hotspot中对象的内存排布

conclusion.txt

hsdb> mem 0x23c02cc8 12
0x23c02cc8: 0x00000001  （标记字）
0x23c02ccc: 0x33ba2ca0  （类对象地址）
0x23c02cd0: 0x00000003  （属性域l）
0x23c02cd4: 0x00000000
0x23c02cd8: 0x66666666  （属性域d）
0x23c02cdc: 0x40166666  
0x23c02ce0: 0x00000001  （属性域i）
0x23c02ce4: 0x00040032  （属性域c和s）
0x23c02ce8: 0x00000038  （属性域cc）
0x23c02cec: 0x23c02cf8  （Object数组的地址）
0x23c02cf0: 0x23c02d10  （Object对象的地址）
0x23c02cf4: 0x23c02eb8  （Integer对象的地址）

从上面的内容可以看到，jvm重新排布了对象的属性域，将对象引用都放到了最后，并且合并了shrot和char类型的属性域。


hsdb> mem 0x23c02cf8 6
0x23c02cf8: 0x00000001  （标记字）
0x23c02cfc: 0x38bc50a0  （类对象地址）
0x23c02d00: 0x00000003  （数组长度）
0x23c02d04: 0x00000000  （数组元素0）
0x23c02d08: 0x00000000  （数组元素1）
0x23c02d0c: 0x00000000  （数组元素2）

从上面的内容可以看到，在数组对象中，数组长度是紧跟在类对象后面的。

environment.cfg

Intel(R) Core(TM) i3 CPU    M 380 @ 2.53GHz
2.00 GB(1.74 GB可用)
32位操作系统

java version "1.7.0_07"
Java(TM) SE Runtime Environment (build 1.7.0_07-b10)
Java HotSpot(TM) Client VM (build 23.3-b01, mixed mode, sharing)

hsdb.out

hsdb> universe
Heap Parameters:
Gen 0:   eden [0x23b80000,0x23c16bb8,0x23fd0000) space capacity = 4521984, 13.653299082880435 used
  from [0x23fd0000,0x23fd0000,0x24050000) space capacity = 524288, 0.0 used
  to   [0x24050000,0x24050000,0x240d0000) space capacity = 524288, 0.0 usedInvocations: 0

Gen 1:   old  [0x290d0000,0x290d0000,0x29b80000) space capacity = 11206656, 0.0 usedInvocations: 0

  perm [0x33b80000,0x33ba3080,0x34780000) space capacity = 12582912, 1.1403401692708333 used  ro space:  [0x37b80000,0x38004650,0x38580000) space capacity = 10485760, 45.171661376953125 used, rw space:  [0x38580000,0x38c07e58,0x39180000) space capacity = 12582912, 54.42371368408203 usedInvocations:0

hsdb> scanoops 0x23b80000 0x23c16bb8 Test
0x23c02cc8 Test

hsdb> inspect 0x23c02cc8
instance of Oop for Test @ 0x23c02cc8 @ 0x23c02cc8 (size = 48)
_mark: 1
_metadata._klass: InstanceKlass for Test @ 0x33ba2ca0 Oop @ 0x33ba2ca0
objectArray: ObjArray @ 0x23c02cf8 Oop for [Ljava/lang/Object; @ 0x23c02cf8
obj: Oop for java/lang/Object @ 0x23c02d10 Oop for java/lang/Object @ 0x23c02d10
i: 1
c: '2'
l: 3
s: 4
d: 5.6
integer: Oop for java/lang/Integer @ 0x23c02eb8 Oop for java/lang/Integer @ 0x23c02eb8
cc: '8'

hsdb> mem 0x23c02cc8 12
0x23c02cc8: 0x00000001
0x23c02ccc: 0x33ba2ca0
0x23c02cd0: 0x00000003
0x23c02cd4: 0x00000000
0x23c02cd8: 0x66666666
0x23c02cdc: 0x40166666
0x23c02ce0: 0x00000001
0x23c02ce4: 0x00040032
0x23c02ce8: 0x00000038
0x23c02cec: 0x23c02cf8
0x23c02cf0: 0x23c02d10
0x23c02cf4: 0x23c02eb8

hsdb> inspect 0x23c02cf8
instance of ObjArray @ 0x23c02cf8 @ 0x23c02cf8 (size = 24)
_mark: 1
_metadata._klass: ObjArrayKlass for InstanceKlass for java/lang/Object @ 0x38bc50a0 Oop @ 0x38bc50a0

0: null null
1: null null
2: null null

hsdb> mem 0x23c02cf8 6
0x23c02cf8: 0x00000001
0x23c02cfc: 0x38bc50a0
0x23c02d00: 0x00000003
0x23c02d04: 0x00000000
0x23c02d08: 0x00000000
0x23c02d0c: 0x00000000

Test.java

class Test{
    private static Object[] staticObjectArray = new Object[3];
    
    private Object[] objectArray = new Object[3];
    private Object obj = new Object();
    private int i = 1;
    private char c = '2';
    private long l = 3L;
    private short s = 4;
    private double d = 5.6;
    private Integer integer = new Integer(7);
    private char cc = '8';
    
    public static int foo(int a) throws Exception{
        while(a != 0){
            Thread.sleep(500);
            a --;
            System.out.println(a);
        }
        return a;
    }
    
    public static void main(String[] args) throws Exception{
        Test t = new Test();
        while (true) {
            foo(10);
        }
    }
}