IntentService浅析
说起IntentService就需要先了解一下Service。
Service 是长期运行在后台的应用程序组件。
Service 不是一个单独的进程,它和应用程序在同一个进程中,Service 也不是一个线程,它和线程没有任何关系,所以它不能直接处理耗时操作。如果直接把耗时操作放在 Service 的 onStartCommand() 中,很容易引起 ANR(ActivityManagerService.java中定义了超时时间,前台service超过20S,后台Service超过200S无响应就会ANR) 。如果有耗时操作就必须开启一个单独的线程来处理。
既然Service不能直接执行耗时操作,那么在Service开启子线程执行耗时操作不就好了。当然,这么想完全没毛病,在Service中执行耗时操作也只能这么干。如:
public int onStartCommand(Intent intent, int flags, int startId) {
Thread thread = new Thread(){
@Override
public void run() {
/**
* 耗时的代码在子线程里面写
*/
}
};
thread.start();
return super.onStartCommand(intent, flags, startId);
}
正如前面说到,Service是长期运行在后台的应用程序组件,那么Service一旦启动就会一直运行下去,必须人为调用stopService()
或者stopSelf()
方法才能让服务停止下来。如果耗时操作只想执行一次,那么必须在执行耗时操作的子线程执行完后就结束Service自身。如:
public void run() {
/**
* 耗时的代码在子线程里面写
*/
stopSelf();
}
或者在代码里的某个地方调用stopService()
或者stopSelf()
方法。
这么做是否很麻烦呢?如果忘记在Service开始子线程呢?如果忘记结束Service呢?
所以引入了今天的话题:IntentService。
IntentService 是继承于 Service 并处理异步请求的一个类,在 IntentService 内有一个工作线程来处理耗时操作,启动 IntentService 的方式和启动传统 Service 一样,同时,当任务执行完后,IntentService 会自动停止,而不需要我们去手动控制。另外,可以启动 IntentService 多次,而每一个耗时操作会以工作队列的方式在IntentService 的 onHandleIntent 回调方法中执行,并且,每次只会执行一个工作线程,执行完第一个再执行第二个,以此类推。
而且,所有请求都在一个单线程中,不会阻塞应用程序的主线程(UI Thread),同一时间只处理一个请求。 那么,用 IntentService 有什么好处呢?首先,我们省去了在 Service 中手动开线程的麻烦,第二,当操作完成时,我们不用手动停止 Service。
IntentService源码分析
package android.app;
import android.annotation.WorkerThread;
import android.content.Intent;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.IBinder;
import android.os.Looper;
import android.os.Message;
/**
* IntentService is a base class for {@link Service}s that handle asynchronous
* requests (expressed as {@link Intent}s) on demand. Clients send requests
* through {@link android.content.Context#startService(Intent)} calls; the
* service is started as needed, handles each Intent in turn using a worker
* thread, and stops itself when it runs out of work.
*
* <p>This "work queue processor" pattern is commonly used to offload tasks
* from an application's main thread. The IntentService class exists to
* simplify this pattern and take care of the mechanics. To use it, extend
* IntentService and implement {@link #onHandleIntent(Intent)}. IntentService
* will receive the Intents, launch a worker thread, and stop the service as
* appropriate.
*
* <p>All requests are handled on a single worker thread -- they may take as
* long as necessary (and will not block the application's main loop), but
* only one request will be processed at a time.
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For a detailed discussion about how to create services, read the
* <a href="{@docRoot}guide/topics/fundamentals/services.html">Services</a> developer guide.</p>
* </div>
*
* @see android.os.AsyncTask
*/
public abstract class IntentService extends Service {
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;
private String mName;
private boolean mRedelivery;
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
onHandleIntent((Intent)msg.obj);
stopSelf(msg.arg1);
}
}
/**
* Creates an IntentService. Invoked by your subclass's constructor.
*
* @param name Used to name the worker thread, important only for debugging.
*/
public IntentService(String name) {
super();
mName = name;
}
/**
* Sets intent redelivery preferences. Usually called from the constructor
* with your preferred semantics.
*
* <p>If enabled is true,
* {@link #onStartCommand(Intent, int, int)} will return
* {@link Service#START_REDELIVER_INTENT}, so if this process dies before
* {@link #onHandleIntent(Intent)} returns, the process will be restarted
* and the intent redelivered. If multiple Intents have been sent, only
* the most recent one is guaranteed to be redelivered.
*
* <p>If enabled is false (the default),
* {@link #onStartCommand(Intent, int, int)} will return
* {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
* dies along with it.
*/
public void setIntentRedelivery(boolean enabled) {
mRedelivery = enabled;
}
@Override
public void onCreate() {
// TODO: It would be nice to have an option to hold a partial wakelock
// during processing, and to have a static startService(Context, Intent)
// method that would launch the service & hand off a wakelock.
super.onCreate();
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
@Override
public void onStart(Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
/**
* You should not override this method for your IntentService. Instead,
* override {@link #onHandleIntent}, which the system calls when the IntentService
* receives a start request.
* @see android.app.Service#onStartCommand
*/
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
onStart(intent, startId);
return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}
@Override
public void onDestroy() {
mServiceLooper.quit();
}
/**
* Unless you provide binding for your service, you don't need to implement this
* method, because the default implementation returns null.
* @see android.app.Service#onBind
*/
@Override
public IBinder onBind(Intent intent) {
return null;
}
/**
* This method is invoked on the worker thread with a request to process.
* Only one Intent is processed at a time, but the processing happens on a
* worker thread that runs independently from other application logic.
* So, if this code takes a long time, it will hold up other requests to
* the same IntentService, but it will not hold up anything else.
* When all requests have been handled, the IntentService stops itself,
* so you should not call {@link #stopSelf}.
*
* @param intent The value passed to {@link
* android.content.Context#startService(Intent)}.
*/
@WorkerThread
protected abstract void onHandleIntent(Intent intent);
}
Service有startService()
和bindService()
两种启动方式,那么IntentService是否也有呢?
/**
* Unless you provide binding for your service, you don't need to implement this
* method, because the default implementation returns null.
* @see android.app.Service#onBind
*/
@Override
public IBinder onBind(Intent intent) {
return null;
}
IntentService 源码中的 onBind()
默认返回 null;不适合 bindService()
启动服务,如果你执意要 bindService()
来启动 IntentService,可能因为你想通过 Binder 或 Messenger 使得 IntentService 和 Activity 可以通信,这样那么 onHandleIntent()
不会被回调,相当于在你使用 Service 而不是 IntentService。因此,并不建议通过 bindService()
启动 IntentService,而是通过startService()
来启动IntentService。
为什么多次启动 IntentService 会顺序执行事件,停止服务后,后续的事件得不到执行?
@Override
public void onStart(Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
@Override
public void onDestroy() {
mServiceLooper.quit();
}
IntentService 中使用的 Handler、Looper、MessageQueue 机制把消息发送到线程中去执行的,所以多次启动 IntentService 不会重新创建新的服务和新的线程,只是把消息加入消息队列中等待执行,而如果服务停止,会清除消息队列中的消息,后续的事件得不到执行。
使用方法:
1、创建一个类并继承IntentService,重写onHandleIntent()
,构造函数super("线程名")
;
2、在AndroidManifest.xml里注册,同Service;
3、通过startService()
启动。
package com.eebbk.synchinese.widgetutil;
import android.app.IntentService;
import android.content.Intent;
import android.support.annotation.Nullable;
/**
* 刷新桌面挂件
* Created by zhangshao on 2018/2/28.
*/
public class WidgetUpdateService extends IntentService {
public WidgetUpdateService() {
super("WidgetUpdateService");
}
@Override
protected void onHandleIntent(@Nullable Intent intent) {
synchronized (this) {
FastOpenBookUtil.refreshLearnBookWidgetBroadCast(this);
}
}
}
附:
ActivityManagerService.java中ANR的超时时间定义:
1、Broadcast超时时间为10秒:
// How long we allow a receiver to run before giving up on it.
static final int BROADCAST_FG_TIMEOUT = 10*1000;
static final int BROADCAST_BG_TIMEOUT = 60*1000;
2、按键无响应的超时时间为5秒
// How long we wait until we timeout on key dispatching.
static final int KEY_DISPATCHING_TIMEOUT = 5*1000;
3、Service的ANR在ActiveServices中定义:
前台service无响应的超时时间为20秒,后台service为200秒
// How long we wait for a service to finish executing.
static final int SERVICE_TIMEOUT = 20*1000;
// How long we wait for a service to finish executing.
static final int SERVICE_BACKGROUND_TIMEOUT = SERVICE_TIMEOUT * 10;
今天的文章IntentService浅析分享到此就结束了,感谢您的阅读。
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