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

Person.java

package cn.itcast.domain;import com.fasterxml.jackson.annotation.JsonFormat;import java.util.Date;public class Person {private String name;private int age ;private String gender;//@JsonIgnore // 忽略该属性@JsonFormat(pattern = "yyyy-MM-dd")private Date birthday;public Date getBirthday() {return birthday;}public void setBirthday(Date birthday) {this.birthday = birthday;}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;}@Overridepublic String toString() {return "Person{" +"name='" + name + '\'' +", age=" + age +", gender='" + gender + '\'' +'}';}
}

JacksonTest.java

package cn.itcast.test;import cn.itcast.domain.Person;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;import java.io.FileWriter;
import java.util.*;public class JacksonTest {//Java对象转为JSON字符串@Testpublic void test1() throws Exception {//1.创建Person对象Person p  = new Person();p.setName("张三");p.setAge(23);p.setGender("男");//2.创建Jackson的核心对象  ObjectMapperObjectMapper 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*/String json = mapper.writeValueAsString(p);//{"name":"张三","age":23,"gender":"男"}//System.out.println(json);//{"name":"张三","age":23,"gender":"男"}//writeValue，将数据写到d://a.txt文件中//mapper.writeValue(new File("d://a.txt"),p);//writeValue.将数据关联到Writer中mapper.writeValue(new FileWriter("d://b.txt"),p);}@Testpublic void test2() throws Exception {//1.创建Person对象Person p = new Person();p.setName("张三");p.setAge(23);p.setGender("男");p.setBirthday(new Date());//2.转换ObjectMapper mapper = new ObjectMapper();String json = mapper.writeValueAsString(p);System.out.println(json);//{"name":"张三","age":23,"gender":"男","birthday":1530958029263}//{"name":"张三","age":23,"gender":"男","birthday":"2018-07-07"}}@Testpublic void test3() throws Exception {//1.创建Person对象Person p = new Person();p.setName("张三");p.setAge(23);p.setGender("男");p.setBirthday(new Date());Person p1 = new Person();p1.setName("张三");p1.setAge(23);p1.setGender("男");p1.setBirthday(new Date());Person p2 = new Person();p2.setName("张三");p2.setAge(23);p2.setGender("男");p2.setBirthday(new Date());//创建List集合List<Person> ps = new ArrayList<Person>();ps.add(p);ps.add(p1);ps.add(p2);//2.转换ObjectMapper mapper = new ObjectMapper();String json = mapper.writeValueAsString(ps);// [{},{},{}]//[{"name":"张三","age":23,"gender":"男","birthday":"2018-07-07"},{"name":"张三","age":23,"gender":"男","birthday":"2018-07-07"},{"name":"张三","age":23,"gender":"男","birthday":"2018-07-07"}]System.out.println(json);}@Testpublic void test4() throws Exception {//1.创建map对象Map<String,Object> map = new HashMap<String,Object>();map.put("name","张三");map.put("age",23);map.put("gender","男");//2.转换ObjectMapper mapper = new ObjectMapper();String json = mapper.writeValueAsString(map);//{"name":"张三","age":23,"gender":"男"}System.out.println(json);//{"gender":"男","name":"张三","age":23}}//演示 JSON字符串转为Java对象@Testpublic void test5() throws Exception {//1.初始化JSON字符串String json = "{\"gender\":\"男\",\"name\":\"张三\",\"age\":23}";//2.创建ObjectMapper对象ObjectMapper mapper = new ObjectMapper();//3.转换为Java对象 Person对象Person person = mapper.readValue(json, Person.class);System.out.println(person);}}