@ConditionalOnProperty的讲解和用法

@ConditionalOnProperty的用法

一、说明

SpringBoot中有时候需要控制配置类是否生效，使用 @ConditionalOnProperty 注解来控制@Bean是否生效。

二、例子

在配置类头部添加@Configuration和@ConditionalOnProperty注解

• 例子1

@Configuration
@ConditionalOnProperty(prefix = "config",name = "enable",havingValue = "true", matchIfMissing = true)
public class CorssConfig {

    @Bean
    public CorsFilter corsFilter(){
        CorsConfiguration corsConfiguration = new CorsConfiguration();
        corsConfiguration.addAllowedOrigin("*");
        corsConfiguration.setAllowCredentials(false);
        corsConfiguration.addAllowedMethod("*");
        corsConfiguration.addAllowedHeader("*");
        corsConfiguration.addExposedHeader(HttpHeaders.ACCEPT);
        UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
        source.registerCorsConfiguration("/**",corsConfiguration);
        return new CorsFilter(source);
    }

• 例子2

@Configuration
@ConditionalOnProperty(value= {"config.enable"}, havingValue = "true", matchIfMissing = true)
public class CorssConfig {

    @Bean
    public CorsFilter corsFilter(){
        //内容同上
    }

三、讲解

• 源码

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {

	/**
	 * Alias for {@link #name()}.
	 * @return the names
	 */
	String[] value() default {};

	/**
	 * A prefix that should be applied to each property. The prefix automatically ends
	 * with a dot if not specified. A valid prefix is defined by one or more words
	 * separated with dots (e.g. {@code "acme.system.feature"}).
	 * @return the prefix
	 */
	String prefix() default "";

	/**
	 * The name of the properties to test. If a prefix has been defined, it is applied to
	 * compute the full key of each property. For instance if the prefix is
	 * {@code app.config} and one value is {@code my-value}, the full key would be
	 * {@code app.config.my-value}
	 * 

	 * Use the dashed notation to specify each property, that is all lower case with a "-"
	 * to separate words (e.g. {@code my-long-property}).
	 * @return the names
	 */
	String[] name() default {};

	/**
	 * The string representation of the expected value for the properties. If not
	 * specified, the property must not be equal to {@code false}.
	 * @return the expected value
	 */
	String havingValue() default "";

	/**
	 * Specify if the condition should match if the property is not set. Defaults to
	 * {@code false}.
	 * @return if should match if the property is missing
	 */
	boolean matchIfMissing() default false;

}

• 含义

源码里这个注解是有5个参数的，首先了解其含义
- value
String数组的配置类别名
- prefix
String类型的配置类前缀
- name
String数组的配置名称，要结合prefix来用
- havingValue
String类型的默认值，与配置的值对比值，当两个值相同返回true，配置类生效
- matchIfMissing
boolean类型的缺省值，缺少配置时，是否可以加载；缺少配置时，true:正常加载，false：报错

• 使用

在配置文件中，以SpringBoot为例，在application.yml中，配置config.enable

config:
    enable:ok

在配置类上可如下配置：

 @ConditionalOnProperty(prefix = "config",name = "enable",havingValue = "ok", matchIfMissing = true)

或者

@ConditionalOnProperty(value= {"config.enable"}, havingValue = "ok", matchIfMissing = true)

四、总结

@ConditionalOnProperty可以很方便去控制不同环境下对配置的要求，方便调试，省去麻烦