首先要明白需要的情景，然后对三种方式进行选择：

（一）可以接收Service的信息(获取Service中的方法)，但不可以给Service发送信息

(二) 使用Messenger既可以接受Service消息，也可以发送Service消息。但是无法调用Service中的方法。因为利用Message,所以不用担心并发

Extending the Binder classIf your service is private to your own application and runs in the same process as the client (which is common), you should create your interface by extending the Binder class and returning an instance of it from onBind(). The client receives the Binder and can use it to directly access public methods available in either the Binder implementation（得到Service对象，从而获取Service中的方法） or even the Service.This is the preferred technique when your service is merely a background worker for your own application. The only reason you would not create your interface this way is because your service is used by other applications or across separate processes.Service代码如下：
Activity代码如下：Using a Messenger

If you need your interface to work across different processes, you can create an interface for the service with a Messenger. In this manner, the service defines a Handler that responds to different types of Message objects. This Handler is the basis for a Messenger that can then share an IBinder with the client, allowing the client to send commands to the service using Message objects. Additionally, the client can define a Messenger of its own so the service can send messages back.

This is the simplest way to perform interprocess communication (IPC), because the Messenger queues all requests into a single thread so that you don't have to design your service to be thread-safe.

If you need your service to communicate with remote processes, then you can use a Messenger to provide the interface for your service. This technique allows you to perform interprocess communication (IPC) without the need to use AIDL.

Here's a summary of how to use a Messenger:

• The service implements a Handler that receives a callback for each call from a client.
• The Handler is used to create a Messenger object (which is a reference to the Handler).
• The Messenger creates an IBinder that the service returns to clients from onBind().
• Clients use the IBinder to instantiate the Messenger (that references the service's Handler), which the client uses to sendMessage objects to the service.
• The service receives each Message in its Handler—specifically, in the handleMessage() method.

  In this way, there are no methods for the client to call on the service. Instead, the client delivers messages (Message objects) that the service receives in its Handler.

  Messenger和AIDL的比较：

  Compared to AIDL

  When you need to perform IPC, using a Messenger for your interface is simpler than implementing it with AIDL, because Messenger queues all calls to the service, whereas, a pure AIDL interface sends simultaneous requests to the service, which must then handle multi-threading.

  For most applications, the service doesn't need to perform multi-threading, so using a Messengerallows the service to handle one call at a time. If it's important that your service be multi-threaded, then you should use AIDL to define your interface.

  Binding to a Service

  Application components (clients) can bind to a service by calling bindService(). The Android system then calls the service's onBind() method, which returns an IBinder for interacting with the service.

  The binding is asynchronous. bindService() returns immediately and does not return the IBinder to the client. To receive the IBinder, the client must create an instance of ServiceConnection and pass it to bindService(). The ServiceConnection includes a callback method that the system calls to deliver the IBinder.

  Managing the Lifecycle of a Bound Service

  ---

  When a service is unbound from all clients, the Android system destroys it (unless it was also started with onStartCommand()). As such, you don't have to manage the lifecycle of your service if it's purely a bound service—the Android system manages it for you based on whether it is bound to any clients.

  However, if you choose to implement the onStartCommand() callback method, then you must explicitly stop the service, because the service is now considered to be started. In this case, the service runs until the service stops itself with stopSelf() or another component calls stopService(), regardless of whether it is bound to any clients.

  Additionally, if your service is started and accepts binding, then when the system calls your onUnbind()method, you can optionally return true if you would like to receive a call to onRebind() the next time a client binds to the service (instead of receiving a call to onBind()). onRebind() returns void, but the client still receives the IBinder in its onServiceConnected() callback. Below, figure 1 illustrates the logic for this kind of lifecycle.

  

  （三） 使用AIDL (1.不需要IPC：implement a Binder; 2.需要IPC,不需要并发：use a Messenger; 3.需要IPC,需要并发：AIDL) Using AIDL is necessary only if you allow clients from different applications to access your service for IPC and want to handle multithreading in your service. If you do not need to perform concurrent IPC across different applications, you should create your interface by implementing a Binder or, if you want to perform IPC, but do not need to handle multithreading, implement your interface using a Messenger.

  1. Create the .aidl file

     This file defines the programming interface with method signatures.

  2. Implement the interface

     The Android SDK tools generate an interface in the Java programming language, based on your .aidl file. This interface has an inner abstract class named Stub that extends Binder and implements methods from your AIDL interface. You must extend the Stub class and implement the methods.

  3. Expose the interface to clients

     Implement a Service and override onBind() to return your implementation of the Stub class.

     IRemoteService.aidl文件：

     // IRemoteService.aidl
     package com.example.boundservice;// Declare any non-default types here with import statements/** Example service interface */
     interface IRemoteService {/** Request the process ID of this service, to do evil things with it. */int getPid();/** Demonstrates some basic types that you can use as parameters* and return values in AIDL.*/void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat,double aDouble, String aString);
     }

     自动生成(eclipse自动，studio需要rebuild)的IRemoteService.java: