Jackson--配置--方法/实例

本是古典何须时尚 2023-09-29 14:01 8阅读 0赞

原文网址：[https://knife.blog.csdn.net/article/details/124690765][https_knife.blog.csdn.net_article_details_124690765]

## 简介 ##

本文介绍Jackson（ObjectMapper）的配置方式。

## 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

访问：[http://localhost:8080/test][http_localhost_8080_test]

后端结果：

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBASVTliKnliIPlh7rpnpg_size_20_color_FFFFFF_t_70_g_se_x_16][]

#### **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**

访问：[http://localhost:8080/test][http_localhost_8080_test]

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;
        }
    }

[https_knife.blog.csdn.net_article_details_124690765]: https://knife.blog.csdn.net/article/details/124690765
[http_localhost_8080_test]: http://localhost:8080/test
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBASVTliKnliIPlh7rpnpg_size_20_color_FFFFFF_t_70_g_se_x_16]: https://img-blog.csdnimg.cn/20210918221150853.png?x-oss-process=image/watermark,type_ZHJvaWRzYW5zZmFsbGJhY2s,shadow_50,text_Q1NETiBASVTliKnliIPlh7rpnpg=,size_20,color_FFFFFF,t_70,g_se,x_16