Samples DataBind Fastjson Circular Reference problem

Fastjson full support DataBind, it's simple-to-use.

Encode

import com.alibaba.fastjson.JSON;Group group = new Group();group.setId(0L);group.setName("admin");User guestUser = new User();guestUser.setId(2L);guestUser.setName("guest");User rootUser = new User();rootUser.setId(3L);rootUser.setName("root");group.addUser(guestUser);group.addUser(rootUser);String jsonString = JSON.toJSONString(group);System.out.println(jsonString);

Output

{"id":0,"name":"admin","users":[{"id":2,"name":"guest"},{"id":3,"name":"root"}]}

Decode

String jsonString = ...;Group group = JSON.parseObject(jsonString, Group.class);

Group.java

public class Group {    private Long       id;    private String     name;    private List<User> users = new ArrayList<User>();    public Long getId() {        return id;    }    public void setId(Long id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public List<User> getUsers() {        return users;    }    public void setUsers(List<User> users) {        this.users = users;    }        public void addUser(User user) {            users.add(user);        }}

User.java

public class User {    private Long   id;    private String name;    public Long getId() {        return id;    }    public void setId(Long id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

Https://github.com/alibaba/fastjson/wiki/Samples-DataBind

Last week, to help colleagues see a problem, is to construct a universal query object, which contains a generic object, in the Spring MVC controller layer to be directly converted through requestbody, Spring MVC code is as follows:

Java code

1. @RequestMapping ("/testbind")
2. @ResponseBody
3. Public String testbind (@RequestBody mgquerycondition<testbean> querycondition) {
4. System.out.println (querycondition);
6. return "Success";
7. }

Java code

2. Import java.io.Serializable;
6. Public class Testbean implements serializable{
8. private Integer ID;
12. Public Integer getId () {
14. return ID;
16. }
20. public void SetId (Integer id) {
22. this.id = ID;
24. }
28. @Override
30. Public String toString () {
32. return "Testbean [id=" + ID + "]";
34. }
36. }

Java code

2. Import Com.alibaba.fastjson.JSON;
4. Import Com.alibaba.fastjson.JSONObject;
6. Import com.alibaba.fastjson.TypeReference;
10. Public class Mgquerycondition<t> {
12. private T model;
16. private Integer PageNo;
20. private Integer Pagenum;
24. Public T Getmodel () {
26. return model;
28. }
32. public void Setmodel (T model) {
34. This.model = model;
36. }
40. Public Integer Getpageno () {
42. return pageno;
44. }
48. public void Setpageno (Integer pageno) {
50. This.pageno = PageNo;
52. }
56. Public Integer Getpagenum () {
58. return pagenum;
60. }
64. public void Setpagenum (Integer pagenum) {
66. this.pagenum = pagenum;
68. }
72. @Override
74. Public String toString () {
76. return "mgquerycondition [model=" + model + ", pageno=" + pageno
78. + ", pagenum=" + Pagenum + "]";
80. }
84. }

If we go to debug, we find that the data type inside the mgquerycondition is jsonobject, if the type conversion problem occurs when the Getmodel is called, then I look at the Fastjson document.

For generic types, use TypeReference for parsing with the following code:

Java code

2. Public static void Main (string[] args) {
4. mgquerycondition<testbean> test = new mgquerycondition<testbean> ();
6. Testbean Testbean = new Testbean ();
8. Testbean.setid (1);
10. Test.setmodel (Testbean);
12. Test.setpageno (1);
14. Test.setpagenum (1);
16. System.out.println (jsonobject.tojsonstring (test));
20. String json = "{\" model\ ": {\" id\ ": 1},\" pageno\ ": 1,\" pagenum\ ": 1}";
22. <strong>MgQueryCondition<TestBean> clazz = Json.parseobject (JSON,
24. New Typereference<mgquerycondition<testbean>> () {});</strong>
26. System.out.println (Clazz.getmodel (). GetClass ());
30. }

So we can't use the @requestbody, so we should use the most basic KV structure for the reference.

Fastjson Circular Reference problems

What are circular references and duplicate references

Duplicate references: Multiple properties in an object reference the same object at the same time
For example:

        Object obj=new Object();        Map<String,Object> map=new HashMap<>(); map.put("1", obj); map.put("2", obj);//引用了同一个对象 System.out.println(JSON.toJSONString(map));

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Circular reference: There is a mutual reference between the properties of an object causing a loop that causes a StackOverflow exception
For example:

        Map<String,Object> map1=new HashMap<>();        Map<String,Object> map2=new HashMap<>(); map1.put("1",map2);//map1引用了map2 map2.put("1",map1);//map2又引用了map1,导致循环引用 System.out.println(JSON.toJSONString(map1));

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Fastjson How to troubleshoot circular references/duplicate references

Fastjson supports circular reference/duplicate references, and is open by default.
* The first example of serialization results in the following output: {"1":{},"2":{"$ref":"$.1"}}
The first object is serialized normally, and the second object is denoted by a reference
* After serialization of the second column, the output is:{"1":{"1":{"$ref":".."}}}

According to the syntax of Fastjson:

Grammar	Describe
{"$ref": "\$"}	Referencing the root object
{"$ref": "@"}	Quote yourself
{"$ref": ".."}	Reference Parent Object
{"$ref": ".. /..”}	References parent objects of parent object
{"$ref": "\$.members[0].reportto"}	Path-based references

It can be concluded that "$.1" represents the first element (obj) that references the root object (map), "..." Represents a reference to a parent object (MAP1).

Turn off circular references/duplicate references

Fastjson by default, the JSON serialization is detected by cyclic reference, thus avoiding the stackoverflow exception. When serialized JSON is transferred to a browser or other language, these JSON parsers do not support circular references, resulting in data loss. You can turn off cyclic reference detection for Fastjson.
Global configuration shutdown

  JSON.DEFAULT_GENERATE_FEATURE |= SerializerFeature.DisableCircularReferenceDetect.getMask();

Non-global shutdown

  JSON.toJSONString(obj, SerializerFeature.DisableCircularReferenceDetect);

Handling StackOverflowException

When the cyclic reference detection function is turned off, a StackOverflow exception occurs when serializing, which requires the user to handle whether there is a circular reference between the attributes:
You can use annotations on a field or Getter method @JSONField(serialize=false) to set some fields to not serialize, thus avoiding circular references.

http://blog.csdn.net/helloxiaoyueyue/article/details/51173168

Samples DataBind Fastjson Circular Reference problem