哈哈
IPowerManager service = IPowerManager.Stub.asInterface(b);//这里我们知道asInterface(b)最终会将参数b传递到IPowerManager.Stub.Proxy的构造函数，将BinderProxy对象赋值给IPowerManager.Stub.Proxy的mRemote；IPowerManager.Stub.asInterface(b)调用最终会返回IPowerManager.Stub.Proxy对象；

有没有真相大白的感觉，困扰你的mRemote终于验明正身了；又没有很兴奋激动，反正我还是比较激动的。

小结：本小结主要阐述了Binder通信从APP层到framework层的通信过程，即
PowerManager powerManager = (PowerManager) getSystemService(Context.POWER_SERVICE)
boolean bool = powerManager.isScreenOn()=========>mService.isInteractive（mService是IPowerManager对象）=========>mRemote.transact(Stub.TRANSACTION_isInteractive, _data, _reply, 0)（mRemote是一个BinderProxy对象）=========>BinderProxy.transact；下一篇文章将从BinderProxy开始分析；

Android Binder——APP->framework(mRemote的前世今生)

1. APP——> framework

mService

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