Java代码执行在没有断点和正常运行的调试中产生不同的结果。ExecutorService是否损坏？

储仲渊

2023-03-14

问题内容：

TL：DR ExecutorService executorService = Executors.newFixedThreadPool(8);
在调试中并发运行，但在正常运行时它会并发运行，但稍后在单线程中运行。

我有一些代码可以在中启动4个不同的任务ExecutorService。这些任务中的两项应几乎立即完成，其他两项应运行一段时间。

这些任务以秒为单位返回执行时间Future<Double>。

此代码负责任务执行和度量：

public Future<Double> measure(int[] arr, ProcessIntArray processIntArray, ExecutorService es) {
    Callable<Double> task = () -> {
        long start = System.nanoTime();
        processIntArray.process(arr);
        long end = System.nanoTime();
        return (end - start) / 1000000000.0;
    };
    return es.submit(task);
}

稍后，在启动这些任务之后，我以相同输入大小的执行顺序从前一次运行打印它们。

    Future<Double> bubbleSortTime = measure(bubbleSortArray, Solution::bubbleSort, executorService);
    Future<Double> insertionSortTime = measure(insertionSortArray, Solution::insertionSort, executorService);
    Future<Double> quickSortTime = measure(quickSortArray, Solution::quickSort, executorService);
    Future<Double> mergeSortTime = measure(mergeSortArray, Solution::mergeSort, executorService);


    System.out.println();
    System.out.println("array size:     " + size);
    System.out.println("quick sort:     " + quickSortTime.get() + "s");
    System.out.println("merge sort:     " + mergeSortTime.get() + "s");
    System.out.println("insertion sort: " + insertionSortTime.get() + "s");
    System.out.println("bubble sort:    " + bubbleSortTime.get() + "s");

当我在调试模式下运行代码时，将立即打印2个结果，并且我需要等待一会儿以获得第3个结果（我不必费心等待第4个结果）。

在调试中启动后（正确和预期的）：

array size: 1000000 quick sort: 0.186892839s merge sort: 0.291950604s insertion sort: 344.534256723s

正常执行会有所不同，看起来好像在long start = System.nanoTime();执行方法measure
，然后线程进入睡眠状态，并且在完成insertSort之后，quickSort返回到执行中，从而产生输出：

array size: 1000000 quick sort: 345.893922141s merge sort: 345.944023095s insertion sort: 345.871908569s

错了所有这些线程应与newFixedThreadPool javadoc中的所有线程同时运行。

/**
 * Creates a thread pool that reuses a fixed number of threads
 * operating off a shared unbounded queue.  At any point, at most
 * {@code nThreads} threads will be active processing tasks.
 * If additional tasks are submitted when all threads are active,
 * they will wait in the queue until a thread is available.
 * If any thread terminates due to a failure during execution
 * prior to shutdown, a new one will take its place if needed to
 * execute subsequent tasks.  The threads in the pool will exist
 * until it is explicitly {@link ExecutorService#shutdown shutdown}.
 *
 * @param nThreads the number of threads in the pool
 * @return the newly created thread pool
 * @throws IllegalArgumentException if {@code nThreads <= 0}
 */
public static ExecutorService newFixedThreadPool(int nThreads)

我附上源代码：

import java.util.Arrays;
import java.util.Random;
import java.util.concurrent.*;

class ThreadedSortingComparsion {
    Random random = new Random(System.currentTimeMillis());

    void popul(int[] array) {
        for (int i = 0; i < array.length; i++) {
            array[i] = random.nextInt();
        }
    }

    interface ArraySorter {
        void sort(int[] array);
    }

    public Future<Double> measureTime(int[] array, ArraySorter arraySorter, ExecutorService executorService) {
        Callable<Double> task = () -> {
            long start = System.nanoTime();
            arraySorter.sort(array);
            long end = System.nanoTime();
            return (end - start) / 1000000000.0;
        };
        return executorService.submit(task);
    }


    public void start() throws ExecutionException, InterruptedException {
        ExecutorService executorService = Executors.newFixedThreadPool(8);

        int size = 1000 * 1000;
        int[] quickSortArray = new int[size];
        popul(quickSortArray);
        int[] bubbleSortArray = Arrays.copyOf(quickSortArray, quickSortArray.length);
        int[] mergeSortArray = Arrays.copyOf(quickSortArray, quickSortArray.length);
        int[] originalArray = Arrays.copyOf(quickSortArray, quickSortArray.length);
        int[] insertionSortArray = Arrays.copyOf(quickSortArray, quickSortArray.length);

        Future<Double> bubbleSortTime = measureTime(bubbleSortArray, ThreadedSortingComparsion::bubbleSort, executorService);
        Future<Double> insertionSortTime = measureTime(insertionSortArray, ThreadedSortingComparsion::insertionSort, executorService);
        Future<Double> quickSortTime = measureTime(quickSortArray, ThreadedSortingComparsion::quickSort, executorService);
        Future<Double> mergeSortTime = measureTime(mergeSortArray, ThreadedSortingComparsion::mergeSort, executorService);


        System.out.println();
        System.out.println("array size:     " + size);
        System.out.println("quick sort:     " + quickSortTime.get() + "s");
        System.out.println("merge sort:     " + mergeSortTime.get() + "s");
        System.out.println("insertion sort: " + insertionSortTime.get() + "s");
        System.out.println("bubble sort:    " + bubbleSortTime.get() + "s");
        executorService.shutdown();

        for (int i = 0; i < quickSortArray.length; i++) {
            if (quickSortArray[i] != bubbleSortArray[i] || quickSortArray[i] != mergeSortArray[i] || quickSortArray[i] != insertionSortArray[i]) {
                throw new RuntimeException(Arrays.toString(originalArray));
            }
        }
    }


    public static void mergeSort(int[] ar) {
        if (ar.length < 5) {
            bubbleSort(ar);
            return;
        }
        int middle = ar.length / 2;
        int[] arrayLeft = new int[middle];
        int[] arrayRight = new int[ar.length - middle];
        for (int i = 0; i < ar.length; i++) {
            if (i < middle) {
                arrayLeft[i] = ar[i];
            } else {
                arrayRight[i - middle] = ar[i];
            }
        }
        mergeSort(arrayLeft);
        mergeSort(arrayRight);
        int indexLeft = 0;
        int indexRight = 0;
        int inputArrayIndex = 0;
        while (true) {
            int whatToPutInAR = 0;
            if (indexLeft != arrayLeft.length && indexRight != arrayRight.length) {
                if (arrayLeft[indexLeft] < arrayRight[indexRight]) {
                    whatToPutInAR = arrayLeft[indexLeft];
                    indexLeft++;
                } else {
                    whatToPutInAR = arrayRight[indexRight];
                    indexRight++;
                }
            } else if (indexLeft != arrayLeft.length) {
                whatToPutInAR = arrayLeft[indexLeft];
                indexLeft++;
            } else if (indexRight != arrayRight.length) {
                whatToPutInAR = arrayRight[indexRight];
                indexRight++;
            }
            if (inputArrayIndex == ar.length) return;
            ar[inputArrayIndex++] = whatToPutInAR;
        }
    }

    private static void quickSort(int[] ar) {
        quickSort(ar, 0, ar.length);
    }

    static public void quickSort(int[] array, int start, int end) {
        boolean changed = false;
        if (end == 0) return;
        int pivot = array[end - 1];
        int partitionCandidate = start;
        for (int i = start; i < end; i++) {
            if (array[i] < pivot) {
                swap(array, partitionCandidate++, i);
                changed = true;
            } else if (pivot < array[i]) {
                swap(array, end - 1, i);
                changed = true;
            }
        }
        if (start < partitionCandidate) {
            quickSort(array, start, partitionCandidate);
        }
        if (partitionCandidate < end) {
            if (partitionCandidate != start || changed) quickSort(array, partitionCandidate, end);
        }
    }


    public static void swap(int[] ar, int from, int to) {
        int old = ar[from];
        ar[from] = ar[to];
        ar[to] = old;
    }

    public static void bubbleSort(int[] array) {
        for (int i = 0; i < array.length; i++) {
            for (int j = 0; j < array.length - 1; j++) {
                if (array[j] > array[j + 1]) {
                    swap(array, j + 1, j);
                }
            }
        }
    }

    private static void insertionSort(int[] ar) {
        for (int i = 0; i < ar.length; i++) {
            for (int j = i; j >= 1; j--) {
                boolean breaker = true;
                if (ar[j] < ar[j - 1]) {
                    breaker = false;
                    swap(ar, j - 1, j);
                }
                if (breaker) break;
            }
        }
    }

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        ThreadedSortingComparsion s = new ThreadedSortingComparsion();
        s.start();
    }
}

编辑：当我在Ideone中运行它时，代码工作正常。 http://ideone.com/1E8C51
Ideone具有Java版本1.8.0_51
在测试我1.8.0_91，1.8.0_92。和1.8.0_45。为什么它可以在ideone上运行，但不能在我测试的其他两台PC上运行呢？

当我执行线程转储时，虽然不在调试中执行，但我会等待一会儿，并且在打印完线程转储后，还会打印结果。因此线程转储是在完成插入排序之后完成的。

"C:\Program Files\Java\jdk1.8.0_45\bin\java" -Xmx8G -Xss1G -Didea.launcher.port=7533 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 2016.1.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_45\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_45\jre\lib\rt.jar;C:\Users\Tomasz_Mielczarski\IdeaProjects\untitled\out\production\untitled;C:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 2016.1.3\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain ThreadedSortingComparsion

array size:     1000000
2016-07-15 13:45:22
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.45-b02 mixed mode):

"pool-1-thread-4" #15 prio=5 os_prio=0 tid=0x00000000696bd000 nid=0x560 runnable [0x00000002fffee000]
   java.lang.Thread.State: RUNNABLE
    at ThreadedSortingComparsion.mergeSort(ThreadedSortingComparsion.java:77)
    at ThreadedSortingComparsion.mergeSort(ThreadedSortingComparsion.java:78)
    at ThreadedSortingComparsion.mergeSort(ThreadedSortingComparsion.java:78)
    at ThreadedSortingComparsion.mergeSort(ThreadedSortingComparsion.java:78)
    at ThreadedSortingComparsion.mergeSort(ThreadedSortingComparsion.java:77)
    at ThreadedSortingComparsion$$Lambda$5/81628611.sort(Unknown Source)
    at ThreadedSortingComparsion.lambda$measureTime$0(ThreadedSortingComparsion.java:21)
    at ThreadedSortingComparsion$$Lambda$2/1023892928.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

"pool-1-thread-3" #14 prio=5 os_prio=0 tid=0x00000000696bb800 nid=0x2634 runnable [0x00000002bffee000]
   java.lang.Thread.State: RUNNABLE
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:123)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:126)
    at ThreadedSortingComparsion.quickSort(ThreadedSortingComparsion.java:105)
    at ThreadedSortingComparsion$$Lambda$4/1989780873.sort(Unknown Source)
    at ThreadedSortingComparsion.lambda$measureTime$0(ThreadedSortingComparsion.java:21)
    at ThreadedSortingComparsion$$Lambda$2/1023892928.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

"pool-1-thread-2" #13 prio=5 os_prio=0 tid=0x00000000696b7800 nid=0x1c70 waiting on condition [0x000000027ffef000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x0000000719d72480> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

"pool-1-thread-1" #12 prio=5 os_prio=0 tid=0x00000000696b6800 nid=0x478 runnable [0x000000023ffee000]
   java.lang.Thread.State: RUNNABLE
    at ThreadedSortingComparsion.bubbleSort(ThreadedSortingComparsion.java:139)
    at ThreadedSortingComparsion$$Lambda$1/990368553.sort(Unknown Source)
    at ThreadedSortingComparsion.lambda$measureTime$0(ThreadedSortingComparsion.java:21)
    at ThreadedSortingComparsion$$Lambda$2/1023892928.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

"Monitor Ctrl-Break" #11 daemon prio=5 os_prio=0 tid=0x0000000068d3d000 nid=0x2f3c runnable [0x00000001fffee000]
   java.lang.Thread.State: RUNNABLE
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
    at java.net.SocketInputStream.read(SocketInputStream.java:170)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
    at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
    at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
    - locked <0x00000007156892b8> (a java.io.InputStreamReader)
    at java.io.InputStreamReader.read(InputStreamReader.java:184)
    at java.io.BufferedReader.fill(BufferedReader.java:161)
    at java.io.BufferedReader.readLine(BufferedReader.java:324)
    - locked <0x00000007156892b8> (a java.io.InputStreamReader)
    at java.io.BufferedReader.readLine(BufferedReader.java:389)
    at com.intellij.rt.execution.application.AppMain$1.run(AppMain.java:93)
    at java.lang.Thread.run(Thread.java:745)

"Service Thread" #10 daemon prio=9 os_prio=0 tid=0x0000000068c81000 nid=0x2d6c runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread3" #9 daemon prio=9 os_prio=2 tid=0x0000000068bea800 nid=0x1ad0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread2" #8 daemon prio=9 os_prio=2 tid=0x0000000068be4000 nid=0x17d0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" #7 daemon prio=9 os_prio=2 tid=0x0000000068bdd800 nid=0x3238 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" #6 daemon prio=9 os_prio=2 tid=0x0000000068bda000 nid=0x1824 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x0000000068bd8800 nid=0x910 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x0000000068bd7800 nid=0x31f8 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x0000000043229800 nid=0x2810 in Object.wait() [0x00000000fffee000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x0000000719d707e0> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
    - locked <0x0000000719d707e0> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000043223000 nid=0xd48 in Object.wait() [0x00000000bffef000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x0000000719d78370> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:502)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:157)
    - locked <0x0000000719d78370> (a java.lang.ref.Reference$Lock)

"main" #1 prio=5 os_prio=0 tid=0x000000000311d800 nid=0x2ed0 waiting on condition [0x000000004311e000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x0000000719d58fe0> (a java.util.concurrent.FutureTask)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:429)
    at java.util.concurrent.FutureTask.get(FutureTask.java:191)
    at ThreadedSortingComparsion.start(ThreadedSortingComparsion.java:48)
    at ThreadedSortingComparsion.main(ThreadedSortingComparsion.java:162)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

"VM Thread" os_prio=2 tid=0x0000000056348800 nid=0x2984 runnable

"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000043147000 nid=0x27e0 runnable

"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x0000000043148800 nid=0x20b4 runnable

"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x000000004314a000 nid=0x1da4 runnable

"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x000000004314c000 nid=0x29e0 runnable

"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x000000004314e000 nid=0xa04 runnable

"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000043150000 nid=0x14b8 runnable

"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x0000000043153800 nid=0xf00 runnable

"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x0000000043154800 nid=0x243c runnable

"VM Periodic Task Thread" os_prio=2 tid=0x0000000068c82000 nid=0x22d8 waiting on condition

JNI global references: 224

Heap
 PSYoungGen      total 76288K, used 13755K [0x0000000715580000, 0x000000071aa80000, 0x00000007c0000000)
  eden space 65536K, 4% used [0x0000000715580000,0x0000000715874910,0x0000000719580000)
  from space 10752K, 99% used [0x0000000719580000,0x0000000719ffa348,0x000000071a000000)
  to   space 10752K, 0% used [0x000000071a000000,0x000000071a000000,0x000000071aa80000)
 ParOldGen       total 175104K, used 33211K [0x00000005c0000000, 0x00000005cab00000, 0x0000000715580000)
  object space 175104K, 18% used [0x00000005c0000000,0x00000005c206ed30,0x00000005cab00000)
 Metaspace       used 4277K, capacity 4790K, committed 4992K, reserved 1056768K
  class space    used 484K, capacity 535K, committed 640K, reserved 1048576K

quick sort:     355.579434803s
merge sort:     355.629940032s
insertion sort: 355.532578023s

问题答案：

TL; DR HotSpot JIT优化消除了计数循环中的安全点检查，这是一个恶作剧。

这是一个非常有趣的问题：一个简单的Java测试揭示了JVM内部的一个重要问题。

没有立即出现线程转储的事实表明该问题不在Java代码中，而是与JVM相关。线程转储在安全点打印。延迟意味着VM无法在短时间内到达安全点。

背景

当没有Java线程在运行时，某些VM操作（GC，Deoptimization，Thread
dump和某些其他操作）在世界停顿时执行。但是Java线程无法在任意点停止，它们只能在称为安全
点的某些位置暂停。在JIT编译的代码中，安全点通常放置在方法出口和后向分支（即循环内部）中。

就性能而言，安全点检查相对便宜，但并非免费。这就是为什么JIT编译器试图在可能的情况下减少安全点的数量。一种这样的优化是消除计数循环中的安全点检查，即已知具有有限次数的迭代的具有整数计数器的循环。

验证理论

让我们回到测试中，检查是否及时到达安全点。

添加-XX:+SafepointTimeout -XX:SafepointTimeoutDelay=1000JVM选项。每当VM在1000毫秒内未达到安全点时，这应该会打印一条调试消息。

# SafepointSynchronize::begin: Timeout detected:
# SafepointSynchronize::begin: Timed out while spinning to reach a safepoint.
# SafepointSynchronize::begin: Threads which did not reach the safepoint:
# "pool-1-thread-2" #12 prio=5 os_prio=0 tid=0x0000000019004800 nid=0x1480 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

# SafepointSynchronize::begin: (End of list)

是的，它打印出线程pool-1-thread-2无法在1000毫秒内停止。这是执行程序池的第二个线程，应运行insertionSort算法。

insertionSort有两个非常长的嵌套计数循环，看起来JVM消除了其中的安全点检查。因此，如果此方法以编译模式运行，则JVM在该方法完成之前无法停止它。如果在方法运行时请求世界暂停，则所有其他线程也将等待。

该怎么办？

这个问题早已为人所知。这是相关的JVM错误：JDK-5014723。这不是一个高度优先的问题，因为该问题很少出现在现实应用中。

JDK 8u92中出现了一个新的JVM标志，以解决此问题。
-XX:+UseCountedLoopSafepoints总是将安全点检查放在循环内。

另一种解决方案是通过修改循环中的计数器变量将长计数循环转换为通用循环。

例如，如果您替换为问题if (breaker) break;，if (breaker) j = 0;该问题也将消失。

那为什么它在调试模式下工作呢？

在启动调试器的情况下启动JVM时，将禁用某些JIT优化，以使调试信息可用。在这种情况下，编译后的代码具有所有安全点检查。

Java代码执行在没有断点和正常运行的调试中产生不同的结果。ExecutorService是否损坏？

背景

验证理论

该怎么办？

那为什么它在调试模式下工作呢？

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