释放线程时,Java会从堆中泄漏内存

Question

我有一个小应用程序,分配大约25000个线程,然后释放它们.当线程被释放时,即使在所有线程退出之后,应用程序的内存消耗也会上升并保持高水平.

top看起来像这样:

 PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
9133 root      20   0 22.601g 8.612g  12080 S   0.0  9.1   1:18.61 java

而jmap -heap看起来像这样:

Attaching to process ID 9133, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.66-b17

using thread-local object allocation.
Parallel GC with 18 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 1073741824 (1024.0MB)
   NewSize                  = 104857600 (100.0MB)
   MaxNewSize               = 104857600 (100.0MB)
   OldSize                  = 968884224 (924.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 1073741824 (1024.0MB)
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 78643200 (75.0MB)
   used     = 1572864 (1.5MB)
   free     = 77070336 (73.5MB)
   2.0% used
From Space:
   capacity = 13107200 (12.5MB)
   used     = 0 (0.0MB)
   free     = 13107200 (12.5MB)
   0.0% used
To Space:
   capacity = 13107200 (12.5MB)
   used     = 0 (0.0MB)
   free     = 13107200 (12.5MB)
   0.0% used
PS Old Generation
   capacity = 968884224 (924.0MB)
   used     = 1264416 (1.205841064453125MB)
   free     = 967619808 (922.7941589355469MB)
   0.13050227970271916% used

808 interned Strings occupying 54648 bytes.

据我所见,jmap报告中没有任何内容可以解释top报告的8.612g.

Java版本是oracle 1.8.0_66

该应用程序在Red Hat Enterprise Linux Server 7.1(Maipo)上运行.

该应用程序的代码如下:

import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;

public class WaitTest {
    static AtomicInteger ai = new AtomicInteger(0);

    public static void main(String[] args) {
        List<WaitingThread> threads = new LinkedList<>();
        while (true) {
            System.out.println("number of threads: " + ai.get());
            String s = System.console().readLine();
            if (s == null)
                System.exit(0);
            s = s.trim();
            if (s.isEmpty())
                continue;
            char command = s.charAt(0);
            if (command != '+' && command != '-') {
                System.out.println("+ or - please");
                continue;
            }
            String num = s.substring(1);
            int iNum;
            try {
                iNum = Integer.parseInt(num.trim());
            } catch (Exception ex) {
                System.out.println("valid number please");
                continue;
            }
            if (command == '+') {
                for (int i = 0; i < iNum; i++) {
                    WaitingThread t = new WaitingThread();
                    t.start();
                    threads.add(t);
                }
            }
            if (command == '-') {
                Set<WaitingThread> threadsToJoin = new HashSet<>();
                for (Iterator<WaitingThread> it = threads.iterator(); it.hasNext(); ) {
                    if (iNum > 0) {
                        WaitingThread t = it.next();
                        threadsToJoin.add(t);

                        synchronized (t.lock) {
                            t.lock.notify();
                        }

                        it.remove();
                        iNum--;
                    } else
                        break;

                }
                for (WaitingThread t : threadsToJoin)
                    try {
                        t.join();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
            }
            System.gc();
        }
    }

    static class WaitingThread extends Thread {
        public final Object lock = new Object();

        @Override
        public void run() {
            ai.incrementAndGet();
            try {
                deepStack(200);
            } catch (InterruptedException ex) {
            } catch (Exception ex) {
                System.out.println("exception in thread " + Thread.currentThread().getName() + ", " + ex.getMessage());
            } finally {
                ai.decrementAndGet();
            }
        }

        private void deepStack(int depth) throws InterruptedException {
            if (depth == 0) {
                synchronized (lock) {
                    lock.wait();
                }

            } else
                deepStack(depth - 1);
        }
    }

}

这是pmap输出的链接:

这是jmap -dump(hprof)的链接

Answer 1

难道所有线程首先都是在列表变量中的命令“+”期间创建的，'threads'而列表变量永远不会超出范围吗？这意味着链表保存了对所有线程的引用，并且在Main()退出之前不会被垃圾收集。此外，LinkedList 本身也可能泄漏内存。即使这不是问题，您也应该考虑使用 ArrayList 而不是 LinkedList。

另外，这里还有一个关于查找 Java 内存泄漏的问题。