JavaEE（15）JSON、Jackson、FastJson
1. JSON
1. JSON概念
（1）JSON(JavaScript Object Notation, JS 对象标记) 是一种轻量级的数据交换格式
（2）采用完全独立于编程语言的文本格式来存储和表示数据
（3）简洁和清晰的层次结构使得 JSON 成为理想的数据交换语言
（4）易于人阅读和编写，同时也易于机器解析和生成，并有效地提升网络传输效率
2. JSON
（1）格式
1. 对象表示为键值对，数据由逗号分隔
2. 花括号保存对象
3. 方括号保存数组
{
"name":"老王"}
{
"age":"12"}
{
"sex","男"}
（2）JSON与JS对象的关系
JSON是JS对象的字符串表示法，他使用文本表示一个JS对象的信息，本质上是一个字符串
var obj = {
a:'hello',b:'world'}; //JS对象
var str = '{"a":"hello","b","world"}'; //JSON字符串
（3）JSON字符串与JS对象互转
1. JSON字符串==>JavaScript对象：JSON.parse()
2. JavaScript对象==>JSON字符串：JSON.stringify()
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Titletitle>
head>
<body>
body>
html>
<script>
//JS对象
var User = {
name : "老王",
age : 13,
sex : "男"
};
//JS对象转化为JSON字符串
var str = JSON.stringify(User);
console.log(str); //{"name":"老王","age":13,"sex":"男"}
//再将字符串转化为JS对象
var obj = JSON.parse(str);
console.log(obj); //Object { name: "老王", age: 13, sex: "男" }
script>
2. Controller返回JSON数据：使用Jackson
1. Jackson概念：JSON解析工具
2. 使用
（1）导入jar包
<dependency>
<groupId>com.fasterxml.jackson.coregroupId>
<artifactId>jackson-databindartifactId>
<version>2.9.8version>
dependency>
（2）配置web.xml
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
<servlet>
<servlet-name>springmvcservlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServletservlet-class>
<init-param>
<param-name>contextConfigLocationparam-name>
<param-value>classpath:springmvc-servlet.xmlparam-value>
init-param>
<load-on-startup>1load-on-startup>
servlet>
<servlet-mapping>
<servlet-name>springmvcservlet-name>
<url-pattern>/url-pattern>
servlet-mapping>
<filter>
<filter-name>encodingfilter-name>
<filter-class>org.springframework.web.filter.CharacterEncodingFilterfilter-class>
<init-param>
<param-name>encodingparam-name>
<param-value>utf-8param-value>
init-param>
filter>
<filter-mapping>
<filter-name>encodingfilter-name>
<url-pattern>/*url-pattern>
filter-mapping>
web-app>
（3）配置spring-servlet.xml
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/mvc https://www.springframework.org/schema/mvc/spring-mvc.xsd">
<context:component-scan base-package="com.nelws.controller"/>
<mvc:default-servlet-handler/>
<mvc:annotation-driven/>
<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver" id="internalResourceViewResolver">
<property name="prefix" value="/WEB-INF/jsp/"/>
<property name="suffix" value=".jsp"/>
bean>
beans>
（4）编写User实体类
@Data
@AllArgsConstructor
@NoArgsConstructor
public class User {
private String name;
private int age;
private String sex;
}
（5）编写Controller类
@Controller
public class UserController {
@RequestMapping("/j01")
@ResponseBody //此注解不会通过视图解析器，会直接返回字符串
public String json01() throws JsonProcessingException {
//创建一个jackson的对象映射器，用来解析数据
ObjectMapper mapper = new ObjectMapper();
//创建一个对象
User user = new User("小王", 12, "男");
//将对象解析成JSON
String str = mapper.writeValueAsString(user);
return str;
}
}
（6）配置tomcat，并测试：http://localhost:8080/j01
发现有乱码
（7）乱码解决：在springmvc-servlet.xml中添加
<mvc:annotation-driven>
<mvc:message-converters register-defaults="true">
<bean class="org.springframework.http.converter.StringHttpMessageConverter">
<constructor-arg value="UTF-8"/>
bean>
<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
<property name="objectMapper">
<bean class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
<property name="failOnEmptyBeans" value="false"/>
bean>
property>
bean>
mvc:message-converters>
mvc:annotation-driven>
（8）返回JSON字符串的统一解决：常用
直接在类上使用@RestController。这样子，里面的所有方法都只会返回JSON字符串了，不用再每个类上添加@ResponseBody
@RestController
public class UserController {
@RequestMapping("/j01")
public String json01() throws JsonProcessingException {
//创建一个jackson的对象映射器，用来解析数据
ObjectMapper mapper = new ObjectMapper();
//创建一个对象
User user = new User("小王", 12, "男");
//将对象解析成JSON
String str = mapper.writeValueAsString(user);
return str;
}
}
（9）测试：http://localhost:8080/j01
3. 测试集合输出
（1）编写Controller类
@RequestMapping("/j02")
public String json02() throws JsonProcessingException {
ObjectMapper mapper = new ObjectMapper();
User user1 = new User("小王1号", 14, "男");
User user2 = new User("小王2号", 14, "男");
User user3 = new User("小王3号", 14, "男");
User user4 = new User("小王4号", 14, "男");
ArrayList<User> list = new ArrayList<User>();
list.add(user1);
list.add(user2);
list.add(user3);
list.add(user4);
String str = mapper.writeValueAsString(list);
return str;
}
（2）测试：http://localhost:8080/j02
4. 测试时间输出
（1）编写Controller类
@RequestMapping("/j03")
public String json03() throws JsonProcessingException {
ObjectMapper mapper = new ObjectMapper();
Date date = new Date();
//自定义时间格式
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String str = mapper.writeValueAsString(sdf.format(date));
return str;
}
（2）测试：http://localhost:8080/j03
5. 抽取JSON工具类
（1）编写工具类
public class JsonUtil {
public static String getJson(Object object){
return getJson(object,"yyyy-MM-dd HH:mm:ss");
}
public static String getJson(Object object,String dataFormat){
ObjectMapper mapper = new ObjectMapper();
//不使用时间戳的方式
mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
//自定义日期格式对象
SimpleDateFormat sdf = new SimpleDateFormat(dataFormat);
//指定日期格式
mapper.setDateFormat(sdf);
try {
return mapper.writeValueAsString(object);
} catch (JsonProcessingException e) {
e.printStackTrace();
}
return null;
}
}
（2）编写Controller类
@RequestMapping("/j04")
public String json04() throws JsonProcessingException {
Date date = new Date();
String str = JsonUtil.getJson(date);
return str; //"2021-01-11 14:11:30"
}
@RequestMapping("/j05")
public String json05() throws JsonProcessingException {
User user = new User("阿萨德", 11, "男");
String str = JsonUtil.getJson(user);
return str; //{"name":"阿萨德","age":11,"sex":"男"}
}
（3）测试
3. FastJson
1. 介绍：fastjson.jar是阿里开发的一款专门用于Java开发的包，可以方便的实现json对象与JavaBean对象的转换，实现JavaBean对象与json字符串的转换，实现json对象与json字符串的转换
2. 使用
（1）添加jar包
<dependency>
<groupId>com.alibabagroupId>
<artifactId>fastjsonartifactId>
<version>1.2.60version>
dependency>
（2）使用
public class FastJsonTest {
public static void main(String[] args) {
User user1 = new User("小王", 14, "男");
User user2 = new User("小李", 14, "女");
ArrayList<User> list = new ArrayList<User>();
list.add(user1);
list.add(user2);
System.out.println("=====java对象 转 JSON字符串=====");
String user_str = JSON.toJSONString(user1);
String list_str = JSON.toJSONString(list);
System.out.println(user_str); //{"age":14,"name":"小王","sex":"男"}
System.out.println(list_str); //[{"age":14,"name":"小王","sex":"男"},
//{"age":14,"name":"小李","sex":"女"}]
System.out.println("=====JSON字符串 转 java对象=====");
User user_obj = JSON.parseObject(user_str, User.class);
System.out.println(user_obj); //User(name=小王, age=14, sex=男)
}
}