jvm优化之：OOM（out of memory）内存溢出

内存溢出

 注意内存溢出不是内存泄漏！！这里主要是介绍如何用jdk自带的jmap工具导出进程堆空间快照。

内存溢出：
Out Of Memory，是指申请的堆内存空间不够用了，比如：你申请了10M空间，但是你要放12M的东西进去；
内存泄漏：
Memory Leak，是指你的堆空间一直有对象不能被GC清理掉，但是你还要放对象进去，还在向堆申请放对象的空间，此时就会报 Memory Leak。
这里主要演示内存溢出 ：模拟代码

package com.distributed.lock.utils;import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;/**1. -Xms30m -Xmx=30m -XX:SurvivorRatio=8 -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=d:\99.hprof2. 模拟OOM*/
public class OOMTest {public static void main(String[] args) {System.out.println("演示开始:>>>>>>>>>>>>>>>>>>>>>>>>>>>");//先向堆空间申请一块内存区域，区域的大小默认是ArrayList的长度List<int[]> arr = new ArrayList<>(10);for (int i = 0; i < 10000; i++) {//不断的新创建对象，对象大小为1Mint[] ints = new int[1024*1024];//一直往ArrayList集合添加arr.add(ints);try {TimeUnit.MILLISECONDS.sleep(100);} catch (Exception e) {e.printStackTrace();}}System.out.println("演示结束:>>>>>>>>>>>>>>>>>>>>>>>>>>>");}
}

异常信息 ：
导出堆空间快照（dump）:
先看jmap语法规则：

C:\Windows\system32>jmap -help
Usage: //语法规则，使用方式jmap [option] <pid>(to connect to running process)jmap [option] <executable <core>(to connect to a core file)jmap [option] [server_id@]<remote server IP or hostname>(to connect to remote debug server)where <option> is one of: //参数选项介绍<none>               to print same info as Solaris pmap-heap                to print java heap summary-histo[:live]        to print histogram of java object heap; if the "live" suboption is specified, only count live objects //只是当前时间节点活着的对象-clstats             to print class loader statistics-finalizerinfo       to print information on objects awaiting finalization-dump:<dump-options> to dump java heap in hprof binary formatdump-options:live         dump only live objects; if not specified,all objects in the heap are dumped.format=b     binary formatfile=<file>  dump heap to <file>Example: jmap -dump:live,format=b,file=heap.bin <pid>-F                   force. Use with -dump:<dump-options> <pid> or -histoto force a heap dump or histogram when <pid> does notrespond. The "live" suboption is not supportedin this mode.-h | -help           to print this help message-J<flag>             to pass <flag> directly to the runtime system

将堆空间信息以标准格式导出到文件：

jmap -dump:format=b,file=d:\7.hprof 14876

将堆空间存活对象的信息以标准格式导出到文件：

jmap -dump:live,format=b,file=d:\7.hprof 14876

显示当前时间节点堆内存信息：

jmap -heap 14876 >d:\a.txt  // 17924 是进程号，输出到d盘的a.txt文件

直接控制台查看堆空间信息：

C:\Windows\system32>jmap -heap 14876
Attaching to process ID 14876, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.291-b10using thread-local object allocation.
Parallel GC with 8 thread(s)Heap Configuration: //堆空间的配置信息MinHeapFreeRatio         = 0MaxHeapFreeRatio         = 100MaxHeapSize              = 8550088704 (8154.0MB)NewSize                  = 178257920 (170.0MB)MaxNewSize               = 2850029568 (2718.0MB)OldSize                  = 356515840 (340.0MB)NewRatio                 = 2SurvivorRatio            = 8MetaspaceSize            = 21807104 (20.796875MB)CompressedClassSpaceSize = 1073741824 (1024.0MB)MaxMetaspaceSize         = 17592186044415 MBG1HeapRegionSize         = 0 (0.0MB)Heap Usage: //堆空间各区使用状况
PS Young Generation
Eden Space://Eden区capacity = 134217728 (128.0MB) used     = 123984136 (118.24048614501953MB)free     = 10233592 (9.759513854980469MB)92.37537980079651% used
From Space: //Survivor from区，或者：S0capacity = 22020096 (21.0MB)used     = 0 (0.0MB)free     = 22020096 (21.0MB)0.0% used
To Space: //Survivor to区，或者：S1capacity = 22020096 (21.0MB)used     = 0 (0.0MB)free     = 22020096 (21.0MB)0.0% used
PS Old Generation //old区，或者：老年代capacity = 356515840 (340.0MB)used     = 0 (0.0MB)free     = 356515840 (340.0MB)0.0% used3196 interned Strings occupying 261920 bytes.使用的：ParallelGC垃圾收集器