Spring Boot框架中的@Conditional系列注解

目录

1. @Conditional 注解

2. Spring boot 扩展

1) @ConditionalOnClass和@ConditionalOnMissingClass注解

2) @ConditionalOnBean 和@ConditionalOnMissingBean注解

3) @ConditionalOnProperty注解

---

1. @Conditional 注解

        @Conditional注解是Spring-context模块提供了一个注解，该注解的作用是可以根据一定的条件来使@Configuration注解标记的配置类是否生效,代码如下:

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
 
package org.springframework.context.annotation;
 
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
 
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
    Classextends Condition>[] value();
}

        value值为实现Condition 接口的一个Class，Spring框架根据实现Conditon接口的matches方法返回true或者false来做以下操作，如果matches方法返回true,那么该配置类会被Spring扫描到容器里， 如果为false,那么Spring框架会自动跳过该配置类不进行扫描装配，使用方法:

        实现Condition接口, 例如在配置文件里配置了dataSource.none=true, 那么表示不需要使用数据源，那么Spring在扫描的时候会自动跳过该配置类。

package com.bing.sh.datasource;
 
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
 
/**
 * 如果为返回false,那么Spring会忽略配置类
 */
public class DataSourceCondition implements Condition {
 
 
    @Override
    public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
        // 此处使用 conditionContext获取Environment即可。
        String configureDataSource = conditionContext.getEnvironment().getProperty("dataSource.none", "false");
        return "false".equals(configureDataSource);
    }
 
 
}

        定义配置类，与@Conditional注解一起使用:

@Configuration
@Conditional(value = DataSourceCondition.class)
public class CustomDataSourceConfig {
 
 
}

        除了Conditional注解，Spring boot 框架提供了 其他conditional系列注解。

2. Spring boot 扩展

        SpringBoot的spring-boot-autoconfigure模块也提供了Conditional系列的相关注解，这些注解能帮助开发者根据一定的条件去装载需要的Bean。 

1) @ConditionalOnClass和@ConditionalOnMissingClass注解

        当Spring加载的Bean被@ConditionOnClass注解标记时，类加载器会先去先找到指定的Class, 如果没有找到目标Class,那么被ConditionOnClass注解标记的类不会被Spring装载,相反ConditionalOnMissingBean是指如果没有找到目标Class, 那么就装载该类。

2) @ConditionalOnBean 和@ConditionalOnMissingBean注解

        当Spring加载的Bean被@ConditionalOnBean注解标记时，接下来会先找到指定的Bean,如果没有找到目标Bean,那么被@ConditionalOnBean标记的类不会被Spring装载,相反ConditionalOnMissingBean是指如果没有Class, 那么就装载该Bean。

        看一个例子, Dubbo与Springboot做自动装配时，先寻找BASE_PACKAGES_BEAN_NAME这个Bean, 如果Bean 不存在，那么serviceAnnotationBeanProcessor这个Bean不会被Spring 装载.

@ConditionalOnProperty(prefix = DUBBO_PREFIX, name = "enabled", matchIfMissing = true)
@Configuration
@AutoConfigureAfter(DubboRelaxedBindingAutoConfiguration.class)
@EnableConfigurationProperties(DubboConfigurationProperties.class)
@EnableDubboConfig
public class DubboAutoConfiguration {
 
    /**
     * Creates {@link ServiceAnnotationPostProcessor} Bean
     * dubbo.scan.base-packages
     * @param packagesToScan the packages to scan
     * @return {@link ServiceAnnotationPostProcessor}
     */
    @ConditionalOnProperty(prefix = DUBBO_SCAN_PREFIX, name = BASE_PACKAGES_PROPERTY_NAME)
    // 先找BASE_PACKAGES_BEAN_NAME 这个bean, 如果没有这个bean, 那么serviceAnnotationBeanProcessor不会被Spring装载。
    @ConditionalOnBean(name = BASE_PACKAGES_BEAN_NAME)
    @Bean
    public ServiceAnnotationPostProcessor serviceAnnotationBeanProcessor(@Qualifier(BASE_PACKAGES_BEAN_NAME)
                                                                       Set packagesToScan) {
        return new ServiceAnnotationPostProcessor(packagesToScan);
    }
 
}

        使用@ConditionalOnMissingBean注解定义BASE_PACKAGES_BEAN_NAME这个Bean

/**
 * Dubbo Relaxed Binding Auto-{@link Configuration} for Spring Boot 2.0
 *
 * @see DubboRelaxedBindingAutoConfiguration
 * @since 2.7.0
 */
@Configuration
@ConditionalOnProperty(prefix = DUBBO_PREFIX, name = "enabled", matchIfMissing = true)
@ConditionalOnClass(name = "org.springframework.boot.context.properties.bind.Binder")
@AutoConfigureBefore(DubboRelaxedBindingAutoConfiguration.class)
public class DubboRelaxedBinding2AutoConfiguration {
 
    public PropertyResolver dubboScanBasePackagesPropertyResolver(ConfigurableEnvironment environment) {
        ConfigurableEnvironment propertyResolver = new AbstractEnvironment() {
            @Override
            protected void customizePropertySources(MutablePropertySources propertySources) {
                Map dubboScanProperties = getSubProperties(environment.getPropertySources(), DUBBO_SCAN_PREFIX);
                propertySources.addLast(new MapPropertySource("dubboScanProperties", dubboScanProperties));
            }
        };
        ConfigurationPropertySources.attach(propertyResolver);
        return propertyResolver;
    }
 
    /**
     * The bean is used to scan the packages of Dubbo Service classes
     * 如果没有就创建
     * @param environment {@link Environment} instance
     * @return non-null {@link Set}
     * @since 2.7.8
     */
    @ConditionalOnMissingBean(name = BASE_PACKAGES_BEAN_NAME)
    @Bean(name = BASE_PACKAGES_BEAN_NAME)
    public Set dubboBasePackages(ConfigurableEnvironment environment) {
        PropertyResolver propertyResolver = dubboScanBasePackagesPropertyResolver(environment);
        return propertyResolver.getProperty(BASE_PACKAGES_PROPERTY_NAME, Set.class, emptySet());
    }
 
    @ConditionalOnMissingBean(name = RELAXED_DUBBO_CONFIG_BINDER_BEAN_NAME, value = ConfigurationBeanBinder.class)
    @Bean(RELAXED_DUBBO_CONFIG_BINDER_BEAN_NAME)
    @Scope(scopeName = SCOPE_PROTOTYPE)
    public ConfigurationBeanBinder relaxedDubboConfigBinder() {
        return new BinderDubboConfigBinder();
    }
 
}

3) @ConditionalOnProperty注解

        该注解的作用是解析application.yml/application.properties 里的配置生成条件来生效，也是与@Configuration注解一起使用。

        使用场景，例如在指定数据源时，指定datasource的type。例如包含如下配置使用Hikari数据源。

spring.datasource.type=com.zaxxer.hikari.HikariDataSource

        在使用时，一般设置matchIfMissing=false， 这样条件没有匹配上的话会Spring在扫描bean时会自动跳过该配置类。 

        也可以设定matchIfMissing=true,这种场景例如缓存，我们可以这样配置默认是开启缓存的。

@ConditionalOnProperty(name={cache.effect},marchIfMissing=true)
 
public class CacheAutoConfiguration{
 
 
 
   // ...
 
}

        如果在application.properties里配置cache.effect=false, 那么该配置类就会跳过，这样配置就能使缓存不生效。