AsyncTask

用途

AsyncTask翻译成中文就是异步任务,这个类可以让我们在不使用多线程或Handler的情况下在后台进行操作并将结果发送到UI线程。

实现方式

AysncTask是通过Handler+Thread来实现,它作为一个工具类,不能当成线程框架来使用。

注意事项

AsyncTask理想状态下是用于较短的耗时操作,如果希望线程长时间运行,建议使用Executor, ThreadPoolExecutor和FutureTask

源码分析

1.创建线程池
AsyncTask会创建两个Executor,一个串行,一个并行
串行Executor

 private static class SerialExecutor implements Executor {
        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);
            }
        }
    }

并行Executor

    /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>(), sThreadFactory);
        threadPoolExecutor.setRejectedExecutionHandler(sRunOnSerialPolicy);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

AsyncTask默认为串行Executor
2.执行方式

//执行AsyncTask,使用默认的串行Executor
task.execute(null)
//执行AsyncTask,使用指定的Executor,可以自定义Executor来指定
task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, null)