spring - @Autowired Spring @Component 和 @ConditionalOnProperty

Question

我可以用@Autowired Spring 4.x @Component s 带有 @ConditionalOnProperty基于featuretoggles.properties 文件选择功能的实现？

public class Controller {
  @Autowired
  private Feature feature;
}

@Component
@ConditionalOnProperty(name = "b", havingValue = "off")
public class A implements Feature {
}

@Component
@ConditionalOnProperty(name = "b", havingValue = "on")
public class B implements Feature {
}

@Configuration
@PropertySource("classpath:featuretoggles.properties")
public class SomeRandomConfig {
}

使用 src/main/resources/featuretoggles.properties 文件:

b = on

(切换的名称“b”和类“B”的名称匹配是巧合；我的目标不是让它们相等，切换可以有任何名称。)

这无法自动连接 feature在 Controller与 UnsatisfiedDependencyException ，说“没有可用的‘功能’类型的合格 bean:预计至少有 1 个 bean 有资格作为 Autowiring 候选”。

我知道我可以通过 @Configuration 实现这一点选择 @Bean 的类视属性(property)而定。但是当我这样做时，我必须在每次添加功能切换时添加一个新的 Configuration 类，这些 Configuration 类将非常相似:

@Configuration
@PropertySource("classpath:featuretoggles.properties")
public class FeatureConfig {

    @Bean
    @ConditionalOnProperty(name = "b", havingValue = "on")
    public Feature useB() {
        return new B();
    }

    @Bean
    @ConditionalOnProperty(name = "b", havingValue = "off")
    public Feature useA() {
        return new A();
    }

}

Answer 1

我通过关注 this guide 做了你想做的事情.第一步是写一个 Condition ...

public class OnEnvironmentPropertyCondition implements Condition
{
  @Override
  public boolean matches(ConditionContext ctx, AnnotatedTypeMetadata meta)
  {
    Environment env = ctx.getEnvironment();
    Map<String, Object> attr = meta.getAnnotationAttributes(
                                 ConditionalOnEnvProperty.class.getName());

    boolean shouldPropExist = (Boolean)attr.get("exists");
    String prop = (String)attr.get("value");

    boolean doesPropExist = env.getProperty(prop) != null;

    // doesPropExist    shouldPropExist    result
    //    true             true             true
    //    true             false            false
    //    false            true             false
    //    true             false            true
    return doesPropExist == shouldPropExist;
  }
}

...然后是使用该条件的注释。

/*
 * Condition returns true if myprop exists:
 * @ConditionalOnEnvProperty("myprop")
 *
 * Condition returns true if myprop does not exist
 * @ConditionalOnEnvProperty(value="myprop", exists=false)
 */
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Conditional(OnEnvironmentPropertyCondition.class)
public @interface ConditionalOnEnvProperty
{
  public String value();
  public boolean exists() default true;
}

您可以添加 featuretoggles.properties与 @PropertySource 的环境注解。