java 代码执行在没有断点的调试和正常 运行 中产生不同的结果。 ExecutorService 坏了吗？
java code execution yields to different results in debug without breakpoints and normal run. Is ExecutorService broken?
TL:DR ExecutorService executorService = Executors.newFixedThreadPool(8);
在调试中 运行s 并发，但在正常 运行 时间它开始并发，但稍后在单线程中 运行s。
我有一些代码可以在 ExecutorService 中启动 4 个不同的任务。其中两个任务应该几乎立即完成，另外两个应该 运行 一段时间。
那些任务 return 的执行时间 Future<Double>。
这段代码负责任务执行和测量：
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，在开始这些任务后，我按照前 运行 秒的执行时间顺序打印它们，输入大小相同。
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编码时，立即打印了2个结果，我必须等待一段时间才能得到第3个结果（我懒得等第4个）。
调试启动后（正确且符合预期）：
array size: 1000000
quick sort: 0.186892839s
merge sort: 0.291950604s
insertion sort: 344.534256723s
正常执行不一样，貌似在方法measure long start = System.nanoTime();执行了，然后thread睡着了，insertionSort完成后，quickSort又开始执行，输出：
array size: 1000000
quick sort: 345.893922141s
merge sort: 345.944023095s
insertion sort: 345.871908569s
这是错误的。从 newFixedThreadPool javadoc 开始，所有这些线程都应该同时 运行ning。
/**
* 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 有 java 版本 1.8.0_51
我在 1.8.0_91、1.8.0_92 上进行了测试。和 1.8.0_45。
为什么它在 ideone 上有效，但在我测试过的其他 2 台 PC 上却无效？
当我做thread dump的时候，虽然没有在debug中执行，然后我等了一会儿，在打印thread dump之后，也打印了结果。所以线程转储是在完成插入排序后完成的。
"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$$Lambda/81628611.sort(Unknown Source)
at ThreadedSortingComparsion.lambda$measureTime[=16=](ThreadedSortingComparsion.java:21)
at ThreadedSortingComparsion$$Lambda/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$$Lambda/1989780873.sort(Unknown Source)
at ThreadedSortingComparsion.lambda$measureTime[=16=](ThreadedSortingComparsion.java:21)
at ThreadedSortingComparsion$$Lambda/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$$Lambda/990368553.sort(Unknown Source)
at ThreadedSortingComparsion.lambda$measureTime[=16=](ThreadedSortingComparsion.java:21)
at ThreadedSortingComparsion$$Lambda/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.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
TL;DR HotSpot JIT optimization to eliminate safepoint checks in counted loops played a sick joke.
这是一个非常有趣的问题：一个简单的 Java 测试揭示了 JVM 深处的一个不平凡的问题。
线程转储没有立即出现的事实表明问题不在 Java 代码中，而是与 JVM 有某种关系。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
某些 VM 操作（GC、去优化、线程转储和 ）在没有 Java 线程 运行ning 时在 stop-the-world 暂停时执行。但是 Java 线程不能在任意点停止，它们只能在称为 safepoints 的某些地方暂停。在 JIT 编译代码中，安全点通常位于方法出口和后向分支，即内部循环。
安全点检查在性能方面相对便宜，但不是免费的。这就是 JIT 编译器尽可能减少安全点数量的原因。一种这样的优化是消除计数循环中的安全点检查，即具有已知迭代次数有限的整数计数器的循环。
验证理论
让我们回到我们的测试并检查是否及时到达安全点。
添加 -XX:+SafepointTimeout -XX:SafepointTimeoutDelay=1000 JVM 选项。每当 VM 未能在 1000 毫秒内到达安全点时，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Executor的pool的第二个线程，应该运行 insertionSort algorithm.
insertionSort 有两个很长的嵌套计数循环，看起来 JVM 已经消除了其中的安全点检查。因此，如果此方法 运行s 处于编译模式，则 JVM 无法在该方法完成之前停止它。如果在方法 运行ning 时请求停止世界暂停，所有其他线程也将等待。
怎么办？
这个问题早已为人所知。这是相关的 JVM 错误：JDK-5014723。这不是一个高优先级问题，因为这个问题在实际应用中很少出现。
JDK 8u92 中出现了一个新的 JVM 标记来解决这个问题。
-XX:+UseCountedLoopSafepoints 将始终将安全点检查放在循环中。
另一种解决方案是通过修改循环内的计数器变量将长计数循环转换为通用循环。
例如如果将 if (breaker) break; 替换为 if (breaker) j = 0;，问题也会消失。
为什么它在调试模式下工作？
当 JVM 在调试器打开的情况下启动时，一些 JIT 优化被禁用以使调试信息可用。在这种情况下，编译后的代码具有所有安全点检查。
TL:DR ExecutorService executorService = Executors.newFixedThreadPool(8);
在调试中 运行s 并发，但在正常 运行 时间它开始并发，但稍后在单线程中 运行s。
我有一些代码可以在 ExecutorService 中启动 4 个不同的任务。其中两个任务应该几乎立即完成，另外两个应该 运行 一段时间。
那些任务 return 的执行时间 Future<Double>。
这段代码负责任务执行和测量：
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，在开始这些任务后，我按照前 运行 秒的执行时间顺序打印它们，输入大小相同。
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编码时，立即打印了2个结果，我必须等待一段时间才能得到第3个结果（我懒得等第4个）。
调试启动后（正确且符合预期）：
array size: 1000000
quick sort: 0.186892839s
merge sort: 0.291950604s
insertion sort: 344.534256723s
正常执行不一样，貌似在方法measure long start = System.nanoTime();执行了，然后thread睡着了，insertionSort完成后，quickSort又开始执行，输出：
array size: 1000000
quick sort: 345.893922141s
merge sort: 345.944023095s
insertion sort: 345.871908569s
这是错误的。从 newFixedThreadPool javadoc 开始，所有这些线程都应该同时 运行ning。
/**
* 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 有 java 版本 1.8.0_51
我在 1.8.0_91、1.8.0_92 上进行了测试。和 1.8.0_45。
为什么它在 ideone 上有效，但在我测试过的其他 2 台 PC 上却无效？
当我做thread dump的时候，虽然没有在debug中执行，然后我等了一会儿，在打印thread dump之后，也打印了结果。所以线程转储是在完成插入排序后完成的。
"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$$Lambda/81628611.sort(Unknown Source)
at ThreadedSortingComparsion.lambda$measureTime[=16=](ThreadedSortingComparsion.java:21)
at ThreadedSortingComparsion$$Lambda/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$$Lambda/1989780873.sort(Unknown Source)
at ThreadedSortingComparsion.lambda$measureTime[=16=](ThreadedSortingComparsion.java:21)
at ThreadedSortingComparsion$$Lambda/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$$Lambda/990368553.sort(Unknown Source)
at ThreadedSortingComparsion.lambda$measureTime[=16=](ThreadedSortingComparsion.java:21)
at ThreadedSortingComparsion$$Lambda/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.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
TL;DR HotSpot JIT optimization to eliminate safepoint checks in counted loops played a sick joke.
这是一个非常有趣的问题：一个简单的 Java 测试揭示了 JVM 深处的一个不平凡的问题。
线程转储没有立即出现的事实表明问题不在 Java 代码中，而是与 JVM 有某种关系。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
某些 VM 操作（GC、去优化、线程转储和
安全点检查在性能方面相对便宜，但不是免费的。这就是 JIT 编译器尽可能减少安全点数量的原因。一种这样的优化是消除计数循环中的安全点检查，即具有已知迭代次数有限的整数计数器的循环。
验证理论
让我们回到我们的测试并检查是否及时到达安全点。
添加 -XX:+SafepointTimeout -XX:SafepointTimeoutDelay=1000 JVM 选项。每当 VM 未能在 1000 毫秒内到达安全点时，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Executor的pool的第二个线程，应该运行 insertionSort algorithm.
insertionSort 有两个很长的嵌套计数循环，看起来 JVM 已经消除了其中的安全点检查。因此，如果此方法 运行s 处于编译模式，则 JVM 无法在该方法完成之前停止它。如果在方法 运行ning 时请求停止世界暂停，所有其他线程也将等待。
怎么办？
这个问题早已为人所知。这是相关的 JVM 错误：JDK-5014723。这不是一个高优先级问题，因为这个问题在实际应用中很少出现。
JDK 8u92 中出现了一个新的 JVM 标记来解决这个问题。
-XX:+UseCountedLoopSafepoints 将始终将安全点检查放在循环中。
另一种解决方案是通过修改循环内的计数器变量将长计数循环转换为通用循环。
例如如果将 if (breaker) break; 替换为 if (breaker) j = 0;，问题也会消失。
为什么它在调试模式下工作？
当 JVM 在调试器打开的情况下启动时，一些 JIT 优化被禁用以使调试信息可用。在这种情况下，编译后的代码具有所有安全点检查。