概要介绍
和HandlerThread一样，IntentService也是Android替我们封装的一个Helper类，用来简化开发流程的。IntentService是一个按需处理用Intent表示的异步请求的基础Service类，本质上还是Service。客户端通过Context#startService(Intent);这样的代码来发起一个请求。Service只在没启动的情况下才启动，并且在一个worker thread
中处理所有的请求，当所有的请求处理完毕时IntentService会自动停止，所以你不需要显式的stop它。关于客户端代码如何正确的使用它，请参看官方文档。
源码分析
接着和以往一样，我们先来看看关键字段和ctor：
private volatile Looper mServiceLooper; // 这2者都是和HandlerThread关联的，只是没明白这里为什么需要volatile关键字
private volatile ServiceHandler mServiceHandler; // 看起来他们都只是在UI线程中被访问了，似乎没有什么并发问题。。。可能是为了更保险
private String mName; // 这里的mName给创建HandlerThread时用的名字
private boolean mRedelivery;
/**
* 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;
}
接下来看点有意思的代码：
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) { // 基于我们前面关于Handler的介绍，这些代码都很容易理解
onHandleIntent((Intent)msg.obj); // 注意这个Template方法，这是我们的子类中真正处理请求的地方
stopSelf(msg.arg1); // 注意看这里调用的是带参数的stopSelf并不是无参版本的stopSelf()，
} // 这是因为IntentService并不是处理完一个请求就退出，而是所有请求。
}
/**
* 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) { // 设置是否重新发送Intent，一般在ctor中设置
mRedelivery = enabled; // 具体内容请详细阅读方法的doc
}
@Override
public void onCreate() { // 此方法只在第一次需要创建service的时候调用
// 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(); // 启动接下来处理客户端异步请求的HandlerThread
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper); // 拿到与之关联的Handler，用来向它发送待处理的消息（即客户端请求）
}
接下来看2个onStartXXX相关的方法：
@Override
public void onStart(Intent intent, int startId) { // 其所作的事情就是根据参数获得一个对应的Message，
Message msg = mServiceHandler.obtainMessage(); // send一个Message而已，消息的处理会在ServiceHandler
msg.arg1 = startId; // 的handleMessage方法中进行
msg.obj = intent; // 稍后我们分析下这里的startId咋来的
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; // 根据mRedelivery返回不同的策略值
}
这里我们解释下int startId的来历。首先我们说下这部分代码位于frameworks/base/services/java/com/android/server/am/这个目录，
它下面有很多对Android内部机制来说很重要的类（比如“很有名”的ANR dialog就在这里）。与startId相关的2个类分别是ServiceRecord和ActiveServices，这里我们看下ServiceRecord中与startId相关的代码，如下：
private int lastStartId; // identifier of most recent start request.
public int getLastStartId() {
return lastStartId;
}
public int makeNextStartId() { // 此方法的调用是在ActiveServices中
lastStartId++;
if (lastStartId < 1) { // 通过代码我们可以看到startId是从1开始的正整数，每次+1
lastStartId = 1; // 你可以理解成客户端请求的次数(即startService调用的次数)
}
return lastStartId;
}
这一点代码就完全解释了我们一直以来的困惑，像我自己一直以来就不理解这里的startId是干嘛用的，咋来的。
接下来我们看一组stopXXX相关的方法：
/**
* Stop the service, if it was previously started. This is the same as
* calling {@link android.content.Context#stopService} for this particular service.
*
* @see #stopSelfResult(int)
*/
public final void stopSelf() { // 内部调用参数为-1的版本，此方法会停止service
stopSelf(-1);
}
/**
* Old version of {@link #stopSelfResult} that doesn't return a result.
*
* @see #stopSelfResult
*/
public final void stopSelf(int startId) { // 参数startId要么是-1要么是从1开始的正整数，只有它等于我们最后一次调用
if (mActivityManager == null) { // startService时，onStartCommand里传递进来的startId值时，
return; // service才会停止，否则并不会停止service。service会在处理完
} // 所有的客户端请求后自动停止。比如客户端调用了10次startService来
try { // 发出多个请求，那么只有当这里的startId == 10的时候，service才会停止，
mActivityManager.stopServiceToken(// 其onDestroy方法才会被调用。另外由于我们的请求总是串行处理的，所以永远不会
new ComponentName(this, mClassName), mToken, startId); // 出现先stopSelf(10)再stopSelf(9)这种情况。
} catch (RemoteException ex) {
}
}
/**
* Stop the service if the most recent time it was started was
* <var>startId</var>. This is the same as calling {@link
* android.content.Context#stopService} for this particular service but allows you to
* safely avoid stopping if there is a start request from a client that you
* haven't yet seen in {@link #onStart}.
*
* <p><em>Be careful about ordering of your calls to this function.</em>.
* If you call this function with the most-recently received ID before
* you have called it for previously received IDs, the service will be
* immediately stopped anyway. If you may end up processing IDs out
* of order (such as by dispatching them on separate threads), then you
* are responsible for stopping them in the same order you received them.</p>
*
* @param startId The most recent start identifier received in {@link
* #onStart}.
* @return Returns true if the startId matches the last start request
* and the service will be stopped, else false.
*
* @see #stopSelf()
*/
public final boolean stopSelfResult(int startId) { // 此方法基本同上，不赘述，后面我们刨根问底下stopServiceToken到底咋实现的，
if (mActivityManager == null) { // 看看这里startId是-1和正整数到底有啥区别。
return false;
}
try {
return mActivityManager.stopServiceToken(
new ComponentName(this, mClassName), mToken, startId);
} catch (RemoteException ex) {
}
return false;
}
@Override
public void onDestroy() { // 处理完所有客户端请求，stop service的时候会被调到，退出looper。
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) { // 当你只是个started service的时候，默认实现就足够了。
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)}.
*/
protected abstract void onHandleIntent(Intent intent); // handleMessage中定义的模板方法，也即我们处理请求的逻辑发生的地方
从上面的代码我们看出，停止Service时都调用了这样的代码：
mActivityManager.stopServiceToken(new ComponentName(this, mClassName), mToken, startId);这里的mActivityManager相关的代码，可以参考ActivityManagerNative.java文件，另外这里实际上是利用了Android的Binder机制，通过IPC调到了system_service进程中的ActivityManagerService#stopServiceToken方法，代码如下：
@Override
public boolean stopServiceToken(ComponentName className, IBinder token, // ActivityManagerService.java中的方法
int startId) {
synchronized(this) {
return mServices.stopServiceTokenLocked(className, token, startId);
}
}
boolean stopServiceTokenLocked(ComponentName className, IBinder token, // ActiveServices.java中的方法
int startId) {
if (DEBUG_SERVICE) Slog.v(TAG, "stopServiceToken: " + className
+ " " + token + " startId=" + startId);
ServiceRecord r = findServiceLocked(className, token, UserHandle.getCallingUserId());
if (r != null) {
if (startId >= 0) { // 注意这个判断，和我们猜测的一样
// Asked to only stop if done with all work. Note that
// to avoid leaks, we will take this as dropping all
// start items up to and including this one.
ServiceRecord.StartItem si = r.findDeliveredStart(startId, false);
if (si != null) {
while (r.deliveredStarts.size() > 0) {
ServiceRecord.StartItem cur = r.deliveredStarts.remove(0);
cur.removeUriPermissionsLocked();
if (cur == si) {
break;
}
}
}
if (r.getLastStartId() != startId) { // 这句代码是所有疑惑的答案
return false; // 如果不是最后一个请求的startId，直接返回了，并没有往下面执行；
} // 这也就解释了为啥非last startId不能让service停止的原因。
if (r.deliveredStarts.size() > 0) {
Slog.w(TAG, "stopServiceToken startId " + startId
+ " is last, but have " + r.deliveredStarts.size()
+ " remaining args");
}
}
synchronized (r.stats.getBatteryStats()) {
r.stats.stopRunningLocked();
}
// 如果是-1，直接往下走，所以一次调用就能停止Service
r.startRequested = false;
if (r.tracker != null) {
r.tracker.setStarted(false, mAm.mProcessStats.getMemFactorLocked(),
SystemClock.uptimeMillis());
}
r.callStart = false;
final long origId = Binder.clearCallingIdentity();
bringDownServiceIfNeededLocked(r, false, false); // 真正让service停止的代码
Binder.restoreCallingIdentity(origId);
return true;
}
return false;
}
至此IntentService相关的所有代码都已经分析完毕了，enjoy。。。
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