何时使用Spring @ConfigurationCondition vs. @Condition？ [英] When to use Spring @ConfigurationCondition vs. @Condition?

问题描述

Spring 4有两个新的注释 @Condition 和 @ConfigurationConditon 用于控制是否将bean添加到弹簧中应用背景。 JavaDoc没有提供足够的上下文/大图来理解 @ConfigurationCondition 的用例。

Spring 4 has two new annotations @Condition and @ConfigurationConditon for controlling whether a bean is added to the spring application context. The JavaDoc does not provide enough context / big picture to understand the use cases for @ConfigurationCondition.

何时应使用 @ConfigurationCondition 与 @Condition ？

public interface ConfigurationCondition extends Condition {

    /**
     * Returns the {@link ConfigurationPhase} in which the condition should be evaluated.
     */
    ConfigurationPhase getConfigurationPhase();

    /**
     * The various configuration phases where the condition could be evaluated.
     */
    public static enum ConfigurationPhase {

        /**
         * The {@link Condition} should be evaluated as a {@code @Configuration} class is
         * being parsed.
         *
         * <p>If the condition does not match at this point the {@code @Configuration}
         * class will not be added.
         */
        PARSE_CONFIGURATION,

        /**
         * The {@link Condition} should be evaluated when adding a regular (non
         * {@code @Configuration}) bean. The condition will not prevent
         * {@code @Configuration} classes from being added.
         *
         * <p>At the time that the condition is evaluated all {@code @Configuration}s
         * will have been parsed.
         */
        REGISTER_BEAN
    }

}

推荐答案

ConfigurationCondition 是条件的专业化 @Configuration 类。

普通条件对99％的用例都很好，所以你应该先考虑一下。专业化实际上是关于确定 @Configuration 类处理的阶段应评估条件。

Plain Condition is just fine for 99% of your use cases so you should consider that first. The specialization is really about determining in which phase of the processing of @Configuration classes the condition should be evaluated.

分为两个阶段：

• PARSE_CONFIGURATION 评估解析 @Configuration -annotated类时的条件。这样就有机会完全排除配置类


• REGISTER_BEAN 在注册配置类中的bean时评估条件。这不会阻止添加配置类，但如果条件不匹配，则允许跳过bean定义（由匹配定义条件接口）

• PARSE_CONFIGURATION evaluates the condition when the @Configuration-annotated class is parsed. This gives a chance to fully exclude the configuration class
• REGISTER_BEAN evaluates the condition when a bean from a configuration class is registered. This does not prevent the configuration class to be added but it allows to skip a bean definition if the condition does not match (as defined by the matches method of the Condition interface)

Spring Boot有一个 OnBeanCondition 在注册阶段基本上检查是否存在另一个bean。这是 ConditionalOnBean 的核心，当bean存在时基本上会做某事

Spring Boot has a OnBeanCondition that basically checks during the registration phase if another bean is present. This is the core of ConditionalOnBean that basically does something when a bean is present

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