AsyncTask通常有以下两种调用方式:
1. AsyncTaskImpl().setTask(RunnableImpl).execute();
2. AsyncTask.execute(RunnableImpl);
- 仅针对第一种方式进行分析:
AsyncTask.AsyncTask():
public AsyncTask() {
/**
* 1. 初始化AsyncTask时会在主线程中创建WorkerRunnable和FutureTask对象;
* 2. 先跳过FutureTask, 直接分析execute(), 然后从execute()分析FutureTask;
*/
mWorker = new WorkerRunnable<Params, Result>() {...};
mFuture = new FutureTask<Result>(mWorker) {...};
}
WorkerRunnable :
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
public interface Callable<V> {
V call() throws Exception;
}
AsyncTask.execute() :
public class AsyncTask {
public final AsyncTask<...> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static class SerialExecutor implements Executor {...}
}
AsyncTask.executeOnExecutor() :
public class AsyncTask {
public final AsyncTask<...> executeOnExecutor(Executor exec, ...) {
/**
* 1. mStatus初始化时被赋值Status.PENDING, 由下文可知没Status只有三个可
* 能的值: PENDING/RUNNING/FINISHED, 当前任务执行完毕, mStatus被赋值
* FINISHED, 当任务执行中时RUNNING, 所以当再次调用execute()时, 会抛出异
* 常, 这种方式下的AsyncTask仅仅适合单个任务, 不适
* 用于多线程的场景;
* 2. 如果想要按顺序的执行多个AsyncTask, 可以使用方式2_AsyncTask.execute(RunnableImpl);
*/
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;
/**
* 在主线程中执行;
*/
onPreExecute();
mWorker.mParams = params;
/**
* FutureTask的分析从此揭开序幕;
*/
exec.execute(mFuture);
return this;
}
}
AsyncTask.mFuture :
public class AsyncTask {
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
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 static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
/**
* 由ArrayDeque内部结构可以知道当执行一个Runnable时, 会先将改Runnable添
* 加至ArrayDeque内部数组的尾部, 然后从数组的头部读取Runnable执行, 即按
* 照FILO的方式;
*/
public synchronized void execute(final Runnable r) {
/**
* 对于方式1, 也仅考虑方式1(方式2直观没有什么地方值得研究); 此时Runnable
* 实际指向mFuture, 而mFuture内部又持有WorkerRunnable的引用;
* mActive实际指向mFuture;
* 这里主要用到了适配器的模式;
*/
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);
}
}
}
}
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
public class FutureTask<V> implements RunnableFuture<V> {
public FutureTask(Callable<V> callable) {
this.callable = callable;
this.state = NEW;
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW;
}
public void run() {
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
result = c.call();
ran = true;
if (ran)
set(result);
}
}
}
}
这里用到了适配器模式, FutureTask有两个构造函数分别可以接收Callable与Runnable, 但是对外之暴露了一个run()的接口供调用, 如果传的是Callbale, run方法调用了c.call()方法. 如果传入的是Runnable, run()方法则会一波三折, 他先调用c.call()方法, 此时c实际是RunnableAdapter implements Callable, 然后调用的是RunnableAdapter中的call()方法, 然后里面又调用了Runnable的run()方法, 此Runnable就是我们传入的Runnable对象;
public class Executors {
public static <T> Callable<T> callable(Runnable task, T result) {
return new RunnableAdapter<T>(task, result);
}
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
}
其实感觉AsyncTask本身是挺简单的, 关键在于他对线程的操控, 这个需要注意一下. 这个现在是真心没法看下去了, 等我把几本关于底层的书看完在说这些事儿吧;
