2.java对象占多少内存?

[TOC]

1.对象内存布局

详见
大致分成三部分，对象头，实例数据，对齐填充

对象所占内存=对象头+实例数据(+对齐填充)

1.1 对象头

由mark word和类型指针组成
markword占8个字节，类型指针占4个字节（开启指针压缩，默认开启，可以通过虚拟机参数设置-XX:-UseCompressedOops关闭）

1.2 实例数据

八大基本类型根据所占字节来计算（例如int 4字节），包装类型占4字节

1.3 对齐填充

如果对象头+实例数据的所占字节，不能被8整除，就自动填充

2.测试

引入依赖

       <dependency>
            <groupId>org.openjdk.jol</groupId>
            <artifactId>jol-core</artifactId>
            <version>0.9</version>
        </dependency>

2.1 空对象大小

public class EmptyObj {

    // 空对象 没有属性的对象
    public static void main(String[] args) {
        EmptyObj obj = new EmptyObj();
        System.out.println(ClassLayout.parseInstance(obj).toPrintable());
    }
}

打印结果
空对象所占内存大小=16B，看到最后一行offset为12，自动填充了4B

cn.summit.jvm.EmptyObj 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)                           05 c1 00 f8 (00000101 11000001 00000000 11111000) (-134168315)
     12     4        (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

设置压缩指针关闭

VM optional = -XX:-UseCompressedOops

cn.summit.jvm.EmptyObj 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)                           28 30 f8 1b (00101000 00110000 11111000 00011011) (469250088)
     12     4        (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
Instance size: 16 bytes

2.2 其它对象

public class ClazzObj {

    int a = 4;
    int b = 4;
    int c = 4;

    // int  f = 4;
   // LocalDateTime a = LocalDateTime.now();
   // String str;
    // 8 + 4 + 4+ 4 = 20 + 4
    // 关闭 8 + 8+ 4*3 = 28 + 4

    public static void main(String[] args) {
        // 8 + 4 +16 = 28 +4 =32
        ClazzObj obj = new ClazzObj();
        System.out.println(ClassLayout.parseInstance(obj).toPrintable());
    }
}

大小计算 8+4+4*3=24 %8=0，所以大小为24b

验证

cn.summit.jvm.ClazzObj 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)                           05 c1 00 f8 (00000101 11000001 00000000 11111000) (-134168315)
     12     4    int ClazzObj.a                                4
     16     4    int ClazzObj.b                                4
     20     4    int ClazzObj.c                                4
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total