使用Jackson进行json转对象，对象转json总结
背景
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
- Company，公司
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Company {
private String companyName;
private List<WebSite> webSites;
}
- WebSite，站点
@Data
@NoArgsConstructor
@AllArgsConstructor
public class WebSite {
private String webSiteName;
private List<User> users;
}
- 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.User
at 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);
}
}
总结
以上是在项目中常用的，当然还有一些小弟没有用过的，希望各位大佬分享一下！