JSON数据和Java对象的相互转换

JSON解析器:

常见的解析器: Jsonlib, Gson, fastjson, jackson

其中应用最广泛的是jackson，阿里的fastjson虽然比jackson快一点，但存在的问题比较多，推荐使用jackson

使用步骤:

1. 导入jackson的相关jar包
2. 创建Jackson核心对象objectMapper
3. 调用objectMapper的相关方法进行转换

---

Java对象转换JSON

person类

import com.fasterxml.jackson.annotation.JsonFormat;

import java.util.Date;

public class Person {
    private String name;
    private int age;
    private String gender;

    //JsonIgnore
    @JsonFormat(pattern = "yyyy-MM-dd")
    private Date bir;

    public Date getBir() {
        return bir;
    }

    public void setBir(Date bir) {
        this.bir = bir;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

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

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

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

    //Java对象转为JSON字符串
    @Test
    public void test1() throws Exception {
        //1.创建Person对象
        Person p  = new Person();
        p.setName("张三");
        p.setAge(22);
        p.setGender("男");
        p.setBir(new Date());

        //2.创建Jackson的核心对象 ObjectMapper
        ObjectMapper mapper = new ObjectMapper();
        //3.转换
        /**
         * 转换方法：
         *       writeValue(参数1，obj):
         *       参数1：
         *           File:将obj对象转换为JSON字符串，并保存到指定的文件中
         *           Writer:将obj对象转换为JSON字符串，并将json数据填充到字符输出流
         *           OutputString：将obj对象转换为JSON字符串，并将json数据填充到字节输出流中
         *
         *        writeValueAsString(obj):将对象转为json字符串
         *
         */
        String json = mapper.writeValueAsString(p);
        System.out.println(json);//{"name":"张三","age":22,"gender":"男"}

        //writeValue，将数据写到a.txt文件中
        mapper.writeValue(new File("a.txt"),p);

        //writeValue.将数据关联到Writer中
        mapper.writeValue(new FileWriter("b.txt"),p);
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注解：

1. @JsonIgnore :排除属性。
2. @JsonFormat :属性值得格式化

    //JsonIgnore
    @JsonFormat(pattern = "yyyy-MM-dd")
    private Date bir;
123

List方式：

 //lIST集合
    @Test
    public void test2() throws Exception{
        Person p2  = new Person();
        p2.setName("张三");
        p2.setAge(22);
        p2.setGender("男");
        p2.setBir(new Date());

        //创建List集合
        List<Person> ps = new ArrayList<Person>();
        ps.add(p2);

        //2.转换
        ObjectMapper mapper = new ObjectMapper();
        String json = mapper.writeValueAsString(ps);
        System.out.println(json);//[{"name":"张三","age":22,"gender":"男","bir":"2020-06-17"}]
    }
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Map方式：

@Test
    public void test3() throws Exception{
        Person p3  = new Person();
        p3.setName("张三");
        p3.setAge(22);
        p3.setGender("男");
        p3.setBir(new Date());

        //创建map集合
        Map<String,Object> map = new HashMap<String,Object>();
        map.put("name","李四");
        map.put("age",22);
        map.put("gender","男");

        //2.转换
        ObjectMapper mapper = new ObjectMapper();
        String json = mapper.writeValueAsString(map);
        System.out.println(json);//{"gender":"男","name":"李四","age":22}
    }

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---

JSON转为Java对象

使用readValue

    //json转java 字符串对象
    @Test
    public void test4() throws Exception{
        //初始化json字符串
        String json = "{\"gender\":\"男\",\"name\":\"李四\",\"age\":22}";

        //创建ObjMapper对象
        ObjectMapper mapper = new ObjectMapper();
        Person person = mapper.readValue(json, Person.class);
        System.out.println(person);
    }

参考文档