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springboot jackson默认配置

​ springboot是约定大约配置，默认就会进行比较合理的配置，自动配置jackson也是如此。下面看一下springboot是如何配置jackson的反序列化特性的。

配置jackson的方法

​ 想要自定义配置jackson，只需实现Jackson2ObjectMapperBuilderCustomizer接口，并注册到spring容器里即可。

平时我们主要关注下面两个配置，一个是序列化时不返回值为null的字段，二是反序列化时遇到不认识的key不报错而是忽略此key。

@Configuration
public class JacksonConfig implements Jackson2ObjectMapperBuilderCustomizer {

    @Override
    public void customize(Jackson2ObjectMapperBuilder jacksonObjectMapperBuilder) {
        jacksonObjectMapperBuilder.serializationInclusion(JsonInclude.Include.NON_NULL);
        jacksonObjectMapperBuilder.failOnUnknownProperties(false);
    }

}

自动配置的jackson配置

如果没提供自定义的配置，将只使用springboot的默认配置，在JacksonAutoConfiguration类中，存在如下代码

@Bean
@Primary
@ConditionalOnMissingBean(ObjectMapper.class)
public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
	return builder.createXmlMapper(false).build();
}

可见其实使用Jackson2ObjectMapperBuilder类来创建ObjectMapper，且关闭了xml功能。

Jackson2ObjectMapperBuilder的创建同样在此类中，因为我们没有传入任何自定义Jackson2ObjectMapperBuilderCustomizer，所以其使用默认的值。

@Bean
@ConditionalOnMissingBean(Jackson2ObjectMapperBuilder.class)
public Jackson2ObjectMapperBuilder jacksonObjectMapperBuilder(
		List<Jackson2ObjectMapperBuilderCustomizer> customizers) {
	Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
	builder.applicationContext(this.applicationContext);
	customize(builder, customizers);
	return builder;
}

查看Jackson2ObjectMapperBuilder类内部的build方法源码，其也是创建一个新的ObjectMapper并对其进行配置，重点就在下面源码的configure(mapper)方法里面。

public <T extends ObjectMapper> T build() {
	ObjectMapper mapper;
	if (this.createXmlMapper) {
		mapper = (this.defaultUseWrapper != null ?
				new XmlObjectMapperInitializer().create(this.defaultUseWrapper) :
				new XmlObjectMapperInitializer().create());
	}
	else {
		mapper = new ObjectMapper();
	}
	configure(mapper);
	return (T) mapper;
}

​ 进入configure方法里，其将此构建器的属性进行非空判断，如果不为空就设置到ObjectMapper里。因为我们没有提供任何自定义配置，所以这些值都是空的，其中此方法里有一行调用了customizeDefaultFeatures(objectMapper)方法，其会设置默认配置。

public void configure(ObjectMapper objectMapper) {
	Assert.notNull(objectMapper, "ObjectMapper must not be null");

	if (this.findModulesViaServiceLoader) {
		// Jackson 2.2+
		objectMapper.registerModules(ObjectMapper.findModules(this.moduleClassLoader));
	}
	else if (this.findWellKnownModules) {
		registerWellKnownModulesIfAvailable(objectMapper);
	}

	if (this.modules != null) {
		for (Module module : this.modules) {
			// Using Jackson 2.0+ registerModule method, not Jackson 2.2+ registerModules
			objectMapper.registerModule(module);
		}
	}
	if (this.moduleClasses != null) {
		for (Class<? extends Module> module : this.moduleClasses) {
			objectMapper.registerModule(BeanUtils.instantiate(module));
		}
	}

	if (this.dateFormat != null) {
		objectMapper.setDateFormat(this.dateFormat);
	}
	if (this.locale != null) {
		objectMapper.setLocale(this.locale);
	}
	if (this.timeZone != null) {
		objectMapper.setTimeZone(this.timeZone);
	}

	if (this.annotationIntrospector != null) {
		objectMapper.setAnnotationIntrospector(this.annotationIntrospector);
	}
	if (this.propertyNamingStrategy != null) {
		objectMapper.setPropertyNamingStrategy(this.propertyNamingStrategy);
	}
	if (this.defaultTyping != null) {
		objectMapper.setDefaultTyping(this.defaultTyping);
	}
	if (this.serializationInclusion != null) {
		objectMapper.setSerializationInclusion(this.serializationInclusion);
	}

	if (this.filters != null) {
		objectMapper.setFilterProvider(this.filters);
	}

	for (Class<?> target : this.mixIns.keySet()) {
		objectMapper.addMixIn(target, this.mixIns.get(target));
	}

	if (!this.serializers.isEmpty() || !this.deserializers.isEmpty()) {
		SimpleModule module = new SimpleModule();
		addSerializers(module);
		addDeserializers(module);
		objectMapper.registerModule(module);
	}

	customizeDefaultFeatures(objectMapper);
	for (Object feature : this.features.keySet()) {
		configureFeature(objectMapper, feature, this.features.get(feature));
	}

	if (this.handlerInstantiator != null) {
		objectMapper.setHandlerInstantiator(this.handlerInstantiator);
	}
	else if (this.applicationContext != null) {
		objectMapper.setHandlerInstantiator(
				new SpringHandlerInstantiator(this.applicationContext.getAutowireCapableBeanFactory()));
	}
}

​ 可以看到当features里面不包含FAIL_ON_UNKNOWN_PROPERTIES时，即我们没有主动将FAIL_ON_UNKNOWN_PROPERTIES设为true或者false，springboot会将其设置为false，这也正符合我们平时的使用习惯。

private void customizeDefaultFeatures(ObjectMapper objectMapper) {
	if (!this.features.containsKey(MapperFeature.DEFAULT_VIEW_INCLUSION)) {
		configureFeature(objectMapper, MapperFeature.DEFAULT_VIEW_INCLUSION, false);
	}
       //列表内找不到该属性，就将此属性设置为false
	if (!this.features.containsKey(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)) {
		configureFeature(objectMapper, DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
	}
}

总结

​ 上面可以看出springboot会为我们自动配置好比较合理的配置，即约定大于配置，我们没有FAIL_ON_UNKNOWN_PROPERTIES的配置时，他会自动将其设为false。

​ 于是我们在下次自定义配置时，不配置此属性也是可以的，将上面的配置改为下面这样，其也是能自动设置FAIL_ON_UNKNOWN_PROPERTIES=false的。

@Configuration
public class JacksonConfig implements Jackson2ObjectMapperBuilderCustomizer {

    @Override
    public void customize(Jackson2ObjectMapperBuilder jacksonObjectMapperBuilder) {
        jacksonObjectMapperBuilder.serializationInclusion(JsonInclude.Include.NON_NULL);
    }
}

​ 但是考虑到不是每个人都看过源码，还是推荐加上，这样其他人看到会更清晰一点。