Android系统源码分析-进程间通信机制binder(三)：从framework层到Native层

太过爱你忘了你带给我的痛 2023-10-16 22:01 105阅读 0赞

#### 1.内容简介： ####

Android Binder机制既提供IPC通信，又具有RPC功能，这样，对应上层应用程序提供了非常友好的API调用接口。实际上，为了支持支撑这一机制，从Android Framework到Native再到Binder Driver，是一个非常复杂的业务支撑系统。本节主要介绍在这个过程中，Binder在框架层的相关组件。

Binder源码分析系列文章：

[Android系统源码分析-进程间通信机制binder(一)：守护进程servicemanager][Android_-_binder_servicemanager]

[Android系统源码分析-进程间通信机制binder(二)：binder内存映射][Android_-_binder_binder]

[Android系统源码分析-进程间通信机制binder(三)：从framework层到Native层][Android_-_binder_framework_Native]

[Android系统源码分析-进程间通信机制binder(四)：从Native层到Driver层][Android_-_binder_Native_Driver]

#### 2.Framework Binder架构： ####

从进程的角度，先看一下简化的Framework Binder结构：

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBAbGlyYW5rZQ_size_20_color_FFFFFF_t_70_g_se_x_16][]

说明：

Manager：属于Android Framework层，被用户App调用，它又调用Server的stub（通过代理proxy）。例如，ActivityManager，PackageManager...等。它运行在App所在的进程中。

Server：属于Android Framework层，向上提供服务，向下与servicemanager进行交互。

servicemanager：即服务管理，是一个守护进程。

Binder Driver：binder驱动。

Framework Binder相关组件：

为了支撑上述架构，Android提供了一系列的组件，如下图：

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBAbGlyYW5rZQ_size_20_color_FFFFFF_t_70_g_se_x_16 1][]

说明：

整个框架分为如下几层：

JAVA，JNI，C++，C层，这与Android系统框架是一致的；

1）JAVA层：

Service: 提供服务的组件，例如，ActivityManagerService,PackageMangerService。

Client：这里的client指的是Service的调用者，例如， ActivityManager，PackageManager。

SeiviceManager:提供service的管理，包括注册，获取service等；它的最大的特点是：它本身也是一个service，遵循C/S架构；同时，它又是其它service的管理类。用户要想访问到其它service，必须通过ServiceManager来实现。

parcel和Parcelable:用于Binder通信信息的数据结构；

IInterface:该接口是binder服务接口的父接口；

IBinder：Java层的IBinder类，提供了transact方法来调用远程服务；

Binder: 实现了IBinder接口，封装了JNI的实现。Java层Binder服务的基类；

IServiceManager：用于ServiceManager管理的基类；

2）JNI层：承上启下，进行各个组件的衔接（java层到Native层）。native即C/C++代码，jni使得程序员可以在C/C++端调用Java代码，也可以在Java端调用C/C++代码。下面这段代码展示了“如何进行java与native的衔接”：

以下代码截取自android\_util\_Binder.cpp：

static const JNINativeMethod gBinderMethods[] = {
         /* name, signature, funcPtr */
        { "getCallingPid", "()I", (void*)android_os_Binder_getCallingPid },
        { "getCallingUid", "()I", (void*)android_os_Binder_getCallingUid },
        { "getOrigCallingUidNative", "()I", (void*)android_os_Binder_getOrigCallingUid },
        { "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 }
    };

以下代码截取自android\_os\_Parcel.cpp注册的方法：

static const JNINativeMethod gParcelMethods[] = {
        {"nativeDataSize",            "(I)I", (void*)android_os_Parcel_dataSize},
        {"nativeDataAvail",           "(I)I", (void*)android_os_Parcel_dataAvail},
        {"nativeDataPosition",        "(I)I", (void*)android_os_Parcel_dataPosition},
        {"nativeDataCapacity",        "(I)I", (void*)android_os_Parcel_dataCapacity},
        {"nativeSetDataSize",         "(II)V", (void*)android_os_Parcel_setDataSize},
        {"nativeSetDataPosition",     "(II)V", (void*)android_os_Parcel_setDataPosition},
        {"nativeSetDataCapacity",     "(II)V", (void*)android_os_Parcel_setDataCapacity},
    
        {"nativePushAllowFds",        "(IZ)Z", (void*)android_os_Parcel_pushAllowFds},
        {"nativeRestoreAllowFds",     "(IZ)V", (void*)android_os_Parcel_restoreAllowFds},
    
        {"nativeWriteByteArray",      "(I[BII)V", (void*)android_os_Parcel_writeNative},
        {"nativeWriteInt",            "(II)V", (void*)android_os_Parcel_writeInt},
        {"nativeWriteLong",           "(IJ)V", (void*)android_os_Parcel_writeLong},
        {"nativeWriteFloat",          "(IF)V", (void*)android_os_Parcel_writeFloat},
        {"nativeWriteDouble",         "(ID)V", (void*)android_os_Parcel_writeDouble},
        {"nativeWriteString",         "(ILjava/lang/String;)V", (void*)android_os_Parcel_writeString},
        {"nativeWriteStrongBinder",   "(ILandroid/os/IBinder;)V", (void*)android_os_Parcel_writeStrongBinder},
        {"nativeWriteFileDescriptor", "(ILjava/io/FileDescriptor;)V", (void*)android_os_Parcel_writeFileDescriptor},
    
        {"nativeCreateByteArray",     "(I)[B", (void*)android_os_Parcel_createByteArray},
        {"nativeReadInt",             "(I)I", (void*)android_os_Parcel_readInt},
        {"nativeReadLong",            "(I)J", (void*)android_os_Parcel_readLong},
        {"nativeReadFloat",           "(I)F", (void*)android_os_Parcel_readFloat},
        {"nativeReadDouble",          "(I)D", (void*)android_os_Parcel_readDouble},
        {"nativeReadString",          "(I)Ljava/lang/String;", (void*)android_os_Parcel_readString},
        {"nativeReadStrongBinder",    "(I)Landroid/os/IBinder;", (void*)android_os_Parcel_readStrongBinder},
        {"nativeReadFileDescriptor",  "(I)Ljava/io/FileDescriptor;", (void*)android_os_Parcel_readFileDescriptor},
    
        {"openFileDescriptor",        "(Ljava/lang/String;I)Ljava/io/FileDescriptor;", (void*)android_os_Parcel_openFileDescriptor},
        {"dupFileDescriptor",         "(Ljava/io/FileDescriptor;)Ljava/io/FileDescriptor;", (void*)android_os_Parcel_dupFileDescriptor},
        {"closeFileDescriptor",       "(Ljava/io/FileDescriptor;)V", (void*)android_os_Parcel_closeFileDescriptor},
        {"clearFileDescriptor",       "(Ljava/io/FileDescriptor;)V", (void*)android_os_Parcel_clearFileDescriptor},
    
        {"nativeCreate",              "()I", (void*)android_os_Parcel_create},
        {"nativeFreeBuffer",          "(I)V", (void*)android_os_Parcel_freeBuffer},
        {"nativeDestroy",             "(I)V", (void*)android_os_Parcel_destroy},
    
        {"nativeMarshall",            "(I)[B", (void*)android_os_Parcel_marshall},
        {"nativeUnmarshall",          "(I[BII)V", (void*)android_os_Parcel_unmarshall},
        {"nativeAppendFrom",          "(IIII)V", (void*)android_os_Parcel_appendFrom},
        {"nativeHasFileDescriptors",  "(I)Z", (void*)android_os_Parcel_hasFileDescriptors},
        {"nativeWriteInterfaceToken", "(ILjava/lang/String;)V", (void*)android_os_Parcel_writeInterfaceToken},
        {"nativeEnforceInterface",    "(ILjava/lang/String;)V", (void*)android_os_Parcel_enforceInterface},
    };

3）C++层（Native层）：用于实现在java层定义的各个API，以及与下面的内核空间进行交互；

4）C层：这里主要指的是Android Binder Driver，相关内容，请参考前两节博文。

#### 3. PackageManager Java层相关类图： ####

在App开始时，经常用到诸如ActivityManager,PackageManager...等组件（或者称为服务）。

本节以PackageManager来进行说明，各个组件的实现，相关类图如下：

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBAbGlyYW5rZQ_size_20_color_FFFFFF_t_70_g_se_x_16 2][]

相关代码：

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

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

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

Client端:  
frameworks/base/core/java/android/content/pm/PackageManager.java  
frameworks/base/core/java/android/app/ApplicationPackageManager.java

Service端：  
frameworks/base/core/java/android/content/pm/IPackageManager.aidl  
out/target/common/obj/JAVA\_LIBRARIES/framework\_intermediates/src/core/java/android/content/pm/IPackageManager.java  
frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java

![watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBAbGlyYW5rZQ_size_20_color_FFFFFF_t_70_g_se_x_16 3][]

这里，重点介绍一下AIDL。

AIDL：AIDL是Android中IPC（Inter-Process Communication）方式中的一种，AIDL是Android Interface definition language的缩写，AIDL的作用是，通过AIDL，可以在自己的APP里绑定一个其他APP的service，从而实现一个APP可以访问另外一个APP提供的服务。

从源码的角度来看，AIDL是以aidl为后缀的文件；在进行编译的时候，系统会将aidl编译为java文件。

可以看到：IPackageManager.接口就被定义在了IPackageManager.aidl文件中。

原始的IPackageManager.aidl文件如下：

interface IPackageManager {
        PackageInfo getPackageInfo(String packageName, int flags, int userId);
        int getPackageUid(String packageName, int userId);
        int[] getPackageGids(String packageName);
        
        String[] currentToCanonicalPackageNames(in String[] names);
        String[] canonicalToCurrentPackageNames(in String[] names);
    
        PermissionInfo getPermissionInfo(String name, int flags);
        
        List<PermissionInfo> queryPermissionsByGroup(String group, int flags);
        
        PermissionGroupInfo getPermissionGroupInfo(String name, int flags);
    ......
    }

经过编译后的对应的java文件：

public interface IPackageManager extends android.os.IInterface{
        //IPackageManager中定义的类Stub，Stub集成自Binder,并实现IPackageManager.aidl中定义的接口
        public static abstract class Stub extends android.os.Binder implements android.content.pm.IPackageManager{
        ...
        public static android.content.pm.IPackageManager asInterface(android.os.IBinder obj){
            if ((obj==null)) {
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            //如果是当前进程，返回的是IPackageManager本身
            if (((iin!=null)&&(iin instanceof android.content.pm.IPackageManager))) {
                return ((android.content.pm.IPackageManager)iin);
            }
            //不是当前进程，返回的是代理类
            return new android.content.pm.IPackageManager.Stub.Proxy(obj);
        }
    
        @Override 
        public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {
            ...
            switch (code) {
            case TRANSACTION_getPackageInfo:{
            data.enforceInterface(DESCRIPTOR);
            java.lang.String _arg0;
            _arg0 = data.readString();
            int _arg1;
            _arg1 = data.readInt();
            int _arg2;
            _arg2 = data.readInt();
            android.content.pm.PackageInfo _result = this.getPackageInfo(_arg0, _arg1, _arg2);
            reply.writeNoException();
            if ((_result!=null)) {
            reply.writeInt(1);
            _result.writeToParcel(reply, android.os.Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
            ...//类似的case方法很多，这里只举例case TRANSACTION_getPackageInfo，其他代码省略
            }
            ...
        }
        ...
        //类Stub中定义的代理类Proxy，Proxy中代理方法很多,这里同样只贴出了getPackageInfo方法
        private static class Proxy implements android.content.pm.IPackageManager {
            private android.os.IBinder mRemote;
            Proxy(android.os.IBinder remote) {
                mRemote = remote;
            }
            ...
            @Override 
            public android.content.pm.PackageInfo getPackageInfo(java.lang.String packageName, int flags, int userId) throws android.os.RemoteException{
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                android.content.pm.PackageInfo _result;
                try {
                _data.writeInterfaceToken(DESCRIPTOR);
                _data.writeString(packageName);
                _data.writeInt(flags);
                _data.writeInt(userId);
                mRemote.transact(Stub.TRANSACTION_getPackageInfo, _data, _reply, 0);
                _reply.readException();
                if ((0!=_reply.readInt())) {
                    _result = android.content.pm.PackageInfo.CREATOR.createFromParcel(_reply);
                } else {
                    _result = null;
                }
                }
                finally {
                _reply.recycle();
                _data.recycle();
                }
                return _result;
            }
            ...
            }
        ...
        //这里的函数就是Stub实现的接口implements android.content.pm.IPackageManager，只列出来三个，别的都省略，继承接口方法必须全部实现，否则会报错
        public void checkPackageStartable(java.lang.String packageName, int userId) throws android.os.RemoteException;
        public boolean isPackageAvailable(java.lang.String packageName, int userId) throws android.os.RemoteException;
        public android.content.pm.PackageInfo getPackageInfo(java.lang.String packageName, int flags, int userId) throws android.os.RemoteException;
        ...
    }

分析：

1）IPackageManager 是继承自 android.os.IInterface的；

2）在IPackageManager中，提供了Stub抽象类。Stub类用于被外部的service来继承，这个类实现了IPackageManager.aidl中定义的接口；

3）在IPackageManager中，定义了私有静态类Proxy，Proxy类用于代理IPackageManager。外部的PackageManger调用的方法最终是由Proxy来实现的；

4）调用Proxy中的方法其实就是间接调用了Stub中的onTransact方法，在onTransact中最终调用了Stub实现的接口方法；

#### 4. 服务注册： ####

我们所说的这些系统级服务，例如PackageMangerService，ActivityMangerService...等，是如何注册在servicemanager中的？本小节一起分析一下。

以PackageManagerService来看，

public static final IPackageManager main(Context context, boolean factoryTest,
                boolean onlyCore) {
            PackageManagerService m = new PackageManagerService(context, factoryTest, onlyCore);
            ServiceManager.addService("package", m);
            return m;
        }

可以看到，通过ServiceManager的addService来进行的注册。其中，第一个参数是要注册的服务的名称，这里是"package",第二个参数就是真正的服务的实例，即PackageManagerService。name和service是有对应关系的。这样，在后面使用service的时候，就可以通过name来找到具体的service了。

addService的代码：

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

public static void addService(String name, IBinder service) {
            try {
                getIServiceManager().addService(name, service, false);
            } catch (RemoteException e) {
                Log.e(TAG, "error in addService", e);
            }
        }

分析：

1）getIServiceManager会获取到一个ServiceManager的实例，再通过这个实例去访问到ServiceManagerNative的对应的addService方法，最终调用到它的内部类ServiceManagerProxy的addService方法中。

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);
            data.writeStrongBinder(service);
            data.writeInt(allowIsolated ? 1 : 0);
            mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0);
            reply.recycle();
            data.recycle();
        }

通过mRemote.transact(ADD\_SERVICE\_TRANSACTION, data, reply, 0);去调用

ServiceManagerNative的onTransact方法。与远端的服务进行通信。

在这个方法中：

public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
        {
            try {
                switch (code) {
                case IServiceManager.GET_SERVICE_TRANSACTION: {
                    data.enforceInterface(IServiceManager.descriptor);
                    String name = data.readString();
                    IBinder service = getService(name);
                    reply.writeStrongBinder(service);
                    return true;
                }
        
                case IServiceManager.CHECK_SERVICE_TRANSACTION: {
                    data.enforceInterface(IServiceManager.descriptor);
                    String name = data.readString();
                    IBinder service = checkService(name);
                    reply.writeStrongBinder(service);
                    return true;
                }
        
                case IServiceManager.ADD_SERVICE_TRANSACTION: {
                    data.enforceInterface(IServiceManager.descriptor);
                    String name = data.readString();
                    IBinder service = data.readStrongBinder();
                    boolean allowIsolated = data.readInt() != 0;
                    addService(name, service, allowIsolated);
                    return true;
                }
        
                case IServiceManager.LIST_SERVICES_TRANSACTION: {
                    data.enforceInterface(IServiceManager.descriptor);
                    String[] list = listServices();
                    reply.writeStringArray(list);
                    return true;
                }
                
                case IServiceManager.SET_PERMISSION_CONTROLLER_TRANSACTION: {
                    data.enforceInterface(IServiceManager.descriptor);
                    IPermissionController controller
                            = IPermissionController.Stub.asInterface(
                                    data.readStrongBinder());
                    setPermissionController(controller);
                    return true;
                }
                }
            } catch (RemoteException e) {
            }
            
            return false;
        }
    
        public IBinder asBinder()
        {
            return this;
        }
    }

这里，只分析case IServiceManager.ADD\_SERVICE\_TRANSACTION的处理：

参数data的类型是Parcel，这个数据结构就是java与native进行数据交换的桥梁。

下面这些方法，最终都对应到native方法，这个对应关系是由android\_os\_Parcel.cpp完成的。如下表：

<table style="width:500px;"> 
 <tbody> 
  <tr> 
   <td style="width:236px;">Parcel.java</td> 
   <td style="width:490px;">android_os_Parcel.cpp</td> 
  </tr> 
  <tr> 
   <td style="width:236px;">data.enforceInterface(IServiceManager.descriptor);</td> 
   <td style="width:490px;"> <pre>{"nativeEnforceInterface",    "(ILjava/lang/String;)V", (void*)android_os_Parcel_enforceInterface},
</pre> </td> 
  </tr> 
  <tr> 
   <td style="width:236px;">data.readString();</td> 
   <td style="width:490px;"> <pre>{"nativeReadString",          "(I)Ljava/lang/String;", (void*)android_os_Parcel_readString},
</pre> </td> 
  </tr> 
  <tr> 
   <td style="width:236px;">data.readStrongBinder();</td> 
   <td style="width:490px;"> <pre>{"nativeReadStrongBinder",    "(I)Landroid/os/IBinder;", (void*)android_os_Parcel_readStrongBinder},
</pre> </td> 
  </tr> 
  <tr> 
   <td style="width:236px;">data.readInt()&nbsp;</td> 
   <td style="width:490px;"> <pre>{"nativeReadInt",             "(I)I", (void*)android_os_Parcel_readInt}</pre> </td> 
  </tr> 
 </tbody> 
</table>

最终，调用到native的Parcel.cpp中对应的函数中。

例如，Parcel::enforceInterface函数的实现：

bool Parcel::enforceInterface(const String16& interface,
                                  IPCThreadState* threadState) const
    {
        int32_t strictPolicy = readInt32();
        if (threadState == NULL) {
            threadState = IPCThreadState::self();
        }
        if ((threadState->getLastTransactionBinderFlags() &
             IBinder::FLAG_ONEWAY) != 0) {
          // For one-way calls, the callee is running entirely
          // disconnected from the caller, so disable StrictMode entirely.
          // Not only does disk/network usage not impact the caller, but
          // there's no way to commuicate back any violations anyway.
          threadState->setStrictModePolicy(0);
        } else {
          threadState->setStrictModePolicy(strictPolicy);
        }
        const String16 str(readString16());
        if (str == interface) {
            return true;
        } else {
            ALOGW("**** enforceInterface() expected '%s' but read '%s'\n",
                    String8(interface).string(), String8(str).string());
            return false;
        }
    }

可见，最终是通过IPCThreadState来实现，这就涉及到native层的具体binder的实现流程了。

下一节，分析native层相关代码。

#### 5. PackageManagerService的初始化： ####

那么，PackageManagerService的main方法是什么时候被调用的呢？

答案就在system\_server进程中。

system\_server进程：在android系统初始化过程中，会有zygote孵化出system\_server进程。可以用ps命令看到这个进程的父进程就是zygote。

在system\_server中，初始化了Android Framework层的许多服务，其中，就包含PackageManagerService服务。

代码如下：

SystemServer.java的代码:

system\_server初始化流程：

1）main -> call

System.loadLibrary("android_servers");
    init1(args);

native public static void init1(String[] args);

2）init1 -> 通过native再调用到java层的init2

init2

3）init2:

public static final void init2() {
            Slog.i(TAG, "Entered the Android system server!");
            Thread thr = new ServerThread();
            thr.setName("android.server.ServerThread");
            thr.start();
        }

4）ServerThread中，对各个组件进行初始化：

class ServerThread extends Thread {
        @Override
        public void run() {
            EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN,
                SystemClock.uptimeMillis());
    
            Looper.prepare();
    
            ......
    
            IPackageManager pm = null;
            Context context = null;
            WindowManagerService wm = null;
            BluetoothService bluetooth = null;
    
            ......
    
            try {
                Slog.i(TAG, "Entropy Mixer");
                ServiceManager.addService("entropy", new EntropyMixer());
    
                Slog.i(TAG, "Power Manager");
                power = new PowerManagerService();
                ServiceManager.addService(Context.POWER_SERVICE, power);
    
                Slog.i(TAG, "Activity Manager");
                context = ActivityManagerService.main(factoryTest);
    
                ......
    
                pm = PackageManagerService.main(context,
                        factoryTest != SystemServer.FACTORY_TEST_OFF,
                        onlyCore);
            }
    
    
        }
        ......
    }

这样，PackageManagerService就初始化完成了。

#### 6. 总结： ####

PackageManager的addService的代码调用流程：

SystemServer.init1/init2->PackageManagerService.main()

\->ServiceManager.addService()->IServiceManager().addService

\->ServiceManagerNative.addService()->ServiceManagerProxy.addService()

\->Binder.onTransact()->ServiceManagerNative.onTransact() //

case IServiceManager.ADD_SERVICE_TRANSACTION: {

\->Parcel.enforceInterface()

\->android\_os\_Parcel.cpp //JNI

\->Parcel::enforceInterface（） //Parcel.cpp Native

以上，就是Binder在JAVA层相关的组件以及其在整个系统中的调用流程的基本介绍。

--------------------

[Android_-_binder_servicemanager]: https://blog.csdn.net/liranke/article/details/4720689
[Android_-_binder_binder]: https://blog.csdn.net/liranke/article/details/4723360
[Android_-_binder_framework_Native]: https://blog.csdn.net/liranke/article/details/4727335
[Android_-_binder_Native_Driver]: https://blog.csdn.net/liranke/article/details/4727339
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