JVM----垃圾收集信息解读实战

本是古典何须时尚 2023-05-29 05:21 12阅读 0赞

## demo1 ##

public class Main {
        public static void main(String[] args) {
            {
                byte[] bytes=new byte[20*1024*1024];
            }
            System.gc();
        }
    }

在 verbose:gc -XX:PrintGCDetails 之后

控制台显示:

[GC (System.gc()) [PSYoungGen: 25398K->952K(56320K)] 25398K->21432K(184832K), 0.0200508 secs] [Times: user=0.01 sys=0.00, real=0.02 secs] 
    
    新生代占用内存减小了,但是整个堆内存使用量并没有减小太多远没有20M,那么这20M去了哪儿?,这说明内存占用转移到了老年代,从之后的FullGC可以验证,老年代20480->21384k,没有被回收掉,一直在老年代中
    [Full GC (System.gc()) [PSYoungGen: 952K->0K(56320K)] [ParOldGen: 20480K->21384K(128512K)] 21432K->21384K(184832K), [Metaspace: 3497K->3497K(1056768K)], 0.0242428 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
    Heap
     PSYoungGen      total 56320K, used 973K [0x0000000781900000, 0x0000000785780000, 0x00000007c0000000)
      eden space 48640K, 2% used [0x0000000781900000,0x00000007819f34c0,0x0000000784880000)
      from space 7680K, 0% used [0x0000000784880000,0x0000000784880000,0x0000000785000000)
      to   space 7680K, 0% used [0x0000000785000000,0x0000000785000000,0x0000000785780000)
     ParOldGen       total 128512K, used 21384K [0x0000000704a00000, 0x000000070c780000, 0x0000000781900000)
      object space 128512K, 16% used [0x0000000704a00000,0x0000000705ee22d8,0x000000070c780000)
     Metaspace       used 3506K, capacity 4496K, committed 4864K, reserved 1056768K
      class space    used 386K, capacity 388K, committed 512K, reserved 1048576K

## demo2 ##

public class Main {
        public static void main(String[] args) {
            {
                byte[] bytes=new byte[20*1024*1024];
            }
            int a=10;
            System.gc();
        }
    }

GC打印:

[GC (System.gc()) [PSYoungGen: 25398K->968K(56320K)] 25398K->976K(184832K), 0.0194207 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
    
    这个明显是新生代内存占用减少的量跟整个堆内存占用减少的量是相等的,说明直接被回收了
    
    [Full GC (System.gc()) [PSYoungGen: 968K->0K(56320K)] [ParOldGen: 8K->909K(128512K)] 976K->909K(184832K), [Metaspace: 3455K->3455K(1056768K)], 0.0084968 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] 
    Heap
     PSYoungGen      total 56320K, used 973K [0x0000000781900000, 0x0000000785780000, 0x00000007c0000000)
      eden space 48640K, 2% used [0x0000000781900000,0x00000007819f3510,0x0000000784880000)
      from space 7680K, 0% used [0x0000000784880000,0x0000000784880000,0x0000000785000000)
      to   space 7680K, 0% used [0x0000000785000000,0x0000000785000000,0x0000000785780000)
     ParOldGen       total 128512K, used 909K [0x0000000704a00000, 0x000000070c780000, 0x0000000781900000)
      object space 128512K, 0% used [0x0000000704a00000,0x0000000704ae34e8,0x000000070c780000)
     Metaspace       used 3477K, capacity 4496K, committed 4864K, reserved 1056768K
      class space    used 381K, capacity 388K, committed 512K, reserved 1048576K

## demo3 ##

public class Main {
        public static void main(String[] args) {
            {
                byte[] bytes=new byte[20*1024*1024];
                bytes=null;
            }
            System.gc();
        }
    }

GC:

[GC (System.gc()) [PSYoungGen: 25398K->1048K(56320K)] 25398K->1056K(184832K), 0.0025668 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
    
    同上
    
    [Full GC (System.gc()) [PSYoungGen: 1048K->0K(56320K)] [ParOldGen: 8K->904K(128512K)] 1056K->904K(184832K), [Metaspace: 3498K->3498K(1056768K)], 0.0356986 secs] [Times: user=0.05 sys=0.00, real=0.04 secs] 
    Heap
     PSYoungGen      total 56320K, used 973K [0x0000000781900000, 0x0000000785780000, 0x00000007c0000000)
      eden space 48640K, 2% used [0x0000000781900000,0x00000007819f34c0,0x0000000784880000)
      from space 7680K, 0% used [0x0000000784880000,0x0000000784880000,0x0000000785000000)
      to   space 7680K, 0% used [0x0000000785000000,0x0000000785000000,0x0000000785780000)
     ParOldGen       total 128512K, used 904K [0x0000000704a00000, 0x000000070c780000, 0x0000000781900000)
      object space 128512K, 0% used [0x0000000704a00000,0x0000000704ae2258,0x000000070c780000)
     Metaspace       used 3511K, capacity 4498K, committed 4864K, reserved 1056768K
      class space    used 387K, capacity 390K, committed 512K, reserved 1048576K

## 总结:    ##

用过的变量如果比较大,最好是置为null,否则,如上的大数组就会一直占用着内存,因为引用一直是存在的.