java - 多线程 newSingleThreadExecutor 饥饿死锁

Question

这直接来自 Java 并发实践。以下情况会导致死锁:

class ThreadDeadlock {
    ExecutorService exec = Executors.newSingleThreadExecutor();
    public class RenderPageTask implements Callable<String> {
        public String call() throws Exception {
            Future<String> header, footer;
            System.out.println(Thread.currentThread().getName());
            header = exec.submit(new LoadFileTask("header.html"));
            footer = exec.submit(new LoadFileTask("footer.html"));
            //String page = renderBody();
            // Will deadlock -- task waiting for result of subtask
            //System.out.println("executed header and footer");
            String headerS = header.get();
            System.out.println("retrieved header");
            String footerS = footer.get();
            return headerS+ footerS;
        }
    }
}



class LoadFileTask implements Callable<String>{
    String name;
    public LoadFileTask(String name) {
        super();
        this.name = name;
    }

    @Override
    public String call() throws Exception {
        /*Scanner s = new Scanner(name);
        StringBuilder str = new StringBuilder();
        while(s.hasNextLine()){
            str.append(s.nextLine());
        }*/
        System.out.println(Thread.currentThread().getName()+this.name);
        return "";
    }
}

/* Call from Main */
         private static void checkDeadLock(){
    ThreadDeadlock.RenderPageTask callable = new ThreadDeadlock().new RenderPageTask();
    ExecutorService es = Executors.newFixedThreadPool(1);
    es.submit(callable);
    es.shutdown();
}

我无法弄清楚为什么这会导致僵局。我已经检查过，确实如此。它甚至返回 header.get() 并打印检索到的 header ，那么它是如何死锁的？

Answer 1

您的代码中没有死锁，JVM 不会关闭，因为它有工作线程 - 用于 exec 服务的执行器线程(在 ThreadDeadlock 中定义)。如果添加

exec.shutdown();

在RenderPageTask中return之前，程序将在所有任务完成后终止。