一、设置垃圾收集器参数

我们设置JVM垃圾收集器参数时，一个参数就同时指定新生代老年代各自的垃圾收集器了。

-XX:+UseSerialGC，虚拟机运行在Client模式下的默认值，Serial+Serial Old。
-XX:+UseParNewGC，ParNew+Serial Old，在JDK1.8被废弃，在JDK1.7还可以使用。
-XX:+UseConcMarkSweepGC，ParNew+CMS+Serial Old。
-XX:+UseParallelGC，Parallel Scavenge+Serial Old(PS Mark Sweep)。该参数是jdk8默认垃圾收集器
-XX:+UseParallelOldGC，Parallel Scavenge+Parallel Old。
-XX:+UseG1GC，G1+G1。

二、用代码查看实际的GC

算是对上述设置后的一种验证

测试环境


java version "1.8.0_192"
Java(TM) SE Runtime Environment (build 1.8.0_192-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.192-b12, mixed mode)

测试代码

package com.alioo.gc;

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.util.List;

public class GarbageCollectorDemo {
    public static void main(String[] args) {


        List<GarbageCollectorMXBean> gcbeans = ManagementFactory.getGarbageCollectorMXBeans();
        for (GarbageCollectorMXBean gcbean : gcbeans) {
            System.out.println("alioolog gcbean："+gcbean.getName());
        }
    }
}

测试结果


alioolog gcbean：PS Scavenge
alioolog gcbean：PS MarkSweep

三、使用-XX:+PrintCommandLineFlags参数查看实际使用的GC

启动时增加JVM参数（打印GC详细信息，打印JVM参数）
-XX:+PrintGCDetails -XX:+PrintCommandLineFlags

测试环境

java version "1.8.0_192"
Java(TM) SE Runtime Environment (build 1.8.0_192-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.192-b12, mixed mode)

测试结果

不手工指定gc收集器，查看默认值


-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 

alioolog gcbean：PS Scavenge
alioolog gcbean：PS MarkSweep
Heap
 PSYoungGen      total 76288K, used 6554K [0x000000076ab00000, 0x0000000770000000, 0x00000007c0000000)
  eden space 65536K, 10% used [0x000000076ab00000,0x000000076b166900,0x000000076eb00000)
  from space 10752K, 0% used [0x000000076f580000,0x000000076f580000,0x0000000770000000)
  to   space 10752K, 0% used [0x000000076eb00000,0x000000076eb00000,0x000000076f580000)
 ParOldGen       total 175104K, used 0K [0x00000006c0000000, 0x00000006cab00000, 0x000000076ab00000)
  object space 175104K, 0% used [0x00000006c0000000,0x00000006c0000000,0x00000006cab00000)
 Metaspace       used 3299K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 336K, capacity 388K, committed 512K, reserved 1048576K

指定-XX:+UseSerialGC


-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseSerialGC 
alioolog gcbean：Copy
alioolog gcbean：MarkSweepCompact
Heap
 def new generation   total 78656K, used 6996K [0x00000006c0000000, 0x00000006c5550000, 0x0000000715550000)
  eden space 69952K,  10% used [0x00000006c0000000, 0x00000006c06d5168, 0x00000006c4450000)
  from space 8704K,   0% used [0x00000006c4450000, 0x00000006c4450000, 0x00000006c4cd0000)
  to   space 8704K,   0% used [0x00000006c4cd0000, 0x00000006c4cd0000, 0x00000006c5550000)
 tenured generation   total 174784K, used 0K [0x0000000715550000, 0x0000000720000000, 0x00000007c0000000)
   the space 174784K,   0% used [0x0000000715550000, 0x0000000715550000, 0x0000000715550200, 0x0000000720000000)
 Metaspace       used 3297K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 336K, capacity 388K, committed 512K, reserved 1048576K

指定-XX:+UseParNewGC

-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParNewGC 
alioolog gcbean：ParNew
alioolog gcbean：MarkSweepCompact
Heap
 par new generation   total 78656K, used 6996K [0x00000006c0000000, 0x00000006c5550000, 0x0000000715550000)
  eden space 69952K,  10% used [0x00000006c0000000, 0x00000006c06d5168, 0x00000006c4450000)
  from space 8704K,   0% used [0x00000006c4450000, 0x00000006c4450000, 0x00000006c4cd0000)
  to   space 8704K,   0% used [0x00000006c4cd0000, 0x00000006c4cd0000, 0x00000006c5550000)
 tenured generation   total 174784K, used 0K [0x0000000715550000, 0x0000000720000000, 0x00000007c0000000)
   the space 174784K,   0% used [0x0000000715550000, 0x0000000715550000, 0x0000000715550200, 0x0000000720000000)
 Metaspace       used 3280K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 336K, capacity 388K, committed 512K, reserved 1048576K

指定-XX:+UseConcMarkSweepGC

-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:MaxNewSize=697933824 -XX:MaxTenuringThreshold=6 -XX:OldPLABSize=16 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseConcMarkSweepGC -XX:+UseParNewGC 
alioolog gcbean：ParNew
alioolog gcbean：ConcurrentMarkSweep
Heap
 par new generation   total 78656K, used 6996K [0x00000006c0000000, 0x00000006c5550000, 0x00000006e9990000)
  eden space 69952K,  10% used [0x00000006c0000000, 0x00000006c06d5168, 0x00000006c4450000)
  from space 8704K,   0% used [0x00000006c4450000, 0x00000006c4450000, 0x00000006c4cd0000)
  to   space 8704K,   0% used [0x00000006c4cd0000, 0x00000006c4cd0000, 0x00000006c5550000)
 concurrent mark-sweep generation total 174784K, used 0K [0x00000006e9990000, 0x00000006f4440000, 0x00000007c0000000)
 Metaspace       used 3322K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 337K, capacity 388K, committed 512K, reserved 1048576K

指定-XX:+UseParallelGC
默认参数就是这个，加不加gc都一样

-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
alioolog gcbean：PS Scavenge
alioolog gcbean：PS MarkSweep
Heap
 PSYoungGen      total 76288K, used 6554K [0x000000076ab00000, 0x0000000770000000, 0x00000007c0000000)
  eden space 65536K, 10% used [0x000000076ab00000,0x000000076b166900,0x000000076eb00000)
  from space 10752K, 0% used [0x000000076f580000,0x000000076f580000,0x0000000770000000)
  to   space 10752K, 0% used [0x000000076eb00000,0x000000076eb00000,0x000000076f580000)
 ParOldGen       total 175104K, used 0K [0x00000006c0000000, 0x00000006cab00000, 0x000000076ab00000)
  object space 175104K, 0% used [0x00000006c0000000,0x00000006c0000000,0x00000006cab00000)
 Metaspace       used 3269K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 334K, capacity 388K, committed 512K, reserved 1048576K

指定-XX:+UseParallelOldGC
默认参数就是这个，加不加gc都一样

-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelOldGC 
alioolog gcbean：PS Scavenge
alioolog gcbean：PS MarkSweep
Heap
 PSYoungGen      total 76288K, used 6554K [0x000000076ab00000, 0x0000000770000000, 0x00000007c0000000)
  eden space 65536K, 10% used [0x000000076ab00000,0x000000076b166900,0x000000076eb00000)
  from space 10752K, 0% used [0x000000076f580000,0x000000076f580000,0x0000000770000000)
  to   space 10752K, 0% used [0x000000076eb00000,0x000000076eb00000,0x000000076f580000)
 ParOldGen       total 175104K, used 0K [0x00000006c0000000, 0x00000006cab00000, 0x000000076ab00000)
  object space 175104K, 0% used [0x00000006c0000000,0x00000006c0000000,0x00000006cab00000)
 Metaspace       used 3251K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 328K, capacity 388K, committed 512K, reserved 1048576K

指定-XX:+UseG1GC
默认参数就是这个，加不加gc都一样

-XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseG1GC 
alioolog gcbean：G1 Young Generation
alioolog gcbean：G1 Old Generation
Heap
 garbage-first heap   total 262144K, used 3072K [0x00000006c0000000, 0x00000006c0100800, 0x00000007c0000000)
  region size 1024K, 4 young (4096K), 0 survivors (0K)
 Metaspace       used 3543K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 359K, capacity 388K, committed 512K, reserved 1048576K

四、使用jinfo命令查看实际使用的GC

上面的第二、三部分使用代码的确是可以查到具体使用的GC，但是生产环境是不允许你值入代码后再重新启动，打印日志来查看的，因为这样你就破坏了现场
所以建议你使用jinfo命令来进行查看

测试代码
这里随便一个java进程即可，为避免进程退出太快，加入了休眠代码

package com.alioo.gc;

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.util.List;

public class GarbageCollectorDemo {
    public static void main(String[] args) {

        List<GarbageCollectorMXBean> gcbeans = ManagementFactory.getGarbageCollectorMXBeans();
        for (GarbageCollectorMXBean gcbean : gcbeans) {
            System.out.println("alioolog gcbean："+gcbean.getName());
        }


        try {
            Thread.sleep(10*60*1000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

测试环境

java version "1.8.0_192"
Java(TM) SE Runtime Environment (build 1.8.0_192-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.192-b12, mixed mode)

测试结果
根据输出结果-XX:+UseParallelOldGC，结合第一部分知道新生代使用Parallel Scavenge ，老年代使用Parallel Old


jps -m
87834 GarbageCollectorDemo

jinfo -flags 87834
VM Flags:
-XX:CICompilerCount=4 -XX:InitialHeapSize=268435456 -XX:MaxHeapSize=4294967296 -XX:MaxNewSize=1431306240 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=89128960 -XX:OldSize=179306496 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseParallelOldGC

GC专题-查看JVM使用的GC

一、设置垃圾收集器参数

二、用代码查看实际的GC

三、使用-XX:+PrintCommandLineFlags参数查看实际使用的GC

四、使用jinfo命令查看实际使用的GC

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