案例：{"xiaoMing":{"courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"},"studentNo":"0001"},"xiaoHua":{"courses":{"$ref":"$.xiaoMing.courses"},"studentNo":"0002"}}

原因：一个对象被重复引用，fastjson默认开启循环引用检查，所以序列化结果会出现($ref. $.)，来标注被重复引用的对象。

package cn.com.dl.bean;

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.io.Serializable;
import java.util.Map;

/**
 * Created by yanshao on 2021-01-13.
 */
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Student implements Serializable {
    private static final long serialVersionUID = 465467703322405014L;
    private String studentNo;
    private Map<String,String> courses;
}

异常案例:

直接com.alibaba.fastjson.JSON#toJSONString(java.lang.Object)来序列化

    @Test
    public void test(){
        Map<String,Student> map = new HashMap<>();
        Map<String,String> courses = new HashMap<>();
        courses.put("English","English");
        courses.put("Chinese","English");
        courses.put("Mathematics","Mathematics");
        Student xiaoMing = Student.builder()
                .studentNo("0001")
                .courses(courses)
                .build();
        map.put("xiaoMing",xiaoMing);
        Student xiaoHua = Student.builder()
                .studentNo("0002")
                .courses(courses)
                .build();
        map.put("xiaoHua",xiaoHua);
        System.out.println(JSONObject.toJSONString(map));
    }

序列化结果：

{"xiaoMing":{"courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"},"studentNo":"0001"},"xiaoHua":{"courses":{"$ref":"$.xiaoMing.courses"},"studentNo":"0002"}}

解决方案1：关闭fastjson的循环引用检查

    @Test
    public void test(){
        Map<String,Student> map = new HashMap<>();
        Map<String,String> courses = new HashMap<>();
        courses.put("English","English");
        courses.put("Chinese","English");
        courses.put("Mathematics","Mathematics");
        Student xiaoMing = Student.builder()
                .studentNo("0001")
                .courses(courses)
                .build();
        map.put("xiaoMing",xiaoMing);
        Student xiaoHua = Student.builder()
                .studentNo("0002")
                .courses(courses)
                .build();
        map.put("xiaoHua",xiaoHua);
        System.out.println( JSONObject.toJSONString(map,SerializerFeature.DisableCircularReferenceDetect));
    }

序列化结果：

{"xiaoMing":{"courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"},"studentNo":"0001"},"xiaoHua":{"courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"},"studentNo":"0002"}}

解决方案2：使用com.google.gson.Gson

    @Test
    public void test(){
        Map<String,Student> map = new HashMap<>();
        Map<String,String> courses = new HashMap<>();
        courses.put("English","English");
        courses.put("Chinese","English");
        courses.put("Mathematics","Mathematics");
        Student xiaoMing = Student.builder()
                .studentNo("0001")
                .courses(courses)
                .build();
        map.put("xiaoMing",xiaoMing);
        Student xiaoHua = Student.builder()
                .studentNo("0002")
                .courses(courses)
                .build();
        map.put("xiaoHua",xiaoHua);
        Gson gson = new Gson();
        System.out.println(gson.toJson(map));
//        System.out.println("fastjson" + JSONObject.toJSONString(map,SerializerFeature.DisableCircularReferenceDetect));
    }

序列化结果：

{"xiaoMing":{"studentNo":"0001","courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"}},"xiaoHua":{"studentNo":"0002","courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"}}}

解决方案3：深拷贝或者新创建对象

    @Test
    public void test(){
        Map<String,Student> map = new HashMap<>();
        Map<String,String> courses = new HashMap<>();
        courses.put("English","English");
        courses.put("Chinese","English");
        courses.put("Mathematics","Mathematics");
        Student xiaoMing = Student.builder()
                .studentNo("0001")
                .courses(courses)
                .build();
        map.put("xiaoMing",xiaoMing);
        Student xiaoHua = Student.builder()
                .studentNo("0002")
                .courses(new HashMap<>(courses))
                .build();
        map.put("xiaoHua",xiaoHua);
        System.out.println(JSONObject.toJSONString(map));
//        Gson gson = new Gson();
//        System.out.println(gson.toJson(map));
//        System.out.println("fastjson" + JSONObject.toJSONString(map,SerializerFeature.DisableCircularReferenceDetect));
    }

注意:  对于循环引用，fastjson和gson处理方式不同，fastjson默认开启循环引用检查，而使用gson序列化时，会堆内存溢出；

fastjson：

fastjson{"map,":{"$ref":"@"},"xiaoMing":{"courses":{"English":"English","Chinese":"English","Mathematics":"Mathematics"},"studentNo":"0001"},"xiaoHua":{"courses":{"$ref":"$.xiaoMing.courses"},"studentNo":"0002"}}

gson：

java.lang.StackOverflowError
	at java.lang.StringBuffer.append(StringBuffer.java:270)
	at java.io.StringWriter.write(StringWriter.java:112)
	at com.google.gson.stream.JsonWriter.string(JsonWriter.java:590)