Service是什么?Service又不是什么?

在Android王国中,Service是一个劳动模范,总是默默的在后台运行,无怨无悔,且总是干最脏最累的活,比如下载文件,倾听音乐,网络操作等这些耗时的操作,所以我们请尊重的叫他一声:"劳模,您辛苦了".

带着这份好尊重,我又重新研读了API的文档,发现老外写东西还是很靠谱的,人家在文档中告诉你Service是什么,又告诉你Service又不是什么,我觉得这种思路不错,那就从这两个个方面开始谈起吧:

Service是什么?

A Service is an application component. ☆ Service 是一个应用程序组件
that can perform long-running operations in the background. ☆ 它能在后台执行一些耗时较长的操作.
and does not provide a user interface. ☆ 并且不提供用户界面
Another application component can start a service and it will continue to run in the background event if the user switches to another application. ☆ 服务能被其它的应用程序组件启动,即使用户切换到其他的应用程序时还能保持在后台运行.
Additionally,a component can bind to a service to interact with it and even perform interprocess communcation(IPC). ☆ 此外,组件还能绑定服务,并与服务交互,甚至执行进程间通信(IPC).
For example,a service might handle network transactions,play music,perform file I/O,or interact with a content provider,all from the background. ☆ 比如,一个服务可以处理网络传输,听音乐,执行文件操作,或者与内容提供者进行交互,所有这些都在后台进行.
A service can essentially tack two forms.☆ 服务有以下两种基本类型

Started --> startService()
Bound --> bindService()

Service又不是什么?

A service is not a separate process.☆ 服务不是一个单独的进程.
A service is not a thread.it runs in the main thread of its hosting process. ☆ 服务不是一个线程,它运行在主线程.
the service does not create its own thread and does not run in a separate process(unless you specify otherwise). ☆ 服务不能自己创建并且不能运行在单独的进程中(除非你明确指定).
This means that, if your service is going to do any CPU intensive work ot blocking operations(such as MP3 playback or network). ☆ 这意味着如果你的服务要执行一些很耗CUP的工作或者阻塞的操作(比如播放mp3或网络操作),you should create a new thread within the service to do that work. ☆ 你应该在服务中创建一个新的线程来执行这些工作.
By using a separate thread, you will reduce the risk of Application Not Responding(ANR) errors and the application's main thread can remain dedicated to user interaction with your activities. ☆ 利用一个分离的进程,将减少你的activities发生应用程序停止响应(ANR)错误的风险.

如何创建一个Started服务

继承service
1. publicclassFirstServiceextendsService{
2. privatestaticfinalString TAG ="--FirstService-->";
3. publicFirstService(){
4. Log.i(TAG,"Service is running.");
5. }
7. @Override
8. publicvoid onCreate(){
9. Log.i(TAG,"onCreate is running.");
10. super.onCreate();
11. }
13. @Override
14. publicint onStartCommand(Intent intent,int flags,int startId){
15. Log.i(TAG,"onStartCommand is running.");
16. returnsuper.onStartCommand(intent, flags, startId);
17. }
19. @Override
20. publicIBinder onBind(Intent intent){
21. Log.i(TAG,"IBinder is running.");
22. returnnull;
23. }
24. }
四大组件都需要在manifests.xml中注册,这个也不例外.
如何启动它
1. Intent intent =newIntent(ServiceActivity.this,FirstService.class);
2. startService(intent);
生命周期onCreate(), onStartCommand(), onDestory()就这三个生命周期
1. --FirstService-->:Service is running.
2. --FirstService-->: onCreate is running.
3. --FirstService-->: onStartCommand is running.
我们在onStartCommand方法中打印下当前线程
1. @Override
2. publicint onStartCommand(Intent intent,int flags,int startId){
3. Log.i(TAG,"onStartCommand is running.Thread:"+Thread.currentThread());
4. returnsuper.onStartCommand(intent, flags, startId);
5. }
打印结果如下:
1. onStartCommand is running.Thread:Thread[main,5,main]
验证了两点:①由于是第二次运行,构造方法和onStart都没有打印,说明服务一旦启动是默默运行在后台的;②服务是运行在主线程的
如何结束服务:①调用stopService()方法 ,会回调service的onDestory()方法;
1. Intent intent2 =newIntent(ServiceActivity.this,FirstService.class);
2. stopService(intent2);
还可以在Android系统设置-->应用-->正在运行-->找到后是如下样式,会告诉我们有一个服务在运行;
onStartCommand的返回值:

START_STICKY:粘性的,被意外中止后自动重启,重新调用onStartCommand(),但会丢失原来激活它的Intent,会用一个null intent来调用onStartCommand(),可以用于播放器.值为1
START_NOT_STICKY:非粘性的,被意外中止后不会重启,除非还存在未发送的Intent,这是避免服务运行的最安全选项; 值为2
START_REDELIVER_INTENT:粘性的且重新发送Intent,被意外中止后重新启动,且该service组件将得到用于激活它的Intent对象,这中服务适用于需要立即恢复工作的活跃服务,比如下载文件; 值为3

onStartCommand的参数:
1. @Override //第一个参数:为我们传入的intent;第二个flags:启动服务的方式,与返回值有关;第三个为我们启动service的次数.
2. publicint onStartCommand(Intent intent,int flags,int startId){
3. Log.i(TAG,"onStartCommand is running.Thread:"+Thread.currentThread());
4. Log.i(TAG,"flags:"+flags);
5. Log.i(TAG,"startId:"+startId);
6. returnsuper.onStartCommand(intent, flags, startId);
7. }
因为前面服务已经启动了,这次我们连续点了三次启动服务的按钮,打印日志如下:
1. 11-1602:56:28.3859039-9039/com.wanghx.androidstudy I/--FirstService-->: onStartCommand is running.Thread:Thread[main,5,main]
2. 11-1602:56:28.3859039-9039/com.wanghx.androidstudy I/--FirstService-->: flags:0
3. 11-1602:56:28.3859039-9039/com.wanghx.androidstudy I/--FirstService-->: startId:2
4. 11-1602:56:33.9859039-9039/com.wanghx.androidstudy I/--FirstService-->: onStartCommand is running.Thread:Thread[main,5,main]
5. 11-1602:56:33.9859039-9039/com.wanghx.androidstudy I/--FirstService-->: flags:0
6. 11-1602:56:33.9859039-9039/com.wanghx.androidstudy I/--FirstService-->: startId:3
7. 11-1602:56:35.5359039-9039/com.wanghx.androidstudy I/--FirstService-->: onStartCommand is running.Thread:Thread[main,5,main]
8. 11-1602:56:35.5359039-9039/com.wanghx.androidstudy I/--FirstService-->: flags:0
9. 11-1602:56:35.5359039-9039/com.wanghx.androidstudy I/--FirstService-->: startId:4
我们发现flags的值没有发生改变,而startId再按顺序增加.

如何启动一个绑定服务

在activity中创建一个内部类,继承ServiceConnection.
1. classMyServiceConnectionimplementsServiceConnection{
2. @Override
3. publicvoid onServiceConnected(ComponentName name,IBinder service){
4. Log.i(TAG,"onServiceConnected");
5. }
7. @Override
8. publicvoid onServiceDisconnected(ComponentName name){
9. Log.i(TAG,"onServiceDisconnected");
10. }
11. }
在activity中定义一个成员connection
1. privateMyServiceConnection connection =newMyServiceConnection();
在绑定服务按钮中加入绑定代码
1. Intent intent3 =newIntent(ServiceActivity.this,FirstService.class);
2. bindService(intent3, connection, BIND_AUTO_CREATE);
按钮点击后打印如下日志:
1. I/--FirstService-->:Service is running.
2. I/--FirstService-->: onCreate is running.
3. I/--FirstService-->:IBinder is running.
在解绑服务中加入代码,这里的connection必须和上边的绑定服务的connection实例一致.
1. unbindService(connection);

Service和Thread的关系

其实他两个没有一毛钱关系.只是因为service需要做耗时操作,需要重新建立一线程来处理工作,而不阻塞主线程;
service是运行在主线程的;
activity启动service后,即使activity被销毁了,如果没有主动关闭服务,服务还是会在后台默默运行的;

如何连接远程的service,只需要在manifests.xml中这样写即可
1. <service
2. android:name="com.example.servicetest.MyService"
3. android:process=":remote">
4. </service>
如何让activity与远程的service进行通信呢?这就要使用AIDL进行跨进程通信(IPC)了.
AIDL:Android Interface Definition Language:Android接口定义语言,它可以用于让多个service与多个应用程序组件之间进行跨进程通信;
这些都不是重点,我们还是弄一下在我们自己的程序中service与activity之间的通信吧;

activity-->service 通过intent传递数据给service;
activity调用onServiceConnected()中的IBind对象来访问service中的方法;
IBinder通信的关键是利用activity中的IBinder对象获得service对象,然后调用方法;
1. publicclassFirstServiceextendsService{
2. privatestaticfinalString TAG ="FirstService-->";
3. privateMyBinder myBinder =newMyBinder();
5. publicFirstService(){
6. Log.i(TAG,"Service is running.");
7. }
9. @Override
10. publicvoid onCreate(){
11. Log.i(TAG,"onCreate is running.");
12. super.onCreate();
13. }
15. @Override
16. publicint onStartCommand(Intent intent,int flags,int startId){
17. String name = intent.getStringExtra("name");
18. Log.i(TAG,"onStartCommand is running.Thread:"+Thread.currentThread());
19. Log.i(TAG,"flags:"+flags);
20. Log.i(TAG,"startId:"+startId);
21. Log.i(TAG,"name:"+name);
22. return START_STICKY;
23. }
25. @Override
26. publicvoid onDestroy(){
27. Log.i(TAG,"onDestroy is running.");
28. super.onDestroy();
29. }
31. @Override
32. publicIBinder onBind(Intent intent){
33. Log.i(TAG,"IBinder is running.");
34. return myBinder;
35. }
37. publicclassMyBinderextendsBinder{
38. publicFirstService getService(){
39. returnFirstService.this;
40. }
41. }
43. publicint getRandomNumber(){
44. returnnewRandom().nextInt(10)+1;
45. }
46. }
1. publicclassServiceActivityextendsAppCompatActivityimplementsView.OnClickListener{
2. privateMyServiceConnection connection =newMyServiceConnection();
3. privatestaticfinalString TAG ="ServiceActivity-->";
4. privateFirstService mFirstService;
5. privateboolean isBinder;// 服务是否绑定
7. @Override
8. protectedvoid onCreate(Bundle savedInstanceState){
9. super.onCreate(savedInstanceState);
10. setContentView(R.layout.activity_service);
11. findViewById(R.id.btn_start_service).setOnClickListener(this);
12. findViewById(R.id.btn_stop_service).setOnClickListener(this);
13. findViewById(R.id.btn_bound_service).setOnClickListener(this);
14. findViewById(R.id.btn_unbound_service).setOnClickListener(this);
15. findViewById(R.id.btn_get_number).setOnClickListener(this);
16. }
18. @Override
19. publicvoid onClick(View v){
20. switch(v.getId()){
21. case R.id.btn_start_service:
22. Intent intent =newIntent(ServiceActivity.this,FirstService.class);
23. intent.putExtra("name","Zhangsan");
24. startService(intent);
25. break;
26. case R.id.btn_stop_service:
27. Intent intent2 =newIntent(ServiceActivity.this,FirstService.class);
28. stopService(intent2);
29. break;
30. case R.id.btn_bound_service:
31. if(!isBinder){
32. Intent intent3 =newIntent(ServiceActivity.this,FirstService.class);
33. bindService(intent3, connection, BIND_AUTO_CREATE);
34. isBinder =true;
35. }
36. break;
37. case R.id.btn_unbound_service:
38. if(isBinder){
39. unbindService(connection);
40. isBinder =false;
41. }
42. break;
43. case R.id.btn_get_number:
44. if(mFirstService ==null){
45. Toast.makeText(getApplicationContext(),"请先绑定服务",Toast.LENGTH_SHORT).show();
46. }else{
47. Toast.makeText(getApplicationContext(),"得到的随机数为:"+ mFirstService.getRandomNumber(),Toast
48. .LENGTH_SHORT).show();
49. }
50. break;
51. }
52. }
54. classMyServiceConnectionimplementsServiceConnection{
55. @Override
56. publicvoid onServiceConnected(ComponentName name,IBinder service){
57. Log.i(TAG,"onServiceConnected");
58. FirstService.MyBinder myBinder =(FirstService.MyBinder) service;
59. mFirstService = myBinder.getService();
60. }
62. @Override
63. publicvoid onServiceDisconnected(ComponentName name){
64. Log.i(TAG,"onServiceDisconnected");
65. }
66. }
67. }
这里有一个小插曲:一起写出来大家分享下:记得以前学java基础时,老师曾说过一个java文件中只能有一个public类,类名称必须与java文件名相同,为什么这个FirstService中有两个public类,只是因为MyBinder虽然是一个public class,但是MyBinder是一个内部类,这里必须要用public修饰,否则其他包就访问不到这个内部类了;

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Service是什么?Service又不是什么?

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