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：

@Configuration
public class ObjectMapperConfig {@Beanpublic 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：

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 {@Beanpublic 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>() {@Overridepublic 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

/*** 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;}