MappingJackson2HttpMessageConverter 分类别处理null

超、凢脫俗 2024-02-18 15:02 53阅读 0赞

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 |= feature.mask;
           }
           nullBooleanJsonSerializer = (config & WriteNullBooleanAsFalse.mask) != 0 ? new NullBooleanSerializer() : null;
           nullNumberJsonSerializer = (config & WriteNullNumberAsZero.mask) != 0 ? new NullNumberSerializer() : null;
           nullListJsonSerializer = (config & WriteNullListAsEmpty.mask) != 0 ? new NullListJsonSerializer() : null;
           nullStringJsonSerializer = (config & WriteNullStringAsEmpty.mask) != 0 ? new NullStringSerializer() : null;
       }
       
       @Override
       public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
           for (BeanPropertyWriter writer : beanProperties) {
               final JavaType javaType = writer.getType();
               final Class<?> rawClass = javaType.getRawClass();
               if (javaType.isArrayType() || javaType.isCollectionLikeType()) {
                   writer.assignNullSerializer(nullListJsonSerializer);
               } else if (Number.class.isAssignableFrom(rawClass) && rawClass.getName().startsWith("java.lang")) {
                   writer.assignNullSerializer(nullNumberJsonSerializer);
               } else if (Boolean.class.equals(rawClass)) {
                   writer.assignNullSerializer(nullBooleanJsonSerializer);
               } else if (String.class.equals(rawClass)) {
                   writer.assignNullSerializer(nullStringJsonSerializer);
               }
           }
           return beanProperties;
       }
       
       private static class NullListJsonSerializer extends JsonSerializer<Object> {
           @Override
           public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
               jgen.writeStartArray();
               jgen.writeEndArray();
           }
       }
       
       private static class NullNumberSerializer extends JsonSerializer<Object> {
           @Override
           public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
               jgen.writeNumber(0);
           }
       }
       
       private static class NullBooleanSerializer extends JsonSerializer<Object> {
           @Override
           public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
               jgen.writeBoolean(false);
           }
       }
       
       private static class NullStringSerializer extends JsonSerializer<Object> {
           @Override
           public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
               jgen.writeString("");
           }
       }
    }

最后把该 `Modifier` 注入到 `ObjectMapper` 中。

objectMapper.setSerializerFactory(objectMapper.getSerializerFactory().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。

objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

### 重新设置SpringMVC的HttpMessageConvertor ###

<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
          <property name="objectMapper">
              <bean class="com.dianping.orderdish.framework.util.CustomJacksonGenerator.CustomObjectMapperFactoryBean">
                  <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) {
         this.features = features;
     }
    
     @Override
     public ObjectMapper getObject() throws Exception {
         ObjectMapper objectMapper = new ObjectMapper();
         objectMapper.setSerializerFactory(objectMapper.getSerializerFactory().withSerializerModifier(new FastJsonSerializerFeatureCompatibleForJackson(features))).configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
         return objectMapper;
     }
    
     @Override
     public Class<?> getObjectType() {
         return ObjectMapper.class;
     }
    
     @Override
     public boolean isSingleton() {
         return false;
     }
    }