Question

使用Executors和callable从调用中转储调用堆栈时，我看到了奇怪的结果。来自callable的方法在调用堆栈中出现两次。

pool-1-thread-1@454, prio=5, in group 'main', status: 'RUNNING'
      at com.test.tracked.ACallable.call(ACallable.java:15)
      at com.test.tracked.ACallable.call(ACallable.java:9)
      at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
      at java.util.concurrent.FutureTask.run(FutureTask.java:166)
      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
      at java.lang.Thread.run(Thread.java:722)

如您所见，方法ACallable在堆栈中出现两次：第9行是ACallable类的声明，第15行是方法签名：

package com.test.tracked;

import java.util.concurrent.Callable;

/**
 *  A callable test class.
 *  
 */
public final class ACallable
    implements Callable<String> {
  private final String who;

  @Override
  public String call() throws Exception {
    Thread.dumpStack();
    return "Hello "+who+" from callable";
  }

  public ACallable(String who) {
    this.who = who;
  }

}

'主'线程：

main@1, prio=5, in group 'main', status: 'WAIT'
      at sun.misc.Unsafe.park(Unsafe.java:-1)
      at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
      at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:834)
      at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:994)
      at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1303)
      at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:248)
      at java.util.concurrent.FutureTask.get(FutureTask.java:111)
      at com.test.tracked.AsynchronousCall.callFuture(AsynchronousCall.java:26)
      at com.test.Main.main(Main.java:21)

调用callable的代码：

package com.test.tracked;

import java.io.Closeable;
import java.io.IOException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeoutException;



/**
 * Aynchronous call
 */
public class AsynchronousCall implements Closeable {

  private final ExecutorService executorService;


  public AsynchronousCall() {
    executorService = Executors.newSingleThreadExecutor();
  }

  public String callFuture(String who) throws InterruptedException, ExecutionException, TimeoutException {
    Thread.dumpStack();
    String ret = executorService.submit(new ACallable(who)).get();
    System.out.println("callFuture from " + getClass().getName() + " return " + ret);
    return ret;
  }

  @Override
  public void close() throws IOException {
    executorService.shutdownNow();
  }
}

Answer 1

编译器添加了一个合成桥接方法来支持泛型。所以

@Override
public String call() throws Exception {
    Thread.dumpStack();
    return "Hello "+who+" from callable";
}

编译好的.class文件中的

实际上是两种方法

// actual method
public Object call() throws Exception {
    return call(); // the other call
} 

// your implementation
public String call() throws Exception {
    Thread.dumpStack();
    return "Hello "+who+" from callable";
}

（请注意，这在源代码中是不可能的，因为两种方法都具有相同的签名。）

在其他问题和答案中进一步解释了这一点：

Java Generics - Bridge method?

Answer 2

这是编译器添加的桥接方法的结果。

在JVM中，方法没有协变返回类型。尝试致电Object call()将不致电String call() - 如果没有Object call()方法，则会抛出NoSuchMethodError。因此，编译器添加类似于：

的桥接方法

public Object call() throws Exception {
    return call(); // the call() method that returns String, not this one
}

为什么会有Object call()方法呢？由于泛型类型擦除 - 字节码不使用泛型类型。在字节码中，Callable看起来像这样：

interface Callable {
    Object call() throws Exception;
}

和ACallable看起来像这样:(这不是有效的Java源代码，因为它包含两个具有相同名称和参数的方法）

class ACallable implements Callable {
    private final String who;

    // does NOT override the method in Callable due to the different return type
    public String call() throws Exception {
        Thread.dumpStack();
        return "Hello "+who+" from callable";
    }

    // overrides the method in Callable
    public Object call() throws Exception {
        return call(); // the version of call() that returns String
    }
}

您可以通过在生成的类文件上运行javap来查看此内容。

使用Callable时出现奇怪的调用堆栈

2 个答案: