Binder之三：ServiceManager的获取

第三部分 Binder之三：ServiceManager的获取

0 入口. getIServiceManager()

ServiceManager.java

    private static IServiceManager sServiceManager;
    private static IServiceManager getIServiceManager() {
        if (sServiceManager != null) {
            return sServiceManager;
        }

        // Find the service manager
        sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());
        return sServiceManager;
    }

1. asInterface

ServiceManagerNative

    static public IServiceManager asInterface(IBinder obj)
    {
        if (obj == null) {
            return null;
        }
        IServiceManager in =
            (IServiceManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }
        
        return new ServiceManagerProxy(obj);
    }

2. ServiceManagerProxy的构造函数

ServiceManagerProxy是SM的代理，必定会参与Binder通信，所以在构造函数中传进来了一个IBinder对象。其实ServiceManagerProxy中的所有操作最终都是通过这个IBinder对象来完成的。

//IServiceManager.java中定义了SM的所有功能
class ServiceManagerProxy implements IServiceManager {
    
    private IBinder mRemote;
    
    public ServiceManagerProxy(IBinder remote) {
        mRemote = remote;
    }
    
    public IBinder asBinder() {
        return mRemote;
    }
    
    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();//从返回中获取
        reply.recycle();
        data.recycle();
        return binder;
    }

    public IBinder checkService(String name) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IServiceManager.descriptor);
        data.writeString(name);
        mRemote.transact(CHECK_SERVICE_TRANSACTION, data, reply, 0);
        IBinder binder = reply.readStrongBinder();
        reply.recycle();
        data.recycle();
        return binder;
    }

    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();
    }
    
    public String[] listServices() throws RemoteException {
       //...
    }

    public void setPermissionController(IPermissionController controller)
            throws RemoteException { 
            //...
    } 
}

3.IBinder的queryLocalInterface方法

我们的目标是为应用程序使用SM服务提供方便，其中的核心是ServiceManagerProxy，其外围又包装了ServiceManager.java和ServiceManagerNative.java。
在创建ServiceManagerProxy的时候，传入了一个IBinder对象，然后借助于它的transact方法，可以方便的与BInderDriver进行通信。
用户希望通过尽可能精简的步骤来使用BInder的功能，Binder的功能可以统一在IBinder中表示

public interface IBinder {
     ...
     public IInterface queryLocalInterface(String descriptor);
     public boolean transact(int code, Parcel data, Parcel reply, int flags)
        throws RemoteException;
     ...
}

4.BInderInternal.getContextObject();

获取IBinder的类是BInderInternal

 public class BinderInternal {
          ...
      public static final native IBinder getContextObject();
          ...
}

getContextObject是native 方法，需要jni实现

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

IBinder只是一个接口，在Native层，它的实现类是BpBinder.cpp，在java层它的实现是Binder.java中的BinderProxy，通过 ProcessState::self()->getContextObject(NULL)返回一个BpBinder，然后通过javaObjectForIBinder方法转换成BinderProxy

6.BinderProxy的transact方法

BinderProxy的transact方法

    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);
    }
//会执行到这里，这里是一个本地方法
public native boolean transactNative(int code, Parcel data, Parcel reply,
            int flags) throws RemoteException;

native实现

static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,
        jint code, jobject dataObj, jobject replyObj, jint flags) // throws RemoteException
{
    //data和reply分别是来自BinderProxy的参数，这里是把他们专程native中的Parcel实现
    Parcel* data = parcelForJavaObject(env, dataObj);
   
    Parcel* reply = parcelForJavaObject(env, replyObj);
    
    IBinder* target = (IBinder*)
        env->GetLongField(obj, gBinderProxyOffsets.mObject);
 

    //IBinder的native实现是BpBinder，所以这里就是调用BpBinder的transact方法
    status_t err = target->transact(code, *data, reply, flags); 

    signalExceptionForError(env, obj, err, true /*canThrowRemoteException*/, data->dataSize());
    return JNI_FALSE;
}

BpBinder的transact方法：最终会调用到IPCThreadState::self()->transact。

status_t BpBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // Once a binder has died, it will never come back to life.
    if (mAlive) {
       //调用IPCThreadState的transact方法
        status_t status = IPCThreadState::self()->transact(
            mHandle, code, data, reply, flags);
        if (status == DEAD_OBJECT) mAlive = 0;
        return status;
    }

    return DEAD_OBJECT;
}

概览

Paste_Image.png

总体分为三大步
1.ProcessState::self()：用于获取ProcessState对象(也是单例模式)，每个进程有且只有一个ProcessState对象，存在则直接返回，不存在则创建(1-3)
2.getContextObject()： 用于获取BpBiner对象，对于handle=0的BpBiner对象，存在则直接返回，不存在才创建(4-7)
3.interface_cast()：创建BpServiceManager对象(8-10)

sp defaultServiceManager()
{
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;
    
    {
        AutoMutex _l(gDefaultServiceManagerLock);
        while (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast(
                ProcessState::self()->getContextObject(NULL));
            if (gDefaultServiceManager == NULL)
                sleep(1);
        }
    }
    
    return gDefaultServiceManager;
}

1-3 ProcessState和IPCThreadState

ProcessState：专门管理进程中Binder操作的类，执行Binder驱动的一系列名对对上层用户透明。ProcessState采用单粒模式创建ProcessState::self，在构造函数的中通过binder_open，binder_mmap等操作建立了与BinderDriver通信的通道。

在BinderDriver的分析中可以看到binder_proc中有threads红黑树用来管理进程中所有线程的IPC业务，进程中每个线程都有与BinderDriver沟通的权利，与BiderDriver驱动实际进行命令通信的是IPCThreadState

1. ProcessState::self

sp ProcessState::self()
{
 Mutex::Autolock _l(gProcessMutex);
 if (gProcess != NULL) {
 return gProcess;
 }

 //实例化ProcessState  
 gProcess = new ProcessState;
 return gProcess;
}

2. new ProcessState

ProcessState::ProcessState()
 : mDriverFD(open_driver()) // 打开Binder驱动 
 , mVMStart(MAP_FAILED)
 , mThreadCountLock(PTHREAD_MUTEX_INITIALIZER)
 , mThreadCountDecrement(PTHREAD_COND_INITIALIZER)
 , mExecutingThreadsCount(0)
 , mMaxThreads(DEFAULT_MAX_BINDER_THREADS)
 , mManagesContexts(false)
 , mBinderContextCheckFunc(NULL)
 , mBinderContextUserData(NULL)
 , mThreadPoolStarted(false)
 , mThreadPoolSeq(1)
{
 if (mDriverFD >= 0) {
 //采用内存映射函数mmap，给binder分配一块虚拟地址空间,用来接收事务
 mVMStart = mmap(0, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);
 if (mVMStart == MAP_FAILED) {
 close(mDriverFD); //没有足够空间分配给/dev/binder,则关闭驱动
 mDriverFD = -1;
 }
 }
}

3.open_driver

static int open_driver()
{
 // 打开/dev/binder设备，建立与内核的Binder驱动的交互通道
 int fd = open("/dev/binder", O_RDWR);
 if (fd >= 0) {
 fcntl(fd, F_SETFD, FD_CLOEXEC);
 int vers = 0;
 status_t result = ioctl(fd, BINDER_VERSION, &vers);
 if (result == -1) {
 close(fd);
 fd = -1;
 }
 if (result != 0 || vers != BINDER_CURRENT_PROTOCOL_VERSION) {
 close(fd);
 fd = -1;
 }
 size_t maxThreads = DEFAULT_MAX_BINDER_THREADS;

 // 通过ioctl设置binder驱动，能支持的最大线程数
 result = ioctl(fd, BINDER_SET_MAX_THREADS, &maxThreads);
 if (result == -1) {
 ALOGE("Binder ioctl to set max threads failed: %s", strerror(errno));
 }
 } else {
 ALOGW("Opening '/dev/binder' failed: %s\n", strerror(errno));
 }
 return fd;

4. getContextObject:获取handle=0的IBinder

sp ProcessState::getContextObject(const sp& /*caller*/)
{
 return getStrongProxyForHandle(0);  
}

5.getStrongProxyForHandle

sp ProcessState::getStrongProxyForHandle(int32_t handle)
{
 sp 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)) {
 if (handle == 0) {
 Parcel data;
 //通过ping操作测试binder是否准备就绪
 status_t status = IPCThreadState::self()->transact(
 0, IBinder::PING_TRANSACTION, data, NULL, 0);
 if (status == DEAD_OBJECT)
 return NULL;
 }
 //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;
}

6. lookupHandleLocked

根据handle值来查找对应的handle_entry,handle_entry是一个结构体，里面记录IBinder和weakref_type两个指针。当handle大于mHandleToObject的Vector长度时，则向该Vector中添加(handle+1-N)个handle_entry结构体，然后再返回handle向对应位置的handle_entry结构体指针

ProcessState::handle_entry* ProcessState::lookupHandleLocked(int32_t handle)
{
 const size_t N=mHandleToObject.size();
 //当handle大于mHandleToObject的长度时，进入该分支
 if (N <= (size_t)handle) {
 handle_entry e;
 e.binder = NULL;
 e.refs = NULL;
 //从mHandleToObject的第N个位置开始，插入(handle+1-N)个e到队列中
 status_t err = mHandleToObject.insertAt(e, N, handle+1-N);
 if (err < NO_ERROR) return NULL;
 }
 return &mHandleToObject.editItemAt(handle);
}

7.new BpBinder

BpBinder::BpBinder(int32_t handle)
 : mHandle(handle)
 , mAlive(1)
 , mObitsSent(0)
 , mObituaries(NULL)
{
 extendObjectLifetime(OBJECT_LIFETIME_WEAK); //延长对象的生命时间
 IPCThreadState::self()->incWeakHandle(handle); //handle所对应的bindle弱引用 + 1
}

8.interface_cast

template
inline sp interface_cast(const sp& obj)
{
 return INTERFACE::asInterface(obj); //【见流程10】
}

9.IServiceManager::asInterface

//位于IServiceManager.h文件
DECLARE_META_INTERFACE(IServiceManager)
//位于IServiceManager.cpp文件
IMPLEMENT_META_INTERFACE(ServiceManager,"android.os.IServiceManager")

9.1

const android::String16 IServiceManager::descriptor(“android.os.IServiceManager”);

const android::String16& IServiceManager::getInterfaceDescriptor() const
{
 return IServiceManager::descriptor;
}

 android::sp IServiceManager::asInterface(const android::sp& obj)
{
 android::sp intr;
 if(obj != NULL) {
 intr = static_cast(
 obj->queryLocalInterface(IServiceManager::descriptor).get());
 if (intr == NULL) {
 intr = new BpServiceManager(obj); //【见流程11】
 }
 }
 return intr;
}

IServiceManager::IServiceManager () { }
IServiceManager::~ IServiceManager() { }

10 new BpServiceManager

10.1初始化BpServiceManager

BpServiceManager(const sp& impl)
 : BpInterface(impl)
{
}

10.2 初始化父类BpInterface

inline BpInterface::BpInterface(const sp& remote)
 :BpRefBase(remote)
{
}

10.3 初始化父类BpRefBase

BpRefBase::BpRefBase(const sp& o)
 : mRemote(o.get()), mRefs(NULL), mState(0)
{
 extendObjectLifetime(OBJECT_LIFETIME_WEAK);

 if (mRemote) {
 mRemote->incStrong(this);
 mRefs = mRemote->createWeak(this);
 }
}

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参考
《深入理解android内核设计》
Binder系列4—获取ServiceManager