android线程与线程池-----AsyncTask（一）《android开发艺术与探索》

线程在android是个重要的概念，从用途上讲，线程分为主线程和子线程，主线程负责页面相关，子线程负责耗时操作。

在android中除了Thread本身还有 AsyncTask  IntentService  HandlerThread。

AsyncTask

    public abstract class AsyncTask<Params, Progress, Result>  

1 Params 参数类型
2 Progress 执行进度类型
3 Result 返回数据类型

不需要参数可以用Void代替

它提供了4个核心方法：

    //异步任务执行之前调用，一般用来执行一些准备操作
           @Override
           protected void onPreExecute() {
               super.onPreExecute();
           }
           //在线程池调用， 用于执行异步任务
           @Override
           protected String doInBackground(String... params) {
               return null;
           }
           //异步任务执行之后会调用
           @Override
           protected void onPostExecute(String result) {
               super.onPostExecute(result);
           }
           //主线程中执行，后台任务执行进度发生改变调用
           @Override
           protected void onProgressUpdate(Void... values) {
               super.onProgressUpdate(values);
        }  

注意：

1 AsyncTask类必须在主线程中加载

2 AsyncTask的对象必须在主线程创建

3 execute必须在UI线程调用

4 一个AsyncTask的对象只能执行一次，即只调用一次execute方法

工作原理：

    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
            return executeOnExecutor(sDefaultExecutor, params);
        }
    execute 方法会调用 executeOnExecutor 方法；看一下 executeOnExecutor 方法：
     public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
                Params... params) {
            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;  

            onPreExecute();  

            mWorker.mParams = params;
            exec.execute(mFuture);  

            return this;
        }  

可以看出封装了一个线程池，接着找我发现了

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();  

    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();
                }
            }  

从SerialExecutor 看出AsyncTask是排队执行的过程。

系统首先会把AsyncTask的参数Params 封装为FutureTask对象，接着会把FutureTask交给SerialExecutor的execute处理，
execute方法把FutureTask交给mTasks任务队列中，如果这时没有AsyncTask任务，SerialExecutor会
scheduleNext()来执行下一个任务。当一个任务执行完以后，SerialExecutor才会执行其他任务，可以看出AsyncTask是串
行的。

AsyncTask构造函数有这样一段代码：

    mWorker = new WorkerRunnable<Params, Result>() {
                public Result call() throws Exception {
                    mTaskInvoked.set(true);  

                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    return postResult(doInBackground(mParams));
                }
            };  

再看FutureTask的run方法：

派生到我的代码片

    public void run() {
            if (state != NEW ||
                !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                             null, Thread.currentThread()))
                return;
            try {
                Callable<V> c = callable;
                if (c != null && state == NEW) {
                    V result;
                    boolean ran;
                    try {
                        result = c.call();
                        ran = true;
                    } catch (Throwable ex) {
                        result = null;
                        ran = false;
                        setException(ex);
                    }
                    if (ran)
                        set(result);
                }
            } 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);
            }
        }  

发现FutureTask的run方法是执行了mWorker的call的，所以call也会在线程池中执行。

mWorker的call中将mTaskInvoked.set(true);表示当前任务已经调用然后执行AsyncTask的doInBackground方法接着将
返回值传给postResult方法；

    private Result postResult(Result result) {
          @SuppressWarnings("unchecked")
          Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                  new AsyncTaskResult<Result>(this, result));
          message.sendToTarget();
          return result;
      }  

可以看出postResult通过一个Handler发送一个MESSAGE_POST_RESULT消息

    private static Handler getHandler() {
            synchronized (AsyncTask.class) {
                if (sHandler == null) {
                    sHandler = new InternalHandler();
                }
                return 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]);
                        break;
                    case MESSAGE_POST_PROGRESS:
                        result.mTask.onProgressUpdate(result.mData);
                        break;
                }
            }
        }  

当这个Handler收到MESSAGE_POST_RESULT消息之后，会调用finish方法了

    private void finish(Result result) {
            if (isCancelled()) {
                onCancelled(result);
            } else {
                onPostExecute(result);
            }
            mStatus = Status.FINISHED;
        }  

如果取消执行就 onCancelled 了，否则就调用onPostExecute这个方法。