//AsyncTask从本质上讲,是对ThreadPool和handler的封装.
在学习线程池相关的知识时,看到书中提到AsyncTask的实现中使用到了ThreadPool,于是把源码翻了出来,
源码并不长,顺便分析了一下.小白挑战,大牛勿喷(好像大牛也不会看到...)
成员变量的解析
我打算先把成员变量分析一下(AsyncTask源码不是很复杂,可以这么做,算是特殊情况吧),算是为后面分析AsyncTask的工作流程做铺垫.
线程池 THREAD_POOL_EXECUTOR
//cpu核心数
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//核心线程数
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
//最大线程数
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
//非核心线程数闲置时的超时时长
private static final int KEEP_ALIVE = 1;
//线程工厂,为线程池提供创建新线程的功能只有一个方法newThread(),具体实现就是创建一个thread并返回
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//线程池中的任务队列,通过线程池的execute方法提交的Runnable对象会存储在这个参数中,此处LinkedBlockingQueue是一个单向链表实现的阻塞队列
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
//使用以上参数创建的线程池
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
sDefaultExecutor (特别强调:串行)
private static class SerialExecutor implements Executor {
//这里使用了一个双端队列
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
这里实现了一个串行的exectuor,这里的实现很巧妙,仔细观察可以发现,每次调用execute()方式都必然会调用scheduleNext(),
使得mTasks中的所有Runnable对象依次执行.
还有一点需要注意的是:
这里的execute()方法的作用只是用于给任务排队,真正执行任务的是THREAD_POOL_EXECUTOR.
然后看一下成员变量的声明:
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
SERIAL_EXECUTOR 唯一的作用是把SerialExecutor对象传递给sDefaultExecutor,我觉得这种写法很奇怪,于是尝试了一下发现
final和volatile不能同时修饰一个变量,其实从它们各自的作用也可以看出同时使用这两个修饰符是矛盾的.
为什么这里要这样使用呢?
我觉得原因在于下面这个隐藏方法:
/** @hide */
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
也就是说AsyncTask中有一个sDefaultExecutor的默认实现,但是还提供了一个隐藏方法,可以对其进行修改.
sHandler
首先先看一个内部类的实现,这个内部类是用来包装执行结果的,其中封装了一个AsyncTask实例和某种类型的数据集.
@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
接下来看InternalHandler的具体实现
private static final int MESSAGE_POST_RESULT = 0x1;//返回结果
private static final int MESSAGE_POST_PROGRESS = 0x2;//返回进度
private static InternalHandler sHandler;
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);//具体实现在下面
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);//这个就是我们自定义AsyncTask时需要实现的onProgressUpdate()
break;
}
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);//这个同样需要用户来实现
} else {
onPostExecute(result);//这个就是我们自定义AsyncTask时需要实现的onPostExecute()
}
mStatus = Status.FINISHED;//任务执行完成的状态
}
//这里的实现是单例模式
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
onProgressUpdate,onPostExecute,onCancelled这三个回调方法会在这里集中处理,结合AsyncTask的使用方法,我们可以得出以下结论:
在doInBackground()执行过程中或者结束后,会由该handler发送消息到主线程,调用相关回调方法,可以认为,handler是AsyncTask整个工作流程的倒数第二站.
mWorker&FutureTask
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
Callable
WorkRunnable实现了Callble接口,Callable与Runnable的功能大致相似,不同的是Callable是一个泛型接口,它有一个泛型参数V,该接口中只有一个call()方法,该方法返回类型为V的值.
Callable可以简单理解为有返回值的Runnable.
可以对比一下
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
public interface Runnable {
/**
* Starts executing the active part of the class' code. This method is
* called when a thread is started that has been created with a class which
* implements {@code Runnable}.
*/
public void run();
}
FutureTask
FutureTask稍微复杂一些,FutureTask实现了 RunnableFuture接口,而RunnableFuture继承Runnable, Future
Future提供了对Runnable或者Callable任务的执行结果进行取消,查询是否完成,获取结果的操作,FutureTask是它的实现类
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
下面是FutureTask的一个构造函数,在AsyncTask的构造函数中会使用其创建一个FutureTask实例
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
下面看一下构造函数对这两个成员变量的初始化(构造函数唯一做的事情)
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);//设置mTask已经被调用过
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
Result result = doInBackground(mParams);//调用doInBackground()!!!!!!!!
Binder.flushPendingCommands();
//调用postResult将结果投递给UI线程
return postResult(result);
}
};
//将mWorker作为参数创建FutureTask实例
//在mFuture实例中,将会调用mWorker做后台任务,完成后调用done方法
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());//确保postResult的执行
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
//确保postResult的执行
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
//这里最终的结果是调用onCancelled()或者onPostExecute()
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
doInBackground和handler的使用都在这里!!!!!
最后的三个成员变量(标志位)
//枚举 执行状态
private volatile Status mStatus = Status.PENDING;
//是否取消
private final AtomicBoolean mCancelled = new AtomicBoolean();
//mTask是否被调用过
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}
至此,AsyncTask中的所有成员变量分析完毕,各个关键回调方法的调用地点也都清楚了,下面我们从execute()方法入手,分析一下AsyncTask的工作流程.
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);//这里的参数现在看起来已经很清晰了,sDefaultExecutor是用于将任务加入队列的executor,params是用户提供的参数.
}
继续往下看executeOnExecutor
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
//首先,判断状态,只有PENDING才能继续执行
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
//修改状态为RUNNING
mStatus = Status.RUNNING;
onPreExecute();//onPreExectue首先执行
//设置mWorker的参数
mWorker.mParams = params;
//这里的exec就是sDefaultExecutor,
exec.execute(mFuture);
return this;
}
通过上文的分析,我们知道这里的exec.execute(mFuture)最终调用到了THREAD_POOL_EXECUTOR中的execute()方法.
线程池内部的具体细节暂不分析,execute()方法执行的最终结果是将mFuture中的run()方法启动.
FutureTask中的run():
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();//这里调用了mWorker的call()方法,而mWorker的call()调用了doInBackground()方法.至此,线程真正启动了.
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);//成功获取结果后,调用set()方法
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
protected void set(V v) {
if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
outcome = v;
U.putOrderedInt(this, STATE, NORMAL); // final state
finishCompletion();//set()方法又调用finishCompletion()方法
}
}
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (U.compareAndSwapObject(this, WAITERS, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();//finishCompletion()方法最终调用了done()方法,而上文中已经提到,调用done()方法最终的结果是通过handler调用onCancelled()或者opPostExecute();
callable = null; // to reduce footprint
}
至此,AsyncTask的整个工作流程分析完毕,一次奇妙的源码之旅get,接下来可以去拜读各位大牛对于AsyncTask更加深入的分析了...