java 进阶笔记线程与并发之AbstractExecutorService
简介
一个正常的submit、invokeAny 和 invokeAll 方法的通用实现。
源码简析
RunnableFuture
/**
* 成功执行run()，才会使Future完成，并得到它的结果。
* @see FutureTask
* @see Executor
* @since 1.6
* @author Doug Lea
* @param The result type returned by this Future's {@code get} method
*/
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
AbstractExecutorService
package java.util.concurrent;
import java.util.*;
/**
* 提供了ExecutorService的执行方法的一些默认实现。
* 此类使用 newTaskFor 返回的 RunnableFuture 实现 submit、invokeAny 和 invokeAll 方法，
* 默认情况下，RunnableFuture 是此包中提供的 FutureTask 类。
* @since 1.5
* @author Doug Lea
*/
public abstract class AbstractExecutorService implements ExecutorService {
/**
* Returns a {@code RunnableFuture} for the given runnable and default
* value.
*
* @param runnable the runnable task being wrapped
* @param value the default value for the returned future
* @param the type of the given value
* @return a RunnableFuture,它将运行Runnable中的任务，且生成给的的值，
* 提供取消任务的功能
* @since 1.6
*/
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
/**
* @since 1.6
*/
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
/**
* invokeAny的主要机制
*/
private <T> T doInvokeAny(Collection<? extends Callable<T>> tasks,
boolean timed, long nanos)
throws InterruptedException, ExecutionException, TimeoutException {
if (tasks == null)
throw new NullPointerException();
//ps:复用，提高性能
int ntasks = tasks.size();
if (ntasks == 0)
throw new IllegalArgumentException();
//ps: 设置ntasks作为初始容量
ArrayList<Future<T>> futures = new ArrayList<Future<T>>(ntasks);
//维护一个阻塞队列，完成的任务加入阻塞队列，所以有完成的任务时立即返回
//否则将会阻塞等待 直到队列中有阻塞任务的加入
ExecutorCompletionService<T> ecs =
new ExecutorCompletionService<T>(this);
// 为了提高效率，特别是在并行度有限的执行器中，在提交更多任务之前，
//请检查以前提交的任务是否已经完成。这种交错和异常机制造成了主循环的混乱。
try {
// 记录异常，这样如果我们不能获得任何结果，我们就可以抛出我们得到的最后一个异常.
ExecutionException ee = null;
//计算截止时间
final long deadline = timed ? System.nanoTime() + nanos : 0L;
Iterator<? extends Callable<T>> it = tasks.iterator();
// Start one task for sure; the rest incrementally
//ps: 这里先提交一个任务，然后继续处理
futures.add(ecs.submit(it.next()));
--ntasks;
int active = 1;
// ps: 循环的作用，在还没有任务完成时，循环启动新的任务。
// 这里的超时只会在启动所有任务且有任务未运行完毕时候才会判断，所以时间不能太短
for (;;) {
//非阻塞获取一个Future
Future<T> f = ecs.poll();
if (f == null) {
if (ntasks > 0) {
--ntasks;
futures.add(ecs.submit(it.next()));
++active;
}
else if (active == 0)
break;
else if (timed) {
f = ecs.poll(nanos, TimeUnit.NANOSECONDS);
if (f == null)
throw new TimeoutException();
nanos = deadline - System.nanoTime();
}
//ps : 若f == null 且 剩余任务<=0 ,且没有正在运行的任务，
// 且timed==false,则调用ecs.take()，阻塞等待获取
else
f = ecs.take();
}
//ps:说明任务已经完成了至少一个,返回对应结果或者抛出异常
if (f != null) {
--active;
try {
return f.get();
} catch (ExecutionException eex) {
ee = eex;
} catch (RuntimeException rex) {
ee = new ExecutionException(rex);
}
}
}
if (ee == null)
ee = new ExecutionException();
throw ee;
} finally {
//ps： 尝试关闭未完成的任务，已完成的不受影响
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(true);
}
}
// 一直等待结果，没有超时
public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException {
try {
return doInvokeAny(tasks, false, 0);
} catch (TimeoutException cannotHappen) {
assert false;
return null;
}
}
public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
return doInvokeAny(tasks, true, unit.toNanos(timeout));
}
//ps：源码为啥这里没注释了。。。
//循环开启任务，循环保存结果（阻塞）
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException {
if (tasks == null)
throw new NullPointerException();
ArrayList<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
boolean done = false;
try {
for (Callable<T> t : tasks) {
RunnableFuture<T> f = newTaskFor(t);
futures.add(f);
execute(f);
}
for (int i = 0, size = futures.size(); i < size; i++) {
Future<T> f = futures.get(i);
//ps: 这里之所以同步的使用future.get()来阻塞，是因为
//任务大部分是异步运行，除开任务的逻辑本身耗时足够少。若是同步，则相当于顺序执行。
if (!f.isDone()) {
try {
f.get();
} catch (CancellationException ignore) {
} catch (ExecutionException ignore) {
}
}
}
done = true;
return futures;
} finally {
if (!done)
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(true);
}
}
ps: 为什么没和上面方法一起封装，也许是为了极限的性能
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException {
if (tasks == null)
throw new NullPointerException();
long nanos = unit.toNanos(timeout);
ArrayList<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
boolean done = false;
try {
for (Callable<T> t : tasks)
futures.add(newTaskFor(t));
final long deadline = System.nanoTime() + nanos;
final int size = futures.size();
// Interleave time checks and calls to execute in case
// executor doesn't have any/much parallelism.
for (int i = 0; i < size; i++) {
execute((Runnable)futures.get(i));
nanos = deadline - System.nanoTime();
if (nanos <= 0L)
return futures;
}
for (int i = 0; i < size; i++) {
Future<T> f = futures.get(i);
if (!f.isDone()) {
if (nanos <= 0L)
return futures;
try {
f.get(nanos, TimeUnit.NANOSECONDS);
} catch (CancellationException ignore) {
} catch (ExecutionException ignore) {
} catch (TimeoutException toe) {
return futures;
}
nanos = deadline - System.nanoTime();
}
}
done = true;
return futures;
} finally {
if (!done)
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(true);
}
}
}