一、前言
AsyncTask是一个异步任务。里面封装了线程池及Handler。所以，它可以方便地实现线程的切换及耗时任务的执行。
二、先上基本使用代码：
class CusAsyncTask extends AsyncTask<Long, Integer, Long>
{
// 第一个执行的方法，可在此做些初始化操作。工作在主线程MainThread
@Override
protected void onPreExecute() {
super.onPreExecute();
}
// 此处执行耗时操作。工作在工作线程WorkerThread
@Override
protected Long doInBackground(Long... values)
{
long startTime = System.currentTimeMillis();
for (int progress = 0; progress < values[0]; progress ++)
{
// 此行代码会触发onProgressUpdate方法，实际使用中可用于进度更新
publishProgress(progress);
}
// 此处return 的结果会当成参数传递到 onPostExecute 中
return System.currentTimeMillis() - startTime;
}
// 进度更新操作，由publishProgress来触发。执行在主线程MainThread
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
Log.e("onProgressUpdate ", values[0] + "");
}
// 参数result实际上是doInBackground返回的结果。执行在主线程MainThread。可把结果
直接显示到相关控件上
@Override
protected void onPostExecute(Long result) {
super.onPostExecute(result);
Log.e("onPostExecute ", "耗时 : " + result+ "ms");
}
}
以下为调用方法：
CusAsyncTask cusAsyncTask = new CusAsyncTask();
// 并行运行
cusAsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, 100L);
// 串行运行
// cusAsyncTask.execute(100L);
三、流程及原理分析
1.先看看CusAsyncTask cusAsyncTask = new CusAsyncTask();都做了什么：
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI
thread.
看到没有，初始化必须在UI线程也就是主线程中执行。
*/
public AsyncTask() {
this((Looper) null);
}
......
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI
thread.
* 这是一个隐藏的函数。用于创建一个新的异步任务，由上面的构造函数所调用
* @hide
*/
public AsyncTask(@Nullable Looper callbackLooper) {
// 这是必须要在主线程调用的原因，mHandler 需要主线程Handler
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
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
// 线程执行的时候，实际上就是执行call方法。此时doInBackground启动
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
// 最终onPostExecute方法被回调，一个任务流程结束
postResult(result);
}
return result;
}
};
// 封装了Callable
mFuture = new FutureTask<Result>(mWorker) {
// 当 Callable 的 call() 方法执行完毕之后调用
@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);
}
}
};
}
2.再看看cusAsyncTask.execute(100L)做了啥事。这样启动的任务是串行执行的
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
其中，sDefaultExecutor实际上是一个自定义的线程池。且该线程池一次只能按排队顺序地执行一个任务（串行）。当然这是高版本，低版本3.0之前默认是并行的。
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
/**
* 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 class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
// 任务排队
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
// 执行下一个任务
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
// 实际上由THREAD_POOL_EXECUTOR线程池来执行任务
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
回到正题executeOnExecutor
@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;
// 这就是为什么回调方法中onPreExecute()最先执行的原因
onPreExecute();
mWorker.mParams = params;
// 把任务排到任务队列队尾。此时，任务正式执行
exec.execute(mFuture);
return this;
}
3.再看看并行启动的任务cusAsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, 100L);
/**
* 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;
}
其中几个常量如下：
// CPU核数，“设置——>关于手机”那里可以看到是几核。一般有4，6，8...
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
// 核心线程数为2到4之间，这些线程通常会一直活着，不会销毁
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
// 最大线程数为：CPU核数 * 2 + 1。按目前的设备来看，一般在10-20左右
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
// 存活时间，即线程空闲时存活时间为30秒
private static final int KEEP_ALIVE_SECONDS = 30;
// 缓存任务数量最多128
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
