17-21堆（线程共享）

17_堆_定义

定义： Heap，通过new关键字创建的对象，都存放在堆内存中。

特点：
线程共享，堆中的对象都存在线程安全的问题
垃圾回收，垃圾回收机制重点区域。

18_堆_内存溢出

一个简单的例子 java.lang.OutOfMemeryError:Java Heap spac
-Xmx8m

19_堆_内存诊断_jmap

1、jps
查看系统有哪些进程。
2、jmap
查看堆内存使用情况 jmap -heap PID
3、jconsole
图形界面，多功能检测工具，连续监测。

public class Demo1 {
    public static  void main(String[] args) throws InterruptedException {
        System.out.println("1....");
        Thread.sleep(30000);
        byte[] array =  new byte[1024 * 1024 * 10];//10M
        System.out.println("2....");
        Thread.sleep(30000);
        array = null;//array没有引用，可以被回收了
        System.gc();
        System.out.println("3...");
        Thread.sleep(1000000L);
    }
}

运行上面代码，首先得到Demo进程PID

D:\openSourceProject\jvm1>jps
8916 Launcher
9876 RemoteMavenServer36
11656
13976 Demo1
13756 Jps

后每次输出后，jmap -heap PID得到如下内容：
1、

D:\openSourceProject\jvm1>jmap -heap 13976
Attaching to process ID 13976, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.221-b11

using thread-local object allocation.
Parallel GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4276092928 (4078.0MB)
   NewSize                  = 89128960 (85.0MB)
   MaxNewSize               = 1425014784 (1359.0MB)
   OldSize                  = 179306496 (171.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 67108864 (64.0MB)
   used     = 6711104 (6.40020751953125MB)
   free     = 60397760 (57.59979248046875MB)
   10.000324249267578% used
From Space:
   capacity = 11010048 (10.5MB)
   used     = 0 (0.0MB)
   free     = 11010048 (10.5MB)
   0.0% used
To Space:
   capacity = 11010048 (10.5MB)
   used     = 0 (0.0MB)
   free     = 11010048 (10.5MB)
   0.0% used
PS Old Generation
   capacity = 179306496 (171.0MB)
   used     = 0 (0.0MB)
   free     = 179306496 (171.0MB)
   0.0% used

3175 interned Strings occupying 260400 bytes.

2、

D:\openSourceProject\jvm1>jmap -heap 13976
Attaching to process ID 13976, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.221-b11

using thread-local object allocation.
Parallel GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4276092928 (4078.0MB)
   NewSize                  = 89128960 (85.0MB)
   MaxNewSize               = 1425014784 (1359.0MB)
   OldSize                  = 179306496 (171.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 67108864 (64.0MB)
   used     = 17196880 (16.400222778320312MB)
   free     = 49911984 (47.59977722167969MB)
   25.62534809112549% used
From Space:
   capacity = 11010048 (10.5MB)
   used     = 0 (0.0MB)
   free     = 11010048 (10.5MB)
   0.0% used
To Space:
   capacity = 11010048 (10.5MB)
   used     = 0 (0.0MB)
   free     = 11010048 (10.5MB)
   0.0% used
PS Old Generation
   capacity = 179306496 (171.0MB)
   used     = 0 (0.0MB)
   free     = 179306496 (171.0MB)
   0.0% used

3176 interned Strings occupying 260456 bytes.

3、

D:\openSourceProject\jvm1>jmap -heap 13976
Attaching to process ID 13976, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.221-b11

using thread-local object allocation.
Parallel GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4276092928 (4078.0MB)
   NewSize                  = 89128960 (85.0MB)
   MaxNewSize               = 1425014784 (1359.0MB)
   OldSize                  = 179306496 (171.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 67108864 (64.0MB)
   used     = 1342200 (1.2800216674804688MB)
   free     = 65766664 (62.71997833251953MB)
   2.0000338554382324% used
From Space:
   capacity = 11010048 (10.5MB)
   used     = 0 (0.0MB)
   free     = 11010048 (10.5MB)
   0.0% used
To Space:
   capacity = 11010048 (10.5MB)
   used     = 0 (0.0MB)
   free     = 11010048 (10.5MB)
   0.0% used
PS Old Generation
   capacity = 179306496 (171.0MB)
   used     = 1106008 (1.0547714233398438MB)
   free     = 178200488 (169.94522857666016MB)
   0.6168253937659905% used

3162 interned Strings occupying 259464 bytes.

20_堆_内存诊断_jconsole

21_堆_内存诊断_jvirsualvm

案例：垃圾回收后，内存占用依然很高
1.控制台输入jvistualvm，在1处选择对应进程，2处点“dump”。
2.点击“查找”，点击第一条占用内存最大的记录。
3.找到问题所在，list中有过多大对象student，无法被清除。

public class Demo2 {
    public static  void main(String[] args) throws InterruptedException {
        List<student> list = new ArrayList<>();
        for (int i = 0; i < 200;i++){
            list.add(new student());
        }
        Thread.sleep(10000000000L);
    }
}

class student{
    private byte[] big = new byte[1024 * 1024];
}

解决方法: 去官网下载即可, 我是win10系统, 官网链接, 下载后, 解压, 打开bin目录下的visualvm.exe即可使用
在这里插入图片描述

生成快照dump日志方法

1、获取JVM的dump文件的两种方式

#出现 OOME 时生成堆 dump: 
-XX:+HeapDumpOnOutOfMemoryError
#生成堆文件地址：
-XX:HeapDumpPath=/home/liuke/jvmlogs/

2、
jmap -dump:format=b,file=xxx.hprof
这会让VM接到dump请求后直接把Java heap里所有对象都dump下来，无论对象是死是活

3、
jmap -dump:format=b,live,file=xxx.hprof
这种则让VM在接到dump请求时先做一次full GC，然后再dump；所以已经死掉的对象刚被GC清理掉，就不会出现在dump里了。

第一种方式是一种事后方式，需要等待当前JVM出现问题后才能生成dmp文件，实时性不高
第二种方式在执行时，JVM是暂停服务的，所以对线上的运行会产生影响。所以建议第一种方式。

4、查看项目启动参数：
ps aux|grep pid