JSON数据和Java对象的相互转换

• JSON解析器
  • 创建的解析器：Jsonlib，Gson，Fastjson，jackson

JSON转为Java对象

• 使用步骤：
  • 导入jackson相关的jar包
  • 创建Jackson革新对象 ObjectMapper
  • 调用ObjectMapper的相关方法进行转换
    • readValue(json字符串，Class)

    @Test
    public void Test05() throws Exception {
        String json = "{\"gender\":\"男\",\"name\":\"张三\",\"age\":19}";
        ObjectMapper mapper = new ObjectMapper();
        Person person = mapper.readValue(json, Person.class);
        System.out.println(person);//Person{name='张三', age=19, gender='男', date=null}
    }

Java对象转换JSON

• 使用步骤：
  • 导入jackson相关的jar包
  • 创建Jackson革新对象 ObjectMapper
  • 调用ObjectMapper的相关方法进行转换

package com.ailyt;

import com.ailyt.domain.Person;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;

import java.io.FileWriter;

public class JacksonTest {
    //Java对象转为Json字符串
    @Test
    public void Test01() throws Exception {
        //创建Person对象
        Person person = new Person();
        person.setName("张三");
        person.setAge(17);
        person.setGender("男");

        //创建Jackson的核心对象ObjectMapper
        ObjectMapper mapper = new ObjectMapper();

        //转换
        /**
         * 转换方法：
         *  writerValue(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中
         *          OutputStream：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
         *  writerValueAsString(obj):将对象转为json字符串
         */
        String json = mapper.writeValueAsString(person);
        System.out.println(json);//{"name":"张三","age":17,"gender":"男"}

        //writeValue 将文件写入到D:\\a.txt
        FileWriter fileWriter = new FileWriter("D:\\a.txt");
        mapper.writeValue(fileWriter,person);
        fileWriter.close();
    }
}

• 注解：
  • @JsonIgnore：排除属性
  • @JsonFormat：属性值得格式化

package com.ailyt.domain;

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonIgnore;

import java.util.Date;

public class Person {
    private String name;
    private int age;
    private String gender;
    //@JsonIgnore//忽略改属性
    @JsonFormat(pattern = "yyyy-MM-dd")//
    private Date date;

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

    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;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    public Person() {
    }

    public Person(String name, int age, String gender, Date date) {
        this.name = name;
        this.age = age;
        this.gender = gender;
        this.date = date;
    }
}

    @Test
    public void Test02() throws Exception {
        //创建Person对象
        Person person = new Person();
        person.setName("张三");
        person.setAge(17);
        person.setGender("男");
        person.setDate(new Date());

        ObjectMapper mapper = new ObjectMapper();
        String value = mapper.writeValueAsString(person);
        System.out.println(value);//{"name":"张三","age":17,"gender":"男","date":1661051661487}
    }

复杂Java对象转换

• List:数组
• Map：对象格式一直

    @Test
    public void Test03() throws Exception {
        //创建Person对象
        Person person1 = new Person();
        person1.setName("张三");
        person1.setAge(17);
        person1.setGender("男");
        person1.setDate(new Date());

        Person person2 = new Person();
        person2.setName("李四");
        person2.setAge(15);
        person2.setGender("女");
        person2.setDate(new Date());

        Person person3 = new Person();
        person3.setName("王五");
        person3.setAge(21);
        person3.setGender("男");
        person3.setDate(new Date());

        ArrayList<Person> people = new ArrayList<>();

        Collections.addAll(people,person1,person2,person3);

        ObjectMapper mapper = new ObjectMapper();

        String s = mapper.writeValueAsString(people);
        System.out.println(s);//[{"name":"张三","age":17,"gender":"男","date":"2022-08-21"},{"name":"李四","age":15,"gender":"女","date":"2022-08-21"},{"name":"王五","age":21,"gender":"男","date":"2022-08-21"}]

    }


    @Test
    public void Test04() throws Exception {
        HashMap<String, Object> map = new HashMap<>();
        map.put("name","张三");
        map.put("age",19);
        map.put("gender",'男');

        ObjectMapper mapper = new ObjectMapper();

        String s = mapper.writeValueAsString(map);
        System.out.println(s);//{"gender":"男","name":"张三","age":19}
    }