MappingJackson2HttpMessageConverter 分类别处理null

jackson并没有提供分类别处理null，jackson的核心类ObjectMapper暴露的NullSerializer无法针对不同类型设定不同的默认值，所幸jackson有完善的扩展机制。BeanSerializerModifier的changeProperties()方法提供了细粒度控制每个Field的序列化行为的可能，代码如下。

public enum SerializerFeature {
   WriteNullListAsEmpty,
   WriteNullStringAsEmpty,
   WriteNullNumberAsZero,
   WriteNullBooleanAsFalse;
   
   public final int mask;
   
   SerializerFeature() {
       mask = (1 << ordinal());
   }
}

final public static class FastJsonSerializerFeatureCompatibleForJackson extends BeanSerializerModifier {
   final private JsonSerializer<Object> nullBooleanJsonSerializer;
   final private JsonSerializer<Object> nullNumberJsonSerializer;
   final private JsonSerializer<Object> nullListJsonSerializer;
   final private JsonSerializer<Object> nullStringJsonSerializer;
   
   FastJsonSerializerFeatureCompatibleForJackson(SerializerFeature... features) {
       int config = 0;
       for (SerializerFeature feature : features) {
           config |= ;
       }
       nullBooleanJsonSerializer = (config & ) != 0 ? new NullBooleanSerializer() : null;
       nullNumberJsonSerializer = (config & ) != 0 ? new NullNumberSerializer() : null;
       nullListJsonSerializer = (config & ) != 0 ? new NullListJsonSerializer() : null;
       nullStringJsonSerializer = (config & ) != 0 ? new NullStringSerializer() : null;
   }
   
   @Override
   public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
       for (BeanPropertyWriter writer : beanProperties) {
           final JavaType javaType = ();
           final Class<?> rawClass = ();
           if (() || ()) {
               (nullListJsonSerializer);
           } else if ((rawClass) && ().startsWith("")) {
               (nullNumberJsonSerializer);
           } else if ((rawClass)) {
               (nullBooleanJsonSerializer);
           } else if ((rawClass)) {
               (nullStringJsonSerializer);
           }
       }
       return beanProperties;
   }
   
   private static class NullListJsonSerializer extends JsonSerializer<Object> {
       @Override
       public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
           ();
           ();
       }
   }
   
   private static class NullNumberSerializer extends JsonSerializer<Object> {
       @Override
       public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
           (0);
       }
   }
   
   private static class NullBooleanSerializer extends JsonSerializer<Object> {
       @Override
       public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
           (false);
       }
   }
   
   private static class NullStringSerializer extends JsonSerializer<Object> {
       @Override
       public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
           ("");
       }
   }
}

最后把该 Modifier 注入到 ObjectMapper 中。

(().withSerializerModifier(new FastJsonSerializerFeatureCompatibleForJackson(features)));

反序列化时忽略不存在的字段

在标准Json规范中，如果尝试反序列化一个Json字符串为指定的POJO，而且字符串中有一个Field在POJO中不存在，应该抛出错误。对于这种情况，fastjson默认是跳过，jackson默认是中止。

(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

重新设置SpringMVC的HttpMessageConvertor

<bean class=".MappingJackson2HttpMessageConverter">
      <property name="objectMapper">
          <bean class="">
              <constructor-arg>
                  <util:list>
                      <value>WriteNullListAsEmpty</value>
                      <value>WriteNullStringAsEmpty</value>
                      <value>WriteNullNumberAsZero</value>
                      <value>WriteNullBooleanAsFalse</value>
                  </util:list>
              </constructor-arg>
          </bean>
      </property>
  </bean>

由于需要编程式的设置全局配置，扩展 FactoryBean （ CustomObjectMapperFactoryBean ）生成自定义的 ObjectMapper 来替代 MappingJackson2HttpMessageConverter 中默认 ObjectMapper 。

final public static class CustomObjectMapperFactoryBean implements FactoryBean<ObjectMapper> {
 SerializerFeature[] features;
 public CustomObjectMapperFactoryBean(SerializerFeature[] features) {
      = features;
 }

 @Override
 public ObjectMapper getObject() throws Exception {
     ObjectMapper objectMapper = new ObjectMapper();
     (().withSerializerModifier(new FastJsonSerializerFeatureCompatibleForJackson(features))).configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
     return objectMapper;
 }

 @Override
 public Class<?> getObjectType() {
     return ;
 }

 @Override
 public boolean isSingleton() {
     return false;
 }
}