Android Binder—APP->framework(mRemote的前世今生)
前言
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();
分析
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 <code>null</code> 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<IBinder> 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<IBinder> ProcessState::getContextObject(const sp<IBinder>& /*caller*/)//caller 上面传递的值为null
{
return getStrongProxyForHandle(0);//参数为0
}
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
sp<IBinder> 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<IBinder> 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<IBinder>& val)
{
if (val == NULL) return NULL;
if (val->checkSubclass(&gBinderOffsets)) {
// One of our own!
jobject object = static_cast<JavaBBinder*>(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<DeathRecipientList> drl = new DeathRecipientList;
drl->incStrong((void*)javaObjectForIBinder);
env->SetLongField(object, gBinderProxyOffsets.mOrgue, reinterpret_cast<jlong>(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<Parcel*>(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<IBinder> Parcel::readStrongBinder() const
{
sp<IBinder> val;
unflatten_binder(ProcessState::self(), *this, &val);//核心，解析Binder
return val;
}
------> Parcel.cpp
status_t unflatten_binder(const sp<ProcessState>& proc,
const Parcel& in, sp<IBinder>* out)
{
const flat_binder_object* flat = in.readObject(false);
if (flat) {
switch (flat->type) {
case BINDER_TYPE_BINDER://Binder实体
*out = reinterpret_cast<IBinder*>(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<BpBinder*>(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
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
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