jvm内存管理之java堆溢出 -实例分析

java堆用于存储对象实例，我们只要不断地创建对象，并且保证GC Roots到对象之间有可达路径来避免垃圾回收机制清楚这些对象，就会在对象数量达到最大堆的容量限制后产生内存溢出异常。

下面给出例子代码体验一下java堆溢出的过程：

1.通过jvm参数（-Xms20M -Xmx20M -Xmn10M ），限制java 堆的大小

2.通过jvm参数（-verbose:gc -XX:+PrintGCDetails ）打印GC的过程信息

3.通过jvm参数（-XX:+HeapDumpOnOutOfMemoryError），可以让虚拟机在出现内存溢出异常时Dump出当前的内存堆转储快照。

package com.jaynol.jvm.oom.sample;

import java.util.ArrayList;
import java.util.List;

/**
 * VM Args:
 *  -verbose:gc -XX:+PrintGCDetails
 *  -Xms20M -Xmx20M -Xmn10M -XX:+HeapDumpOnOutOfMemoryError
 * @author Johnny
 *
 */
public class HeapOOM {

    static class OOMObject{}

    /**
     * @param args
     */
    public static void main(String[] args) {
        List<OOMObject> list = new ArrayList<OOMObject>();

        while(true){
            list.add(new OOMObject());
        }
    }

}

运行结果：

[GC [DefNew: 7669K->1024K(9216K), 0.0146870 secs] 7669K->4602K(19456K), 0.0147209 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 
[GC [DefNew: 9216K->1024K(9216K), 0.0200416 secs] 12794K->10520K(19456K), 0.0200646 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
[GC [DefNew: 9216K->9216K(9216K), 0.0000159 secs][Tenured: 9496K->5712K(10240K), 0.0564110 secs] 18712K->14138K(19456K), [Perm : 166K->166K(12288K)], 0.0564723 secs] [Times: user=0.06 sys=0.00, real=0.06 secs] 
[Full GC [Tenured: 5712K->5712K(10240K), 0.0396698 secs] 14625K->14625K(19456K), [Perm : 166K->166K(12288K)], 0.0396968 secs] [Times: user=0.05 sys=0.00, real=0.04 secs] 
[Full GC [Tenured: 5712K->5700K(10240K), 0.0441324 secs] 14625K->14612K(19456K), [Perm : 166K->166K(12288K)], 0.0441718 secs] [Times: user=0.05 sys=0.00, real=0.04 secs] 
java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid1340.hprof ...
Heap dump file created [29756494 bytes in 0.462 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Unknown Source)
at java.util.Arrays.copyOf(Unknown Source)
at java.util.ArrayList.grow(Unknown Source)
at java.util.ArrayList.ensureCapacityInternal(Unknown Source)
at java.util.ArrayList.add(Unknown Source)
at com.jaynol.jvm.oom.sample.HeapOOM.main(HeapOOM.java:24)
Heap
 def new generation   total 9216K, used 8920K [0x33210000, 0x33c10000, 0x33c10000)
  eden space 8192K, 100% used [0x33210000, 0x33a10000, 0x33a10000)
  from space 1024K,  71% used [0x33a10000, 0x33ac6038, 0x33b10000)
  to   space 1024K,   0% used [0x33b10000, 0x33b10000, 0x33c10000)
 tenured generation   total 10240K, used 5700K [0x33c10000, 0x34610000, 0x34610000)
   the space 10240K,  55% used [0x33c10000, 0x341a10b8, 0x341a1200, 0x34610000)
 compacting perm gen  total 12288K, used 168K [0x34610000, 0x35210000, 0x38610000)
   the space 12288K,   1% used [0x34610000, 0x3463a390, 0x3463a400, 0x35210000)
    ro space 10240K,  42% used [0x38610000, 0x38a4f150, 0x38a4f200, 0x39010000)
    rw space 12288K,  54% used [0x39010000, 0x3968fad8, 0x3968fc00, 0x39c10000)

从运行结果可以看出，经过JVM的3次Minor GC，再经过两次full GC后已经无法再GC了，空间已经无法再扩展了，最终导致了OutOfMemoryError