修改Springboot默认序列化工具Jackson的配置

如果我们在Spring Boot应用中手动定义并注入了一个ObjectMapper Bean，那么这个自定义的ObjectMapper实例会替换掉Spring Boot默认配置的ObjectMapper。当Spring容器中存在多个同类型的Bean时，默认情况下最后一个创建的Bean将作为首选Bean（如果未明确指定@Primary注解），因此我们的自定义ObjectMapper将会被所有依赖于ObjectMapper的地方使用。

例如：

java 复制代码

@Configuration
public class ObjectMapperConfig {

    @Bean
    public ObjectMapper customObjectMapper() {
        ObjectMapper objectMapper = new ObjectMapper();
        // 添加自定义配置...
        return objectMapper;
    }
}

上述代码定义了一个自定义的ObjectMapper Bean，并将其注册到了Spring容器中。这样一来，在整个应用中需要ObjectMapper的地方，包括HTTP请求和响应的JSON转换等场景，都会使用到这个自定义配置的ObjectMapper，而非Spring Boot默认提供的那个。

因此，如果我们只想修改Spring Boot默认ObjectMapper的一些配置，而不是完全替换掉它，使用Jackson2ObjectMapperBuilderCustomizer接口是一个更好的选择。通过实现这个接口并注册一个定制器Bean，我们可以对默认的ObjectMapper进行扩展和修改，而不会覆盖其他默认配置。

下面是一个例子：

java 复制代码

import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.boot.autoconfigure.jackson.Jackson2ObjectMapperBuilderCustomizer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class JacksonConfig {

    @Bean
    public Jackson2ObjectMapperBuilderCustomizer jacksonCustomizer() {
        return builder -> {
            // 修改日期格式化
            builder.dateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ");
            
            // 关闭未知属性导致反序列化失败
            builder.featuresToDisable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
            
            // 其他自定义配置...
        };
    }
}

这样，我们的配置将应用于所有的ObjectMapper实例，包括那些由Spring Boot自动配置创建的实例。这意味着在HTTP请求响应处理、消息转换等任何使用到ObjectMapper的地方，都会采用我们自定义的配置。

另外，Jackson2ObjectMapperBuilderCustomizer接口并不能配置空值序列化操作，因此我们可以这样：（该方法无效）

java 复制代码

	// 该方式不会完全替换Springboot默认的ObjectMapper，并且可以设置空值序列化器
    @Bean
    @Primary
    @ConditionalOnMissingBean(ObjectMapper.class)
    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        builder.featuresToDisable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, DeserializationFeature.ADJUST_DATES_TO_CONTEXT_TIME_ZONE, DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
        builder.modules(getJavaLongSimpleModule(), getJavaTimeSimpleModule());
        
        ObjectMapper objectMapper = builder.createXmlMapper(false).build();
        objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
            @Override
            public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
                jsonGenerator.writeString("");
            }
        });
        return objectMapper;
    }

该方式不会完全替换Springboot默认的ObjectMapper，并且可以设置空值序列化器。

注：JsonSerializer无法在序列化时对空值操作，因为其serialize方法接收到的被序列化对象永远不为null

java 复制代码

/**
 * Abstract class that defines API used by {@link ObjectMapper} (and
 * other chained {@link JsonSerializer}s too) to serialize Objects of
 * arbitrary types into JSON, using provided {@link JsonGenerator}.
 * {@link com.fasterxml.jackson.databind.ser.std.StdSerializer} instead
 * of this class, since it will implement many of optional
 * methods of this class.
 *<p>
 * NOTE: various <code>serialize</code> methods are never (to be) called
 * with null values -- caller <b>must</b> handle null values, usually
 * by calling {@link SerializerProvider#findNullValueSerializer} to obtain
 * serializer to use.
 * This also means that custom serializers cannot be directly used to change
 * the output to produce when serializing null values.
 *<p>
 * If serializer is an aggregate one -- meaning it delegates handling of some
 * of its contents by using other serializer(s) -- it typically also needs
 * to implement {@link com.fasterxml.jackson.databind.ser.ResolvableSerializer},
 * which can locate secondary serializers needed. This is important to allow dynamic
 * overrides of serializers; separate call interface is needed to separate
 * resolution of secondary serializers (which may have cyclic link back
 * to serializer itself, directly or indirectly).
 *<p>
 * In addition, to support per-property annotations (to configure aspects
 * of serialization on per-property basis), serializers may want
 * to implement 
 * {@link com.fasterxml.jackson.databind.ser.ContextualSerializer},
 * which allows specialization of serializers: call to
 * {@link com.fasterxml.jackson.databind.ser.ContextualSerializer#createContextual}
 * is passed information on property, and can create a newly configured
 * serializer for handling that particular property.
 *<p>
 * If both
 * {@link com.fasterxml.jackson.databind.ser.ResolvableSerializer} and
 * {@link com.fasterxml.jackson.databind.ser.ContextualSerializer}
 * are implemented, resolution of serializers occurs before
 * contextualization.
 */
 public abstract class JsonSerializer<T> implements JsonFormatVisitable // since 2.1
 {
     /**
     * Method that can be called to ask implementation to serialize
     * values of type this serializer handles.
     *
     * @param value Value to serialize; can <b>not</b> be null.
     * @param gen Generator used to output resulting Json content
     * @param serializers Provider that can be used to get serializers for
     *   serializing Objects value contains, if any.
     */
    public abstract void serialize(T value, JsonGenerator gen, SerializerProvider serializers)
        throws IOException;
 }