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【深入浅出-JVM】（36）：System.gc()

默认system.gc() 是单线程执行即指定了 G1,CMS 回收器,除非 -XX:+ExplicitGCInvokesConcurrent 改变这种默认行为

并行 GC 额外触发一次新生代 GC

package com.mousycoder.mycode.thinking_in_jvm;

/**
 * @version 1.0
 * @author: mousycoder
 * @date: 2019-07-15 17:57
 */
public class ScavengeBeforeFullGC {

    public static void main(String[] args) {
        System.gc();
    }
}

虚拟机参数

-XX:+PrintGCDetails -XX:+UseSerialGC

输出

[Full GC (System.gc()) [Tenured: 0K->493K(129792K), 0.0025750 secs] 910K->493K(130944K), [Metaspace: 2735K->2735K(1056768K)], 0.0028750 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap
 def new generation total 58496K, used 1561K [0x0000000740000000, 0x0000000743f70000, 0x000000076aaa0000)
  eden space 52032K, 3% used [0x0000000740000000, 0x00000007401864f8, 0x00000007432d0000)
  from space 6464K, 0% used [0x00000007432d0000, 0x00000007432d0000, 0x0000000743920000)
  to space 6464K, 0% used [0x0000000743920000, 0x0000000743920000, 0x0000000743f70000)
 tenured generation total 129792K, used 493K [0x000000076aaa0000, 0x0000000772960000, 0x00000007c0000000)
   the space 129792K, 0% used [0x000000076aaa0000, 0x000000076ab1b5c0, 0x000000076ab1b600, 0x0000000772960000)
 Metaspace used 2744K, capacity 4486K, committed 4864K, reserved 1056768K
  class space used 297K, capacity 386K, committed 512K, reserved 1048576K

虚拟机参数

-XX:+PrintGCDetails -XX:+UseParallelOldGC

输出

[GC (System.gc()) [PSYoungGen: 797K->512K(1024K)] 805K->544K(130560K), 0.0007440 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (System.gc()) [PSYoungGen: 512K->0K(1024K)] [ParOldGen: 32K->493K(129536K)] 544K->493K(130560K), [Metaspace: 2738K->2738K(1056768K)], 0.0038530 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 
Heap
 PSYoungGen total 1024K, used 5K [0x0000000795580000, 0x0000000795700000, 0x00000007c0000000)
  eden space 512K, 1% used [0x0000000795580000,0x00000007955815f0,0x0000000795600000)
  from space 512K, 0% used [0x0000000795680000,0x0000000795680000,0x0000000795700000)
  to space 512K, 0% used [0x0000000795600000,0x0000000795600000,0x0000000795680000)
 ParOldGen total 129536K, used 493K [0x0000000740000000, 0x0000000747e80000, 0x0000000795580000)
  object space 129536K, 0% used [0x0000000740000000,0x000000074007b5c0,0x0000000747e80000)
 Metaspace used 2744K, capacity 4486K, committed 4864K, reserved 1056768K
  class space used 297K, capacity 386K, committed 512K, reserved 1048576K

可见并行 GC 的时候，在 FULL GC 前多了一次新生代的 GC，好处是为了缩短停顿，避免所有工作交给 FULL GC