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
	
	1. JSON转为Java对象
		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
			1. readValue(json字符串数据,Class)
	2. Java对象转换JSON
		1. 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
				1. 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
	                * writeValueAsString(obj):将对象转为json字符串

				2. 注解：
					1. @JsonIgnore：排除属性。
					2. @JsonFormat：属性值得格式化
						* @JsonFormat(pattern = "yyyy-MM-dd")

				3. 复杂java对象转换
					1. List：数组
					2. Map：对象格式一致

测试类

import com.code_g.domain.Person;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;

import java.io.File;
import java.util.ArrayList;
import java.util.Date;
import java.util.HashMap;

public class TestJackSon {

    //将java对象转换为JavaScript
    @Test
    public void testJack1() throws Exception {
        Person person = new Person();
        person.setName("code_g");
        person.setAge(18);
        person.setGender("male");
        ObjectMapper mapper = new ObjectMapper();
//        String s = mapper.writeValueAsString(person);   //将对象转换为json字符串
//        System.out.println(s);  //{"name":"code_g","age":18,"gender":"male"}

        //将对象转为json在写入文件中
        mapper.writeValue(new File("E:\\a.txt"),person);
    }

    //测试注解
    @Test
    public void testJack2() throws Exception {
        Person person = new Person();
        person.setName("code_g");
        person.setAge(18);
        person.setGender("male");
        person.setBirthday(new Date());
        ObjectMapper mapper = new ObjectMapper();
        String s = mapper.writeValueAsString(person);
        System.out.println(s);
        //{"name":"code_g","age":18,"gender":"male","birthday":1620825617349}
        //加@JsonIgnore注解 {"name":"code_g","age":18,"gender":"male"}
        //加@JsonFormat注解 {"name":"code_g","age":18,"gender":"male","birthday":"2021-05-12"}
    }

    //数组转换
    @Test
    public void testJack3() throws Exception {
        Person person1 = new Person();
        person1.setName("code_g");
        person1.setAge(18);
        person1.setGender("male");
        person1.setBirthday(new Date());

        Person person2 = new Person();
        person2.setName("code_g");
        person2.setAge(18);
        person2.setGender("male");
        person2.setBirthday(new Date());

        Person person3 = new Person();
        person3.setName("code_g");
        person3.setAge(18);
        person3.setGender("male");
        person3.setBirthday(new Date());

        ArrayList<Person> list = new ArrayList<>();
        list.add(person1);
        list.add(person2);
        list.add(person3);

        ObjectMapper mapper = new ObjectMapper();
        String s = mapper.writeValueAsString(list);
        System.out.println(s);
        //[
        // {"name":"code_g","age":18,"gender":"male","birthday":"2021-05-12"},
        // {"name":"code_g","age":18,"gender":"male","birthday":"2021-05-12"},{
        // "name":"code_g","age":18,"gender":"male","birthday":"2021-05-12"}
        // ]
    }

    //map
    @Test
    public void testJack4() throws Exception {
        HashMap<String, Object> map = new HashMap<>();
        map.put("name","code_g");
        map.put("age",18);
        map.put("gender","female");
        ObjectMapper mapper = new ObjectMapper();
        String s = mapper.writeValueAsString(map);
        System.out.println(s);
        //{"gender":"female","name":"code_g","age":18}
    }

    //将json转换为java对象
    @Test
    public void testJack5() throws Exception {
        String s = "{\"gender\":\"female\",\"name\":\"code_g\",\"age\":18}";
        ObjectMapper mapper = new ObjectMapper();
        Person person = mapper.readValue(s, Person.class);
        System.out.println(person);     //Person{name=‘code_g‘, age=18, gender=‘female‘, birthday=null}
    }
}

Person类

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

    public Date getBirthday() {
        return birthday;
    }

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

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

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

    public Person() {
    }

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

    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;
    }

}

Jackson解析Json