Java查看对象头大小

添加依赖

        <dependency>
            <groupId>org.openjdk.jol</groupId>
            <artifactId>jol-core</artifactId>
            <version>0.17</version>
        </dependency>

查看对象头大小

    @Test
    void header() {
        ClassLayout layout = ClassLayout.parseInstance(new Object());
        System.out.println(layout.toPrintable());
        ClassLayout layout2 = ClassLayout.parseInstance(new int[]{});
        System.out.println(layout2.toPrintable());
        User user = new User();
        user.setAge(2);
        user.setName("hello");
        ClassLayout layout3 = ClassLayout.parseInstance(user);
        System.out.println(layout3.toPrintable());
    }

    @Data
    public static class User {
        private String name;
        private Integer age;
        private String remark;
    }

输出：

JavaObjectHeaderTest$User object internals:
OFF  SZ                TYPE DESCRIPTION               VALUE
  0   8                     (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   4                     (object header: class)    0x00d00eb0
 12   4    java.lang.String User.name                 (object)
 16   4   java.lang.Integer User.age                  2
 20   4    java.lang.String User.remark               null
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

创建一个自定义User对象，对象头大小为24 Bytes