Android之IPC6————Binder3 Framework层分析

Android之IPC6————Binder3 Framework层分析

文章目录

• Android之IPC6————Binder3 Framework层分析
• • • 一.概述
    • 二.BinderJNI初始化
    • • 1.注册JNI方法
      • 2.注册Binder类
      • 3.注册BinderInternal
      • 4. 注册BinderProxy
    • 三.注册服务
    • • 1.SM.addService
      • 2.获取SMP
      • 3.SMP.addService()
      • 4.writeStrongBinder
      • 5.mRemote.transact
      • 6.小结
    • 四.获取服务
    • • 1. SM.getService
      • 2. SMP.getService
      • 3.mRemote.transact
      • 4.readStrongBinder
      • 5.总结
    • 五.总结
    • 六.参考资料

一.概述

在前两篇我们分析了Binder在Native层的实现，我们今天来看在Framework层，Binder相关的内容，这一篇也是以注册服务和获取服务来看Binder的流程。

bidner在Framework层，采用JNI技术来调用native层的binder架构，从而为上层应用提供服务。

java JNI本意是Java native interface,是为了方便javad调用C,C++等本地代码所封装的异常接口。jni也是JVM规范中的一部份，因此可以将我们写的 JNI 程序在任何实现了 JNI 规范的 Java 虚拟机中运行。
这篇文章主要讲下面几个方面：

• BinderJNI初始化(注册)
• FrameWork层的注册服务
• FrameWork层的获取服务

下面这个是framework的binder类关系图：

二.BinderJNI初始化

在Android系统开机的过程中，Zygote启动时会有一个虚拟机注册过程，该过程调用AndroidRuntime::startReg方法来完成jni方法的注册。

1.注册JNI方法

int AndroidRuntime::startReg(JNIEnv* env)
{androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);env->PushLocalFrame(200);//注册jni方法 【见下】if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {env->PopLocalFrame(NULL);return -1;}env->PopLocalFrame(NULL);return 0;
}

注册jni方法

int register_android_os_Binder(JNIEnv* env) {// 注册Binder类的jni方法【见下】if (int_register_android_os_Binder(env) < 0)return -1;// 注册BinderInternal类的jni方法【见下】if (int_register_android_os_BinderInternal(env) < 0)return -1;// 注册BinderProxy类的jni方法【见下】if (int_register_android_os_BinderProxy(env) < 0)return -1;...return 0;
}

2.注册Binder类

static int int_register_android_os_Binder(JNIEnv* env)
{//其中kBinderPathName = "android/os/Binder";查找kBinderPathName路径所属类jclass clazz;clazz = env->FindClass(kBinderPathName);LOG_FATAL_IF(clazz == NULL, "Unable to find class android.os.Binder");//将java层的Binder类保存到mClass变量；gBinderOffsets.mClass = (jclass) env->NewGlobalRef(clazz);//将Java层execTransact()方法保存到mExecTransact变量；gBinderOffsets.mExecTransact= env->GetMethodID(clazz, "execTransact", "(IJJI)Z");assert(gBinderOffsets.mExecTransact);//将Java层mObject属性保存到mObject变量gBinderOffsets.mObject= env->GetFieldID(clazz, "mObject", "J");assert(gBinderOffsets.mObject);return AndroidRuntime::registerNativeMethods(env, kBinderPathName,gBinderMethods, NELEM(gBinderMethods));
}

gBinderoffers是全局静态结构体，其定义如下：

static struct bindernative_offsets_t
{// Class state.jclass mClass; //记录Binder类jmethodID mExecTransact;//记录execTransact()方法// Object state.jfieldID mObject; //记录mObject属性} gBinderOffsets;

gBinderoffers保存了Binder.java类本身的以及其成员方法，execTransact()和成员属性mObject，这为JNI层访问Java层提供通道。另外通过查询获取Java层 binder信息后保存到gBinderOffsets。

再看看registerNativeMethods中第三个参数，即 gBinderMethods

//为Java层访问JNI层提供通道。static const JNINativeMethod gBinderMethods[] = {/* name, signature, funcPtr */{ "getCallingPid", "()I", (void*)android_os_Binder_getCallingPid },{ "getCallingUid", "()I", (void*)android_os_Binder_getCallingUid },{ "clearCallingIdentity", "()J", (void*)android_os_Binder_clearCallingIdentity },{ "restoreCallingIdentity", "(J)V", (void*)android_os_Binder_restoreCallingIdentity },{ "setThreadStrictModePolicy", "(I)V", (void*)android_os_Binder_setThreadStrictModePolicy },{ "getThreadStrictModePolicy", "()I", (void*)android_os_Binder_getThreadStrictModePolicy },{ "flushPendingCommands", "()V", (void*)android_os_Binder_flushPendingCommands },{ "init", "()V", (void*)android_os_Binder_init },{ "destroy", "()V", (void*)android_os_Binder_destroy }
};

在int_register_android_os_Binder中：

• 通过gBinderOffsets，保存Java层Binder类的信息，为JNI层访问Java层提供通道；
• 通过RegisterMethodsOrDie，将gBinderMethods数组完成映射关系，从而为Java层访问JNI层提供通道。

3.注册BinderInternal

static int int_register_android_os_BinderInternal(JNIEnv* env) {//其中kBinderInternalPathName = "com/android/internal/os/BinderInternal"jclass clazz = FindClassOrDie(env, kBinderInternalPathName);gBinderInternalOffsets.mClass = MakeGlobalRefOrDie(env, clazz);gBinderInternalOffsets.mForceGc = GetStaticMethodIDOrDie(env, clazz, "forceBinderGc", "()V");return RegisterMethodsOrDie(env, kBinderInternalPathName,gBinderInternalMethods, NELEM(gBinderInternalMethods));
}

上面是BinderInternal类的jni方法，gBinderInternalOffsets保存了BinderInternal的forceBinderGc()方法。

gBinderInternalOffsets:

//BinderInternal类的JNI方法注册：
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 }
};

和上面的类似，也是给Native层与framework层之间的相互调用的桥梁。

4. 注册BinderProxy

static int int_register_android_os_BinderProxy(JNIEnv* env) {//gErrorOffsets保存了Error类信息jclass clazz = FindClassOrDie(env, "java/lang/Error");gErrorOffsets.mClass = MakeGlobalRefOrDie(env, clazz);//gBinderProxyOffsets保存了BinderProxy类的信息//其中kBinderProxyPathName = "android/os/BinderProxy"clazz = FindClassOrDie(env, kBinderProxyPathName);gBinderProxyOffsets.mClass = MakeGlobalRefOrDie(env, clazz);gBinderProxyOffsets.mConstructor = GetMethodIDOrDie(env, clazz, "", "()V");gBinderProxyOffsets.mSendDeathNotice = GetStaticMethodIDOrDie(env, clazz, "sendDeathNotice", "(Landroid/os/IBinder$DeathRecipient;)V");gBinderProxyOffsets.mObject = GetFieldIDOrDie(env, clazz, "mObject", "J");gBinderProxyOffsets.mSelf = GetFieldIDOrDie(env, clazz, "mSelf", "Ljava/lang/ref/WeakReference;");gBinderProxyOffsets.mOrgue = GetFieldIDOrDie(env, clazz, "mOrgue", "J");//gClassOffsets保存了Class.getName()方法clazz = FindClassOrDie(env, "java/lang/Class");gClassOffsets.mGetName = GetMethodIDOrDie(env, clazz, "getName", "()Ljava/lang/String;");return RegisterMethodsOrDie(env, kBinderProxyPathName,gBinderProxyMethods, NELEM(gBinderProxyMethods));
}

gBinderProxyMethods中:

//下面BinderProxy类的JNI方法注册：  
static const JNINativeMethod gBinderProxyMethods[] = {/* name, signature, funcPtr */{"pingBinder",          "()Z", (void*)android_os_BinderProxy_pingBinder},{"isBinderAlive",       "()Z", (void*)android_os_BinderProxy_isBinderAlive},{"getInterfaceDescriptor", "()Ljava/lang/String;", (void*)android_os_BinderProxy_getInterfaceDescriptor},{"transactNative",      "(ILandroid/os/Parcel;Landroid/os/Parcel;I)Z", (void*)android_os_BinderProxy_transact},{"linkToDeath",         "(Landroid/os/IBinder$DeathRecipient;I)V", (void*)android_os_BinderProxy_linkToDeath},{"unlinkToDeath",       "(Landroid/os/IBinder$DeathRecipient;I)Z", (void*)android_os_BinderProxy_unlinkToDeath},{"destroy",             "()V", (void*)android_os_BinderProxy_destroy},
};

和上面一样，也是Native层与framework层之间的相互调用的桥梁

三.注册服务

1.SM.addService

从framework的ServiceManager.java开始

public static void addService(String name, IBinder service, boolean allowIsolated) {try {//先获取SMP对象，则执行注册服务操作【见小节3.2/3.4】getIServiceManager().addService(name, service, allowIsolated); } catch (RemoteException e) {Log.e(TAG, "error in addService", e);}
}

在addService中先获去ServiceManagerProxy

2.获取SMP

getIServiceManager

private static IServiceManager getIServiceManager() {if (sServiceManager != null) {return sServiceManager;}sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());return sServiceManager;
}

获取SMP有两步关键步骤，即BinderInternal.getContextObject()，和ServiceManagerNative.asInterface()

BinderInternal.getContextObject()中最终会去获取JNI( android_util_binder.cpp)中android_os_BinderInternal_getContextObject方法，

static jobject android_os_BinderInternal_getContextObject(JNIEnv* env, jobject clazz)
{sp<IBinder> b = ProcessState::self()->getContextObject(NULL);return javaObjectForIBinder(env, b); 
}

对于ProcessState::self()->getContextObject()，在上一节中以及解决过，即ProcessState::self()->getContextObject()等价于 new BpBinder(0);

继续看javaObjectForIBinder():

object javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val) {if (val == NULL) return NULL;if (val->checkSubclass(&gBinderOffsets)) { //返回falsejobject object = static_cast<JavaBBinder*>(val.get())->object();return object;}AutoMutex _l(mProxyLock);jobject object = (jobject)val->findObject(&gBinderProxyOffsets);if (object != NULL) { //第一次object为nulljobject res = jniGetReferent(env, object);if (res != NULL) {return res;}android_atomic_dec(&gNumProxyRefs);val->detachObject(&gBinderProxyOffsets);env->DeleteGlobalRef(object);}//创建BinderProxy对象object = env->NewObject(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mConstructor);if (object != NULL) {//BinderProxy.mObject成员变量记录BpBinder对象env->SetLongField(object, gBinderProxyOffsets.mObject, (jlong)val.get());val->incStrong((void*)javaObjectForIBinder);jobject refObject = env->NewGlobalRef(env->GetObjectField(object, gBinderProxyOffsets.mSelf));//将BinderProxy对象信息附加到BpBinder的成员变量mObjects中val->attachObject(&gBinderProxyOffsets, refObject,jnienv_to_javavm(env), proxy_cleanup);sp<DeathRecipientList> drl = new DeathRecipientList;drl->incStrong((void*)javaObjectForIBinder);//BinderProxy.mOrgue成员变量记录死亡通知对象env->SetLongField(object, gBinderProxyOffsets.mOrgue, reinterpret_cast<jlong>(drl.get()));android_atomic_inc(&gNumProxyRefs);incRefsCreated(env);}return object;
}

根据BpBinder(C++)生成BinderProxy(Java)对象. 主要工作是创建BinderProxy对象,并把BpBinder对象地址保存到BinderProxy.mObject成员变量. 到此，可知ServiceManagerNative.asInterface(BinderInternal.getContextObject()) 等价于

ServiceManagerNative.asInterface(new BinderProxy())

回到上面的ServiceManagerNative.asInterface()中

 static public IServiceManager asInterface(IBinder obj) {if (obj == null) { //obj为BpBinderreturn null;}//由于obj为BpBinder，该方法默认返回nullIServiceManager in = (IServiceManager)obj.queryLocalInterface(descriptor);if (in != null) {return in;}return new ServiceManagerProxy(obj); 
}

所以ServiceManagerNative.asInterface(BinderInternal.getContextObject())等价于

new ServiceManagerProxy(new BinderProxy()).

来看看 ServiceManagerProxy()的构造函数

class ServiceManagerProxy implements IServiceManager {public ServiceManagerProxy(IBinder remote) {mRemote = remote;}
}

mRemote为BinderProxy对象，该BinderProxy对象对应于BpBinder(0)，其作为binder代理端，指向native层大管家service Manager。

3.SMP.addService()

通过上面的分析可知getIServiceManager()最终获取的是ServiceManagerProxy()，所以我们来看看ServiceManagerProxy()的addService方法。

public void addService(String name, IBinder service, boolean allowIsolated) throws RemoteException {Parcel data = Parcel.obtain();Parcel reply = Parcel.obtain();data.writeInterfaceToken(IServiceManager.descriptor);data.writeString(name);//【见下文】data.writeStrongBinder(service);data.writeInt(allowIsolated ? 1 : 0);//mRemote为BinderProxy【见下文】mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0);reply.recycle();data.recycle();
}

这里我们发现，和Native层注册服务的步骤几乎一模一样。
所以问题变为FrameWork层的Binder转换为Native层。

当时我们分析Native层的时候，主要看的是writeStrongBinder和transact方法，这里我们继续看这两个方法是如何从java到c++的。

4.writeStrongBinder

public writeStrongBinder(IBinder val){//此处为Native调用nativewriteStrongBinder(mNativePtr, val);
}

看调用JNI的方法

static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr, jobject object) {//将java层Parcel转换为native层ParcelParcel* parcel = reinterpret_cast<Parcel*>(nativePtr);if (parcel != NULL) {//ibinderForJavaObject【见下】const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object));if (err != NO_ERROR) {signalExceptionForError(env, clazz, err);}}
}

继续看ibinderForJavaObject

sp<IBinder> ibinderForJavaObject(JNIEnv* env, jobject obj)
{if (obj == NULL) return NULL;//Java层的Binder对象if (env->IsInstanceOf(obj, gBinderOffsets.mClass)) {JavaBBinderHolder* jbh = (JavaBBinderHolder*)env->GetLongField(obj, gBinderOffsets.mObject);return jbh != NULL ? jbh->get(env, obj) : NULL; }//Java层的BinderProxy对象if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) {return (IBinder*)env->GetLongField(obj, gBinderProxyOffsets.mObject);}return NULL;
}

在这里面将Binde(Java)生成JavaBBinderHolder(C++)对象. 主要工作是创建JavaBBinderHolder对象,并把JavaBBinderHolder对象地址保存到Binder.mObject成员变量。

我们看看 ibinderForJavaObject最终返回的结果,即JavaBBinderHolder.get()

sp<JavaBBinder> get(JNIEnv* env, jobject obj) {AutoMutex _l(mLock);sp<JavaBBinder> b = mBinder.promote();if (b == NULL) {//首次进来，创建JavaBBinder对象b = new JavaBBinder(env, obj);mBinder = b;}return b;
}

所以最终data.writeStrongBinder(service);转换为parcel->writeStrongBinder(new JavaBBinder(env, obj));

最后来看看parcel->writeStrongBinder。

status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
{return flatten_binder(ProcessState::self(), val, this);
}

没错，这就是我们上一篇博客中分析的writeStrongBinder的内容。

5.mRemote.transact

在第一节中，我们知道mRemote 其实就BinderProxy。

public boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException {//用于检测Parcel大小是否大于800kBinder.checkParcel(this, code, data, "Unreasonably large binder buffer");return transactNative(code, data, reply, flags); 
}

transactNative经过jni调用，进入方法 jni的android_os_BinderProxy_transact

static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,jint code, jobject dataObj, jobject replyObj, jint flags)
{...//java Parcel转为native ParcelParcel* data = parcelForJavaObject(env, dataObj);Parcel* reply = parcelForJavaObject(env, replyObj);...//gBinderProxyOffsets.mObject中保存的是new BpBinder(0)对象IBinder* target = (IBinder*)env->GetLongField(obj, gBinderProxyOffsets.mObject);...//此处便是BpBinder::transact(), 经过native层，进入Binder驱动程序status_t err = target->transact(code, *data, reply, flags);...return JNI_FALSE;
}

Java层的BinderProxy.transact()最终交由Native层的BpBinder::transact()完成。之后的内容在就是上一篇博客的内容，也就是交给了Native。

6.小结

在framework中，也是先获取ServiceManagerm,他最终获得的是一个ServiceManagermProxy,在它之中含有一个BinderProxy的成员变量，它含有Native层的BpBInder，而BpBinder含有Binder驱动中ServiceManager的binder对象。

之后和Native层一样，调用Parcel的writeStrongBinder和BinderProxy的transact
，最终经过JNI，调用Native层对应的方法。

四.获取服务

和注册服务一样，调用ServiceManager中的getService方法。

1. SM.getService

public static IBinder getService(String name) {try {IBinder service = sCache.get(name); //先从缓存中查看if (service != null) {return service;} else {return getIServiceManager().getService(name); }} catch (RemoteException e) {Log.e(TAG, "error in getService", e);}return null;
}

return getIServiceManager().getService(name)

这行代码和获取服务一样，最终转换为ServiceManagerNative.getService(name);所以我们直接看对应代码

2. SMP.getService

lass ServiceManagerProxy implements IServiceManager {public IBinder getService(String name) throws RemoteException {Parcel data = Parcel.obtain();Parcel reply = Parcel.obtain();data.writeInterfaceToken(IServiceManager.descriptor);data.writeString(name);//mRemote为BinderProxy mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0); //从reply里面解析出获取的IBinder对象IBinder binder = reply.readStrongBinder(); reply.recycle();data.recycle();return binder;}
}

这里的代码还是Native层几乎一样，我们一样关注reply.readStrongBinder和 mRemote.transact如何调用到Native层的代码

3.mRemote.transact

final class BinderProxy implements IBinder {public boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException {Binder.checkParcel(this, code, data, "Unreasonably large binder buffer");return transactNative(code, data, reply, flags);}
}

transactNative最后会调用jni中的android_os_BinderProxy_transact方法

static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,jint code, jobject dataObj, jobject replyObj, jint flags)
{...//java Parcel转为native ParcelParcel* data = parcelForJavaObject(env, dataObj);Parcel* reply = parcelForJavaObject(env, replyObj);...//gBinderProxyOffsets.mObject中保存的是new BpBinder(0)对象IBinder* target = (IBinder*)env->GetLongField(obj, gBinderProxyOffsets.mObject);...//此处便是BpBinder::transact()status_t err = target->transact(code, *data, reply, flags);...return JNI_FALSE;
}

在这里的最后，发现调用了BpBinder的transact方法，之后就是上一篇博客的内容了。

4.readStrongBinder

java层的

public final IBinder readStrongBinder() {return nativeReadStrongBinder(mNativePtr);}

jni层

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;
}

parcel->readStrongBinder()此处就是Native层的方法，也就是我们上一篇文章分析的地方

来看看javaObjectForIBinder是干什么的

jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val)
{if (val == NULL) return NULL;if (val->checkSubclass(&gBinderOffsets)) {// One of our own!//本线程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.//加锁AutoMutex _l(mProxyLock);// Someone else's...  do we know about it?//其他线程的jobject object = (jobject)val->findObject(&gBinderProxyOffsets);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);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->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.//本机对象需要将一个弱引用保留回代理，以便我们可以检索相同的代理(如果它仍然是活动的)。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//还请记住在此代理上注册的死亡收件人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.//注意，已经创建了一个新的对象引用android_atomic_inc(&gNumProxyRefs);incRefsCreated(env);}return object;
}

在这里主要是将Native层返回的Binder，转换为Framework的Binder。

5.总结

获取服务时，也是先获得ServiceManagerProxy，它其中包含BinderProxy的成员变量，它含有Native层的BpBInder，而BpBinder指向Binder驱动中ServiceManager的binder对象。

在SMP中，获取服务的代码和Native层中几乎一样，最终经过JNI，转换为NAtive层的代码

五.总结

这一篇来说，相对容易理解一下，特别是ServiceManager和BinderProxy在Native层都有对应的对象，可以类比。

在ServiceManagerProxy中的AddService和getService的代码几乎和Native层一样，之后利用JNI调用到Native层的代码。

六.参考资料

Binder系列7—framework层分析

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