java - 如何在 Spring Boot 项目中添加新的 ObjectMapper

Question

在我的 Spring Boot 项目中，我使用默认的 Jackson ObjectMapper。我想将新的 ObjectMapper 添加到 Spring Context 并开始在新的地方使用它，但也保留默认的。添加新的 @Bean 定义将覆盖默认的 ObjectMapper。如何添加新的 ObjectMapper Bean 而不覆盖前一个？

Answer 1

是的，@ConditionalOnMissingBean 是[很难]被破解的。通过一个简单的技巧(亚洲哲学)，我们可以绕过这个问题/让它不再有问题:

将您的(1+，自动配置，@ConditionalOnMissing...)bean 包装在其他/自定义/“包装器”中。 (代价是:引用1+/思考差异/更复杂)

提到MappingJackson2HttpMessageConverter ( auto-config here ) 具有这种(内置)功能(和目的)，可以根据“http 转换”映射到多个对象映射器。

因此，使用(通用的，例如基于 java.util.Map 的)类似的东西:

class MyWrapper<K, V> {
  final Map<K, V> map;
  public MyWrapper(Map<K, V> map) {
    this.map = map;
  }
  public Map<K, V> getMap() {
    return map;
  }
}

我们可以接线:

@Bean
MyWrapper<String, ObjectMapper> myStr2OMWrapper(/*ObjectMapper jacksonOm*/) {
  return new MyWrapper(Map.of(
      // DEFAULT, jacksonOm,
      "foo", fooMapper(), 
      "bar", barMapper()
  ));
}

..其中 fooMapper() 和 barMapper() 可以引用(静态/实例)no-bean 方法:

private static ObjectMapper fooMapper() {
  return new ObjectMapper()
      .configure(SerializationFeature.INDENT_OUTPUT, true) // just a demo...
      .configure(SerializationFeature.WRAP_ROOT_VALUE, true); // configure/set  as see fit...
}
private static ObjectMapper barMapper() {
  return new ObjectMapper()
      .configure(SerializationFeature.INDENT_OUTPUT, false) // just a demo...
      .configure(SerializationFeature.WRAP_ROOT_VALUE, false); // configure/set more...
}

(已经)测试/使用时间:

package com.example.demo;

import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

@SpringBootTest
class DemoAppTests {

  @Autowired
  MyWrapper<String, ObjectMapper> my;
  @Autowired
  ObjectMapper jacksonOM;

  @Test
  void contextLoads() {
    System.err.println(jacksonOM);
    Assertions.assertNotNull(jacksonOM);
    my.getMap().entrySet().forEach(e -> {
      System.err.println(e);
      Assertions.assertNotNull(e.getValue());
    });
  }
}

打印(例如)

...
com.fasterxml.jackson.databind.ObjectMapper@481b2f10
bar=com.fasterxml.jackson.databind.ObjectMapper@577bf0aa
foo=com.fasterxml.jackson.databind.ObjectMapper@7455dacb
...
Results:

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
...

抱歉，此测试不验证(单独)配置，而只是:(视觉上不同)不是空对象映射器。

---

如何启用(多个!)my.custom.jackson.*自动配置，是一个更复杂的问题...(它不像 my.custom.datasource.* config ;(

与:

@Bean
@Primary // ! for auto config, we need one primary (whether it is "spring.jackson"  ... adjust;)
@ConfigurationProperties("spring.jackson")
JacksonProperties springJacksonProps() {
  return new JacksonProperties();
}

@Bean
@ConfigurationProperties("foo.jackson")
JacksonProperties fooProps() {
  return new JacksonProperties();
}

@Bean
@ConfigurationProperties("bar.jackson")
JacksonProperties barProps() {
  return new JacksonProperties();
}

我们已经可以加载和区分(完整的)配置，例如:

spring.jackson.locale=en_US
spring.jackson.time-zone=UTC
# ... all of spring.jackson @see org.springframework.boot.autoconfigure.jackson.JacksonProperties
foo.jackson.locale=en_US
foo.jackson.time-zone=PST
# ... just for demo purpose
bar.jackson.locale=de_DE
bar.jackson.time-zone=GMT+1

并且(没问题)将它们(props)传递给相应的(static [foo|bar]Mapper)方法......但是然后呢？ (如果你擅长的话，你可以停止阅读这里!:)

不幸的是according ("state of art") code (将 JacksonProperties 与“om builder”连接)不是公开的(即不可扩展/可插入)。

相反，自动配置提供(如果没有定义/@ConditionalOnMissingBean):

• 一个原型(prototype) Jackson2ObjectMapperBuilder bean，它(每次请求时):
  • 应用(即接收)所有(已知)Jackson2ObjectMapperBuilderCustomizer beans 的定制。
  • 其中one (auto configured, order(0), package private) is the "standard"负责连接(仅限 spring.jackson.*)JacksonProperties 到 Jackson2ObjectMapperBuilder...

所以最简单的方法似乎(最新)是:

• 钢/采用the code (未-/实现Jackson2ObjectMapperBuilderCustomizer)
• 根据构建器/映射器构建(从“被盗”+属性)，如认为合适。

例如(在生产之前回顾+测试!)非接口(interface)，返回一个Jackson2ObjectMapperBuilder，模仿自动配置，不应用(其他)定制器/-ation:

// ...
import com.fasterxml.jackson.databind.Module; // !! not java.lang.Module ;)
// ...
private static class MyStolenCustomizer {

  private final JacksonProperties jacksonProperties;
  private final Collection<Module> modules;
  // additionally need/want this:
  private final ApplicationContext applicationContext;
  // copy/adopt from spring-boot:
  private static final Map<?, Boolean> FEATURE_DEFAULTS = Map.of(
      SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false,
      SerializationFeature.WRITE_DURATIONS_AS_TIMESTAMPS, false
  );

  public MyStolenCustomizer(
    ApplicationContext applicationContext,
    JacksonProperties jacksonProperties,
    Collection<Module> modules
  ) {
      this.applicationContext = applicationContext;
      this.jacksonProperties = jacksonProperties;
      this.modules = modules;
  }
  // changed method signature!!
  public Jackson2ObjectMapperBuilder buildCustom() {
    // mimic original (spring-boot) bean:
    Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
    builder.applicationContext(applicationContext);
    // without (additional!) customizers:
    if (this.jacksonProperties.getDefaultPropertyInclusion() != null) {
      builder.serializationInclusion(this.jacksonProperties.getDefaultPropertyInclusion());
    }
    if (this.jacksonProperties.getTimeZone() != null) {
      builder.timeZone(this.jacksonProperties.getTimeZone());
    }
    configureFeatures(builder, FEATURE_DEFAULTS);
    configureVisibility(builder, this.jacksonProperties.getVisibility());
    configureFeatures(builder, this.jacksonProperties.getDeserialization());
    configureFeatures(builder, this.jacksonProperties.getSerialization());
    configureFeatures(builder, this.jacksonProperties.getMapper());
    configureFeatures(builder, this.jacksonProperties.getParser());
    configureFeatures(builder, this.jacksonProperties.getGenerator());
    configureDateFormat(builder);
    configurePropertyNamingStrategy(builder);
    configureModules(builder);
    configureLocale(builder);
    configureDefaultLeniency(builder);
    configureConstructorDetector(builder);
    // custom api:
    return builder; // ..alternatively: builder.build();
  }
  // ... rest as in https://github.com/spring-projects/spring-boot/blob/main/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/JacksonAutoConfiguration.java#L223-L341

要连接模块，我们可以(希望像最初一样)依赖:

@Autowired
ObjectProvider<com.fasterxml.jackson.databind.Module> modules

像这样初始化它们:

@Bean
MyStolenCustomizer fooCustomizer(ApplicationContext context, @Qualifier("fooProps") JacksonProperties fooProperties, ObjectProvider<Module> modules) {
  return new MyStolenCustomizer(context, fooProperties, modules.stream().toList());
}

@Bean
MyStolenCustomizer barCustomizer(ApplicationContext context, @Qualifier("barProps") JacksonProperties barProperties, ObjectProvider<Module> modules) {
  return new MyStolenCustomizer(context, barProperties, modules.stream().toList());
}

..并像这样使用它们:

@Bean
MyWrapper<String, Jackson2ObjectMapperBuilder> myStr2OMBuilderWrapper(
    @Qualifier("fooCustomizer") MyStolenCustomizer fooCustomizer,
    @Qualifier("barCustomizer") MyStolenCustomizer barCustomizer) {
  return new MyWrapper(
      Map.of(
          "foo", fooCustomizer.buildCustom(),
          "bar", barCustomizer.buildCustom()
      )
  );
}

...避免“双重定制”/保持 JacksonAutoConfiguration 启用/完整/Activity 。

问题:时间/更新(/外部代码)!