Android源码分析—带你认识不一样的AsyncTask

转载请注明出处:http://blog.csdn.net/singwhatiwanna/article/details/17596225

前言

什么是AsyncTask,相信搞过android开发的朋友们都不陌生。AsyncTask内部封装了Thread和Handler,可以让我们在后台进行计算并且把计算的结果及时更新到UI上,而这些正是Thread+Handler所做的事情,没错,AsyncTask的作用就是简化Thread+Handler,让我们能够通过更少的代码来完成一样的功能,这里,我要说明的是:AsyncTask只是简化Thread+Handler而不是替代,实际上它也替代不了。同时,AsyncTask从最开始到现在已经经过了几次代码修改,任务的执行逻辑慢慢地发生了改变,并不是大家所想象的那样:AsyncTask是完全并行执行的就像多个线程一样,其实不是的,所以用AsyncTask的时候还是要注意,下面会一一说明。另外本文主要是分析AsyncTask的源代码以及使用时候的一些注意事项,如果你还不熟悉AsyncTask,请先阅读android之AsyncTask来了解其基本用法。

这里先给出AsyncTask的一个例子:

private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i / (float) count) * 100));
             // Escape early if cancel() is called
             if (isCancelled()) break;
         }
         return totalSize;
     }

     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }

     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
     }
 }

使用AsyncTask的规则

  • AsyncTask的类必须在UI线程加载(从4.1开始系统会帮我们自动完成)
  • AsyncTask对象必须在UI线程创建
  • execute方法必须在UI线程调用
  • 不要在你的程序中去直接调用onPreExecute(), onPostExecute, doInBackground, onProgressUpdate方法
  • 一个AsyncTask对象只能执行一次,即只能调用一次execute方法,否则会报运行时异常
  • AsyncTask不是被设计为处理耗时操作的,耗时上限为几秒钟,如果要做长耗时操作,强烈建议你使用Executor,ThreadPoolExecutor以及FutureTask
  • 在1.6之前,AsyncTask是串行执行任务的,1.6的时候AsyncTask开始采用线程池里处理并行任务,但是从3.0开始,为了避免AsyncTask所带来的并发错误,AsyncTask又采用一个线程来串行执行任务

AsyncTask到底是串行还是并行?

给大家做一下实验,请看如下实验代码:代码很简单,就是点击按钮的时候同时执行5个AsyncTask,每个AsyncTask休眠3s,同时把每个AsyncTask执行结束的时间打印出来,这样我们就能观察出到底是串行执行还是并行执行。

    @Override
    public void onClick(View v) {
        if (v == mButton) {
            new MyAsyncTask("AsyncTask#1").execute("");
            new MyAsyncTask("AsyncTask#2").execute("");
            new MyAsyncTask("AsyncTask#3").execute("");
            new MyAsyncTask("AsyncTask#4").execute("");
            new MyAsyncTask("AsyncTask#5").execute("");
        }

    }

    private static class MyAsyncTask extends AsyncTask<String, Integer, String> {

        private String mName = "AsyncTask";

        public MyAsyncTask(String name) {
            super();
            mName = name;
        }

        @Override
        protected String doInBackground(String... params) {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return mName;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Log.e(TAG, result + "execute finish at " + df.format(new Date()));
        }
    }
我找了2个手机,系统分别是4.1.1和2.3.3,按照我前面的描述,AsyncTask在4.1.1应该是串行的,在2.3.3应该是并行的,到底是不是这样呢?请看Log

Android 4.1.1上执行:从下面Log可以看出,5个AsyncTask共耗时15s且时间间隔为3s,很显然是串行执行的

Android源码分析—带你认识不一样的AsyncTask

Android 2.3.3上执行:从下面Log可以看出,5个AsyncTask的结束时间是一样的,很显然是并行执行

Android源码分析—带你认识不一样的AsyncTask
结论:从上面的两个Log可以看出,我前面的描述是完全正确的。下面请看源码,让我们去了解下其中的原理。

源码分析

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import java.util.ArrayDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

	//获取当前的cpu核心数
    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
	//线程池核心容量
    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
	//线程池最大容量
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
	//过剩的空闲线程的存活时间
    private static final int KEEP_ALIVE = 1;
	//ThreadFactory 线程工厂,通过工厂方法newThread来获取新线程
    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
		//原子整数,可以在超高并发下正常工作
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };
	//静态阻塞式队列,用来存放待执行的任务,初始容量:128个
    private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);

    /**
     * 静态并发线程池,可以用来并行执行任务,尽管从3.0开始,AsyncTask默认是串行执行任务
	 * 但是我们仍然能构造出并行的AsyncTask
     */
    public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

    /**
     * 静态串行任务执行器,其内部实现了串行控制,
	 * 循环的取出一个个任务交给上述的并发线程池去执行
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
	//消息类型:发送结果
    private static final int MESSAGE_POST_RESULT = 0x1;
	//消息类型:更新进度
    private static final int MESSAGE_POST_PROGRESS = 0x2;
	/**静态Handler,用来发送上述两种通知,采用UI线程的Looper来处理消息
	 * 这就是为什么AsyncTask必须在UI线程调用,因为子线程
	 * 默认没有Looper无法创建下面的Handler,程序会直接Crash
	 */
    private static final InternalHandler sHandler = new InternalHandler();
	//默认任务执行器,被赋值为串行任务执行器,就是它,AsyncTask变成串行的了
    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
	//如下两个变量我们先不要深究,不影响我们对整体逻辑的理解
    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;
	//任务的状态 默认为挂起,即等待执行,其类型标识为易变的(volatile)
    private volatile Status mStatus = Status.PENDING;
    //原子布尔型,支持高并发访问,标识任务是否被取消
    private final AtomicBoolean mCancelled = new AtomicBoolean();
	//原子布尔型,支持高并发访问,标识任务是否被执行过
    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

	/*串行执行器的实现,我们要好好看看,它是怎么把并行转为串行的
	 *目前我们需要知道,asyncTask.execute(Params ...)实际上会调用
	 *SerialExecutor的execute方法,这一点后面再说明。也就是说:当你的asyncTask执行的时候,
	 *首先你的task会被加入到任务队列,然后排队,一个个执行
	 */
    private static class SerialExecutor implements Executor {
		//线性双向队列,用来存储所有的AsyncTask任务
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
		//当前正在执行的AsyncTask任务
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
			//将新的AsyncTask任务加入到双向队列中
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
						//执行AsyncTask任务
                        r.run();
                    } finally {
						//当前AsyncTask任务执行完毕后,进行下一轮执行,如果还有未执行任务的话
						//这一点很明显体现了AsyncTask是串行执行任务的,总是一个任务执行完毕才会执行下一个任务
                        scheduleNext();
                    }
                }
            });
			//如果当前没有任务在执行,直接进入执行逻辑
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
			//从任务队列中取出队列头部的任务,如果有就交给并发线程池去执行
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

    /**
     * 任务的三种状态
     */
    public enum Status {
        /**
         * 任务等待执行
         */
        PENDING,
        /**
         * 任务正在执行
         */
        RUNNING,
        /**
         * 任务已经执行结束
         */
        FINISHED,
    }

    /** 隐藏API:在UI线程中调用,用来初始化Handler */
    public static void init() {
        sHandler.getLooper();
    }

    /** 隐藏API:为AsyncTask设置默认执行器 */
    public static void setDefaultExecutor(Executor exec) {
        sDefaultExecutor = exec;
    }

    /**
     * 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);

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

        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                try {
                    postResultIfNotInvoked(get());
                } catch (InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occured while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
    }

    private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }
	//doInBackground执行完毕,发送消息
    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

    /**
     * 返回任务的状态
     */
    public final Status getStatus() {
        return mStatus;
    }

    /**
	 * 这个方法是我们必须要重写的,用来做后台计算
	 * 所在线程:后台线程
     */
    protected abstract Result doInBackground(Params... params);

    /**
	 * 在doInBackground之前调用,用来做初始化工作
	 * 所在线程:UI线程
     */
    protected void onPreExecute() {
    }

    /**
	 * 在doInBackground之后调用,用来接受后台计算结果更新UI
	 * 所在线程:UI线程
     */
    protected void onPostExecute(Result result) {
    }

    /**
     * Runs on the UI thread after {@link #publishProgress} is invoked.
     /**
	 * 在publishProgress之后调用,用来更新计算进度
	 * 所在线程:UI线程
     */
    protected void onProgressUpdate(Progress... values) {
    }

     /**
	 * cancel被调用并且doInBackground执行结束,会调用onCancelled,表示任务被取消
	 * 这个时候onPostExecute不会再被调用,二者是互斥的,分别表示任务取消和任务执行完成
	 * 所在线程:UI线程
     */
    @SuppressWarnings({"UnusedParameters"})
    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);
    }

    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
        return mFuture.get(timeout, unit);
    }

    /**
     * 这个方法如何执行和系统版本有关,在AsyncTask的使用规则里已经说明,如果你真的想使用并行AsyncTask,
	 * 也是可以的,只要稍作修改
	 * 必须在UI线程调用此方法
     */
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
		//串行执行
        return executeOnExecutor(sDefaultExecutor, params);
		//如果我们想并行执行,这样改就行了,当然这个方法我们没法改
		//return executeOnExecutor(THREAD_POOL_EXECUTOR, params);
    }

    /**
     * 通过这个方法我们可以自定义AsyncTask的执行方式,串行or并行,甚至可以采用自己的Executor
	 * 为了实现并行,我们可以在外部这么用AsyncTask:
	 * asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params... params);
	 * 必须在UI线程调用此方法
     */
    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;
		//这里#onPreExecute会最先执行
        onPreExecute();

        mWorker.mParams = params;
		//然后后台计算#doInBackground才真正开始
        exec.execute(mFuture);
		//接着会有#onProgressUpdate被调用,最后是#onPostExecute

        return this;
    }

    /**
     * 这是AsyncTask提供的一个静态方法,方便我们直接执行一个runnable
     */
    public static void execute(Runnable runnable) {
        sDefaultExecutor.execute(runnable);
    }

    /**
	 * 打印后台计算进度,onProgressUpdate会被调用
     */
    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

	//任务结束的时候会进行判断,如果任务没有被取消,则onPostExecute会被调用
    private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

	//AsyncTask内部Handler,用来发送后台计算进度更新消息和计算完成消息
    private static class InternalHandler extends Handler {
        @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;
            }
        }
    }

    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }

    @SuppressWarnings({"RawUseOfParameterizedType"})
    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = data;
        }
    }
}

让你的AsyncTask在3.0以上的系统中并行起来

通过上面的源码分析,我已经给出了在3.0以上系统中让AsyncTask并行执行的方法,现在,让我们来试一试,代码还是之前采用的测试代码,我们要稍作修改,调用AsyncTask的executeOnExecutor方法而不是execute,请看:

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    @Override
    public void onClick(View v) {
        if (v == mButton) {
            new MyAsyncTask("AsyncTask#1").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
            new MyAsyncTask("AsyncTask#2").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
            new MyAsyncTask("AsyncTask#3").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
            new MyAsyncTask("AsyncTask#4").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
            new MyAsyncTask("AsyncTask#5").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");
        }

    }

    private static class MyAsyncTask extends AsyncTask<String, Integer, String> {

        private String mName = "AsyncTask";

        public MyAsyncTask(String name) {
            super();
            mName = name;
        }

        @Override
        protected String doInBackground(String... params) {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return mName;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Log.e(TAG, result + "execute finish at " + df.format(new Date()));
        }
    }
下面是系统为4.1.1手机打印出的Log:很显然,我们的目的达到了,成功的让AsyncTask在4.1.1的手机上并行起来了,很高兴吧!希望这篇文章对你有用。

Android源码分析—带你认识不一样的AsyncTask

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