java - 多线程环境下Executor Service清除map值

Question

我这里有一个情况。我正在使用执行器服务来利用多线程。所以现在我试图向映射添加一个值，并在完成该循环后立即清除每个线程的值。我为此编写了以下代码。

我创建了一个 map ，并保存每次迭代的值并清除它。但当我使用 Executor Service 时，我相信它创建了 10 个线程并将值(value)添加到 map 中。这就是为什么我能够看到多个值，即使我只是添加一个值并清除它。

那么我如何清除循环中每个事务的 map :(

代码:

public class Test1 {
public static void main(String[] args){
    ExecutorService executor = Executors.newFixedThreadPool(10);
    final Multimap<Object, Object> map = Multimaps.synchronizedMultimap(ArrayListMultimap.create());

    final List<String> listA = new ArrayList<String>();
    listA.add("e");
    listA.add("f");
    listA.add("g");
    final List<String> listB = new ArrayList<String>();
    listB.add("e");
    listB.add("f");
    listB.add("g");
    final List<String> listC = new ArrayList<String>();
    listC.add("e");
    listC.add("f");
    listC.add("g");


    for (final String stateId : listA) {
        for (final String gradeList : listB) {
            for (final String subjectList : listC) {
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                map.clear();
                map.put("f", "hi");
                System.out.println("map before "+map);

                System.out.println("map after "+map);
            }
        });
    }

    }
    }
    executor.shutdown();
    try {
        executor.awaitTermination(24L, TimeUnit.HOURS);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

}

输出:

map before {f=[hi, hi]}
map before {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
 map before {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map before {f=[hi, hi]}
map before {f=[hi, hi]}
map after {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
 map after {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map after {f=[hi, hi]}
map after {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
 map after {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
 map after {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map before {f=[hi]}
map before {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}
map after {f=[hi]}

Answer 1

简单 - javadoc明确指出:

This class is not threadsafe when any concurrent operations update the multimap. Concurrent read operations will work correctly. To allow concurrent update operations, wrap your multimap with a call to Multimaps.synchronizedListMultimap(com.google.common.collect.ListMultimap).

换句话说:您不能只采用任何方便的集合类并向其抛出多个线程 - 而不采取进一步的预防措施。