Android8.0 Binder之面向系统服务(二)

上一篇文章我们着重分析了 1.Binder在Native层如何巧妙的将通讯与业务结合起来； 2.服务是如何路由的。这两个方都是从服务管理和提供的角度分析的。从整个Android系统来看Service包含两层一层是以Java语言铸成的，另外一层是以C++铸成的；今天我们来分析Framework层的Binder框架，Java层的Binder框架是Native中Binder框架的镜像，其实现结构上是类似的，废话少说直接上代码。

1. Binder 顶层协议

首先我们还是从IBinder开始分析

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

public interface IBinder {

    ......

    public @Nullable String getInterfaceDescriptor() throws RemoteException;

    public boolean pingBinder();

    public boolean isBinderAlive();

    public @Nullable IInterface queryLocalInterface(@NonNull String descriptor);

    ......

    //顶层中比较重要的方法
    public boolean transact(int code, @NonNull Parcel data, @Nullable Parcel reply, int flags)
        throws RemoteException;
    ......
}

Binder类实现了IBinder接口，重写了transact()方法

 public final boolean transact(int code, @NonNull Parcel data, @Nullable Parcel reply,
            int flags) throws RemoteException {
        if (false) Log.v("Binder", "Transact: " + code + " to " + this);

        if (data != null) {
            data.setDataPosition(0);
        }
        boolean r = onTransact(code, data, reply, flags); //接受协议
        if (reply != null) {
            reply.setDataPosition(0);
        }
        return r;
    }

onTransact() 将交给子类重写

protected boolean onTransact(int code, @NonNull Parcel data, @Nullable Parcel reply,
            int flags) throws RemoteException {
        if (code == INTERFACE_TRANSACTION) {
            reply.writeString(getInterfaceDescriptor());
            return true;
        } else if (code == DUMP_TRANSACTION) {
            ......
            return true;
        } else if (code == SHELL_COMMAND_TRANSACTION) {
            ......
            return true;
        }
        return false;
    }

同时BinderProxy作为Binder内部类，同样实现了IBinder接口以及transact()方法

final class BinderProxy implements IBinder {

    ......

    public boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException {

       ......

        try {
            return transactNative(code, data, reply, flags); //调用native方法
        } finally {
            if (tracingEnabled) {
                Trace.traceEnd(Trace.TRACE_TAG_ALWAYS);
            }
        }
    }
}

至此我们先稍微打断一下，转而从ServiceManager的角度看一看。

2. 通讯与业务组合

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

public interface IInterface
{
    /**
     * Retrieve the Binder object associated with this interface.
     * You must use this instead of a plain cast, so that proxy objects
     * can return the correct result.
     */
    public IBinder asBinder(); // 仅此一个方法申明
}

IServiceManager继承自IInterface，申明了一些通用方法
/frameworks/base/core/java/android/os/IServiceManager.java

public interface IServiceManager extends IInterface
{

    public IBinder getService(String name) throws RemoteException;

    public IBinder checkService(String name) throws RemoteException;

    public void addService(String name, IBinder service, boolean allowIsolated)
                throws RemoteException;

    public String[] listServices() throws RemoteException;

    ......

}

服务管理最终交给ServiceManagerNative，ServiceManagerNative继承子Binder实现自IServiceManager，内部类ServiceManagerProxy同样实现了IServiceManager。

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

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

        // Find the service manager
        sServiceManager = ServiceManagerNative
                .asInterface(Binder.allowBlocking(
                BinderInternal.getContextObject())); //BinderInternal获取上下文
        return sServiceManager;
    }

BinderInternal 将调用native的方法

public class BinderInternal {
    ......

    public static final native IBinder getContextObject(); //获取上下文

    ......
}

在android_util_Binder.cpp文件中可以查到JNI函数

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 },
    { "setMaxThreads", "(I)V", (void*)android_os_BinderInternal_setMaxThreads },
    { "handleGc", "()V", (void*)android_os_BinderInternal_handleGc }
};

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

指向一个IBinder对象

jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val)
{

    ......
    // gBinderProxyOffsets 是一个结构体
    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()); //存入
        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)); //全局
        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;
}

返回一个ServiceManagerProxy，并将Native层对应的BpBinder传进去

 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);
    }

将传入的BpBinder 由 mRemote 引用

class ServiceManagerProxy implements IServiceManager {
    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); //Parcel类调用写入
        mRemote.transact(GET_SERVICE_TRANSACTION, data, reply, 0); //执行IBinder的transact
        IBinder binder = reply.readStrongBinder();
        reply.recycle();
        data.recycle();
        return binder;
    }
    ......
    private IBinder mRemote;
}

回到BinderProxy的transact()方法会调用transactNative()方法，由上述JNI注册表可以查到此函数,将调用native层的BpBinder执行驱动交互

static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,
        jint code, jobject dataObj, jobject replyObj, jint flags) // throws RemoteException
{
    ......

    IBinder* target = (IBinder*)
        env->GetLongField(obj, gBinderProxyOffsets.mObject); //取出 BpBinder
    if (target == NULL) {
        jniThrowException(env, "java/lang/IllegalStateException", "Binder has been finalized!");
        return JNI_FALSE;
    }

    .......

    //printf("Transact from Java code to %p sending: ", target); data->print();
    status_t err = target->transact(code, *data, reply, flags); // 执行transact

    return JNI_FALSE;
}

至此Binder在Java层的框架就讲完了，整体上Java层的Binder框架最终还是要借助Native层的Binder框架来完成与驱动层的交互，两层交互时需要结束JNI技术进行相互调用，JNI的函数注册就跳过不讲了，分为动态和静态两种方式，有兴趣可以自行了解。

在下一篇的Binder系列中我们将讲述Binder三角架的另一重要实现HAL层的Binder实现，它主要为HAL层而生，更多的是涉及到驱动相关，敬请期待。