AsyncTask解析
AsyncTask解析
AsyncTask原理简介
- AsyncTask通过Handler实现异步消息处理。
- AsyncTask是一个抽象类，使用的时候必须要实现抽象方法doInBackground(Params… params)，onPreExecute()和PostExecute(Result result)和onProgressUpdate(Progress… values)都是空方法，当业务需要的时候可以选择重写该方法。
- AsyncTask初始化流程图
- AsyncTask执行流程图
- AsyncTask默认单任务执行模式，同一时间只能有一个任务在执行。
关键源码解析
public abstract class AsyncTask {
private static final String LOG_TAG = "AsyncTask";
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE = 1;
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());
}
};
private static final BlockingQueue sPoolWorkQueue =
new LinkedBlockingQueue(128);
// 线程池，静态变量，所有的AsyncTask对象共享
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
// 自定义Executor，保证同一时间只有一个任务执行，静态变量所有的AsyncTask对象共享
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
// 任务完成，消息标志
private static final int MESSAGE_POST_RESULT = 0x1;
// 任务进度，消息标志
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler;
private final WorkerRunnable mWorker;
private final FutureTask mFuture;
// 表示任务当前的状态
private volatile Status mStatus = Status.PENDING;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
// 自定义Executor，保证同一时间只有一个任务执行
private static class SerialExecutor implements Executor {
// 自增长队列，多线程不安全
final ArrayDeque mTasks = new ArrayDeque();
Runnable mActive;
// 加锁
public synchronized void execute(final Runnable r) {
// 向队列中插入任务
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
// 当前任务执行完毕后，执行下一个任务
scheduleNext();
}
}
});
// mActive为null,代表当前没有任务执行，执行刚才插入队列的任务
if (mActive == null) {
scheduleNext();
}
}
// 如果队列中没有任务，mActive则变为null；有任务则赋值给mActive，线程池来执行该任务
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
public enum Status {
// 初始化状态
PENDING,
// 正在运行状态
RUNNING,
// 结束状态
FINISHED,
}
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
/** @hide */
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
// AsyncTask构造方法
public AsyncTask() {
// 初始化Callable对象
mWorker = new WorkerRunnable() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// 调用doInBackground()方法
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
// 执行完毕发送结果
return postResult(result);
}
};
// 初始化FutureTask对象，mWorker传递进去
mFuture = new FutureTask(mWorker) {
@Override
protected void done() {
try {
//如果mWorker的call()异常未执行，在这边发送结果
postResultIfNotInvoked(get());
} 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);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
// 发送任务完成消息
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult(this, result));
message.sendToTarget();
return result;
}
public final Status getStatus() {
return mStatus;
}
// 需要实现的方法，耗时操作的代码块
protected abstract Result doInBackground(Params... params);
// 任务开始前的回调方法体
protected void onPreExecute() {
}
// 任务完成后回调方法体
protected void onPostExecute(Result result) {
}
// 进度更新回调方法体
protected void onProgressUpdate(Progress... values) {
}
// 任务取消回调方法体
protected void onCancelled(Result result) {
onCancelled();
}
protected void onCancelled() {
}
public final boolean isCancelled() {
return mCancelled.get();
}
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
// 获取任务结果
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
// 获取任务结果，设置等待结果的超时时间
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
// 开始执行任务，参数最终会传递给doInBackground(Params... params)方法
public final AsyncTask execute(Params... params) {
// sDefaultExecutor默认是SerialExecutor对象，如果不希望同一时间只有一个任务在执行,可以设置sDefaultExecutor为自定义的Executor对象
return executeOnExecutor(sDefaultExecutor, params);
}
// 开始执行任务，参数最终会传递给doInBackground(Params... params)方法
public final AsyncTask execute(Params... params) {
// sDefaultExecutor默认是具有单线程池效果的Executor对象(下面会分析该对象源码)，如果不希望同一时间只有一个任务在执行,可以设置sDefaultExecutor为自定义的Executor对象
return executeOnExecutor(sDefaultExecutor, params);
}
public final AsyncTask executeOnExecutor(Executor exec,
Params... params) {
// 如果不是初始状态，抛出异常。说明一个AsynyTask对象只能调用一次execute(Params... params)方法。
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)");
}
}
mStatus = Status.RUNNING;
// 任务开始执行前回调
onPreExecute();
mWorker.mParams = params;
// 线程池执行FutureTask对象
exec.execute(mFuture);
return this;
}
// 静态方法，可以当做单线程池来用
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
// 进度更新时，通过Handler发送通知
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult