大家好，又见面了，我是你们的朋友全栈君。
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：
代码语言：javascript复制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
代码语言：javascript复制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？
代码语言：javascript复制@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：
代码语言：javascript复制// 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
代码语言：javascript复制// 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
代码语言：javascript复制// 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;发布者：全栈程序员栈长，转载请注明出处：https://javaforall.cn/125861.html原文链接：https://javaforall.cn
