首先引入依赖包，这里使用的是alibaba的fastjson；

    <!-- fastjson依赖 -->
    <dependency>
        <groupId>com.alibaba</groupId>
        <artifactId>fastjson</artifactId>
        <version>1.2.76</version>
    </dependency>

Java对象转成JSON格式

import com.alibaba.fastjson.JSON;
import com.example.study.entity.Student;

public class JsonStudy {
    public static void main(String[] args) {
        Student student = new Student("123","张三",18);
        //直接输出，结果为：Student(id=123, name=张三, age=18)
        System.out.println(student);
       //转换为JSON格式输出，以下两种方法只有返回值不同
        Object objectJson = JSON.toJSON(student);
        String stringJSON = JSON.toJSONString(student);
        //结果为：{"name":"张三","id":"123","age":18}
        System.out.println(objectJson);
        //结果为：{"name":"张三","id":"123","age":18}
        System.out.println(stringJSON);
    }
}

JSON格式字符串转换成Java对象

直接将JSON字符串转换成Java对象

import com.alibaba.fastjson.JSONObject;
import com.example.study.entity.Student;

public class JsonStudy {
    public static void main(String[] args) {
        //定义一个student类型的JSON字符串
        String json = "{\"name\":\"张三\",\"id\":\"123\",\"age\":18}";
        //将这个JSON字符串转换成Student对象
        Student student = JSONObject.parseObject(json, Student.class);
        //输出结果为：Student(id=123, name=张三, age=18)
        System.out.println(student);
        //输出结果为：张三
        System.out.println(student.getName());
    }
}

 先将JSON字符串转换成JSON对象，再转换成Java对象

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.example.study.entity.Student;

public class JsonStudy {
    public static void main(String[] args) {
        //定义一个student类型的JSON字符串
        String json = "{\"name\":\"张三\",\"id\":\"123\",\"age\":18}";
        //将这个JSON字符串转换成JSON对象
        JSONObject jsonObject = JSONObject.parseObject(json);
        //将JSON对象转换成Java对象
        Student student1 = JSONObject.toJavaObject(jsonObject, Student.class);
        Student student2 = JSON.toJavaObject(jsonObject, Student.class);

        //输出结果为：Student(id=123, name=张三, age=18)
        System.out.println(student1);
        //输出结果为：张三
        System.out.println(student1.getName());

        //输出结果为：Student(id=123, name=张三, age=18)
        System.out.println(student2);
        //输出结果为：张三
        System.out.println(student2.getName());
    }
}

如果JSON字符串是一个JSON数组，并且数组里面存放的同一种类型的对象，可以将这个JSON数组转换成Java的List对象；

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONObject;
import com.example.study.entity.Student;

import java.util.ArrayList;
import java.util.List;

public class JsonStudy {
    public static void main(String[] args) {
        //new一个list对象
        List<Student> studentList = new ArrayList<>();
        studentList.add(new Student("111","张三",18));
        studentList.add(new Student("222","李四",20));
        studentList.add(new Student("333","王五",23));

        //获取list的JSON数组形式字符串
        String listJsonString = JSON.toJSONString(studentList);
        //结果为：[{"age":18,"id":"111","name":"张三"},{"age":20,"id":"222","name":"李四"},{"age":23,"id":"333","name":"王五"}]
        System.out.println(listJsonString);

        //将JSON字符串转换成List对象，List里面放的Student对象
        List<Student> students = JSONObject.parseArray(listJsonString, Student.class);
        //结果为：[Student(id=111, name=张三, age=18), Student(id=222, name=李四, age=20), Student(id=333, name=王五, age=23)]
        System.out.println(students);
        //结果为：3
        System.out.println(students.size());
        //结果为：Student(id=333, name=王五, age=23)
        System.out.println(students.get(2));
    }
}