System.gc()日志分析

打开日志：运行配置---XX:+PrintGCDetails

示例程序：

package com.test;

public class Test
{
    private Object instance = null;
    private String str;
    
    private static final int _1MB = 1024 * 1024;
    /** 这个成员属性唯一的作用就是占用一点内存 */
    private byte[] bigSize = new byte[2 * _1MB];
    
    public static void main(String[] args)
    {
        Test objectA = new Test();
        objectA.str = objectA.bigSize.toString();
        Test objectB = new Test();
        objectB.str = objectB.bigSize.toString();
        objectA.instance = objectB;
        objectB.instance = objectA;
        
        objectA = null;
        objectB = null;
        
        System.gc();
    }
}

打印：

[GC [PSYoungGen: 9912K->568K(229376K)] 9912K->568K(491520K), 0.0052670 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 
[Full GC [PSYoungGen: 568K->0K(229376K)] [ParOldGen: 0K->465K(262144K)] 568K->465K(491520K) [PSPermGen: 2573K->2572K(131072K)], 0.0105626 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
Heap
 PSYoungGen      total 229376K, used 5898K [0x00000000f0000000, 0x0000000100000000, 0x0000000100000000)
  eden space 196608K, 3% used [0x00000000f0000000,0x00000000f05c29c0,0x00000000fc000000)
  from space 32768K, 0% used [0x00000000fc000000,0x00000000fc000000,0x00000000fe000000)
  to   space 32768K, 0% used [0x00000000fe000000,0x00000000fe000000,0x0000000100000000)
 ParOldGen       total 262144K, used 465K [0x00000000e0000000, 0x00000000f0000000, 0x00000000f0000000)
  object space 262144K, 0% used [0x00000000e0000000,0x00000000e00744a0,0x00000000f0000000)
 PSPermGen       total 131072K, used 2579K [0x00000000d0000000, 0x00000000d8000000, 0x00000000e0000000)
  object space 131072K, 1% used [0x00000000d0000000,0x00000000d0284e98,0x00000000d8000000)

注释掉objectA = null; 和 objectB = null;

打印：

[GC [PSYoungGen: 9912K->4664K(229376K)] 9912K->4664K(491520K), 0.0053971 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC [PSYoungGen: 4664K->0K(229376K)] [ParOldGen: 0K->4561K(262144K)] 4664K->4561K(491520K) [PSPermGen: 2573K->2572K(131072K)], 0.0118699 secs] [Times: user=0.01 sys=0.03, real=0.01 secs] 
Heap
 PSYoungGen      total 229376K, used 5898K [0x00000000f0000000, 0x0000000100000000, 0x0000000100000000)
  eden space 196608K, 3% used [0x00000000f0000000,0x00000000f05c29c0,0x00000000fc000000)
  from space 32768K, 0% used [0x00000000fc000000,0x00000000fc000000,0x00000000fe000000)
  to   space 32768K, 0% used [0x00000000fe000000,0x00000000fe000000,0x0000000100000000)
 ParOldGen       total 262144K, used 4561K [0x00000000e0000000, 0x00000000f0000000, 0x00000000f0000000)
  object space 262144K, 1% used [0x00000000e0000000,0x00000000e0474570,0x00000000f0000000)
 PSPermGen       total 131072K, used 2579K [0x00000000d0000000, 0x00000000d8000000, 0x00000000e0000000)
  object space 131072K, 1% used [0x00000000d0000000,0x00000000d0284e90,0x00000000d8000000)

日志信息解释：

YoungGC日志解释如下

FullGC