17-21堆（線程共享）

17_堆_定義

定義： Heap，通過new關鍵字創建的對象，都存放在堆內存中。

特點：
線程共享，堆中的對象都存在線程安全的問題
垃圾回收，垃圾回收機制重點區域。

18_堆_內存溢出

一個簡單的例子 java.lang.OutOfMemeryError:Java Heap spac
-Xmx8m

19_堆_內存診斷_jmap

1、jps
查看系統有哪些進程。
2、jmap
查看堆內存使用情況 jmap -heap PID
3、jconsole
圖形界面，多功能檢測工具，連續監測。

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沒有引用，可以被回收了
        System.gc();
        System.out.println("3...");
        Thread.sleep(1000000L);
    }
}

運行上面代碼，首先得到Demo進程PID

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

後每次輸出後，jmap -heap PID得到如下內容：
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_內存診斷_jconsole

21_堆_內存診斷_jvirsualvm

案例：垃圾回收後，內存佔用依然很高
1.控制檯輸入jvistualvm，在1處選擇對應進程，2處點“dump”。
2.點擊“查找”，點擊第一條佔用內存最大的記錄。
3.找到問題所在，list中有過多大對象student，無法被清除。

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];
}

解決方法: 去官網下載即可, 我是win10系統, 官網鏈接, 下載後, 解壓, 打開bin目錄下的visualvm.exe即可使用
在這裏插入圖片描述

生成快照dump日誌方法

1、獲取JVM的dump文件的兩種方式

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

2、
jmap -dump:format=b,file=xxx.hprof
這會讓VM接到dump請求後直接把Java heap裏所有對象都dump下來，無論對象是死是活

3、
jmap -dump:format=b,live,file=xxx.hprof
這種則讓VM在接到dump請求時先做一次full GC，然後再dump；所以已經死掉的對象剛被GC清理掉，就不會出現在dump裏了。

第一種方式是一種事後方式，需要等待當前JVM出現問題後才能生成dmp文件，實時性不高
第二種方式在執行時，JVM是暫停服務的，所以對線上的運行會產生影響。所以建議第一種方式。

4、查看項目啓動參數：
ps aux|grep pid