查看java对象占用内存大小

引入maven依赖：

        <dependency>
            <groupId>org.openjdk.jolgroupId>
            <artifactId>jol-coreartifactId>
            <version>0.9version>
        dependency>

用org.openjdk.jol.info.ClassLayout的方法输出对象占用大小信息：

package com.pilaf.classlayout;

import lombok.Data;

/**
 * @description:
 * @author: pilaf
 * @create: 2019-10-31 21:17
 */
@Data
public class MyTest {
    private int a;

    private boolean b;
}

package com.pilaf.classlayout;

import org.openjdk.jol.info.ClassLayout;

/**
 * @description:
 * @author: pilaf
 * @create: 2019-10-31 21:17
 */
public class LayoutDemo {
    public static void main(String[] args) {
        MyTest myTest = new MyTest();
        System.out.println("myTest:"+myTest);
        System.out.println(ClassLayout.parseInstance(myTest).toPrintable());
    }
}

输出的内容如下：

myTest:MyTest(a=0, b=false)
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
com.pilaf.classlayout.MyTest 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)                           43 c1 00 f8 (01000011 11000001 00000000 11111000) (-134168253)
     12     4       int MyTest.a                                  0
     16     1   boolean MyTest.b                                  false
     17     7           (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 7 bytes external = 7 bytes total

因为jvm要求对象所占的内存空间大小是8字节的整数倍，这个对象本来占17字节，为了对齐，补上了7字节，一共占24字节(8字节*3)。