源码分析之AsyncTask
AsyncTask在Android中是很常用的异步线程，那么AsyncTask和Thread有什么区别呢？这里将从源码角度深入理解AsyncTask的设计和工作原理，这里的AsyncTask基于SDK-25
分析知识准备
首先我们来看一个生产者与消费者模型的例子
public class ThreadTest {
//产品
static class ProductObject{
public volatile static String value; //volatile线程操作变量可见
}
//生产者线程
static class Producer extends Thread{
Object lock;
public Producer(Object lock) {
this.lock = lock;
}
@Override
public void run() {
while(true){
synchronized (lock) {
if(ProductObject.value != null){
try {
lock.wait(); //产品还没有被消费，等待
} catch (InterruptedException e) {
e.printStackTrace();
}
}
ProductObject.value = "NO:"+System.currentTimeMillis();
System.out.println("生产产品："+ProductObject.value);
lock.notify(); //生产完成，通知消费者消费
}
}
}
}
//消费者线程
static class Consumer extends Thread{
Object lock;
public Consumer(Object lock) {
this.lock = lock;
}
@Override
public void run() {
while(true){
synchronized (lock) {
if(ProductObject.value == null){
try {
lock.wait(); //等待，阻塞
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("消费产品："+ProductObject.value);
ProductObject.value = null;
lock.notify(); //消费完成，通知生产者，继续生产
}
}
}
}
public static void main(String\[\] args) {
Object lock = new Object();
new Producer(lock).start();
new Consumer(lock).start();
} }
上面的例子关键点在于两个，其一是 volatile，使得线程间可见，第二个点在于互斥锁，这样就可以使得有商品的时候就要通知消费者消费，同时 wait，那么消费者收到消息开始消费，消费完毕通知生产者继续生产，从而不断生产，这样比轮询方式更加节省资源
在了解完上面的例子以后，我们就可以着手分析AsyncTask的源代码了
首先，我们在AsyncTask首先看其构造方法
private final WorkerRunnable
private final FutureTask
···
public AsyncTask() {
mWorker = new WorkerRunnable
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
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 occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
}; }
这里首先给WorkerRunnable和Future进行了初始化，那么为何要初始化这两个变量呢？
这里就要说到常用的两个方法了，doInBackground()，这个方法是在子线程里面完成的，另一个方法就是onPostExecute()，而这个方法是存在于主线程的，那么也就是说子线程执行完将执行的结果传递到了主线程中，实现了线程间的通信，那么最关键的问题来了，这个通信是怎么实现的呢？
通常在子线程中执行的任务，是没有返回结果的，例如Runnable的源代码如下，就没有返回结果
public interface Runnable {
/**
* When an object implementing interface Runnable is used
* to create a thread, starting the thread causes the object's
* run method to be called in that separately executing
* thread.
*
* The general contract of the method run is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}
那么，要怎么才能得到返回值呢，这里首先想到的就是Callable接口，那么再看看Callable的源代码
@FunctionalInterface
public interface Callable
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
可以看到，这是一个泛型方法，是有返回值的，但是其本身确是不能直接执行的，需要借助其他类，接下来再看一看源代码中涉及到的Future接口
public interface Future
/\*\*
\* Attempts to cancel execution of this task. This attempt will
\* fail if the task has already completed, has already been cancelled,
\* or could not be cancelled for some other reason. If successful,
\* and this task has not started when {@code cancel} is called,
\* this task should never run. If the task has already started,
\* then the {@code mayInterruptIfRunning} parameter determines
\* whether the thread executing this task should be interrupted in
\* an attempt to stop the task.
\*
\* <p>After this method returns, subsequent calls to {@link #isDone} will
\* always return {@code true}. Subsequent calls to {@link #isCancelled}
\* will always return {@code true} if this method returned {@code true}.
\*
\* @param mayInterruptIfRunning {@code true} if the thread executing this
\* task should be interrupted; otherwise, in-progress tasks are allowed
\* to complete
\* @return {@code false} if the task could not be cancelled,
\* typically because it has already completed normally;
\* {@code true} otherwise
\*/
boolean cancel(boolean mayInterruptIfRunning);
/\*\*
\* Returns {@code true} if this task was cancelled before it completed
\* normally.
\*
\* @return {@code true} if this task was cancelled before it completed
\*/
boolean isCancelled();
/\*\*
\* Returns {@code true} if this task completed.
\*
\* Completion may be due to normal termination, an exception, or
\* cancellation -- in all of these cases, this method will return
\* {@code true}.
\*
\* @return {@code true} if this task completed
\*/
boolean isDone();
/\*\*
\* Waits if necessary for the computation to complete, and then
\* retrieves its result.
\*
\* @return the computed result
\* @throws CancellationException if the computation was cancelled
\* @throws ExecutionException if the computation threw an
\* exception
\* @throws InterruptedException if the current thread was interrupted
\* while waiting
\*/
V get() throws InterruptedException, ExecutionException;
/\*\*
\* Waits if necessary for at most the given time for the computation
\* to complete, and then retrieves its result, if available.
\*
\* @param timeout the maximum time to wait
\* @param unit the time unit of the timeout argument
\* @return the computed result
\* @throws CancellationException if the computation was cancelled
\* @throws ExecutionException if the computation threw an
\* exception
\* @throws InterruptedException if the current thread was interrupted
\* while waiting
\* @throws TimeoutException if the wait timed out
\*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException; }
在Future类中，有好几个方法，而这些方法都是有返回值的，那么Runnable与Future和FutureTask有什么关系呢，产看源码便可得知，FutureTask实际上是实现了RunnableFuture接口
public class FutureTask
而RunnableFuture又继承了Runnable和Future
public interface RunnableFuture
那也就是说，FutureTask既可以在子线程中执行，也可以获得执行结果，下面使用一个例子来说明FutureTask
public class FutureTest {
public static void main(String\[\] args) {
Task work = new Task();
FutureTask<Integer> future = new FutureTask<Integer>(work){
@Override
protected void done() { //异步任务执行完成，回调
try {
System.out.println("done:" + get()); //get()获取异步任务的返回值，这是个阻塞方法
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
};
//线程池（使用了预定义的配置）
ExecutorService executor = Executors.newCachedThreadPool();
executor.execute(future);
}
//异步任务
static class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {//返回异步任务的执行结果
int i = 0;
for (; i < 10; i++) {
try {
System.out.println(Thread.currentThread().getName() + "\_" + i);
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return i;
}
} }
上面的例子可以看出，在使用了Callable的时候，需要借助FutureTask来包装，然后使用Executor的execute()方法来执行，那么是怎么得到异步任务的返回值呢，在上面的例子中，我们可以看到，其返回值的获取是通过future.get()得到的，然而这个get()方法确是被阻塞的，只有在异步任务完成的时候才能获取到其结果，那我们怎么才能知道异步任务时候执行完毕呢，这里就可以实现FutureTask的done()方法，当异步任务执行完毕以后会回调这个方法，上述例子其实解释了AsyncTask的实现逻辑，call()方法是在子线程中完成，这也就是doInBackground()的实现，在主线程中获得结果，这是在onPostExecute()使用了get()方法，那也就是说AsyncTask就是通过这一套方法去实现的
从这里我们可以总结出FutureTask为异步任务提供了诸多便利性，包括
- 获取异步任务的返回值
- 监听异步任务的执行情况
- 取消异步任务
那么在AsyncTask中，WorkerRunnable又是啥呢，其实就是一个内部类，对Callable进行了封装
private static abstract class WorkerRunnable
源代码分析
有了以上知识储备，我们就可以动手分析AsyncTask源代码了
拿到源代码，不同的人有不同的分析习惯，这里我按照我的习惯对源代码进行一次分析
首先，因为我们分析源代码是为了更好的去使用，而使用的话，第一个关注的就应该是构造方法，回到之前的的构造方法，这里要开始对构造方法开始入手分析了
private final WorkerRunnable
private final FutureTask
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
···
public AsyncTask() {
mWorker = new WorkerRunnable
public Result call() throws Exception {
//设置线程调用
mTaskInvoked.set(true);
Result result = null;
try {
//设置线程优先级，其给定值为10
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//调用doInBackground()方法得到返回值
result = doInBackground(mParams);
//将当前线程中的Binder命令发送至kernel
Binder.flushPendingCommands();
} catch (Throwable tr) {
//发生异常则取消线程调用设置
mCancelled.set(true);
throw tr;
} finally {
//执行postResult()方法
postResult(result);
}
return result;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
//执行postResultIfNotInvoked()方法
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);
}
}
}; }
以上就是AsyncTask的构造方法了，在构造方法上有一句说明，这个构造方法必须在UI线程中创建，这一点很好理解，因为其有需要再主线程中执行的地方，后面会说到，那么这个构造方法干了什么事情呢，很简单，这里新建了两个对象，首先是WorkerRunnable，而这个WorkerRunnable则是实现自Callable接口，主要是要使用其call()方法，为了返回参数，并没有什么特别之处，再其内部则实现了call()方法，而其自生是无法执行的，需要找一个包装类，而这个包装类就是FutureTask，通过之前的分析，这里就不再多赘述关于FutureTask的东西了，这里实现了done()方法，也就是线程执行完毕调用的方法，简单点来说就是在call()方法中执行，在done()中获得执行的返回结果，上述涉及到一个内部类和三个自定义的方法，那么接下来我们看一看这个内部类和三个方法都干了啥
这里的WorkerRunnable，正如前面所说，这里除了实现Callable就啥也没干，还是个抽象方法，这里将实现放在了构造方法中
private static abstract class WorkerRunnable
首先是doInBackground()方法，前面讲到，这个方法在子线程中完成，那么这里的子线程是哪个呢，其实就是WorkerThread，这个方法是一个抽象方法，放在子线程中执行，其具体实现由调用者完成
@WorkerThread protected abstract Result doInBackground(Params… params);
再看postResult()方法，这里获取了一个Handler，然后发送了一个消息，这里就是子线程能够通信主线程的地方了
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult
message.sendToTarget();
return result;
}
那么到这里，我们关注的重点就来了，子线程是怎么告诉主线程的呢，要知道其中的原因，我们就需要去看看代码里面是怎么实现的
我们先看发送了什么消息，也就是AsyncTaskResult里面干了啥，查看代码发现，其就是做了参数传递的任务
@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
} }
那么接下来的重点就是getHandler()方法了，这里拿到AsyncTask.class就上了锁了，这也很好理解，不上锁其他线程走到这里会产生安全隐患，然后返回sHandler，那再继续看看InternalHandler又是个什么吧
private static InternalHandler sHandler;
···
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
这里就是了，在构造方法里面super(Looper.getMainLooper())，也就说明了这个方法是在主线程中执行的，在主线程中对Message进行处理，这里又涉及到两个方法，一个是finish()，还有一个是onProgressUpdate()，那么好吧，再去看看这两个方法在干啥
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
···
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;
}
} }
首先是又调了isCancelled()，判断是否取消，前面构造函数的时候见过这个，这是在异常发生的时候才设置为true的，那么如果不发生异常，这里应该就是为false的，但在找源代码时发现，另一个方法也对这个参数进行了设置，那就是cancel()，所以在不发生异常和取消的时候应该是为true的，接下来是onCancelled()方法，这里是不做任何操作的，这也是主线程中的方法，还有就是onPostExecute()方法，然后会设置状态，其默认状态是PENDING
private volatile Status mStatus = Status.PENDING;
···
public enum Status {
PENDING,
RUNNING,
FINISHED,
}
···
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}
@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
}
@MainThread
protected void onCancelled() {
}
public final boolean isCancelled() {
return mCancelled.get();
}
···
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
然后是onProgressUpdate()方法，那么这里做了啥呢，嗯~啥也没有，交给调用者在继承时可以使用
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress… values) {
}
来看构造方法中涉及到的最后一个方法postResultIfNotInvoked()，这个方法又干了啥了，首先获得了mTaskInvoked的状态，整个AsyncTask只有构造方法处设置了这个值，然后判断是否执行postResult()方法
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
至此构造方法分析完成，可以看到在构造方法中，其主要做的工作最主要的就是搭建好了子线程和主线程沟通的桥梁
在新建AsyncTask对象以后，要执行的话，需要使用execute()去开始执行
那么我们就从这里入手，看看其具体是怎么工作的，可以看到无论是构造方法还是启动方法，都是需要在主线程中完成的，在execute()中，做了些什么呢，在这之前我们先看看传递的sDefaultExecutor是啥
@MainThread
public final AsyncTask
return executeOnExecutor(sDefaultExecutor, params);
}
这其实是一个线程池，任务调度的线程池，可以看到SerialExecutor实际上是实现了Executor接口，其作用就是将任务添加到双向队列，然后不断地取出执行取出执行，那么THREAD_POOL_EXECUTOR也应该是一个线程池，那这又是啥呢，去看一看这个玩意儿就是到了
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
···
private static class SerialExecutor implements Executor {
//定义了一个双向队列，用来存储线程
final ArrayDeque
Runnable mActive;
public synchronized void execute(final Runnable r) {
//向队列中添加线程
mTasks.offer(new Runnable() {
public void run() {
try {
//线程运行
r.run();
} finally {
//执行scheduleNext()方法
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
//从队列中取出线程并执行
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD\_POOL\_EXECUTOR.execute(mActive);
}
} }
下列代码就是初始化了线程池的参数，指定了线程数量
//获得可用CPU数量
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//设置核心线程池数量其范围[2,4]，无论是否使用都存在
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
//设置最大线程数量
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
//设置闲置回收时间，也就是说线程在这个时间内没有活动的话，会被回收
private static final int KEEP_ALIVE_SECONDS = 30;
//设置线程工厂，通过这个创建线程
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
//创建线程安全的线程个数计数器
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
} };
//设置任务队列大小
private static final BlockingQueue
new LinkedBlockingQueue
//设置线程池
public static final Executor THREAD_POOL_EXECUTOR;
//初始化线程池
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
//打开核心线程池的超时时间
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
所以在执行scheduleNext()的时候，会将THREAD_POOL_EXECUTOR中设置好的线程全部取出来，用来执行后面的任务，其执行的任务就是execute()方法所指定的任务，在executeOnExecutor()方法中，由于前面初始化完成，这里的状态应该是PENDING，之后还设置了mWorker的参数，然后会执行线程池的方法，然后据开始执行任务了，前面没有涉及到的方法还有一个，那我们接下来看看
@MainThread
public final AsyncTask
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;
}
onPreExecute()，这个方法由调用者在继承时候能够使用
@MainThread
protected void onPreExecute() {
}
至此，AsyncTask的执行方法也分析完了，那么我们接下来看看还有什么方法没有涉及到，没有涉及到的方法都是public属性和方法
AsyncTask的公共方法
//这个方法设置为public，那么就意味着我们可以自定义线程池
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
//这个方法意味着我们可以获得其状态，配合枚举值使用
public enum Status {
PENDING,
RUNNING,
FINISHED,
}
···
public final Status getStatus() {
return mStatus;
}
//查看是非被取消
public final boolean isCancelled() {
return mCancelled.get();
}
//取消异步任务
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
//获取返回结果，注意：这个方法是阻塞式的
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
//同上
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
//自定义的线程池可以从这个方法启动
@MainThread
public final AsyncTask
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;
}
//使用默认线程池启动异步任务
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
总结
AsyncTask的实例化过程，其本质上就是实例化了一个FutureTask
其执行过程Executor.execute(mFuture) -> SerialExecutor.mTasks(队列) -> (线程池)THREAD_POOL_EXECUTOR.execute
线程池中的所有线程，为了执行异步任务
如果当前线程池中的数量小于corePoolSize，创建并添加的任务
如果当前线程池中的数量等于corePoolSize，缓冲队列workQueue未满，那么任务被放入缓冲队列、等待任务调度执行
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量小于maximumPoolSize，新提交任务会创建新线程执行任务
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量等于maximumPoolSize，新提交任务由Handler处理
当线程池中的线程大于corePoolSize时，多余线程空闲时间超过keepAliveTime时，会关闭这部分线程
线程池在添加时候是串行的，在执行任务的时候是并行的
附录(源代码)
package android.os;
import android.annotation.MainThread;
import android.annotation.WorkerThread;
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;
/**
*
AsyncTask enables proper and easy use of the UI thread. This class allows you * to perform background operations and publish results on the UI thread without * having to manipulate threads and/or handlers.
*
*
AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
* and does not constitute a generic threading framework. AsyncTasks should ideally be
* used for short operations (a few seconds at the most.) If you need to keep threads
* running for long periods of time, it is highly recommended you use the various APIs
* provided by the java.util.concurrent package such as {@link Executor},
* {@link ThreadPoolExecutor} and {@link FutureTask}.
*
*
An asynchronous task is defined by a computation that runs on a background thread and
* whose result is published on the UI thread. An asynchronous task is defined by 3 generic
* types, called Params, Progress and Result,
* and 4 steps, called onPreExecute, doInBackground,
* onProgressUpdate and onPostExecute.
*
*
Developer Guides
*For more information about using tasks and threads, read the * Processes and * Threads developer guide.
**
*
Usage
*
AsyncTask must be subclassed to be used. The subclass will override at least * one method ({@link #doInBackground}), and most often will override a * second one ({@link #onPostExecute}.)
*
*
Here is an example of subclassing:
*
* 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");
* }
* }
* *
*
Once created, a task is executed very simply:
*
* new DownloadFilesTask().execute(url1, url2, url3); *
*
*
AsyncTask's generic types
*
The three types used by an asynchronous task are the following:
*
-
*
Params, the type of the parameters sent to the task upon * execution.
* Progress, the type of the progress units published during * the background computation.
* Result, the type of the result of the background * computation.
*
*
Not all types are always used by an asynchronous task. To mark a type as unused, * simply use the type {@link Void}:
*
* private class MyTask extends AsyncTask<Void, Void, Void> { … }
* *
*
The 4 steps
*
When an asynchronous task is executed, the task goes through 4 steps:
*
-
*
- {@link #onPreExecute()}, invoked on the UI thread before the task * is executed. This step is normally used to setup the task, for instance by * showing a progress bar in the user interface. *
- {@link #doInBackground}, invoked on the background thread * immediately after {@link #onPreExecute()} finishes executing. This step is used * to perform background computation that can take a long time. The parameters * of the asynchronous task are passed to this step. The result of the computation must * be returned by this step and will be passed back to the last step. This step * can also use {@link #publishProgress} to publish one or more units * of progress. These values are published on the UI thread, in the * {@link #onProgressUpdate} step. *
- {@link #onProgressUpdate}, invoked on the UI thread after a * call to {@link #publishProgress}. The timing of the execution is * undefined. This method is used to display any form of progress in the user * interface while the background computation is still executing. For instance, * it can be used to animate a progress bar or show logs in a text field. *
- {@link #onPostExecute}, invoked on the UI thread after the background * computation finishes. The result of the background computation is passed to * this step as a parameter. *
*
*
Cancelling a task
*
A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking * this method will cause subsequent calls to {@link #isCancelled()} to return true. * After invoking this method, {@link #onCancelled(Object)}, instead of * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])} * returns. To ensure that a task is cancelled as quickly as possible, you should always * check the return value of {@link #isCancelled()} periodically from * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)
*
*
Threading rules
*
There are a few threading rules that must be followed for this class to * work properly:
*
-
*
- The AsyncTask class must be loaded on the UI thread. This is done * automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}. *
- The task instance must be created on the UI thread. *
- {@link #execute} must be invoked on the UI thread. *
- Do not call {@link #onPreExecute()}, {@link #onPostExecute}, * {@link #doInBackground}, {@link #onProgressUpdate} manually. *
- The task can be executed only once (an exception will be thrown if * a second execution is attempted.) *
*
*
Memory observability
*
AsyncTask guarantees that all callback calls are synchronized in such a way that the following * operations are safe without explicit synchronizations.
*
-
*
- Set member fields in the constructor or {@link #onPreExecute}, and refer to them * in {@link #doInBackground}. *
- Set member fields in {@link #doInBackground}, and refer to them in * {@link #onProgressUpdate} and {@link #onPostExecute}. *
*
*
Order of execution
*
When first introduced, AsyncTasks were executed serially on a single background * thread. Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed * to a pool of threads allowing multiple tasks to operate in parallel. Starting with * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single * thread to avoid common application errors caused by parallel execution.
*
If you truly want parallel execution, you can invoke * {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with * {@link #THREAD_POOL_EXECUTOR}.
*/
public abstract class AsyncTask
private static final String LOG_TAG = "AsyncTask";
private static final int CPU\_COUNT = Runtime.getRuntime().availableProcessors();
// We want at least 2 threads and at most 4 threads in the core pool,
// preferring to have 1 less than the CPU count to avoid saturating
// the CPU with background work
private static final int CORE\_POOL\_SIZE = Math.max(2, Math.min(CPU\_COUNT - 1, 4));
private static final int MAXIMUM\_POOL\_SIZE = CPU\_COUNT \* 2 + 1;
private static final int KEEP\_ALIVE\_SECONDS = 30;
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<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
/\*\*
\* 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,
sPoolWorkQueue, sThreadFactory);
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD\_POOL\_EXECUTOR = threadPoolExecutor;
}
/\*\*
\* An {@link Executor} that executes tasks one at a time in serial
\* order. This serialization is global to a particular process.
\*/
public static final Executor SERIAL\_EXECUTOR = new SerialExecutor();
private static final int MESSAGE\_POST\_RESULT = 0x1;
private static final int MESSAGE\_POST\_PROGRESS = 0x2;
private static volatile Executor sDefaultExecutor = SERIAL\_EXECUTOR;
private static InternalHandler sHandler;
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private volatile Status mStatus = Status.PENDING;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
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);
}
}
}
/\*\*
\* Indicates the current status of the task. Each status will be set only once
\* during the lifetime of a task.
\*/
public enum Status {
/\*\*
\* Indicates that the task has not been executed yet.
\*/
PENDING,
/\*\*
\* Indicates that the task is running.
\*/
RUNNING,
/\*\*
\* Indicates that {@link AsyncTask#onPostExecute} has finished.
\*/
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;
}
/\*\*
\* 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);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD\_PRIORITY\_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
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 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);
}
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE\_POST\_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
/\*\*
\* Returns the current status of this task.
\*
\* @return The current status.
\*/
public final Status getStatus() {
return mStatus;
}
/\*\*
\* Override this method to perform a computation on a background thread. The
\* specified parameters are the parameters passed to {@link #execute}
\* by the caller of this task.
\*
\* This method can call {@link #publishProgress} to publish updates
\* on the UI thread.
\*
\* @param params The parameters of the task.
\*
\* @return A result, defined by the subclass of this task.
\*
\* @see #onPreExecute()
\* @see #onPostExecute
\* @see #publishProgress
\*/
@WorkerThread
protected abstract Result doInBackground(Params... params);
/\*\*
\* Runs on the UI thread before {@link #doInBackground}.
\*
\* @see #onPostExecute
\* @see #doInBackground
\*/
@MainThread
protected void onPreExecute() {
}
/\*\*
\* <p>Runs on the UI thread after {@link #doInBackground}. The
\* specified result is the value returned by {@link #doInBackground}.</p>
\*
\* <p>This method won't be invoked if the task was cancelled.</p>
\*
\* @param result The result of the operation computed by {@link #doInBackground}.
\*
\* @see #onPreExecute
\* @see #doInBackground
\* @see #onCancelled(Object)
\*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}
/\*\*
\* Runs on the UI thread after {@link #publishProgress} is invoked.
\* The specified values are the values passed to {@link #publishProgress}.
\*
\* @param values The values indicating progress.
\*
\* @see #publishProgress
\* @see #doInBackground
\*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}
/\*\*
\* <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
\* {@link #doInBackground(Object\[\])} has finished.</p>
\*
\* <p>The default implementation simply invokes {@link #onCancelled()} and
\* ignores the result. If you write your own implementation, do not call
\* super.onCancelled(result).</p>
\*
\* @param result The result, if any, computed in
\* {@link #doInBackground(Object\[\])}, can be null
\*
\* @see #cancel(boolean)
\* @see #isCancelled()
\*/
@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
}
/\*\*
\* <p>Applications should preferably override {@link #onCancelled(Object)}.
\* This method is invoked by the default implementation of
\* {@link #onCancelled(Object)}.</p>
\*
\* <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
\* {@link #doInBackground(Object\[\])} has finished.</p>
\*
\* @see #onCancelled(Object)
\* @see #cancel(boolean)
\* @see #isCancelled()
\*/
@MainThread
protected void onCancelled() {
}
/\*\*
\* Returns <tt>true</tt> if this task was cancelled before it completed
\* normally. If you are calling {@link #cancel(boolean)} on the task,
\* the value returned by this method should be checked periodically from
\* {@link #doInBackground(Object\[\])} to end the task as soon as possible.
\*
\* @return <tt>true</tt> if task was cancelled before it completed
\*
\* @see #cancel(boolean)
\*/
public final boolean isCancelled() {
return mCancelled.get();
}
/\*\*
\* <p>Attempts to cancel execution of this task. This attempt will
\* fail if the task has already completed, already been cancelled,
\* or could not be cancelled for some other reason. If successful,
\* and this task has not started when <tt>cancel</tt> is called,
\* this task should never run. If the task has already started,
\* then the <tt>mayInterruptIfRunning</tt> parameter determines
\* whether the thread executing this task should be interrupted in
\* an attempt to stop the task.</p>
\*
\* <p>Calling this method will result in {@link #onCancelled(Object)} being
\* invoked on the UI thread after {@link #doInBackground(Object\[\])}
\* returns. Calling this method guarantees that {@link #onPostExecute(Object)}
\* is never invoked. After invoking this method, you should check the
\* value returned by {@link #isCancelled()} periodically from
\* {@link #doInBackground(Object\[\])} to finish the task as early as
\* possible.</p>
\*
\* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
\* task should be interrupted; otherwise, in-progress tasks are allowed
\* to complete.
\*
\* @return <tt>false</tt> if the task could not be cancelled,
\* typically because it has already completed normally;
\* <tt>true</tt> otherwise
\*
\* @see #isCancelled()
\* @see #onCancelled(Object)
\*/
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
/\*\*
\* Waits if necessary for the computation to complete, and then
\* retrieves its result.
\*
\* @return The computed result.
\*
\* @throws CancellationException If the computation was cancelled.
\* @throws ExecutionException If the computation threw an exception.
\* @throws InterruptedException If the current thread was interrupted
\* while waiting.
\*/
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
/\*\*
\* Waits if necessary for at most the given time for the computation
\* to complete, and then retrieves its result.
\*
\* @param timeout Time to wait before cancelling the operation.
\* @param unit The time unit for the timeout.
\*
\* @return The computed result.
\*
\* @throws CancellationException If the computation was cancelled.
\* @throws ExecutionException If the computation threw an exception.
\* @throws InterruptedException If the current thread was interrupted
\* while waiting.
\* @throws TimeoutException If the wait timed out.
\*/
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
/\*\*
\* Executes the task with the specified parameters. The task returns
\* itself (this) so that the caller can keep a reference to it.
\*
\* <p>Note: this function schedules the task on a queue for a single background
\* thread or pool of threads depending on the platform version. When first
\* introduced, AsyncTasks were executed serially on a single background thread.
\* Starting with {@link android.os.Build.VERSION\_CODES#DONUT}, this was changed
\* to a pool of threads allowing multiple tasks to operate in parallel. Starting
\* {@link android.os.Build.VERSION\_CODES#HONEYCOMB}, tasks are back to being
\* executed on a single thread to avoid common application errors caused
\* by parallel execution. If you truly want parallel execution, you can use
\* the {@link #executeOnExecutor} version of this method
\* with {@link #THREAD\_POOL\_EXECUTOR}; however, see commentary there for warnings
\* on its use.
\*
\* <p>This method must be invoked on the UI thread.
\*
\* @param params The parameters of the task.
\*
\* @return This instance of AsyncTask.
\*
\* @throws IllegalStateException If {@link #getStatus()} returns either
\* {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
\*
\* @see #executeOnExecutor(java.util.concurrent.Executor, Object\[\])
\* @see #execute(Runnable)
\*/
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
/\*\*
\* Executes the task with the specified parameters. The task returns
\* itself (this) so that the caller can keep a reference to it.
\*
\* <p>This method is typically used with {@link #THREAD\_POOL\_EXECUTOR} to
\* allow multiple tasks to run in parallel on a pool of threads managed by
\* AsyncTask, however you can also use your own {@link Executor} for custom
\* behavior.
\*
\* <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
\* a thread pool is generally <em>not</em> what one wants, because the order
\* of their operation is not defined. For example, if these tasks are used
\* to modify any state in common (such as writing a file due to a button click),
\* there are no guarantees on the order of the modifications.
\* Without careful work it is possible in rare cases for the newer version
\* of the data to be over-written by an older one, leading to obscure data
\* loss and stability issues. Such changes are best
\* executed in serial; to guarantee such work is serialized regardless of
\* platform version you can use this function with {@link #SERIAL\_EXECUTOR}.
\*
\* <p>This method must be invoked on the UI thread.
\*
\* @param exec The executor to use. {@link #THREAD\_POOL\_EXECUTOR} is available as a
\* convenient process-wide thread pool for tasks that are loosely coupled.
\* @param params The parameters of the task.
\*
\* @return This instance of AsyncTask.
\*
\* @throws IllegalStateException If {@link #getStatus()} returns either
\* {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
\*
\* @see #execute(Object\[\])
\*/
@MainThread
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;
}
/\*\*
\* Convenience version of {@link #execute(Object...)} for use with
\* a simple Runnable object. See {@link #execute(Object\[\])} for more
\* information on the order of execution.
\*
\* @see #execute(Object\[\])
\* @see #executeOnExecutor(java.util.concurrent.Executor, Object\[\])
\*/
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
/\*\*
\* This method can be invoked from {@link #doInBackground} to
\* publish updates on the UI thread while the background computation is
\* still running. Each call to this method will trigger the execution of
\* {@link #onProgressUpdate} on the UI thread.
\*
\* {@link #onProgressUpdate} will not be called if the task has been
\* canceled.
\*
\* @param values The progress values to update the UI with.
\*
\* @see #onProgressUpdate
\* @see #doInBackground
\*/
@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE\_POST\_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
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;
}
}
}
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;
}
} }
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