查看JVM默认垃圾收集器

柔光的暖阳◎ 2022-11-26 09:45 236阅读 0赞

## 一、查看JVM默认垃圾收集器 ##

java -XX:+PrintCommandLineFlags -version

[root@jeespring ~]# java -XX:+PrintCommandLineFlags -version
    -XX:InitialHeapSize=132500800 -XX:MaxHeapSize=2120012800 -XX:+PrintCommandLineFlags 
    -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC
    java version "1.8.0_211"
    Java(TM) SE Runtime Environment (build 1.8.0_211-b12)
    Java HotSpot(TM) 64-Bit Server VM (build 25.211-b12, mixed mode)

垃圾收集相关的常用参数

<table> 
 <thead> 
  <tr> 
   <th>参数</th> 
   <th>描述</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>UseSerialGC</td> 
   <td>虚拟机运行再Client模式下的默认值，打开此开关后，使用Serial+Serial Old的收集器组合进行内存回收</td> 
  </tr> 
  <tr> 
   <td>UseParNewGC</td> 
   <td>打开此开关后，使用ParNew+Serial Old的收集器组合进行内存回收</td> 
  </tr> 
  <tr> 
   <td>UseConcMarkSweepGC</td> 
   <td>打开此开关后，使用ParNew+CMS+Serial Old的收集器组合进行内存</td> 
  </tr> 
  <tr> 
   <td>UseParallelGC</td> 
   <td>虚拟机运行在Server模式下的默认值，打开此开关后，使用Parallel Scavenge+Serial Old(PS MarkSweep)的收集器组合进行内存回收</td> 
  </tr> 
  <tr> 
   <td>UseParallelOldGC</td> 
   <td>打开此开关后，使用ParallelSeavenge+ParallelOld的收集器组合进行内存回收</td> 
  </tr> 
  <tr> 
   <td>SurvivorRatio</td> 
   <td>新生代中Eden区域与Survivor区域的容量比值，默认为8，代表Eden：Survivor=8：1</td> 
  </tr> 
  <tr> 
   <td>PretenureSizeThreshold</td> 
   <td>直接晋升到老年代的对象大小，设置这个参数后，大于这个參数的对象将直接在老年代分配</td> 
  </tr> 
  <tr> 
   <td>MaxTenuringThreshold</td> 
   <td>晋升到老年代的对象年龄。每个对象在坚持过一次MinorGC之后，年龄就增加1，当超过这个参数值时就进人老年代</td> 
  </tr> 
 </tbody> 
</table>

## 二、 命令**查看java垃圾收集器** ##

jmap -heap pid

输出结果中，可以查看当前线程使用垃圾收集器

using parallel threads in the new generation.
    
    using thread-local object allocation.
    
    Concurrent Mark-Sweep GC

输出结果

[root@jeespring ~]# jmap -heap 30620
    Attaching to process ID 30620, please wait...
    Debugger attached successfully.
    Server compiler detected.
    JVM version is 25.261-b12
    
    using thread-local object allocation.
    Mark Sweep Compact GC
    
    Heap Configuration:
       MinHeapFreeRatio         = 40
       MaxHeapFreeRatio         = 70
       MaxHeapSize              = 1048576000 (1000.0MB)
       NewSize                  = 10485760 (10.0MB)
       MaxNewSize               = 349503488 (333.3125MB)
       OldSize                  = 20971520 (20.0MB)
       NewRatio                 = 2
       SurvivorRatio            = 8
       MetaspaceSize            = 21807104 (20.796875MB)
       CompressedClassSpaceSize = 1073741824 (1024.0MB)
       MaxMetaspaceSize         = 17592186044415 MB
       G1HeapRegionSize         = 0 (0.0MB)
    
    Heap Usage:
    New Generation (Eden + 1 Survivor Space):
       capacity = 9437184 (9.0MB)
       used     = 3119576 (2.9750595092773438MB)
       free     = 6317608 (6.024940490722656MB)
       33.05621676974826% used
    Eden Space:
       capacity = 8388608 (8.0MB)
       used     = 2071008 (1.975067138671875MB)
       free     = 6317600 (6.024932861328125MB)
       24.688339233398438% used
    From Space:
       capacity = 1048576 (1.0MB)
       used     = 1048568 (0.9999923706054688MB)
       free     = 8 (7.62939453125E-6MB)
       99.99923706054688% used
    To Space:
       capacity = 1048576 (1.0MB)
       used     = 0 (0.0MB)
       free     = 1048576 (1.0MB)
       0.0% used
    tenured generation:
       capacity = 20971520 (20.0MB)
       used     = 4970272 (4.740020751953125MB)
       free     = 16001248 (15.259979248046875MB)
       23.700103759765625% used
    
    4606 interned Strings occupying 362992 bytes.

## 三、各个版本JDK默认的垃圾回收器 ##

jdk1.7 默认垃圾收集器Parallel Scavenge（新生代）+Parallel Old（老年代）

jdk1.8 默认垃圾收集器Parallel Scavenge（新生代）+Parallel Old（老年代）

jdk1.9 默认垃圾收集器G1