ArrayList即使使用同步块也不是线程安全的？

Question

我尝试使用ArrayList解决生产者和消费者问题（我知道ArrayList是nt threadsafe），我确保使用synchronized关键字放置列表，但仍然进入ConcurrentModificationException。这就是错误

启动生产者请提供作业详细信息：TestJob作业完成：TestJob Exception位于java.util.ArrayList$itr.checkforcoModification（未知源）位于test.thread.consumer.run(consumer.java:25)位于java.lang.thread.run（未知源）的java.util.arrayList$itr.next（未知源）

下面是我的代码：

    package test.thread;

import java.util.List;
import java.util.Scanner;

public class Producer implements Runnable {

    private List<String> jobs = null;

    public Producer(List<String> jobs) {
        this.jobs = jobs;
    }

    @Override
    public void run() {

        System.out.println("Starting Producer");
        while(true)
        {
            synchronized (jobs) {
                try {
                if (jobs.isEmpty()) {


                        System.out.println("Please provide the job details: ");
                        Scanner scanner = new Scanner(System.in);
                        String job = scanner.nextLine();
                        jobs.add(job);
                        scanner.close();
                        jobs.notifyAll();
                        Thread.sleep(4000);

                } else {
                    jobs.wait();
                }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        }

    }

}


package test.thread;

import java.util.List;

public class Consumer implements Runnable {

    private List<String> jobs = null;

    public Consumer(List<String> list) {
        this.jobs = list;
    }

    @Override
    public void run() {

        while(true)
        {
            synchronized (jobs) {
                try {
                    if (jobs.isEmpty()) {

                        jobs.wait();

                    } else {
                        for (String job : jobs) {
                            System.out.println("Job completed: " + job);
                            jobs.remove(job);
                            Thread.sleep(2000);
                        }

                        jobs.notifyAll();
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

            }

        }

    }

}



package test.thread;

import java.util.ArrayList;
import java.util.List;

public class TestThread {

    public static void main(String...strings )
    {

        List<String> list = new ArrayList<String>();
        Producer producer = new Producer(list);
        Consumer consumer = new Consumer(list);
        Thread prodThread = new Thread(producer, "producer");
        Thread consThread = new Thread(consumer, "consumer");
        prodThread.start();
        consThread.start();
    }




}

Answer 1

尽管您得到了ConcurrentModificationException，但问题根本不是由多线程引起的。

事实上，这是有问题的代码：

for (String job : jobs) {
    System.out.println("Job completed: " + job);
    jobs.remove(job);
    Thread.sleep(2000);
}

for循环将在作业上使用迭代器，同时直接（从结构上）修改作业（即不通过迭代器)。这意味着迭代器不再定义良好，它抛出这个异常来告诉您有关它的情况。

Iterator<String> jobIterator = job.iterator();
while (jobIterator.hasNext()) {
  String job = jobIterator.next();
  // ... stuff ...
  jobIterator.remove(); // remove the last object that was returned by next()
}