查看JVM使用的什么垃圾收集器

一、方法一

打印虚拟机所有参数

[root@localhost ~]# java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=263475648 -XX:MaxHeapSize=4215610368 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (build 1.8.0_212-b04)
OpenJDK 64-Bit Server VM (build 25.212-b04, mixed mode)
[root@iZ2ze1224b1em0jij7qlssZ ~]# java -XX:+PrintFlagsFinal -version | grep :
     intx CICompilerCount                          := 2                                   {product}
    uintx InitialHeapSize                          := 264241152                           {product}
    uintx MaxHeapSize                              := 4217372672                          {product}
    uintx MaxNewSize                               := 1405616128                          {product}
    uintx MinHeapDeltaBytes                        := 524288                              {product}
    uintx NewSize                                  := 88080384                            {product}
    uintx OldSize                                  := 176160768                           {product}
     bool PrintFlagsFinal                          := true                                {product}
     bool UseCompressedClassPointers               := true                                {lp64_product}
     bool UseCompressedOops                        := true                                {lp64_product}
     bool UseParallelGC                            := true                                {product}
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (build 1.8.0_212-b04)
OpenJDK 64-Bit Server VM (build 25.212-b04, mixed mode)

第三列”=”表示第四列是参数的默认值，而”:=” 表明了参数被用户或者JVM赋值了。

二、方法二

[root@localhost ~]# java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=263475648 -XX:MaxHeapSize=4215610368 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
openjdk version "1.8.0_212"
OpenJDK Runtime Environment (build 1.8.0_212-b04)
OpenJDK 64-Bit Server VM (build 25.212-b04, mixed mode)

针对上述的-XX：UseParallelGC，这边我们引用《深入理解Java虚拟机：JVM高级特性与最佳实践》的介绍：

也就是说，打开此开关，使用的垃圾收集器是：新生代（Parallel Scavenge），老年代（Ps MarkSweep）组合。

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

三、方法三:

[root@localhost ~]# jmap -heap      25523 
Attaching to process ID 25523, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.212-b04

using thread-local object allocation.
Parallel GC with 2 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4217372672 (4022.0MB)
   NewSize                  = 88080384 (84.0MB)
   MaxNewSize               = 1405616128 (1340.5MB)
   OldSize                  = 176160768 (168.0MB)
   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 = 39321600 (37.5MB)
   used     = 3902632 (3.7218399047851562MB)
   free     = 35418968 (33.778160095214844MB)
   9.924906412760416% used
From Space:
   capacity = 524288 (0.5MB)
   used     = 98304 (0.09375MB)
   free     = 425984 (0.40625MB)
   18.75% used
To Space:
   capacity = 524288 (0.5MB)
   used     = 0 (0.0MB)
   free     = 524288 (0.5MB)
   0.0% used
PS Old Generation
   capacity = 121634816 (116.0MB)
   used     = 101585632 (96.87960815429688MB)
   free     = 20049184 (19.120391845703125MB)
   83.5169035812904% used

11612 interned Strings occupying 947640 bytes.

Concurrent Mark-Sweep GC:CMS回收器

Mark Sweep Compact GC:串行GC（Serial GC）

Parallel GC with 2 thread(s):并行GC（ParNew）

参考:

https://www.cnblogs.com/pc-boke/articles/10247068.html

https://blog.csdn.net/earthhour/article/details/76468084