Binder是Android中的跨进程通信方式，bindService的时候，服务端返回Binder对象，通过该对象客户端可以从服务端获取数据。
在进程间通信IPC——AIDL中创建了ICustomAidlInterface.aidl。
以下是根据ICustomAidlInterface.aidl生成的ICustomAidlInterface.Java接口类。

public interface ICustomAidlInterface extends android.os.IInterface {
/**
* Local-side IPC implementation stub class.
*/
public static abstract class Stub extends android.os.Binder implements com.sjl.exercise.ICustomAidlInterface {
private static final java.lang.String DESCRIPTOR = "com.sjl.exercise.ICustomAidlInterface";

/**
* Construct the stub at attach it to the interface.
*/
public Stub() {
this.attachInterface(this, DESCRIPTOR);
}

/**
* Cast an IBinder object into an com.sjl.exercise.ICustomAidlInterface interface,
* generating a proxy if needed.
*/
public static com.sjl.exercise.ICustomAidlInterface asInterface(android.os.IBinder obj) {
if ((obj == null)) {
return null;
}
//④
android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
if (((iin != null) && (iin instanceof com.sjl.exercise.ICustomAidlInterface))) {
return ((com.sjl.exercise.ICustomAidlInterface) iin);
}
return new com.sjl.exercise.ICustomAidlInterface.Stub.Proxy(obj);
}

@Override
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 {
//③
java.lang.String descriptor = DESCRIPTOR;
switch (code) {
case INTERFACE_TRANSACTION: {
reply.writeString(descriptor);
return true;
}
case TRANSACTION_getCurrentTime: {
data.enforceInterface(descriptor);
java.lang.String _result = this.getCurrentTime();
reply.writeNoException();
reply.writeString(_result);
return true;
}
case TRANSACTION_insertUser: {
data.enforceInterface(descriptor);
com.sjl.exercise.bean.UserBean _arg0;
if ((0 != data.readInt())) {
_arg0 = com.sjl.exercise.bean.UserBean.CREATOR.createFromParcel(data);
} else {
_arg0 = null;
}
this.insertUser(_arg0);
reply.writeNoException();
return true;
}
case TRANSACTION_getUsers: {
data.enforceInterface(descriptor);
java.util.List<com.sjl.exercise.bean.UserBean> _result = this.getUsers();
reply.writeNoException();
reply.writeTypedList(_result);
return true;
}
case TRANSACTION_clearUser: {
data.enforceInterface(descriptor);
this.clearUser();
reply.writeNoException();
return true;
}
default: {
return super.onTransact(code, data, reply, flags);
}
}
}

private static class Proxy implements com.sjl.exercise.ICustomAidlInterface {
private android.os.IBinder mRemote;

Proxy(android.os.IBinder remote) {
mRemote = remote;
}

@Override
public android.os.IBinder asBinder() {
return mRemote;
}

public java.lang.String getInterfaceDescriptor() {
return DESCRIPTOR;
}

@Override
public java.lang.String getCurrentTime() throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
java.lang.String _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
//⑤
mRemote.transact(Stub.TRANSACTION_getCurrentTime, _data, _reply, 0);
_reply.readException();
_result = _reply.readString();
} finally {
_reply.recycle();
_data.recycle();
}
return _result;
}

@Override
public void insertUser(com.sjl.exercise.bean.UserBean userBean) throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
try {
_data.writeInterfaceToken(DESCRIPTOR);
if ((userBean != null)) {
_data.writeInt(1);
userBean.writeToParcel(_data, 0);
} else {
_data.writeInt(0);
}
mRemote.transact(Stub.TRANSACTION_insertUser, _data, _reply, 0);
_reply.readException();
} finally {
_reply.recycle();
_data.recycle();
}
}

@Override
public java.util.List<com.sjl.exercise.bean.UserBean> getUsers() throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
java.util.List<com.sjl.exercise.bean.UserBean> _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
mRemote.transact(Stub.TRANSACTION_getUsers, _data, _reply, 0);
_reply.readException();
_result = _reply.createTypedArrayList(com.sjl.exercise.bean.UserBean.CREATOR);
} finally {
_reply.recycle();
_data.recycle();
}
return _result;
}

@Override
public void clearUser() throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
try {
_data.writeInterfaceToken(DESCRIPTOR);
mRemote.transact(Stub.TRANSACTION_clearUser, _data, _reply, 0);
_reply.readException();
} finally {
_reply.recycle();
_data.recycle();
}
}
}
//②
static final int TRANSACTION_getCurrentTime = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
static final int TRANSACTION_insertUser = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
static final int TRANSACTION_getUsers = (android.os.IBinder.FIRST_CALL_TRANSACTION + 2);
static final int TRANSACTION_clearUser = (android.os.IBinder.FIRST_CALL_TRANSACTION + 3);
}
//①
public java.lang.String getCurrentTime() throws android.os.RemoteException;

public void insertUser(com.sjl.exercise.bean.UserBean userBean) throws android.os.RemoteException;

public java.util.List<com.sjl.exercise.bean.UserBean> getUsers() throws android.os.RemoteException;

public void clearUser() throws android.os.RemoteException;
}
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ICustomAidlInterface类最下面的①就是对应aidl文件里声明的几个方法，②所在声明了对应方法的int值，在抽象方法Stub#onTransact方法中可以看到③处正是用②中声明的int值区分调用的对应方法的。

public interface ICustomAidlInterface extends android.os.IInterface {

public static abstract class Stub extends android.os.Binder implements com.sjl.exercise.ICustomAidlInterface {
private static final java.lang.String DESCRIPTOR = "com.sjl.exercise.ICustomAidlInterface";

...
@Override
public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {
//③
java.lang.String descriptor = DESCRIPTOR;
switch (code) {
case INTERFACE_TRANSACTION: {
reply.writeString(descriptor);
return true;
}
case TRANSACTION_getCurrentTime: {
data.enforceInterface(descriptor);
java.lang.String _result = this.getCurrentTime();
reply.writeNoException();
reply.writeString(_result);
return true;
}
case TRANSACTION_insertUser: {
data.enforceInterface(descriptor);
com.sjl.exercise.bean.UserBean _arg0;
if ((0 != data.readInt())) {
_arg0 = com.sjl.exercise.bean.UserBean.CREATOR.createFromParcel(data);
} else {
_arg0 = null;
}
this.insertUser(_arg0);
reply.writeNoException();
return true;
}
case TRANSACTION_getUsers: {
data.enforceInterface(descriptor);
java.util.List<com.sjl.exercise.bean.UserBean> _result = this.getUsers();
reply.writeNoException();
reply.writeTypedList(_result);
return true;
}
case TRANSACTION_clearUser: {
data.enforceInterface(descriptor);
this.clearUser();
reply.writeNoException();
return true;
}
default: {
return super.onTransact(code, data, reply, flags);
}
}
}

private static class Proxy implements com.sjl.exercise.ICustomAidlInterface {
...
//②
static final int TRANSACTION_getCurrentTime = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
static final int TRANSACTION_insertUser = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
static final int TRANSACTION_getUsers = (android.os.IBinder.FIRST_CALL_TRANSACTION + 2);
static final int TRANSACTION_clearUser = (android.os.IBinder.FIRST_CALL_TRANSACTION + 3);
}
//①
public java.lang.String getCurrentTime() throws android.os.RemoteException;

public void insertUser(com.sjl.exercise.bean.UserBean userBean) throws android.os.RemoteException;

public java.util.List<com.sjl.exercise.bean.UserBean> getUsers() throws android.os.RemoteException;

public void clearUser() throws android.os.RemoteException;
}
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Stub构造函数里传入IInterface和DESCRIPTOR，客户端通过绑定服务获取到aidl接口时使用了ICustomAidlInterface.Stub.asInterface(service)方法，在静态抽象类Stub中④处Stub#asInterface方法中可以看到使用Binder#queryLocalInterface方法获取本地aidl接口，如果有则返回本地找到的aidl接口，如果没有就返回定义的Proxy代理类（queryLocalInterface方法就是比对DESCRIPTOR来确认是否返回对应的IInterface，在不同进程时无法找到对应的IInterface，只能通过代理类Proxy来进行aidl方法调用）。

public interface ICustomAidlInterface extends android.os.IInterface {
/**
* Local-side IPC implementation stub class.
*/
public static abstract class Stub extends android.os.Binder implements com.sjl.exercise.ICustomAidlInterface {
private static final java.lang.String DESCRIPTOR = "com.sjl.exercise.ICustomAidlInterface";
...
public static com.sjl.exercise.ICustomAidlInterface asInterface(android.os.IBinder obj) {
if ((obj == null)) {
return null;
}
//④
android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
if (((iin != null) && (iin instanceof com.sjl.exercise.ICustomAidlInterface))) {
return ((com.sjl.exercise.ICustomAidlInterface) iin);
}
return new com.sjl.exercise.ICustomAidlInterface.Stub.Proxy(obj);
}

private static class Proxy implements com.sjl.exercise.ICustomAidlInterface {
...
}
}
}
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查看Proxy类可以看到它的IPC都是mRemote实现的，而在对应的方法中都使用了mRemote.transact方法，传入方法对应的int值、客户端传入参数data和服务端返回结果reply

private static class Proxy implements com.sjl.exercise.ICustomAidlInterface {
private android.os.IBinder mRemote;

Proxy(android.os.IBinder remote) {
mRemote = remote;
}

@Override
public android.os.IBinder asBinder() {
return mRemote;
}

public java.lang.String getInterfaceDescriptor() {
return DESCRIPTOR;
}

@Override
public java.lang.String getCurrentTime() throws android.os.RemoteException {
android.os.Parcel _data = android.os.Parcel.obtain();
android.os.Parcel _reply = android.os.Parcel.obtain();
java.lang.String _result;
try {
_data.writeInterfaceToken(DESCRIPTOR);
//⑤
mRemote.transact(Stub.TRANSACTION_getCurrentTime, _data, _reply, 0);
_reply.readException();
_result = _reply.readString();
} finally {
_reply.recycle();
_data.recycle();
}
return _result;
}

}

}
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Binder#transact代码如下，最终调用了Binder#onTransact，也就是Binder#onTransact

/**
* Default implementation rewinds the parcels and calls onTransact. On
* the remote side, transact calls into the binder to do the IPC.
*/
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);
if (reply != null) {
reply.setDataPosition(0);
}
return r;
}
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Binder#onTransact方法如下，通过code确定调用的对应方法，data获取传入参数，reply设置返回值

@Override
public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {
java.lang.String descriptor = DESCRIPTOR;
switch (code) {
case TRANSACTION_insertUser: {
data.enforceInterface(descriptor);
com.sjl.exercise.bean.UserBean _arg0;
if ((0 != data.readInt())) {
_arg0 = com.sjl.exercise.bean.UserBean.CREATOR.createFromParcel(data);
} else {
_arg0 = null;
}
this.insertUser(_arg0);
reply.writeNoException(http://www.my516.com);
return true;
}
case TRANSACTION_getUsers: {
data.enforceInterface(descriptor);
java.util.List<com.sjl.exercise.bean.UserBean> _result = this.getUsers();
reply.writeNoException();
reply.writeTypedList(_result);
return true;
}
}
}
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总结：
服务端：服务端返回实现IBinder接口的对象；
客户端：客户端如果在本地找到aidl实例直接转换使用，否则通过Proxy代理调用；
通讯：本地客户端强制转换可调用调用方法，其他进程客户端需要通过Proxy代理调用方法；
---------------------

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