java - 同步方法 block 执行

Question

我正在实现一个包含三个类的虚拟程序，以便更直观地了解 Future 的工作原理。我的问题是，有时程序会被锁定在同步方法上，并且无法继续。而且我找不到原因。有人能找出为什么没有一个战士会打印“我赢了”这行字的原因吗？

阻塞时我当前的输出:

Fighter1

Fighter1 has entered the sync method

Fighter2

Fighter2 has entered the sync method

代码如下。

主类:

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class TmpTest {
private static final ExecutorService executorService = 
  Executors.newFixedThreadPool(2);

  public static void main(final String... args) {

    final Fighter fighter1 = new Fighter("Fighter1");
    final Fighter fighter2 = new Fighter("Fighter2");

    final Future<String> submitFighter1 = executorService.submit(fighter1);
    final Future<String> submitFighter2 = executorService.submit(fighter2);

    while (!submitFighter1.isDone() || !submitFighter2.isDone()) {
        if (submitFighter1.isDone()) {
            System.out.println("Fighter 1 wins!");
            submitFighter2.cancel(true);
            executorService.shutdown();
        } else if (submitFighter2.isDone()) {
            submitFighter1.cancel(true);
            System.out.println("Fighter 2 wins!");
            executorService.shutdown();
        }
    }
  }
}

战斗机类别:

class Fighter implements Callable<String> {

  private final String fighterName;
  private final ClassWithSyncMethod classWithSyncMethod;

  public Fighter(final String fighterName) {
    this.fighterName = fighterName;
    classWithSyncMethod = new ClassWithSyncMethod(fighterName);
  }

  @Override
  public String call() throws Exception {
    return classWithSyncMethod.syncMethod();
  }
}

具有同步方法的虚拟类:

class ClassWithSyncMethod {

  private final String fighterName;

  public ClassWithSyncMethod(final String fighterName) {
    this.fighterName = fighterName;
  }

  public synchronized String syncMethod() {
    System.out.println(fighterName + " has entered the sync method");
    try {
        Thread.sleep(1000);
    } catch (final InterruptedException e) {
        System.out.println("Exception trying to sleep the fighter " + fighterName + ";" + e);
    }
    return fighterName + " shouts: I won!";
  }
}

Answer 1

这与同步无关。每个战斗机在不同的对象上同步，因此它们不会互相干扰。

您看不到“我赢了”行的原因要简单得多 - 您从不打印它。ClassWithSyncMethod#syncMethod() 方法(由 Fighter#call() 返回)不会打印任何内容，它只是返回一个值。如果您想打印它，您必须自己从 main 中执行此操作。

例如:

while (!submitFighter1.isDone() || !submitFighter2.isDone()) {
    if (submitFighter1.isDone()) {
        System.out.println("Fighter 1 wins!");
        System.out.println(submitFighter1.get()); // Here!
        submitFighter2.cancel(true);
        executorService.shutdown();
    } else if (submitFighter2.isDone()) {
        submitFighter1.cancel(true);
        System.out.println("Fighter 2 wins!");
        System.out.println(submitFighter2.get()); // And here!
        executorService.shutdown();
    }
}