是否有可能将执行者执行的任务设置为优先级?我已经在JCIP中找到了一些关于可能的陈述,但是我找不到任何示例,也找不到文档中的任何相关内容。
从JCIP:
执行策略指定了任务的“什么,在哪里,何时何地”执行,包括:
- ...
- 应按什么顺序执行任务(FIFO,LIFO,优先顺序)?
- ...
UPD:我意识到我并没有完全问自己想问的问题。我真正想要的是:
如何在执行程序框架中使用/模拟设置线程优先级(即thread.setPriority()
是什么?
[the Executor interface当前唯一的具体实现是the ThreadPoolExecutor和the ScheduledThreadpoolExecutor
而不是使用实用程序/工厂类Executors,而应使用构造函数创建实例。
您可以将BlockingQueue传递给ThreadPoolExecutor的构造函数。
BlockingQueue的一种实现,the PriorityBlockingQueue使您可以将Comparator传递给构造函数,从而使您能够确定执行顺序。
这里的想法是在执行程序中使用PriorityBlockingQueue。为此:
首先,您需要优先考虑未来:
class PriorityFuture<T> implements RunnableFuture<T> {
private RunnableFuture<T> src;
private int priority;
public PriorityFuture(RunnableFuture<T> other, int priority) {
this.src = other;
this.priority = priority;
}
public int getPriority() {
return priority;
}
public boolean cancel(boolean mayInterruptIfRunning) {
return src.cancel(mayInterruptIfRunning);
}
public boolean isCancelled() {
return src.isCancelled();
}
public boolean isDone() {
return src.isDone();
}
public T get() throws InterruptedException, ExecutionException {
return src.get();
}
public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
return src.get();
}
public void run() {
src.run();
}
}
接下来,您需要定义比较器,以对优先级期货进行正确排序:
class PriorityFutureComparator implements Comparator<Runnable> {
public int compare(Runnable o1, Runnable o2) {
if (o1 == null && o2 == null)
return 0;
else if (o1 == null)
return -1;
else if (o2 == null)
return 1;
else {
int p1 = ((PriorityFuture<?>) o1).getPriority();
int p2 = ((PriorityFuture<?>) o2).getPriority();
return p1 > p2 ? 1 : (p1 == p2 ? 0 : -1);
}
}
}
接下来让我们假设我们有一个冗长的工作,像这样:
class LenthyJob implements Callable<Long> {
private int priority;
public LenthyJob(int priority) {
this.priority = priority;
}
public Long call() throws Exception {
System.out.println("Executing: " + priority);
long num = 1000000;
for (int i = 0; i < 1000000; i++) {
num *= Math.random() * 1000;
num /= Math.random() * 1000;
if (num == 0)
num = 1000000;
}
return num;
}
public int getPriority() {
return priority;
}
}
然后为了优先执行这些作业,代码将如下所示:
public class TestPQ {
public static void main(String[] args) throws InterruptedException, ExecutionException {
int nThreads = 2;
int qInitialSize = 10;
ExecutorService exec = new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS,
new PriorityBlockingQueue<Runnable>(qInitialSize, new PriorityFutureComparator())) {
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
RunnableFuture<T> newTaskFor = super.newTaskFor(callable);
return new PriorityFuture<T>(newTaskFor, ((LenthyJob) callable).getPriority());
}
};
for (int i = 0; i < 20; i++) {
int priority = (int) (Math.random() * 100);
System.out.println("Scheduling: " + priority);
LenthyJob job = new LenthyJob(priority);
exec.submit(job);
}
}
}
这是很多代码,但这几乎是完成此任务的唯一方法。
在我的机器上,输出如下:
Scheduling: 39
Scheduling: 90
Scheduling: 88
Executing: 39
Scheduling: 75
Executing: 90
Scheduling: 15
Scheduling: 2
Scheduling: 5
Scheduling: 24
Scheduling: 82
Scheduling: 81
Scheduling: 3
Scheduling: 23
Scheduling: 7
Scheduling: 40
Scheduling: 77
Scheduling: 49
Scheduling: 34
Scheduling: 22
Scheduling: 97
Scheduling: 33
Executing: 2
Executing: 3
Executing: 5
Executing: 7
Executing: 15
Executing: 22
Executing: 23
Executing: 24
Executing: 33
Executing: 34
Executing: 40
Executing: 49
Executing: 75
Executing: 77
Executing: 81
Executing: 82
Executing: 88
Executing: 97
您可以实现自己的ThreadFactory,并在ThreadPoolExecutor中像这样设置它:
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(1, numOfWorkerThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());
threadPool.setThreadFactory(new OpJobThreadFactory(Thread.NORM_PRIORITY-2));
我的OpJobThreadFactory如下所示:
public final static class OpJobThreadFactory implements ThreadFactory {
private int priority;
private boolean daemon;
private final String namePrefix;
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final AtomicInteger threadNumber = new AtomicInteger(1);
public OpJobThreadFactory(int priority) {
this(priority, true);
}
public OpJobThreadFactory(int priority, boolean daemon) {
this.priority = priority;
this.daemon = daemon;
namePrefix = "jobpool-" +poolNumber.getAndIncrement() + "-thread-";
}
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r, namePrefix + threadNumber.getAndIncrement());
t.setDaemon(daemon);
t.setPriority(priority);
return t;
}
}
您可以将ThreadPoolExecutor与Priority阻止队列一起使用How to implement PriorityBlockingQueue with ThreadPoolExecutor and custom tasks
您可以在ThreadFactory
构造函数中指定ThreadFactory
(或ThreadPoolExecutor
工厂方法)。这使您可以为执行程序提供具有给定线程优先级的线程。
要为不同的作业获得不同的线程优先级,您需要将它们发送给具有不同线程工厂的执行者。
请注意,setPriority(..)通常在Linux下运行not。有关完整的详细信息,请参见以下链接:
只需在此讨论中加点我的贡献。我已经为特定目的实现了此http://www.akshaal.info/2008/04/javas-thread-priorities-in-linux.html,它可以在我需要的时候显式地显示在执行者的BlockingQueue(在本例中为LinkedBlockingDeque)的前面,而不必处理优先级(这可能导致死锁)并且是固定的)。
我正在使用它来管理(在Android应用程序内部)需要下载许多显示在长列表视图中的图像的情况。每当用户快速向下滚动时,执行器队列就会涌入大量图像下载请求:通过将最新请求移到队列顶部,我在加载实际在屏幕上的图像方面获得了更好的性能,从而延迟了下载以后可能需要的那些。请注意,我使用内部并发映射键(可以与图像URL字符串一样简单)将任务添加到执行程序,以便稍后可以检索它们以进行重新排序。
[还有许多其他方法可以做到这一点,并且可能过于复杂,但是它可以正常工作,而且他的Android SDK中的Facebook在其自己的工作线程队列中也做了类似的事情。
随意看一下代码并给我建议,它在一个Android项目中,但是剥离一些日志和注释将使该类成为纯Java 6。