# 76.3 自定义Jackson ObjectMapper

在一个HTTP交互中，Spring MVC（客户端和服务端）使用`HttpMessageConverters`协商内容转换。如果classpath下存在Jackson，你就获取到`Jackson2ObjectMapperBuilder`提供的默认转换器，这是Spring Boot为你自动配置的实例。

创建的`ObjectMapper`（或用于Jackson XML转换的`XmlMapper`）实例默认有以下自定义属性：

* `MapperFeature.DEFAULT_VIEW_INCLUSION`，默认是禁用的
* `DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES`，默认是禁用的
* `SerializationFeature.WRITE_DATES_AS_TIMESTAMPS`，默认是禁用的

Spring Boot也有一些用于简化自定义该行为的特性。

你可以使用当前的environment配置`ObjectMapper`和`XmlMapper`实例。Jackson提供一个扩展套件，可以用来关闭或开启一些特性，你可以用它们配置Jackson以处理不同方面。这些特性在Jackson中是使用6个枚举进行描述的，并被映射到environment的属性上：

| Jackson枚举                                               | Environment属性                                   |           |             |              |              |
| ------------------------------------------------------- | ----------------------------------------------- | --------- | ----------- | ------------ | ------------ |
| `com.fasterxml.jackson.databind.DeserializationFeature` | \`spring.jackson.deserialization.=true          | false\`   |             |              |              |
| `com.fasterxml.jackson.core.JsonGenerator.Feature`      | \`spring.jackson.generator.=true                | false\`   |             |              |              |
| `com.fasterxml.jackson.databind.MapperFeature`          | \`spring.jackson.mapper.=true                   | false\`   |             |              |              |
| `com.fasterxml.jackson.core.JsonParser.Feature`         | \`spring.jackson.parser.=true                   | false\`   |             |              |              |
| `com.fasterxml.jackson.databind.SerializationFeature`   | \`spring.jackson.serialization.=true            | false\`   |             |              |              |
| `com.fasterxml.jackson.annotation.JsonInclude.Include`  | \`spring.jackson.serialization-inclusion=always | non\_null | non\_absent | non\_default | non\_empty\` |

例如，设置`spring.jackson.serialization.indent_output=true`可以美化打印输出（pretty print）。注意，由于[松散绑定](https://docs.spring.io/spring-boot/docs/2.0.0.RELEASE/reference/htmlsingle/#boot-features-external-config-relaxed-binding)的使用，`indent_output`不必匹配对应的枚举常量`INDENT_OUTPUT`。

基于environment的配置会应用到自动配置的`Jackson2ObjectMapperBuilder` bean，然后应用到通过该builder创建的mappers，包括自动配置的`ObjectMapper` bean。

`ApplicationContext`中的`Jackson2ObjectMapperBuilder`可以通过`Jackson2ObjectMapperBuilderCustomizer` bean自定义。这些customizer beans可以排序（Spring Boot自己的customizer序号为0）。其他自定义可以应用到Spring Boot自定义之前或之后。

所有类型为`com.fasterxml.jackson.databind.Module`的beans都会自动注册到自动配置的`Jackson2ObjectMapperBuilder`，并应用到它创建的任何`ObjectMapper`实例。这提供了一种全局机制，用于在为应用添加新特性时贡献自定义模块。

如果想完全替换默认的`ObjectMapper`，你既可以定义该类型的`@Bean`并注解`@Primary`，也可以定义`Jackson2ObjectMapperBuilder` `@Bean`，通过builder构建。注意不管哪种方式都会禁用所有的自动配置`ObjectMapper`。

如果你提供`MappingJackson2HttpMessageConverter`类型的`@Bean`，它们将替换MVC配置中的默认值。Spring Boot也提供了一个`HttpMessageConverters`类型的便利bean（如果你使用MVC默认配置，那它就总是可用的），它提供了一些有用的方法来获取默认和用户增强的消息转换器（message converters）。具体详情可参考[76.4 自定义@ResponseBody渲染](https://docs.spring.io/spring-boot/docs/2.0.0.RELEASE/reference/htmlsingle/#howto-customize-the-responsebody-rendering)及[WebMvcAutoConfiguration](https://github.com/spring-projects/spring-boot/tree/v2.0.0.RELEASE/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.java)源码。


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