对象大小与指针压缩--JVM

1.导入依赖

  
        
        
            org.openjdk.jol
            jol-core
            0.9

        

2.编写测试代码

public class JOLSample {
    public static void main(String[] args) {
        ClassLayout layout = ClassLayout.parseInstance(new Object());
        System.out.println(layout.toPrintable());
        System.out.println();
        ClassLayout layout1 = ClassLayout.parseInstance(new int[]{});
        System.out.println(layout1.toPrintable());
        System.out.println();
        ClassLayout layout2 = ClassLayout.parseInstance(new K());
        System.out.println(layout2.toPrintable());
    }

    public static class K{
        int id;
        String name;
        byte b;
        Object o;
    }
}

3.结果分析：

java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) //mark word
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) //mark word
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243) //Klass Pointer
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
[I object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 6d 01 00 f8 (01101101 00000001 00000000 11111000) (-134217363)
12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
16 0 int [I. N/A
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
com.tuling.jvm.JOLSample$K object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 65 cc 00 f8 (01100101 11001100 00000000 11111000) (-134165403)
12 4 int K.id 0
16 1 byte K.b 0
17 3 (alignment/padding gap)
20 4 java.lang.String K.name null
24 4 java.lang.Object K.o null
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 3 bytes internal + 4 bytes external = 7 bytes total

Process finished with exit code 0