一 概述

GC(Garbage Collection)，在程序运行过程中内存空间是有限的，为了更好的的使用有限的内存空间，GC会将不再使用的对象清除然后将其所占用的内存释放出来。
触发GC的条件：
1. 程序调用System.gc的时候触发GC。

  1.     /**
  2.      * Runs the garbage collector.
  3.      * <p>
  4.      * Calling the <code>gc</code> method suggests that the Java Virtual
  5.      * Machine expend effort toward recycling unused objects in order to
  6.      * make the memory they currently occupy available for quick reuse.
  7.      * When control returns from the method call, the Java Virtual
  8.      * Machine has made a best effort to reclaim space from all discarded
  9.      * objects.
  10.      * <p>
  11.      * The call <code>System.gc()</code> is effectively equivalent to the
  12.      * call:
  13.      * <blockquote><pre>
  14.      * Runtime.getRuntime().gc()
  15.      * </pre></blockquote>
  16.      *
  17.      * @see     java.lang.Runtime#gc()
  18.      */
  19.     public static void gc() {
  20.         Runtime.getRuntime().gc();
  21.     }
  1. 系统自身决定GC的触发时机，主要根据Eden区和From Space区的内存大小来决定，当内存大小不足时，则会启动GC线程（Daemon线程）并停止应用线程。
    image.png

二 查看Java8的默认GC

1. cmd命令行查看Java8的GC：

  1. java -XX:+PrintCommandLineFlags -version

结果如下：

  1. -XX:InitialHeapSize=132397312 // JVM默认初始化堆大小
  2. -XX:MaxHeapSize=2118356992 //JVM堆的默认最大值
  3. -XX:+PrintCommandLineFlags 
  4. -XX:+UseCompressedClassPointers 
  5. -XX:+UseCompressedOops 
  6. -XX:-UseLargePagesIndividualAllocation 
  7. -XX:+UseParallelGC //Java8使用的GC类型
  8. java version "1.8.0_20" //使用的java版本
  9. Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
  10. Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)

结果分析：由结果可以看出Java8的GC情况是：-XX:+UseParallelGC，即Parallel Scavenge（新生代） + Parallel Old（老生代），实际上几个主流Java版本的GC情况如下：

jdk1.7 默认垃圾收集器Parallel Scavenge（新生代【标记-复制算法】）+Parallel Old（老年代【标记整理算法】）
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【从局部(两个Region之间)来看是基于”标记—复制”算法实现，从整体来看是基于”标记-整理”算法实现】

  1. cmd命令行查看Java8的GC详细情况：
    1. java -XX:+PrintGCDetails -version
    结果如下：
    1. java version "1.8.0_20"
    2. Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
    3. Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)
    4. Heap
    5. PSYoungGen      total 38400K, used 2678K [0x00000000d5e00000, 0x00000000d8880000, 0x0000000100000000)
    6. eden space 33280K, 8% used [0x00000000d5e00000,0x00000000d609dbc0,0x00000000d7e80000)
    7. from space 5120K, 0% used [0x00000000d8380000,0x00000000d8380000,0x00000000d8880000)
    8. to   space 5120K, 0% used [0x00000000d7e80000,0x00000000d7e80000,0x00000000d8380000)
    9. ParOldGen       total 87552K, used 0K [0x0000000081a00000, 0x0000000086f80000, 0x00000000d5e00000)
    10. object space 87552K, 0% used [0x0000000081a00000,0x0000000081a00000,0x0000000086f80000)
    11. Metaspace       used 2257K, capacity 4480K, committed 4480K, reserved 1056768K
    12. class space    used 244K, capacity 384K, committed 384K, reserved 1048576K
    在Java中使用Metaspace（元空间）而移除了PermGenspace（永久区）。也就意味着这部分内存空间将全部移除。设置JVM的参数PermSize和MaxPermSize时会被忽略并给出警告。但是类的元数据信息（metadata）还在，只是不再存储在连续的堆空间中，而是移动到了被称为”Metaspace（元空间）”的本地内存（Native memory）中。

补充JDK1.6，JDK1.7与JDK1.9的信息：
JDK1.6

  1. -XX:InitialHeapSize=132397312 
  2. -XX:MaxHeapSize=2118356992 
  3. -XX:ParallelGCThreads=4 
  4. -XX:+PrintCommandLineFlags 
  5. -XX:+UseCompressedOops 
  6. -XX:-UseLargePagesIndividualAllocation 
  7. -XX:+UseParallelGC
  8. java version "1.6.0_43"
  9. Java(TM) SE Runtime Environment (build 1.6.0_43-b01)
  10. Java HotSpot(TM) 64-Bit Server VM (build 20.14-b01, mixed mode)
  12. java version "1.6.0_43"
  13. Java(TM) SE Runtime Environment (build 1.6.0_43-b01)
  14. Java HotSpot(TM) 64-Bit Server VM (build 20.14-b01, mixed mode)
  15. Heap
  16.  PSYoungGen      total 37696K, used 646K [0x00000007d5e00000, 0x00000007d8810000, 0x0000000800000000)
  17.   eden space 32320K, 2% used [0x00000007d5e00000,0x00000007d5ea19a8,0x00000007d7d90000)
  18.   from space 5376K, 0% used [0x00000007d82d0000,0x00000007d82d0000,0x00000007d8810000)
  19.   to   space 5376K, 0% used [0x00000007d7d90000,0x00000007d7d90000,0x00000007d82d0000)
  20.  PSOldGen        total 86272K, used 0K [0x0000000781a00000, 0x0000000786e40000, 0x00000007d5e00000)
  21.   object space 86272K, 0% used [0x0000000781a00000,0x0000000781a00000,0x0000000786e40000)
  22.  PSPermGen       total 21248K, used 2709K [0x000000077c800000, 0x000000077dcc0000, 0x0000000781a00000)
  23.   object space 21248K, 12% used [0x000000077c800000,0x000000077caa5738,0x000000077dcc0000)

JDK1.7

  1. -XX:InitialHeapSize=132397312 
  2. -XX:MaxHeapSize=2118356992 
  3. -XX:+PrintCommandLineFlags 
  4. -XX:+UseCompressedOops 
  5. -XX:-UseLargePagesIndividualAllocation 
  6. -XX:+UseParallelGC
  7. java version "1.7.0_80"
  8. Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
  9. Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)
  11. //1.7中的PSPermGen永久代到1.8中是Metaspace
  12. java version "1.7.0_80"
  13. Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
  14. Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)
  15. Heap
  16.  PSYoungGen      total 38400K, used 1997K [0x00000007d5e00000, 0x00000007d8880000, 0x0000000800000000)
  17.   eden space 33280K, 6% used [0x00000007d5e00000,0x00000007d5ff3480,0x00000007d7e80000)
  18.   from space 5120K, 0% used [0x00000007d8380000,0x00000007d8380000,0x00000007d8880000)
  19.   to   space 5120K, 0% used [0x00000007d7e80000,0x00000007d7e80000,0x00000007d8380000)
  20.  ParOldGen       total 86016K, used 0K [0x0000000781a00000, 0x0000000786e00000, 0x00000007d5e00000)
  21.   object space 86016K, 0% used [0x0000000781a00000,0x0000000781a00000,0x0000000786e00000)
  22.  PSPermGen       total 21504K, used 2222K [0x000000077c800000, 0x000000077dd00000, 0x0000000781a00000)
  23.   object space 21504K, 10% used [0x000000077c800000,0x000000077ca2b8c0,0x000000077dd00000)

JDK1.9

  1. -XX:G1ConcRefinementThreads=4 
  2. -XX:InitialHeapSize=132397312 
  3. -XX:MaxHeapSize=2118356992 
  4. -XX:+PrintCommandLineFlags 
  5. -XX:ReservedCodeCacheSize=251658240 
  6. -XX:+SegmentedCodeCache 
  7. -XX:+UseCompressedClassPointers 
  8. -XX:+UseCompressedOops 
  9. -XX:+UseG1GC 
  10. -XX:-UseLargePagesIndividualAllocation
  11. java version "9.0.4"
  12. Java(TM) SE Runtime Environment (build 9.0.4+11)
  13. Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)
  15. C:\Users\pengu>java -Xlog:Gc* -version
  16. [0.015s][info][gc,heap] Heap region size: 1M
  17. [0.021s][info][gc     ] Using G1
  18. [0.021s][info][gc,heap,coops] Heap address: 0x0000000081a00000, size: 2022 MB, Compressed Oops mode: 32-bit
  19. java version "9.0.4"
  20. Java(TM) SE Runtime Environment (build 9.0.4+11)
  21. Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)
  22. [0.123s][info][gc,heap,exit ] Heap
  23. [0.124s][info][gc,heap,exit ]  garbage-first heap   total 131072K, used 1024K [0x0000000081a00000, 0x0000000081b00400, 0x0000000100000000)
  24. [0.124s][info][gc,heap,exit ]   region size 1024K, 2 young (2048K), 0 survivors (0K)
  25. [0.125s][info][gc,heap,exit ]  Metaspace       used 3453K, capacity 4480K, committed 4480K, reserved 1056768K
  26. [0.126s][info][gc,heap,exit ]   class space    used 352K, capacity 384K, committed 384K, reserved 1048576Kjava

原文

https://blog.csdn.net/calm_encode/article/details/106621152