Binder系列7—framework层分析

copy from :http://gityuan.com/2015/11/21/binder-framework/

主要分析Binder在java framework层的框架，相关源码：

framework/base/core/java/android/os/
  - IInterface.java
  - IServiceManager.java
  - ServiceManager.java
  - ServiceManagerNative.java(内含ServiceManagerProxy类)

framework/base/core/java/android/os/
  - IBinder.java
  - Binder.java(内含BinderProxy类)
  - Parcel.java

framework/base/core/java/com/android/internal/os/
  - BinderInternal.java

framework/base/core/jni/
  - AndroidRuntime.cpp
  - android_os_Parcel.cpp
  - android_util_Binder.cpp

一、概述

1.1 Binder架构

binder在framework层，采用JNI技术来调用native(C/C++)层的binder架构，从而为上层应用程序提供服务。看过binder系列之前的文章，我们知道native层中，binder是C/S架构，分为Bn端(Server)和Bp端(Client)。对于java层在命名与架构上非常相近，同样实现了一套IPC通信架构。

framework Binder架构图：查看大图

java_binder

图解：

图中红色代表整个framework层 binder架构相关组件；
- Binder类代表Server端，BinderProxy类代码Client端；
图中蓝色代表Native层Binder架构相关组件；
上层framework层的Binder逻辑是建立在Native层架构基础之上的，核心逻辑都是交予Native层方法来处理。
framework层的ServiceManager类与Native层的功能并不完全对应，framework层的ServiceManager类的实现最终是通过BinderProxy传递给Native层来完成的，后面会详细说明。

1.2 Binder类图

下面列举framework的binder类关系图：查看大图

class_java_binder

图解：(图中浅蓝色都是Interface，其余都是Class)

ServiceManager：通过getIServiceManager方法获取的是ServiceManagerProxy对象； ServiceManager的addService, getService实际工作都交由ServiceManagerProxy的相应方法来处理；
ServiceManagerProxy：其成员变量mRemote指向BinderProxy对象，ServiceManagerProxy的addService, getService方法最终是交由mRemote来完成。
ServiceManagerNative：其方法asInterface()返回的是ServiceManagerProxy对象，ServiceManager便是借助ServiceManagerNative类来找到ServiceManagerProxy；
Binder：其成员变量mObject和方法execTransact()用于native方法
BinderInternal：内部有一个GcWatcher类，用于处理和调试与Binder相关的垃圾回收。
IBinder：接口中常量FLAG_ONEWAY：客户端利用binder跟服务端通信是阻塞式的，但如果设置了FLAG_ONEWAY，这成为非阻塞的调用方式，客户端能立即返回，服务端采用回调方式来通知客户端完成情况。另外IBinder接口有一个内部接口DeathDecipient(死亡通告)。

1.3 Binder类分层

整个Binder从kernel至，native，JNI，Framework层所涉及的全部类

点击查看大图

java_binder_framework

二、初始化

在Android系统开机过程中，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。

2.1 startReg

==> AndroidRuntime.cpp

 
  int AndroidRuntime::startReg(JNIEnv* env)
{
    androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);

    env->PushLocalFrame(200);

    //注册jni方法【见2.2】
    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL); return -1; } env->PopLocalFrame(NULL); return 0; }  
 

注册JNI方法，其中gRegJNI是一个数组，记录所有需要注册的jni方法，其中有一项便是REG_JNI(register_android_os_Binder)，下面说说register_android_os_Binder过程。

2.2 register_android_os_Binder

==> android_util_Binder.cpp

 
  int register_android_os_Binder(JNIEnv* env) {
    // 注册Binder类的jni方法【见2.3】 if (int_register_android_os_Binder(env) < 0) return -1; // 注册BinderInternal类的jni方法【见2.4】 if (int_register_android_os_BinderInternal(env) < 0) return -1; // 注册BinderProxy类的jni方法【见2.5】 if (int_register_android_os_BinderProxy(env) < 0) return -1; ... return 0; }

2.3 注册Binder

==> android_util_Binder.cpp

 
  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 = FindClassOrDie(env, kBinderPathName); //将Java层Binder类保存到mClass变量； gBinderOffsets.mClass = MakeGlobalRefOrDie(env, clazz); //将Java层execTransact()方法保存到mExecTransact变量； gBinderOffsets.mExecTransact = GetMethodIDOrDie(env, clazz, "execTransact", "(IJJI)Z"); //将Java层mObject属性保存到mObject变量 gBinderOffsets.mObject = GetFieldIDOrDie(env, clazz, "mObject", "J"); //注册JNI方法 return RegisterMethodsOrDie(env, kBinderPathName, gBinderMethods, NELEM(gBinderMethods)); }  
 

FindClassOrDie(env, kBinderPathName) 基本等价于 env->FindClass(kBinderPathName)
MakeGlobalRefOrDie() 等价于 env->NewGlobalRef()
GetMethodIDOrDie() 等价于 env->GetMethodID()
GetFieldIDOrDie() 等价于 env->GeFieldID()
RegisterMethodsOrDie() 等价于 Android::registerNativeMethods();

(1)gBinderOffsets

gBinderOffsets是全局静态结构体(struct)，定义如下：

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

gBinderOffsets保存了Binder.java类本身以及其成员方法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，而不再需要每次查找binder类信息的方式能大幅度提高效率，是由于每次查询需要花费较多的CPU时间，尤其是频繁访问时，但用额外的结构体来保存这些信息，是以空间换时间的方法。

(2)gBinderMethods

 
  static const JNINativeMethod gBinderMethods[] = {
     /* 名称, 签名, 函数指针 */
    { "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 }, { "blockUntilThreadAvailable", "()V", (void*)android_os_Binder_blockUntilThreadAvailable } };  
 

通过RegisterMethodsOrDie()，将为gBinderMethods数组中的方法建立了一一映射关系，从而为Java层访问JNI层提供通道。

总之，int_register_android_os_Binder方法的主要功能：

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。

也就是说该过程建立了Binder类在Native层与framework层之间的相互调用的桥梁。

2.4 注册BinderInternal

==> android_util_Binder.cpp

 
  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)); }  
 

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

下面是BinderInternal类的JNI方法注册：

 
  static const JNINativeMethod gBinderInternalMethods[] = {
    { "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 } };

该过程其【2.3】非常类似，也就是说该过程建立了是BinderInternal类在Native层与framework层之间的相互调用的桥梁。

2.5 注册BinderProxy

==> android_util_Binder.cpp

 
  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, "<init>", "()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)); }  
 

注册BinderProxy类的jni方法，gBinderProxyOffsets保存了BinderProxy的构造方法，sendDeathNotice(), mObject, mSelf, mOrgue信息。

下面BinderProxy类的JNI方法注册：

 
  static const JNINativeMethod gBinderProxyMethods[] = {
     /* 名称, 签名, 函数指针 */
    {"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}, };  
 

该过程其【2.3】非常类似，也就是说该过程建立了是BinderProxy类在Native层与framework层之间的相互调用的桥梁。

三、注册服务

3.1 SM.addService

[-> ServiceManager.java]

 
  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); } }  
 

先来看看getIServiceManager()过程，如下：

3.2 getIServiceManager

[-> ServiceManager.java]

 
  private static IServiceManager getIServiceManager() { if (sServiceManager != null) { return sServiceManager; } //【分别见3.2.1和3.3】 sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject()); return sServiceManager; }  
 

采用了单例模式获取ServiceManager getIServiceManager()返回的是ServiceManagerProxy(简称SMP)对象

3.2.1 getContextObject()

[-> android_util_binder.cpp]

 
  static jobject android_os_BinderInternal_getContextObject(JNIEnv* env, jobject clazz)
{
    sp<IBinder> b = ProcessState::self()->getContextObject(NULL);
    return javaObjectForIBinder(env, b);  //【见3.2.2】
}
 
 

BinderInternal.java中有一个native方法getContextObject()，JNI调用执行上述方法。

对于ProcessState::self()->getContextObject()，在获取ServiceManager的第3节已详细解决，即ProcessState::self()->getContextObject()等价于 new BpBinder(0);

3.2.2 javaObjectForIBinder

[-> android_util_binder.cpp]

 
  jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val) {
    if (val == NULL) return NULL; if (val->checkSubclass(&gBinderOffsets)) { //返回false jobject 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null jobject 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())

3.3 SMN.asInterface

[-> ServiceManagerNative.java]

 static public IServiceManager asInterface(IBinder obj) { if (obj == null) { //obj为BpBinder return null; } //由于obj为BpBinder，该方法默认返回null IServiceManager in = (IServiceManager)obj.queryLocalInterface(descriptor); if (in != null) { return in; } return new ServiceManagerProxy(obj); //【见小节3.3.1】 }

由此，可知ServiceManagerNative.asInterface(new BinderProxy()) 等价于new ServiceManagerProxy(new BinderProxy()). 为了方便，ServiceManagerProxy简称为SMP。

3.3.1 ServiceManagerProxy初始化

[-> ServiceManagerNative.java ::ServiceManagerProxy]

 
  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。

ServiceManager.getIServiceManager最终等价于new ServiceManagerProxy(new BinderProxy()),意味着【3.1】中的getIServiceManager().addService()，等价于SMP.addService().

framework层的ServiceManager的调用实际的工作确实交给SMP的成员变量BinderProxy；而BinderProxy通过jni方式，最终会调用BpBinder对象；可见上层binder架构的核心功能依赖native架构的服务来完成的。

3.4 SMP.addService

[-> ServiceManagerNative.java ::ServiceManagerProxy]

 
  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); //【见小节3.5】 data.writeStrongBinder(service); data.writeInt(allowIsolated ? 1 : 0); //mRemote为BinderProxy【见小节3.7】 mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0); reply.recycle(); data.recycle(); }  
 

3.5 writeStrongBinder(Java)

[-> Parcel.java]

 
  public writeStrongBinder(IBinder val){
    //此处为Native调用【见3.5.1】 nativewriteStrongBinder(mNativePtr, val); }

3.5.1 android_os_Parcel_writeStrongBinder

[-> android_os_Parcel.cpp]

 
  static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr, jobject object) { //将java层Parcel转换为native层Parcel Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr); if (parcel != NULL) { //【见3.5.2】 const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object)); if (err != NO_ERROR) { signalExceptionForError(env, clazz, err); } } }  
 

3.5.2 ibinderForJavaObject

[-> android_util_Binder.cpp]

 
  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; //【见3.5.3】 } //Java层的BinderProxy对象 if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) { return (IBinder*)env->GetLongField(obj, gBinderProxyOffsets.mObject); } return NULL; }

根据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.

3.5.3 JavaBBinderHolder.get()

[-> android_util_Binder.cpp]

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【见3.5.4】 b = new JavaBBinder(env, obj); mBinder = b; } return b; }

JavaBBinderHolder有一个成员变量mBinder，保存当前创建的JavaBBinder对象，这是一个wp类型的，可能会被垃圾回收器给回收，所以每次使用前，都需要先判断是否存在。

3.5.4 JavaBBinder初始化

==> [-> android_util_Binder.cpp]

 
  JavaBBinder(JNIEnv* env, jobject object)
    : mVM(jnienv_to_javavm(env)), mObject(env->NewGlobalRef(object))
{
    android_atomic_inc(&gNumLocalRefs);
    incRefsCreated(env);
}
 
 

创建JavaBBinder，该对象继承于BBinder对象。

data.writeStrongBinder(service)最终等价于parcel->writeStrongBinder(new JavaBBinder(env, obj));

3.6 writeStrongBinder(C++)

[-> parcel.cpp]

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

3.6.1 flatten_binder

[-> parcel.cpp]

 
  status_t flatten_binder(const sp<ProcessState>& /*proc*/,
    const sp<IBinder>& binder, Parcel* out)
{
    flat_binder_object obj;

    obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS; if (binder != NULL) { IBinder *local = binder->localBinder(); if (!local) { BpBinder *proxy = binder->remoteBinder(); const int32_t handle = proxy ? proxy->handle() : 0; obj.type = BINDER_TYPE_HANDLE; //远程Binder obj.binder = 0; obj.handle = handle; obj.cookie = 0; } else { obj.type = BINDER_TYPE_BINDER; //本地Binder，进入该分支 obj.binder = reinterpret_cast<uintptr_t>(local->getWeakRefs()); obj.cookie = reinterpret_cast<uintptr_t>(local); } } else { obj.type = BINDER_TYPE_BINDER; //本地Binder obj.binder = 0; obj.cookie = 0; } //【见小节3.6.2】 return finish_flatten_binder(binder, obj, out); }  
 

将Binder对象扁平化，转换成flat_binder_object对象。

对于Binder实体，则cookie记录Binder实体的指针；
对于Binder代理，则用handle记录Binder代理的句柄；

关于localBinder，代码见Binder.cpp。

 
  BBinder* BBinder::localBinder()
{
    return this;
}

BBinder* IBinder::localBinder()
{
    return NULL;
}
 
 

3.6.2 finish_flatten_binder

 
  inline static status_t finish_flatten_binder(
    const sp<IBinder>& , const flat_binder_object& flat, Parcel* out) { return out->writeObject(flat, false); }

再回到小节3.4的addService过程，则接下来进入transact。

3.7 BinderProxy.transact

[-> Binder.java ::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800k Binder.checkParcel(this, code, data, "Unreasonably large binder buffer"); return transactNative(code, data, reply, flags); //【见3.8】 }  
 

回到ServiceManagerProxy.addService，其成员变量mRemote是BinderProxy。transactNative经过jni调用，进入下面的方法

3.8 android_os_BinderProxy_transact

[-> android_util_Binder.cpp]

 
  static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,
    jint code, jobject dataObj, jobject replyObj, jint flags)
{
    ...
    //java Parcel转为native Parcel
    Parcel* 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。Native Binder的注册服务(addService)中有详细说明BpBinder执行过程。另外，该方法可抛出RemoteException。

3.9 小结

addService的核心过程：

 
  public void addService(String name, IBinder service, boolean allowIsolated) throws RemoteException { ... Parcel data = Parcel.obtain(); //此处还需要将java层的Parcel转为Native层的Parcel data->writeStrongBinder(new JavaBBinder(env, obj)); BpBinder::transact(ADD_SERVICE_TRANSACTION, *data, reply, 0); //与Binder驱动交互 ... }  
 

注册服务过程就是通过BpBinder来发送ADD_SERVICE_TRANSACTION命令，与实现与binder驱动进行数据交互。

四、获取服务

4.1 SM.getService

[-> ServiceManager.java]

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); 【见4.2】 } } catch (RemoteException e) { Log.e(TAG, "error in getService", e); } return null; }

关于getIServiceManager()，在前面小节3.2已经讲述了，等价于new ServiceManagerProxy(new BinderProxy())。其中sCache = new HashMap<String, IBinder>()以hashmap格式缓存已组成的名称。请求获取服务过程中，先从缓存中查询是否存在，如果缓存中不存在的话，再通过binder交互来查询相应的服务。

4.2 SMP.getService

[-> ServiceManagerNative.java ::ServiceManagerProxy]

 
  class 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 【见4.3】 mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0); //从reply里面解析出获取的IBinder对象【见4.8】 IBinder binder = reply.readStrongBinder(); reply.recycle(); data.recycle(); return binder; } }  
 

4.3 BinderProxy.transact

[-> Binder.java]

 
  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); } }  
 

4.4 android_os_BinderProxy_transact

[-> android_util_Binder.cpp]

 
  static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,
    jint code, jobject dataObj, jobject replyObj, jint flags)
{
    ...
    //java Parcel转为native Parcel
    Parcel* 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[见小节4.5]
    status_t err = target->transact(code, *data, reply, flags);
    ...
    return JNI_FALSE; }  
 

4.5 BpBinder.transact

[-> BpBinder.cpp]

 
  status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    if (mAlive) {
        // [见小节4.6]
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0; return status; } return DEAD_OBJECT; }  
 

4.6 IPC.transact

[-> IPCThreadState.cpp]

 
  status_t IPCThreadState::transact(int32_t handle,
                                  uint32_t code, const Parcel& data,
                                  Parcel* reply, uint32_t flags) { status_t err = data.errorCheck(); //数据错误检查 flags |= TF_ACCEPT_FDS; .... if (err == NO_ERROR) { // 传输数据 err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL); } ... // 默认情况下,都是采用非oneway的方式, 也就是需要等待服务端的返回结果 if ((flags & TF_ONE_WAY) == 0) { if (reply) { //等待回应事件 err = waitForResponse(reply); }else { Parcel fakeReply; err = waitForResponse(&fakeReply); } } else { err = waitForResponse(NULL, NULL); } return err; }  
 

4.7 IPC.waitForResponse

 
  status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult)
{
    int32_t cmd;
    int32_t err;
    while (1) { if ((err=talkWithDriver()) < NO_ERROR) break; ... cmd = mIn.readInt32(); switch (cmd) { case BR_REPLY: { binder_transaction_data tr; err = mIn.read(&tr, sizeof(tr)); if (reply) { if ((tr.flags & TF_STATUS_CODE) == 0) { //当reply对象回收时，则会调用freeBuffer来回收内存 reply->ipcSetDataReference( reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer), tr.data_size, reinterpret_cast<const binder_size_t*>(tr.data.ptr.offsets), tr.offsets_size/sizeof(binder_size_t), freeBuffer, this); } else { ... } } } case :... } } ... return err; }  
 

那么这个reply是哪来的呢，在文章Binder系列3—启动ServiceManager

4.7.1 binder_send_reply

[-> servicemanager/binder.c]

 
  void binder_send_reply(struct binder_state *bs, struct binder_io *reply, binder_uintptr_t buffer_to_free, int status) { struct { uint32_t cmd_free; binder_uintptr_t buffer; uint32_t cmd_reply; struct binder_transaction_data txn; } __attribute__((packed)) data; data.cmd_free = BC_FREE_BUFFER; //free buffer命令 data.buffer = buffer_to_free; data.cmd_reply = BC_REPLY; // reply命令 data.txn.target.ptr = 0; data.txn.cookie = 0; data.txn.code = 0; if (status) { ... } else {= data.txn.flags = 0; data.txn.data_size = reply->data - reply->data0; data.txn.offsets_size = ((char*) reply->offs) - ((char*) reply->offs0); data.txn.data.ptr.buffer = (uintptr_t)reply->data0; data.txn.data.ptr.offsets = (uintptr_t)reply->offs0; } //向Binder驱动通信 binder_write(bs, &data, sizeof(data)); }  
 

binder_write将BC_FREE_BUFFER和BC_REPLY命令协议发送给驱动，进入驱动。binder_ioctl -> binder_ioctl_write_read -> binder_thread_write，由于是BC_REPLY命令协议，则进入binder_transaction，该方法会向请求服务的线程Todo队列插入事务。

接下来，请求服务的进程在执行talkWithDriver的过程执行到binder_thread_read()，处理Todo队列的事务。

4.8 readStrongBinder

[-> Parcel.java]

readStrongBinder的过程基本是writeStrongBinder逆过程。

 
  static jobject android_os_Parcel_readStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr) {
    Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr); if (parcel != NULL) { //【见小节4.8.1】 return javaObjectForIBinder(env, parcel->readStrongBinder()); } return NULL; }

javaObjectForIBinder 将native层BpBinder对象转换为Java层BinderProxy对象。

4.8.1 readStrongBinder(C++)

[-> Parcel.cpp]

 
  sp<IBinder> Parcel::readStrongBinder() const
{
    sp<IBinder> val;
    //【见小节4.8.2】
    unflatten_binder(ProcessState::self(), *this, &val);
    return val; }  
 

4.8.2 unflatten_binder

[-> 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: *out = reinterpret_cast<IBinder*>(flat->cookie); return finish_unflatten_binder(NULL, *flat, in); case BINDER_TYPE_HANDLE: //进入该分支【见4.8.3】 *out = proc->getStrongProxyForHandle(flat->handle); //创建BpBinder对象 return finish_unflatten_binder( static_cast<BpBinder*>(out->get()), *flat, in); } } return BAD_TYPE; }

4.8.3 getStrongProxyForHandle

[-> ProcessState.cpp]

 
  sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
    sp<IBinder> result;

    AutoMutex _l(mLock);
    //查找handle对应的资源项
    handle_entry* e = lookupHandleLocked(handle);

    if (e != NULL) {
        IBinder* b = e->binder;
        if (b == NULL || !e->refs->attemptIncWeak(this)) { ... //当handle值所对应的IBinder不存在或弱引用无效时，则创建BpBinder对象 b = new BpBinder(handle); e->binder = b; if (b) e->refs = b->getWeakRefs(); result = b; } else { result.force_set(b); e->refs->decWeak(this); } } return result; }  
 

经过该方法，最终创建了指向Binder服务端的BpBinder代理对象。回到[小节4.8] 经过javaObjectForIBinder将native层BpBinder对象转换为Java层BinderProxy对象。也就是说通过getService()最终获取了指向目标Binder服务端的代理对象BinderProxy。

4.9 小结

getService的核心过程：

 
  public static IBinder getService(String name) {
    ...
    Parcel reply = Parcel.obtain(); //此处还需要将java层的Parcel转为Native层的Parcel
    BpBinder::transact(GET_SERVICE_TRANSACTION, *data, reply, 0);  //与Binder驱动交互 IBinder binder = javaObjectForIBinder(env, new BpBinder(handle)); ... }  
 

javaObjectForIBinder作用是创建BinderProxy对象，并将BpBinder对象的地址保存到BinderProxy对象的mObjects中。获取服务过程就是通过BpBinder来发送GET_SERVICE_TRANSACTION命令，与实现与binder驱动进行数据交互。

五. 实例

以IWindowManager为例

 
  public interface IWindowManager extends android.os.IInterface { public static abstract class Stub extends android.os.Binder implements android.view.IWindowManager { private static final java.lang.String DESCRIPTOR = "android.view.IWindowManager"; public Stub() { this.attachInterface(this, DESCRIPTOR); } public static android.view.IWindowManager asInterface(android.os.IBinder obj) { if ((obj == null)) { return null; } android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR); if (((iin != null) && (iin instanceof android.view.IWindowManager))) { return ((android.view.IWindowManager) iin); } return new android.view.IWindowManager.Stub.Proxy(obj); } public android.os.IBinder asBinder() { return this; } private static class Proxy implements android.view.IWindowManager { private android.os.IBinder mRemote; Proxy(android.os.IBinder remote) { mRemote = remote; } public android.os.IBinder asBinder() { return mRemote; } } ... } }  
 

5.1 Binder

[-> Binder.java]

 
  public class Binder implements IBinder { public void attachInterface(IInterface owner, String descriptor) { mOwner = owner; mDescriptor = descriptor; } public IInterface queryLocalInterface(String descriptor) { if (mDescriptor.equals(descriptor)) { return mOwner; } return null; } }  
 

5.2 BinderProxy

 
  final class BinderProxy implements IBinder { public IInterface queryLocalInterface(String descriptor) { return null; } } 
 