Jackson中json格式的字符串与对象的互相转换方式
作者:hestyle
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。
/** * 序列化:对象转json字符串,包含多个数组 * */ @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
/** * 序列化:数组对象转json数组类型的字符串 * */ @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!
/** * 反序列化:数组类型的json字符串转对象数组 * */ @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:
/** * 反序列化:数组类型的json字符串转对象数组 * */ @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脚本之家。