jackson一些用法总结

闲话就不多说了，之所以想起来看这个是因为我们的项目采用前后端分离，有些参数需要序列化为前端需要的格式，一般java定义参数采用驼峰格式，前端需要的是xxx_xxx，这时候使用jackson的序列化方式就很好实现了。

引入jar包

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.5.3</version>
</dependency>

2个pojo

package com.vincent.jacksonlearn;

import java.util.List;

public class School {

    private String schoolName;

    private List<Student> students;

    public String getSchoolName() {
        return schoolName;
    }

    public void setSchoolName(String schoolName) {
        this.schoolName = schoolName;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "School{" +
                "schoolName='" + schoolName + '\'' +
                ", students=" + students +
                '}';
    }
}

package com.vincent.jacksonlearn;

import java.util.Date;

public class Student {

    private String name;

    private Integer age;

    private Date birthdat;

    public String getName() {
        return name;
    }

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

    public Integer getAge() {
        return age;
    }

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

    public Date getBirthdat() {
        return birthdat;
    }

    public void setBirthdat(Date birthdat) {
        this.birthdat = birthdat;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", birthdat=" + birthdat +
                '}';
    }
}

先测试下java对象转成json字符串

@Test
    public void pojo2Json() throws JsonProcessingException {
        School school = new School();
        List<Student> list = new ArrayList<>();
        Student student = new Student();
        student.setAge(10);
        student.setName("abc");
        student.setBirthdat(new Date());
        school.setSchoolName("MIT");
        list.add(student);
        school.setStudents(list);
        ObjectMapper objectMapper = new ObjectMapper();
        String jsonStr = objectMapper.writeValueAsString(school);
        System.out.println(jsonStr);
    }

输出结果为：{"schoolName":"MIT","students":[{"name":"abc","age":10,"birthdat":1504160502692}]}

测试下json字符串转成java对象

@Test
    public void json2Pojo() throws IOException {
        String jsonStr = "{\"schoolName\":\"MIT\",\"students\":[{\"name\":\"abc\",\"age\":10,\"birthdat\":1504160502692}]}";
        ObjectMapper objectMapper = new ObjectMapper();
        School school = objectMapper.readValue(jsonStr,School.class);
        System.out.println(school);
    }

输出结果为：School{schoolName='MIT', students=[Student{name='abc', age=10, birthdat=Thu Aug 31 14:21:42 CST 2017}]}

基本用法已经介绍完毕，下面开始根据实际需求进行相应改变。

情景一：

以第一个java对象转为json字符的示例来说，后端代码写完了，这时候我们将结果输出给前端（spring开发的小伙伴别忘了@ResponseBody注解），前端一看，不对啊，schoolName怎么是驼峰命名呢？日期怎么没有按照我们约定的yyyy-MM-dd的格式呢。

这时候我们在这两个属性下加上这两个注解就ok了。

@JsonProperty("school_name")
    private String schoolName;

@JsonFormat(pattern = "yyyy-MM-dd")
    private Date birthdat;

再运行下之前的测试方法会发现结果变成了前端想要的结果了。

{"students":[{"name":"abc","age":10,"birthdat":"2017-08-31"}],"school_name":"MIT"}

情景二：

有时候有些值的结果并不想返回给前端，这时候在这个属性上加上@JsonIgnore就ok了

在student的age上加上这个注解
@JsonIgnore
    private Integer age;

执行ojo2Json()方法返回结果如下

{"students":[{"name":"abc","birthdat":"2017-08-31"}],"school_name":"MIT"}

 

说了这么多，大家一定有疑问，问什么这些注解能应用于spring呢？

原因在于spring的AbstractMessageConverterMethodArgumentResolver类的readWithMessageConverters方法

仔细看下去会发现他最终会调用这个一段代码

if (inputMessage instanceof MappingJacksonInputMessage) {
                Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
                if (deserializationView != null) {
                    return this.objectMapper.readerWithView(deserializationView).forType(javaType).
                            readValue(inputMessage.getBody());
                }
            }
            return this.objectMapper.readValue(inputMessage.getBody(), javaType);

 

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