1. 背景

在部分使用场景（如元数据驱动，后台接口仅返回序列化后的json字符串，前端需要根据每个字段在前端呈现），需要手动设置字段的长度。通常情况下，框架是有默认的顺序，如 jackson 默认使用字段声明的顺序， fastjson 默认是使用字典序。在这种业务场景下，就需要我们可以手动指定序列化后字段的顺序。

这里分别使用 jackson 和 fastjson 两种框架。

2. 使用 jackson

使用注解 @JsonPropertyOrder 声明具体的字段顺序。如 @JsonPropertyOrder({"city", "age", "name"}) 。具体参考实例程序。

pom依赖：

<dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>2.14.3</version>
</dependency>

Java 验证程序：

package com.ysx.utils.json.jackson;import com.fasterxml.jackson.annotation.JsonPropertyOrder;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Getter;
import lombok.Setter;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;/*** @author youngbear* @email youngbear@aliyun.com* @date 2023-10-15 22:15* @blog <a href="https://blog.csdn.net/next_second">...</a>* @github <a href="https://github.com/YoungBear">...</a>* @description*/
@DisplayName("使用jackson设置序列化时字段的顺序")
public class JsonFieldOrderTest {@Test@DisplayName("默认顺序:字段声明的顺序")public void defaultOrderTest() throws JsonProcessingException {Student1 student = new Student1();student.setName("John");student.setAge(25);student.setCity("Beijing");ObjectMapper objectMapper = new ObjectMapper();String json = objectMapper.writeValueAsString(student);// {"name":"John","age":25,"city":"Beijing"}Assertions.assertEquals("{\"name\":\"John\",\"age\":25,\"city\":\"Beijing\"}", json);}@Test@DisplayName("使用@JsonPropertyOrder指定顺序")public void userJsonPropertyOrderTest() throws JsonProcessingException {Student2 student = new Student2();student.setName("John");student.setAge(25);student.setCity("Beijing");ObjectMapper objectMapper = new ObjectMapper();String json = objectMapper.writeValueAsString(student);// {"city":"Beijing","age":25,"name":"John"}Assertions.assertEquals("{\"city\":\"Beijing\",\"age\":25,\"name\":\"John\"}", json);}@Getter@Setterpublic static class Student1 {private String name;private Integer age;private String city;}@Getter@Setter@JsonPropertyOrder({"city", "age", "name"})public static class Student2 {private String name;private Integer age;private String city;}
}

3. 使用 fastjson

有两种方法，

• 使用 @JsonField 的 ordinal 指定顺序，数越小优先级越高，默认为0。
• 使用@JSONType的orders属性指定字段顺序

详细参考实例程序：

pom依赖：

<!-- https://mvnrepository.com/artifact/com.alibaba/fastjson -->
<dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>2.0.41</version>
</dependency>

程序：

package com.ysx.utils.json.fastjson;import com.alibaba.fastjson2.JSON;
import com.alibaba.fastjson2.annotation.JSONField;
import com.alibaba.fastjson2.annotation.JSONType;
import lombok.Getter;
import lombok.Setter;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;/*** @author youngbear* @email youngbear@aliyun.com* @date 2023-10-15 21:28* @blog <a href="https://blog.csdn.net/next_second">...</a>* @github <a href="https://github.com/YoungBear">...</a>* @description*/
@DisplayName("使用fastjson设置序列化时字段的顺序")
public class JsonFieldOrderTest {@Test@DisplayName("默认顺序:字典序")public void defaultOrderTest() {Student1 student = new Student1();student.setName("John");student.setAge(25);student.setCity("Beijing");String json = JSON.toJSONString(student);// {"age":25,"city":"Beijing","name":"John"}Assertions.assertEquals("{\"age\":25,\"city\":\"Beijing\",\"name\":\"John\"}", json);}@Test@DisplayName("使用@JsonField的ordinal指定顺序，数越小优先级越高，默认为0")public void userJsonFieldOrdinalTest() {Student2 student = new Student2();student.setName("John");student.setAge(25);student.setCity("Beijing");String json = JSON.toJSONString(student);// {"name":"John","age":25,"city":"Beijing"}Assertions.assertEquals("{\"name\":\"John\",\"age\":25,\"city\":\"Beijing\"}", json);}@Test@DisplayName("使用@JSONType的orders属性指定字段顺序")public void userJsonPropertyOrderTest() {Student3 student = new Student3();student.setName("John");student.setAge(25);student.setCity("Beijing");String json = JSON.toJSONString(student);// {"name":"John","age":25,"city":"Beijing"}Assertions.assertEquals("{\"name\":\"John\",\"age\":25,\"city\":\"Beijing\"}", json);}@Getter@Setterpublic static class Student1 {private String name;private Integer age;private String city;}@Getter@Setterpublic static class Student2 {@JSONField(ordinal = 1)private String name;@JSONField(ordinal = 2)private Integer age;@JSONField(ordinal = 3)private String city;}@Getter@Setter@JSONType(orders = {"name", "age", "city"})public static class Student3 {private String name;private Integer age;private String city;}
}

源代码github地址