Java中线程的多线程一致性

Question

这里是Java新手。 只是对这个示例程序不一致的输出感到困惑。 有时，任务队列。size（）是零，有时不是。 这可能是什么原因

输出：

t4:[Task9999]已执行

t1:[Task9969]已执行

等待

完成

剩余任务数：4

进程已完成，退出代码为0

import java.util.ArrayList;

public class TaskExecutor implements Runnable {

    private ArrayList<String> arr;

    public TaskExecutor(ArrayList<String> arr) {
        this.arr = arr;
    }

    @Override
    public void run() {

            String task;
            while (arr.size() != 0 &&  (task = arr.remove(0))!=null) {
                System.out.println(Thread.currentThread().getName() + " : [" + task + "] has been executed");
            }
    }
}

import java.util.ArrayList;

public class Main {

    public static void main(String[] args) throws InterruptedException {

        ArrayList<String> task_queue = new ArrayList<>();

        for(int i = 0; i < 10000; i++) {
            task_queue.add("Task" + i);
        }

        Thread t1 = new Thread(new TaskExecutor(task_queue));
        Thread t2 = new Thread(new TaskExecutor(task_queue));
        Thread t3 = new Thread(new TaskExecutor(task_queue));
        Thread t4 = new Thread(new TaskExecutor(task_queue));
        Thread t5 = new Thread(new TaskExecutor(task_queue));

        t1.setName("t1");
        t2.setName("t2");
        t3.setName("t3");
        t4.setName("t4");
        t5.setName("t5");

        t1.start();
        t2.start();
        t3.start();
        t4.start();
        t5.start();

        System.out.println();

        while(t1.isAlive() || t2.isAlive() || t3.isAlive() || t4.isAlive() || t5.isAlive()) {
        }

        System.out.println("waiting...");
        Thread.currentThread().sleep(2000);
        System.out.println("finished");
        System.out.println("Number of Tasks left : " + task_queue.size());
    }

}

Answer 1

例如，在不使用同步集合的情况下进行同步的一种可能方法是：

public class TaskExecutor implements Runnable {

    private ArrayList<String> arr;

    public TaskExecutor(ArrayList<String> arr) {
        this.arr = arr;
    }

    @Override
    public void run() {

            String task;

                while (true) 
                {
                    synchronized(arr)
                    {
                        if(arr.size()==0)
                           break;

                        task = arr.remove(0);
                        if(task==null)
                           break;
                    }

                    System.out.println(Thread.currentThread().getName() + " : [" + task + "] has been executed");

                }//end while    

    }//end run

}//end class

Answer 2

原因是ArrayList不是线程安全的

使用线程安全的集合之一，比如CopyOnWriteArrayList。或者更好：Collections.synchronizedList(new ArrayList<String>());

Some explanation of thread-safe