Java8 默認垃圾回收器(GC)

一 概述

GC(Garbage Collection)，在程序運行過程中內存空間是有限的，爲了更好的的使用有限的內存空間，GC會將不再使用的對象清除然後將其所佔用的內存釋放出來。

觸發GC的條件：

1. 程序調用System.gc的時候觸發GC。

    /**
     * Runs the garbage collector.
     * <p>
     * Calling the <code>gc</code> method suggests that the Java Virtual
     * Machine expend effort toward recycling unused objects in order to
     * make the memory they currently occupy available for quick reuse.
     * When control returns from the method call, the Java Virtual
     * Machine has made a best effort to reclaim space from all discarded
     * objects.
     * <p>
     * The call <code>System.gc()</code> is effectively equivalent to the
     * call:
     * <blockquote><pre>
     * Runtime.getRuntime().gc()
     * </pre></blockquote>
     *
     * @see     java.lang.Runtime#gc()
     */
    public static void gc() {
        Runtime.getRuntime().gc();
    }

2. 系統自身決定GC的觸發時機，主要根據Eden區和From Space區的內存大小來決定，當內存大小不足時，則會啓動GC線程（Daemon線程）並停止應用線程。

          

二 查看Java8的默認GC

1. cmd命令行查看Java8的GC：

java -XX:+PrintCommandLineFlags -version

結果如下：

-XX:InitialHeapSize=132397312 // JVM默認初始化堆大小
-XX:MaxHeapSize=2118356992 //JVM堆的默認最大值
-XX:+PrintCommandLineFlags 
-XX:+UseCompressedClassPointers 
-XX:+UseCompressedOops 
-XX:-UseLargePagesIndividualAllocation 
-XX:+UseParallelGC //Java8使用的GC類型
java version "1.8.0_20" //使用的java版本
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)

結果分析：由結果可以看出Java8的GC情況是：-XX:+UseParallelGC，即Parallel Scavenge（新生代） + Parallel Old（老生代），實際上幾個主流Java版本的GC情況如下：

• jdk1.7 默認垃圾收集器Parallel Scavenge（新生代）+Parallel Old（老年代）
• jdk1.8 默認垃圾收集器Parallel Scavenge（新生代）+Parallel Old（老年代）
• jdk1.9 默認垃圾收集器G1

2. cmd命令行查看Java8的GC詳細情況：

java -XX:+PrintGCDetails -version

結果如下：

java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)
Heap
 PSYoungGen      total 38400K, used 2678K [0x00000000d5e00000, 0x00000000d8880000, 0x0000000100000000)
  eden space 33280K, 8% used [0x00000000d5e00000,0x00000000d609dbc0,0x00000000d7e80000)
  from space 5120K, 0% used [0x00000000d8380000,0x00000000d8380000,0x00000000d8880000)
  to   space 5120K, 0% used [0x00000000d7e80000,0x00000000d7e80000,0x00000000d8380000)
 ParOldGen       total 87552K, used 0K [0x0000000081a00000, 0x0000000086f80000, 0x00000000d5e00000)
  object space 87552K, 0% used [0x0000000081a00000,0x0000000081a00000,0x0000000086f80000)
 Metaspace       used 2257K, capacity 4480K, committed 4480K, reserved 1056768K
  class space    used 244K, capacity 384K, committed 384K, reserved 1048576K

在Java中使用Metaspace（元空間）而移除了PermGenspace（永久區）。也就意味着這部分內存空間將全部移除。設置JVM的參數PermSize和MaxPermSize時會被忽略並給出警告。但是類的元素據信息（metadata）還在，只是不再是存儲在連續的堆空間中，而是移動到了被稱爲"Metaspace（元空間）"的本地內存（Native memory）中。

補充JDK1.6，JDK1.7與JDK1.9的信息：

JDK1.6

-XX:InitialHeapSize=132397312 
-XX:MaxHeapSize=2118356992 
-XX:ParallelGCThreads=4 
-XX:+PrintCommandLineFlags 
-XX:+UseCompressedOops 
-XX:-UseLargePagesIndividualAllocation 
-XX:+UseParallelGC
java version "1.6.0_43"
Java(TM) SE Runtime Environment (build 1.6.0_43-b01)
Java HotSpot(TM) 64-Bit Server VM (build 20.14-b01, mixed mode)

java version "1.6.0_43"
Java(TM) SE Runtime Environment (build 1.6.0_43-b01)
Java HotSpot(TM) 64-Bit Server VM (build 20.14-b01, mixed mode)
Heap
 PSYoungGen      total 37696K, used 646K [0x00000007d5e00000, 0x00000007d8810000, 0x0000000800000000)
  eden space 32320K, 2% used [0x00000007d5e00000,0x00000007d5ea19a8,0x00000007d7d90000)
  from space 5376K, 0% used [0x00000007d82d0000,0x00000007d82d0000,0x00000007d8810000)
  to   space 5376K, 0% used [0x00000007d7d90000,0x00000007d7d90000,0x00000007d82d0000)
 PSOldGen        total 86272K, used 0K [0x0000000781a00000, 0x0000000786e40000, 0x00000007d5e00000)
  object space 86272K, 0% used [0x0000000781a00000,0x0000000781a00000,0x0000000786e40000)
 PSPermGen       total 21248K, used 2709K [0x000000077c800000, 0x000000077dcc0000, 0x0000000781a00000)
  object space 21248K, 12% used [0x000000077c800000,0x000000077caa5738,0x000000077dcc0000)

JDK1.7

-XX:InitialHeapSize=132397312 
-XX:MaxHeapSize=2118356992 
-XX:+PrintCommandLineFlags 
-XX:+UseCompressedOops 
-XX:-UseLargePagesIndividualAllocation 
-XX:+UseParallelGC
java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)

//1.7中的PSPermGen永久代到1.8中是Metaspace
java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)
Heap
 PSYoungGen      total 38400K, used 1997K [0x00000007d5e00000, 0x00000007d8880000, 0x0000000800000000)
  eden space 33280K, 6% used [0x00000007d5e00000,0x00000007d5ff3480,0x00000007d7e80000)
  from space 5120K, 0% used [0x00000007d8380000,0x00000007d8380000,0x00000007d8880000)
  to   space 5120K, 0% used [0x00000007d7e80000,0x00000007d7e80000,0x00000007d8380000)
 ParOldGen       total 86016K, used 0K [0x0000000781a00000, 0x0000000786e00000, 0x00000007d5e00000)
  object space 86016K, 0% used [0x0000000781a00000,0x0000000781a00000,0x0000000786e00000)
 PSPermGen       total 21504K, used 2222K [0x000000077c800000, 0x000000077dd00000, 0x0000000781a00000)
  object space 21504K, 10% used [0x000000077c800000,0x000000077ca2b8c0,0x000000077dd00000)

JDK1.9

-XX:G1ConcRefinementThreads=4 
-XX:InitialHeapSize=132397312 
-XX:MaxHeapSize=2118356992 
-XX:+PrintCommandLineFlags 
-XX:ReservedCodeCacheSize=251658240 
-XX:+SegmentedCodeCache 
-XX:+UseCompressedClassPointers 
-XX:+UseCompressedOops 
-XX:+UseG1GC 
-XX:-UseLargePagesIndividualAllocation
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

C:\Users\pengu>java -Xlog:Gc* -version
[0.015s][info][gc,heap] Heap region size: 1M
[0.021s][info][gc     ] Using G1
[0.021s][info][gc,heap,coops] Heap address: 0x0000000081a00000, size: 2022 MB, Compressed Oops mode: 32-bit
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)
[0.123s][info][gc,heap,exit ] Heap
[0.124s][info][gc,heap,exit ]  garbage-first heap   total 131072K, used 1024K [0x0000000081a00000, 0x0000000081b00400, 0x0000000100000000)
[0.124s][info][gc,heap,exit ]   region size 1024K, 2 young (2048K), 0 survivors (0K)
[0.125s][info][gc,heap,exit ]  Metaspace       used 3453K, capacity 4480K, committed 4480K, reserved 1056768K
[0.126s][info][gc,heap,exit ]   class space    used 352K, capacity 384K, committed 384K, reserved 1048576Kjava