java实体类中给引用类型对象直接赋值报错记录

package com.sinochem.it.model;

import com.alibaba.fastjson.JSONObject;

public class TestModel {
    int age;
    String name;
    JSONObject obj;
    Attachment attachment;

    public Attachment getAttachment() {
        return attachment;
    }

    public void setAttachment(Attachment attachment) {
        this.attachment = attachment;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public JSONObject getObj() {
        return obj;
    }

    public void setObj(JSONObject obj) {
        this.obj = obj;
    }

    @Override
    public String toString() {
        return "TestModel{" +
                "age=" + age +
                ", name='" + name + '\'' +
                ", obj=" + obj +
                ", attachment=" + attachment +
                '}';
    }
}

 public static void main(String[] args) {
        TestModel model = new TestModel();
        model.setAge(18);
        model.setName("Jhon");
        //model.getObj().put("name","mack");
        model.getAttachment().setBucketName("aa");
        System.out.println(model.toString());
    }