如果你提交一个runnable给一个多线程的executor service，那么多线程会执行那个runnable吗？

Question

我很难理解 ExecutorService 在 Java 中的工作原理 8. 我试图理解这个网站上的一些代码：https://crunchify.com/hashmap-vs-concurrenthashmap-vs-synchronizedmap-how-a-hashmap-can-be-synchronized-in-java/

特别是在他测试不同地图的运行时间的最后。这是代码：

public class CrunchifyConcurrentHashMapVsSynchronizedMap {

public final static int THREAD_POOL_SIZE = 5;

public static Map<String, Integer> crunchifyHashTableObject = null;
public static Map<String, Integer> crunchifySynchronizedMapObject = null;
public static Map<String, Integer> crunchifyConcurrentHashMapObject = null;

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

    // Test with Hashtable Object
    crunchifyHashTableObject = new Hashtable<String, Integer>();
    crunchifyPerformTest(crunchifyHashTableObject);

    // Test with synchronizedMap Object
    crunchifySynchronizedMapObject = Collections.synchronizedMap(new HashMap<String, Integer>());
    crunchifyPerformTest(crunchifySynchronizedMapObject);

    // Test with ConcurrentHashMap Object
    crunchifyConcurrentHashMapObject = new ConcurrentHashMap<String, Integer>();
    crunchifyPerformTest(crunchifyConcurrentHashMapObject);

}

public static void crunchifyPerformTest(final Map<String, Integer> crunchifyThreads) throws InterruptedException {

    System.out.println("Test started for: " + crunchifyThreads.getClass());
    long averageTime = 0;
    for (int i = 0; i < 5; i++) {

        long startTime = System.nanoTime();
        ExecutorService crunchifyExServer = Executors.newFixedThreadPool(THREAD_POOL_SIZE);

        for (int j = 0; j < THREAD_POOL_SIZE; j++) {
            crunchifyExServer.execute(new Runnable() {
                @SuppressWarnings("unused")
                @Override
                public void run() {

                    for (int i = 0; i < 500000; i++) {
                        Integer crunchifyRandomNumber = (int) Math.ceil(Math.random() * 550000);

                        // Retrieve value. We are not using it anywhere
                        Integer crunchifyValue = crunchifyThreads.get(String.valueOf(crunchifyRandomNumber));

                        // Put value
                        crunchifyThreads.put(String.valueOf(crunchifyRandomNumber), crunchifyRandomNumber);
                    }
                }
            });
        }

        // Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation
        // has no additional effect if already shut down.
        // This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.
        crunchifyExServer.shutdown();

        // Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is
        // interrupted, whichever happens first.
        crunchifyExServer.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);

        long entTime = System.nanoTime();
        long totalTime = (entTime - startTime) / 1000000L;
        averageTime += totalTime;
        System.out.println("500K entried added/retrieved in " + totalTime + " ms");
    }
    System.out.println("For " + crunchifyThreads.getClass() + " the average time is " + averageTime / 5 + " ms\n");
}

}

所以在 crunchifyPerformTest class 中，他启动了一个有 5 个线程的 ExecutorService，然后每次提交 5 个不同的 runnables 和 500k 读写到 hashmap？执行器服务会自动让 5 个线程执行每个可运行对象吗？

Answer 1

没有。每个 Runnable 只在一个线程上执行。这意味着所有 Runnable 将并行执行，因为 Runnable 的数量与可用线程的数量匹配。

您还可以提交 6 Runnable。在这种情况下，其中 5 个将并行执行，一旦一个 Runnable 完成执行，第六个将被执行。

顺便说一下，我认为 docs 对行为非常清楚 ExecutorService.

如果你提交一个runnable给一个多线程的executor service，那么多线程会执行那个runnable吗？

If you submit one runnable to an executor service with multiple threads, will multiple threads execute that runnable?

java

multithreading

concurrency

runnable

executorservice