简介
本文介绍Jackson的配置方式。（配置ObjectMapper）。
配置示例
ObjectMapper objectMapper = new ObjectMapper(); // 反序列化：JSON字段中有Java对象中没有的字段时不报错 objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); // 反序列化：不允许基本类型为null //objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, true); // 序列化：序列化BigDecimal时不使用科学计数法输出 objectMapper.configure(JsonGenerator.Feature.WRITE_BIGDECIMAL_AS_PLAIN, true); // 序列化：Java对象为空的字段不拼接JSON //objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
反序列化的配置
忽略未知的JSON字段
有时候，Java对象的字段比JSON字符串中的字段少。 默认情况下，Jackson会抛出异常，报不知道xxx字段异常，因为在Java对象中找不到该字段。
大多数情况下，我们想允许JSON中的字段多于相应的Java对象中的字段。 例如，要从REST服务解析JSON，而该REST服务包含的数据远远超出所需的。 在这种情况下，可以使用Jackson配置忽略这些额外的字段。
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
不允许基本类型为null
简介
将JSON字符串转为Java对象时，如果JSON字符串值为null的字段对应Java对象的基本数据类型（int，long，float，double等），Jackson默认会将这个字段设置为初始值，比如：对于int，则将其设置为0。
我们可以进行自定义配置：基本数据为null让其报错，转换失败。
实例：默认情况
Entity
package com.example.entity;
import lombok.Data;
@Data
public class User {
private String userName;
private int age;
}
Controller
package com.example.controller;
import com.example.entity.User;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class HelloController {
@GetMapping("/test")
public String test() throws JsonProcessingException {
String json = "{\"userName\":\"Tony\", \"age\":null}";
ObjectMapper objectMapper = new ObjectMapper();
User user = objectMapper.readValue(json, User.class);
System.out.println(user);
return "test success";
}
}
测试
后端结果：
实例：修改配置
Entity
package com.example.entity;
import lombok.Data;
@Data
public class User {
private String userName;
private int age;
}
Controller
package com.example.controller;
import com.example.entity.User;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class HelloController {
@GetMapping("/test")
public String test() throws JsonProcessingException {
String json = "{\"userName\":\"Tony\", \"age\":null}";
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, true);
User user = objectMapper.readValue(json, User.class);
System.out.println(user);
return "test success";
}
}
测试
后端结果
2021-09-18 22:12:46.132 ERROR 22264 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot map `null` into type int (set DeserializationConfig.DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES to 'false' to allow)
at [Source: (String)"{"userName":"Tony", "age":null}"; line: 1, column: 27] (through reference chain: com.example.entity.User["age"])
at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1438) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.deser.std.NumberDeserializers$PrimitiveOrWrapperDeserializer.getNullValue(NumberDeserializers.java:161) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:127) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:293) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:156) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4482) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3434) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3402) ~[jackson-databind-2.11.0.jar:2.11.0]
at com.example.controller.HelloController.test(HelloController.java:18) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_201]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_201]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_201]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_201]
at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) ~[spring-web-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105) ~[spring-webmvc-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:879) ~[spring-webmvc-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793) ~[spring-webmvc-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) ~[spring-webmvc-5.2.6.RELEASE.jar:5.2.6.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.6.RELEASE.jar:5.2.6.RELEASE]
...
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.35.jar:9.0.35]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_201]
自定义反序列化
可以将自定义反序列化器添加到ObjectMapper。
在Jackson的ObjectMapper中注册和使用自定义反序列化器的方式：
String json = "{\"userName\": \"Tony\", \"age\" : 22 }";
SimpleModule module = new SimpleModule("UserDeserializer", new Version(3, 1, 8, null, null, null));
module.addDeserializer(User.class, new UserDeserializer(User.class));
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);
User user = mapper.readValue(json, User.class);
自定义反序列化器UserDeserializer类：
public class UserDeserializer extends StdDeserializer<User> {
public UserDeserializer(Class<?> vc) {
super(vc);
}
@Override
public User deserialize(JsonParser parser, DeserializationContext deserializer) throws IOException {
User user = new User();
while(!parser.isClosed()){
JsonToken jsonToken = parser.nextToken();
if(JsonToken.FIELD_NAME.equals(jsonToken)){
String fieldName = parser.getCurrentName();
System.out.println(fieldName);
jsonToken = parser.nextToken();
if("userName".equals(fieldName)){
user.setUserName(parser.getValueAsString());
} else if ("age".equals(fieldName)){
user.setAge(parser.getValueAsInt());
}
}
}
return user;
}
}
请先 ！