尽管使用了WeakHashMap，但仍发生OutOfMemoryException [英] OutOfMemoryException despite using WeakHashMap

问题描述

如果不调用 System.gc()，则系统将引发OutOfMemoryException.我不知道为什么我需要显式调用 System.gc()；JVM应该自己调用 gc()，对吗?请告知.

If do not call System.gc(), the system will throw an OutOfMemoryException. I do not know why I need to call System.gc() explicitly; the JVM should call gc() itself, right? Please advise.

以下是我的测试代码:

public static void main(String[] args) throws InterruptedException {
    WeakHashMap<String, int[]> hm = new WeakHashMap<>();
    int i  = 0;
    while(true) {
        Thread.sleep(1000);
        i++;
        String key = new String(new Integer(i).toString());
        System.out.println(String.format("add new element %d", i));
        hm.put(key, new int[1024 * 10000]);
        key = null;
        //System.gc();
    }
}

如下所示，添加 -XX:+ PrintGCDetails 以打印出GC信息；如您所见，实际上，JVM尝试执行完整的GC运行，但是失败了；我仍然不知道原因.很奇怪，如果我取消注释 System.gc(); 行，结果是肯定的:

As following, add -XX:+PrintGCDetails to print out the GC info; as you see, actually, the JVM tries to do a full GC run, but fails; I still do not know the reason. It is very strange that if I uncomment the System.gc(); line, the result is positive:

add new element 1
add new element 2
add new element 3
add new element 4
add new element 5
[GC (Allocation Failure) --[PSYoungGen: 48344K->48344K(59904K)] 168344K->168352K(196608K), 0.0090913 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 
[Full GC (Ergonomics) [PSYoungGen: 48344K->41377K(59904K)] [ParOldGen: 120008K->120002K(136704K)] 168352K->161380K(196608K), [Metaspace: 5382K->5382K(1056768K)], 0.0380767 secs] [Times: user=0.09 sys=0.03, real=0.04 secs] 
[GC (Allocation Failure) --[PSYoungGen: 41377K->41377K(59904K)] 161380K->161380K(196608K), 0.0040596 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (Allocation Failure) [PSYoungGen: 41377K->41314K(59904K)] [ParOldGen: 120002K->120002K(136704K)] 161380K->161317K(196608K), [Metaspace: 5382K->5378K(1056768K)], 0.0118884 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at test.DeadLock.main(DeadLock.java:23)
Heap
 PSYoungGen      total 59904K, used 42866K [0x00000000fbd80000, 0x0000000100000000, 0x0000000100000000)
  eden space 51712K, 82% used [0x00000000fbd80000,0x00000000fe75c870,0x00000000ff000000)
  from space 8192K, 0% used [0x00000000ff800000,0x00000000ff800000,0x0000000100000000)
  to   space 8192K, 0% used [0x00000000ff000000,0x00000000ff000000,0x00000000ff800000)
 ParOldGen       total 136704K, used 120002K [0x00000000f3800000, 0x00000000fbd80000, 0x00000000fbd80000)
  object space 136704K, 87% used [0x00000000f3800000,0x00000000fad30b90,0x00000000fbd80000)
 Metaspace       used 5409K, capacity 5590K, committed 5760K, reserved 1056768K
  class space    used 576K, capacity 626K, committed 640K, reserved 1048576K

推荐答案

JVM将自行调用GC，但是在这种情况下，它太少了太晚了.在这种情况下，清除内存的不仅是GC.映射值很容易达到，并且在对映射值进行某些操作时会被映射本身清除.

JVM will call GC on it's own, but in this case it will be too little too late. It's not only GC who is responsible in clearing memory in this case. Map values are strongly reachable and are cleared by map itself when certain operations are invoked on it.

如果您打开GC事件(XX:+ PrintGC)，则为以下输出:

Here's the output if you turn on GC events (XX:+PrintGC):

add new element 1
add new element 2
add new element 3
add new element 4
add new element 5
add new element 6
add new element 7
[GC (Allocation Failure)  2407753K->2400920K(2801664K), 0.0123285 secs]
[GC (Allocation Failure)  2400920K->2400856K(2801664K), 0.0090720 secs]
[Full GC (Allocation Failure)  2400856K->2400805K(2590720K), 0.0302800 secs]
[GC (Allocation Failure)  2400805K->2400805K(2801664K), 0.0069942 secs]
[Full GC (Allocation Failure)  2400805K->2400753K(2620928K), 0.0146932 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

GC直到最后一次尝试将值放入映射中时才被触发.

GC is not triggered up until the last attempt to put value into the map.

WeakHashMap在映射键出现在参考队列上之前无法清除陈旧的条目.映射键只有在被垃圾回收后才会出现在参考队列上.在地图有机会自行清除之前，会触发为新地图值分配内存的操作.当内存分配失败并触发GC时，将收集映射键.但这太少了，太晚了-没有足够的内存被释放来分配新的映射值.如果减少有效负载，则可能会获得足够的内存来分配新的映射值，并且过时的条目将被删除.

WeakHashMap can't clear stale entries until map keys occur on a reference queue. And the map keys do not occur on a reference queue until they are garbage collected. Memory allocation for new map value is triggered before map has any chance to clear itself. When memory allocation fails and triggers GC, map keys do get collected. But it's too little too late - not enough memory has been freed to allocate new map value. If you reduce the payload, you probably end up with enough memory to allocate new map value and stale entries will get removed.

另一种解决方案是将值本身包装到WeakReference中.这将允许GC清除资源，而无需等待地图自行完成.这是输出:

Another solution could be wrapping values themselves into WeakReference. This will allow GC clear resources without waiting for map to do it on it's own. Here's the output:

add new element 1
add new element 2
add new element 3
add new element 4
add new element 5
add new element 6
add new element 7
[GC (Allocation Failure)  2407753K->2400920K(2801664K), 0.0133492 secs]
[GC (Allocation Failure)  2400920K->2400888K(2801664K), 0.0090964 secs]
[Full GC (Allocation Failure)  2400888K->806K(190976K), 0.1053405 secs]
add new element 8
add new element 9
add new element 10
add new element 11
add new element 12
add new element 13
[GC (Allocation Failure)  2402096K->2400902K(2801664K), 0.0108237 secs]
[GC (Allocation Failure)  2400902K->2400838K(2865664K), 0.0058837 secs]
[Full GC (Allocation Failure)  2400838K->1024K(255488K), 0.0863236 secs]
add new element 14
add new element 15
...
(and counting)

好多了.

这篇关于尽管使用了WeakHashMap，但仍发生OutOfMemoryException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！