json格式的字符串与对象的互相转换
Jackson 简介
Jackson是一个简单基于Java应用库，Jackson可以轻松的将Java对象转换成json字符串和xml文档，同样也可以将json、xml转换成Java对象。Jackson所依赖的jar包较少，简单易用并且性能也要相对高些，并且Jackson社区相对比较活跃，更新速度也比较快。
jackson特点
- 容易使用 - jackson API提供了一个高层次外观，以简化常用的用例。
- 无需创建映射 - API提供了默认的映射大部分对象序列化。
- 性能高 - 快速，低内存占用，适合大型对象图表或系统。
- 干净的JSON - jackson创建一个干净和紧凑的JSON结果，这是让人很容易阅读。
- 不依赖 - 库不需要任何其他的库，除了JDK。
- 开源代码 - jackson是开源的，可以免费使用。
json格式的字符串与对象的转换
使用Jackson必须要在项目导入对应的jar包。
Book.java文件
package cn.hestyle.bean;
public class Book {
private String id;
private String name;
private double price;
private int pnum;
private String category;
private String description;
public Book() {
super();
// TODO Auto-generated constructor stub
}
public Book(String id, String name, double price, int pnum, String category, String description) {
super();
this.id = id;
this.name = name;
this.price = price;
this.pnum = pnum;
this.category = category;
this.description = description;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
public int getPnum() {
return pnum;
}
public void setPnum(int pnum) {
this.pnum = pnum;
}
public String getCategory() {
return category;
}
public void setCategory(String category) {
this.category = category;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
@Override
public String toString() {
return "Book [id=" + id + ", name=" + name + ", price=" + price + ", pnum=" + pnum + ", category=" + category
+ ", description=" + description + "]";
}
}
package cn.hestyle.utils;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import cn.hestyle.bean.Book;
public class JacksonDemo01 {
public static void main(String[] args) throws JsonGenerationException, JsonMappingException, IOException {
ObjectMapper objectMapper = new ObjectMapper();
//将对象转换为json格式的字符串
Book book = new Book("1001", "Java入门到入土", 99.9, 100, "编程开发", "你懂得！");
String bookJsonStr = objectMapper.writeValueAsString(book);
System.out.println(bookJsonStr);
//将集合、数组对象转换为json格式的字符串
List<Book> bookList = new ArrayList<Book>();
bookList.add(new Book("1001", "Java入门到入土", 99.9, 100, "编程开发", "你懂得！"));
bookList.add(new Book("1002", "Python入门到入土", 89.9, 100, "编程开发", "你懂得！"));
bookList.add(new Book("1003", "C++入门到入土", 89.9, 200, "编程开发", "你懂得！"));
String bookListJsonStr = objectMapper.writeValueAsString(bookList);
System.out.println(bookListJsonStr);
//将json格式的字符串转化为对象
//字符串中含有双引号，需要使用反斜杠转义
String bookString = "{\"id\":\"1001\",\"name\":\"Java入门到入土\",\"price\":99.9,\"pnum\":100}";
//Book必须有无参的构造方法
Book book2 = objectMapper.readValue(bookString, Book.class);
System.out.println(book2);
}
}
控制台输出：
Jackson进行json转对象,对象转json总结
在前后端分离的项目中，后端经常涉及到json与对象互转的场景。阿里巴巴的Fastjson是好用，但是，因为技术领导的原因（可能因为fastjson1的各种问题很多吧），不让用，所以就需要选择其他技术栈。当前比较常用的是SpringBoot自带的Jackson或者谷歌的Gson。
下面，做一下使用Jackson的总结。
JavaBean准备
1.Company，公司
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Company {
private String companyName;
private List<WebSite> webSites;
}
2.WebSite，站点
@Data
@NoArgsConstructor
@AllArgsConstructor
public class WebSite {
private String webSiteName;
private List<User> users;
}
3.User，用户
@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
private String userId;
private String username;
private String password;
}
json字符串和对象互转
对象转json字符串
不管你是什么结构的对象，想转json，直接objectMapper.writeValueAsString()搞定。
@Test
public void testObject2Json2() {
User user2 = new User("1","上海辟谣专属队","职业辟谣，不信谣，不传谣，呵呵");
List<User> users = new ArrayList<>();
users.add(user2);
WebSite webSite = new WebSite();
webSite.setWebSiteName("xxxxxx.com");
webSite.setUsers(users);
List<WebSite> webSites = new ArrayList<>();
webSites.add(webSite);
Company company = new Company();
company.setCompanyName("yyyyyy");
company.setWebSites(webSites);
ObjectMapper objectMapper = new ObjectMapper();
try {
String jsonString = objectMapper.writeValueAsString(company);
System.out.println(jsonString);
} catch (JsonProcessingException e) {
log.error("error: ", e);
}
}
输出：
{"companyName":"yyyyyy","webSites":[{"webSiteName":"xxxxxx.com","users":[{"userId":"1","username":"上海辟谣专属队","password":"职业辟谣，不信谣，不传谣，呵呵"}]}]}
json字符串转对象
注意：这种情形是json对象字符串，不能是json数组字符串，后面会说到。
@Test
public void testJson2Object() {
String json = "{\"companyName\":\"yyyyyy\",\"webSites\":[" +
"{\"webSiteName\":\"xxxxxx.com\",\"users\":[{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}]}" +
"]}";
ObjectMapper objectMapper = new ObjectMapper();
try {
Company company = objectMapper.readValue(json, Company.class);
System.out.println(company.getWebSites().get(0).getUsers().get(0).getUsername());
} catch (JsonProcessingException e) {
log.error("error: ", e);
}
}
输出：
上海辟谣专属队
json数组字符串和数组（集合）对象互转
有个大坑
数组对象转json数组字符串
@Test
public void testObjectArray2JsonArrayString() {
User user1 = new User("1","上海带刀沪卫","带刀大佬");
User user2 = new User("1","上海辟谣专属队","职业辟谣，不信谣，不传谣，呵呵");
List<User> users = new ArrayList<>();
users.add(user1);
users.add(user2);
ObjectMapper objectMapper = new ObjectMapper();
try {
String json = objectMapper.writeValueAsString(users);
System.out.println(json);
} catch (JsonProcessingException e) {
log.error("error: ", e);
}
}
输出：
[{"userId":"1","username":"上海带刀沪卫","password":"带刀大佬"},{"userId":"1","username":"上海辟谣专属队","password":"职业辟谣，不信谣，不传谣，呵呵"}]
json数组字符串转数组对象
先演示一下错误的写法，先演示一下错误的写法，先演示一下错误的写法，重要的事情说三遍！
@Test
public void testJsonArrayString2ObjectArray() {
String json = "[" +
"{\"userId\":\"1\",\"username\":\"上海带刀沪卫\",\"password\":\"带刀大佬\"}" +
",{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}" +
"]";
ObjectMapper objectMapper = new ObjectMapper();
try {
//错误写法
List<User> list = objectMapper.readValue(json, List.class);
list.forEach(user -> {
System.out.println(user.getUsername());
});
} catch (JsonProcessingException e) {
log.error("error: ", e);
}
}
输出异常：
java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.xywei.springboot.springtestdemo.entity.Userat java.util.ArrayList.forEach(ArrayList.java:1259)
at com.xywei.springboot.springtestdemo.junit.TestUseJackson.testJsonArrayString2ObjectArray(TestUseJackson.java:160)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at#省略大段……
Process finished with exit code -1
因为实际转成了map类型，所以使用List方式取值是错的！
正确做法：
@Test
public void testJsonArrayString2ObjectArray() {
String json = "[" +
"{\"userId\":\"1\",\"username\":\"上海带刀沪卫\",\"password\":\"带刀大佬\"}" +
",{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}" +
"]";
ObjectMapper objectMapper = new ObjectMapper();
try {
TypeReference<List<User>> typeReference = new TypeReference<List<User>>() {
};
List<User> list = objectMapper.readValue(json, typeReference);
list.forEach(user -> {
System.out.println(user.getUsername());
});
} catch (JsonProcessingException e) {
log.error("error: ", e);
}
}
最终输出：
上海带刀沪卫
上海辟谣专属队
同时，对多层嵌套的数组也适用：
@Test
public void testJsonArrayString2ObjectArray2() {
String json = "[" +
"{" +
"\"companyName\":\"yyyyyy\",\"webSites\":[" +
"{\"webSiteName\":\"xxxxxx.com\",\"users\":[" +
"{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}" +
"]" +
"}" +
"]" +
"}" +
"]";
ObjectMapper objectMapper = new ObjectMapper();
try {
TypeReference<List<Company>> typeReference = new TypeReference<List<Company>>() {
};
List<Company> list = objectMapper.readValue(json, typeReference);
list.forEach(company -> {
System.out.println(company.getWebSites().get(0).getUsers().get(0).getUsername());
});
} catch (JsonProcessingException e) {
log.error("error: ", e);
}
}
以上为个人经验，希望能给大家一个参考，也希望大家多多支持编程网。
