详解jackson注解（三）jackson包含属性、忽略属性的注解

jackson中，指定包含哪些属性、忽略哪些属性的注解：

	注解类	描述
1	JsonIgnoreProperties	用于标记忽略一个或多个属性。可以注解在类上、构造函数、方法、字段上。
2	JsonIgnore	@JsonIgnore注解用于在字段级别标记要忽略的属性。注意：系列化和反系列化时都会被忽略。
3	JsonInclude	使用@JsonInclude可以只包含非空的属性，也即排除值为empty、null的属性。
4	JsonAutoDetect	默认情况下，jackson获取public权限的字段进行系列化和反系列化。如果没有public修饰的字段，就会去获取public修饰的getter/setter。使用 JsonAutoDetect注解，我们就可以修改默认的行为。

一、JsonIgnoreProperties

用于标记忽略一个或多个属性。可以注解在类上、构造函数、方法、字段上。

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

/**
 * @author chushiyan
 * @email  chushiyan0415@163.com
 * @description 自关联的权限表。一个权限有一个父权限、多个子权限
 */
@Data
public class Permission implements Serializable {

    private String id;

    private String name;

    @ManyToOne
    @JoinColumn(name = "permission_parent_id", referencedColumnName = "permission_id")
    private Permission parent;

    // 必须使用JsonIgnoreProperties忽略parent属性
    // 否则查询到子权限，子权限有parent属性，又去查父权限，会导致系列化时出现死循环
    @JsonIgnoreProperties(value = "parent")
    @OneToMany(mappedBy = "parent")
    private Set<Permission> children = new HashSet<Permission>();
   
}

二、JsonIgnore

@JsonIgnore注解用于在字段级别标记要忽略的属性。注意：系列化和反系列化时都会被忽略。

1、bean

import com.fasterxml.jackson.annotation.JsonIgnore;
import lombok.Data;

import java.io.Serializable;

/**
 * @author chushiyan
 * @email  chushiyan0415@163.com
 * @description
 */
@Data
public class Article implements Serializable {

    private String id;

    private String name;

    @JsonIgnore
    private String category;
}

2、测试

    @Test
    public void testJsonIgnore() throws JsonProcessingException {
        String json = "{"
                + "\"id\" : \"100\","
                + "\"name\" : \"详解jackson注解","
                + "\"category\" : \"jackson\""
                + "}";
        ObjectMapper mapper = new ObjectMapper();
        Article article = mapper.readValue(json,Article.class);
        System.out.println(article);

        Article article2 = new Article();
        article2.setId("100");
        article2.setName("详解jackson注解");
        article2.setCategory("jackson");
        String json2 = mapper.writeValueAsString(article2);
        System.out.println(json2);
}

3、控制台输出

Article(id=100, name=详解jackson注解, category=null)
{"id":"100","name":"详解jackson注解"}

反系列化时，json数据中明明包含了category值，但是最后Article对象的category属性值依然为null。就是因为使用了JsonIgnore。

三、JsonInclude

我们可以使用@JsonInclude排除某些empty、null、默认值的属性。

import com.fasterxml.jackson.annotation.JsonInclude;

/**
 * @author chushiyan
 * @email chushiyan0415@163.com
 * @description
 */
// 使用JsonInclude注解指明只包含非null的属性，即排除值为null的属性
@JsonInclude(JsonInclude.Include.NON_NULL)
public class Result {

    private boolean flag;// 是否成功

    private Integer code;// 返回码

    private String message;// 返回信息

    private Object data;// 返回数据

    public Result() {
    }

    public Result(boolean flag, Integer code, String message) {
        this.flag = flag;
        this.code = code;
        this.message = message;
    }

    public Result(boolean flag, Integer code, String message, Object data) {
        this.flag = flag;
        this.code = code;
        this.message = message;
        this.data = data;
    }

    public boolean isFlag() {
        return flag;
    }

    public void setFlag(boolean flag) {
        this.flag = flag;
    }

    public Integer getCode() {
        return code;
    }

    public void setCode(Integer code) {
        this.code = code;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }
    
    @Override
    public String toString() {
        return "Result{" +
                "flag=" + flag +
                ", code=" + code +
                ", message='" + message + '\'' +
                ", data=" + data +
                '}';
    }

}

2、controller

    @GetMapping("/test8")
    public Result testJsonInclude(){
        return new  Result(true,200,"查询成功");
    }

没有在Result类上加 @JsonInclude(JsonInclude.Include.NON_NULL) 注解时，返回的响应：

{
    "flag": true,
    "code": 200,
    "message": "查询成功",
    "data": null
}

加了之后的返回的响应：

{
    "flag": true,
    "code": 200,
    "message": "查询成功"
}

四、 JsonAutoDetect

默认情况下，jackson获取public权限的字段进行系列化和反系列化。如果没有public权限的字段，就会去获取public修饰的getter/setter。使用 JsonAutoDetect注解，我们就可以修改默认的行为。比如，下面的案例，即使字段都是private的、也没有getter/setter，也照样获取字段。

1、bean

首先，我们定义一个属性id/name都私有的而且没有getter/setter的一个bean

import com.fasterxml.jackson.annotation.JsonAutoDetect;

import java.io.Serializable;

/**
 * @author chushiyan
 * @email chushiyan0415@163.com
 * @description
 */
//@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
public class PrivateBean implements Serializable {

    private String id;

    private String name;

    public PrivateBean() {
    }

    public PrivateBean(String id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        return "PrivateBean{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}

2、反系列化测试：

    @Test
    public void testJsonAutoDetect() throws JsonProcessingException {
        String json = "{"
                + "\"id\" : \"10\","
                + "\"name\" : \"chushiyan\""
                + "}";
        ObjectMapper mapper = new ObjectMapper();
        PrivateBean bean = mapper.readValue(json, PrivateBean.class);
        System.out.println(bean);
    }

报错：

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "id" (class com.chushiyan.test.entity.PrivateBean), not marked as ignorable (0 known properties: ])
 at [Source: (String)"{"id" : "10","name" : "chushiyan"}"; line: 1, column: 10] (through reference chain: com.chushiyan.test.entity.PrivateBean["id"])

3、系列化测试

    @Test
    public void testJsonAutoDetect2() throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        PrivateBean bean = new PrivateBean("11", "chushiyan");
        String json = mapper.writeValueAsString(bean);
        System.out.println(json);
    }

报错：

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class com.chushiyan.test.entity.PrivateBean and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)

4、bean上加上注解

import com.fasterxml.jackson.annotation.JsonAutoDetect;

import java.io.Serializable;

/**
 * @author chushiyan
 * @email chushiyan0415@163.com
 * @description
 */
@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY)
public class PrivateBean implements Serializable {

    private String id;

    private String name;

    public PrivateBean() {
    }

    public PrivateBean(String id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
        return "PrivateBean{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}

5、再次分别测试反系列化、系列化

都运行正常，分别打印：

PrivateBean{id='10', name='chushiyan'}

{"id":"11","name":"chushiyan"}