Java 7 Synchronized 方法似乎允许多线程访问

Question

我在 Java 7 应用程序上实现 Synchronized 接口(interface)时遇到问题。我认为问题可能在于我正在使用同步方法中写入的静态最终数组实例化多个对象。似乎线程没有确认监视器？我什至可以做我想做的事，还是我需要某种具有数组和 writeIndex 增量器的缓冲区对象？ (如果有帮助，代码在 https://github.com/dschaper/CS112_Dan_Schaper/tree/master/Week14/Labs，这不是家庭作业，这只是一个不评分的实验。)

public class SingleDice extends Dice implements Runnable {
    String threadName;
    private static final int[] valuesArray = new int[9];
    private static int writeIndex = 0;

    public SingleDice(String tName) {
        super(1);
        threadName = tName;
    }

    public synchronized void add(int value) {
        int pos = writeIndex;
        System.out.printf("%s starting sync add(%d), writeIndex = %d\n",
            Thread.currentThread().getName(), value,
            writeIndex);
        valuesArray[pos] = value;
        System.out.printf("%s wrote %d to index %d\n",        
            Thread.currentThread().getName(), value, writeIndex);
        ++writeIndex;
        System.out.printf("Next writeIndex %d\n", writeIndex);
    }
    @Override
    public void run() {
    for (int i = 0; i < 3; i++) {
        add(super.Throw());
        }
    }
}

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

public class SyncDiceExecutor {

public static void main(String[] args) {
    System.out.println("Creating newCachedThreadPool");
    SingleDice SD1 = new SingleDice("SD1");
    SingleDice SD2 = new SingleDice("SD2");
    SingleDice SD3 = new SingleDice("SD3");
    ExecutorService executor = Executors.newCachedThreadPool();
    executor.execute(SD1);
    System.out.printf("Starting Thread SD1 %s\n", executor.toString());
    executor.execute(SD2);
    System.out.printf("Starting Thread SD2 %s\n", executor.toString());
    executor.execute(SD3);
    System.out.printf("Starting Thread SD3 %s\n", executor.toString());
    executor.shutdown();
    System.out.printf("Execeutor shutdown called: %s\n", executor.toString());

    }
}

输出

Creating newCachedThreadPool
Starting Thread SD1 java.util.concurrent.ThreadPoolExecutor@544ec1[Running, pool size = 1, active threads = 1, queued tasks = 0, completed tasks = 0]
pool-1-thread-1 starting sync add(4), writeIndex = 0
pool-1-thread-2 starting sync add(4), writeIndex = 0
Starting Thread SD2 java.util.concurrent.ThreadPoolExecutor@544ec1[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0]
pool-1-thread-2 wrote 4 to index 0
pool-1-thread-1 wrote 4 to index 0
pool-1-thread-3 starting sync add(4), writeIndex = 1
Next writeIndex 1
pool-1-thread-2 starting sync add(6), writeIndex = 2
Starting Thread SD3 java.util.concurrent.ThreadPoolExecutor@544ec1[Running, pool size = 3, active threads = 3, queued tasks = 0, completed tasks = 0]
pool-1-thread-2 wrote 6 to index 2
Next writeIndex 3
pool-1-thread-3 wrote 4 to index 2
Next writeIndex 4
Next writeIndex 2
pool-1-thread-3 starting sync add(1), writeIndex = 4
pool-1-thread-2 starting sync add(6), writeIndex = 3
Execeutor shutdown called: java.util.concurrent.ThreadPoolExecutor@544ec1[Shutting down, pool size = 3, active threads = 3, queued tasks = 0, completed tasks = 0]
pool-1-thread-2 wrote 6 to index 4
Next writeIndex 5
pool-1-thread-3 wrote 1 to index 4
Next writeIndex 6
pool-1-thread-1 starting sync add(3), writeIndex = 4
pool-1-thread-3 starting sync add(1), writeIndex = 6
pool-1-thread-1 wrote 3 to index 6
pool-1-thread-3 wrote 1 to index 6
Next writeIndex 8
Next writeIndex 7
pool-1-thread-1 starting sync add(1), writeIndex = 8
pool-1-thread-1 wrote 1 to index 8
Next writeIndex 9

Process finished with exit code 0

Answer 1

SD1、SD2 和 SD3 是不同对象，即它们不共享同一个监视器。每个对象都有自己的监视器。如果您想在所有实例之间共享监视器，您可以使用 synchronized(SingleDice.class) {...}。