Java 服务接口的定义和解析

Java 通过 Android 接口描述语言(Android Interface Definition Language, AIDL) 来定义 Java 服务接口.

// framework/base/core/java/android/os/IFregService.aidl
package android.os;

interface IFregService {
    void setVal(int val);
    int getVal();
}

aidl 文件编译后会生成一个中间文件 IFregService.java

/*
 * This file is auto-generated.  DO NOT MODIFY.
 * Original file: frameworks/base/core/java/android/os/IFregService.aidl
 */
package android.os;
 
public interface IFregService extends android.os.IInterface {
    /** Local-side IPC implementation stub class. */
    public static abstract class Stub extends android.os.Binder implements
            android.os.IFregService {
        private static final java.lang.String DESCRIPTOR = "android.os.IFregService";
 
        /** Construct the stub at attach it to the interface. */
        public Stub() {
            this.attachInterface(this, DESCRIPTOR);
        }
 
        /**
         * Cast an IBinder object into an android.os.IFregService interface,
         * generating a proxy if needed.
         */
        public static android.os.IFregService asInterface(android.os.IBinder obj) {
            if ((obj == null)) {
                return null;
            }
            android.os.IInterface iin = (android.os.IInterface) obj
                    .queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof android.os.IFregService))) {
                return ((android.os.IFregService) iin);
            }
            return new android.os.IFregService.Stub.Proxy(obj);
        }
 
        public android.os.IBinder asBinder() {
            return this;
        }
 
        @Override
        public boolean onTransact(int code, android.os.Parcel data,
                android.os.Parcel reply, int flags)
                throws android.os.RemoteException {
            switch (code) {
            case INTERFACE_TRANSACTION: {
                reply.writeString(DESCRIPTOR);
                return true;
            }
            case TRANSACTION_setVal: {
                data.enforceInterface(DESCRIPTOR);
                int _arg0;
                _arg0 = data.readInt();
                this.setVal(_arg0);
                reply.writeNoException();
                return true;
            }
            case TRANSACTION_getVal: {
                data.enforceInterface(DESCRIPTOR);
                int _result = this.getVal();
                reply.writeNoException();
                reply.writeInt(_result);
                return true;
            }
            }
            return super.onTransact(code, data, reply, flags);
        }
 
        private static class Proxy implements android.os.IFregService {
            private android.os.IBinder mRemote;
 
            Proxy(android.os.IBinder remote) {
                mRemote = remote;
            }
 
            public android.os.IBinder asBinder() {
                return mRemote;
            }
 
            public java.lang.String getInterfaceDescriptor() {
                return DESCRIPTOR;
            }
 
            public void setVal(int val) throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    _data.writeInt(val);
                    mRemote.transact(Stub.TRANSACTION_setVal, _data, _reply, 0);
                    _reply.readException();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
            }
 
            public int getVal() throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                int _result;
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    mRemote.transact(Stub.TRANSACTION_getVal, _data, _reply, 0);
                    _reply.readException();
                    _result = _reply.readInt();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
                return _result;
            }
        }
 
        static final int TRANSACTION_setVal = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
        static final int TRANSACTION_getVal = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
    }
 
    public void setVal(int val) throws android.os.RemoteException;
 
    public int getVal() throws android.os.RemoteException;
}

• IFregService interface
• 内部抽象类: IFregService.Stub
• 内部抽象类: IFregService.Stub.Proxy

IFregService.Stub 和 IFregService.Stub.Proxy 都实现了 IFregService 接口。

• IFregService.Stub 描述一个 Java 服务
• IFregService.Stub.proxy 描述这个 Java 服务的代理对象。

我们要实现的是服务是以 IFregService.Stub 为父类，并且实现 setVal 和 getVal 方法。

• IFregService.Stub 的 asBinder 方法，是将 BinderProxy 封装成 IFregService.Stub.Proxy 对象。
• IFregService.Stub 的 onTransact 方法用来接收和分发进程间通信请求。当 Client 进程使用 通过 Proxy 发送 TRANSACTION_setVal， TRANSACTION_getVal 的通信请求时，IFregService.Stub 的onTransact 就会被调用。
• IFregService.Stub.Proxy 内部成员变量 mRemote，指向了 JavaBinderProxy。