Android AsyncTask 分析内部实现

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mob604757069565 2017-05-23 17:27:00

sdk3.0前，使用内部的线程池，多线程并发运行。线程池大小等于5，最大达128

sdk3.0后，使用默认的serial线程池。运行完一个线程，再顺序运行下一个线程。sdk3.0<=current version <= sdk4.3时线程池大小等于5，最大达128

sdk4.4后线程池大小等于 cpu count + 1。最大值为cpu count * 2 + 1

sdk3.0后有两种线程池的实现，默觉得 Serial 线程池

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static void setDefaultExecutor(Executor exec) {//设置默认线程池
sDefaultExecutor = exec;
}

SerialExecutor ，使用同步锁，一次运行一个线程

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);
            }
        }
    }