一、接口Condition、Conditional（原理）
主要提供一下方法
boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
true：表示装配
false：表示不装配
注解：Conditional() 参数是数组，数组内的都是true才装配
@Target({ElementType.TYPE, ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface Conditional { /** * All { @link Condition}s that must { @linkplain Condition#matches match} * in order for the component to be registered. */ Class [] value();} 通常配合使用。
示例：
接口：EncodingConvert
package com.lhx.spring.springboot_auto_config;public interface EncodingConvert {} 接口实现一：UTF8EncodingConvert
package com.lhx.spring.springboot_auto_config;public class UTF8EncodingConvert implements EncodingConvert {} 接口实现二：GBKEncodingConvert
package com.lhx.spring.springboot_auto_config;public class GBKEncodingConvert implements EncodingConvert {} 配置类：
package com.lhx.spring.springboot_auto_config;import org.springframework.boot.SpringBootConfiguration;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Conditional;@SpringBootConfigurationpublic class EncodingConvertConfiguration { @Bean public EncodingConvert createGBKEncodingConvert() { return new GBKEncodingConvert(); } @Bean public EncodingConvert createUTF8EncodingConvert() { return new UTF8EncodingConvert(); }} App：
package com.lhx.spring.springboot_auto_config;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.context.ConfigurableApplicationContext;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Condition;import org.springframework.context.annotation.Conditional;@SpringBootApplicationpublic class App { @Bean public Runnable createRunnable() { return () -> { System.out.println("spring boot is running"); }; } public static void main(String[] args) { ConfigurableApplicationContext context = SpringApplication.run(App.class, args); context.getBean(Runnable.class).run(); //可以通过启动参数修改-Dfile.encoding=GBK System.out.println(System.getProperty("file.encoding")); System.out.println(context.getBeansOfType(EncodingConvert.class)); context.close(); }} 此时，会发现连个接口实现都会被装配进来。
UTF8Condition实现Condition接口
package com.lhx.spring.springboot_auto_config;import org.springframework.context.annotation.Condition;import org.springframework.context.annotation.ConditionContext;import org.springframework.core.type.AnnotatedTypeMetadata;public class UTF8Condition implements Condition { @Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { String encoding = System.getProperty("file.encoding"); if (encoding != null) { return "utf-8".equalsIgnoreCase(encoding); } return false; }} GBKCondition实现Condition接口
package com.lhx.spring.springboot_auto_config;import org.springframework.context.annotation.Condition;import org.springframework.context.annotation.ConditionContext;import org.springframework.core.type.AnnotatedTypeMetadata;public class GBKCondition implements Condition { @Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { String encoding = System.getProperty("file.encoding"); if (encoding != null) { return "gbk".equalsIgnoreCase(encoding); } return false; }} 修改配置类
package com.lhx.spring.springboot_auto_config;import org.springframework.boot.SpringBootConfiguration;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Conditional;@SpringBootConfigurationpublic class EncodingConvertConfiguration { @Bean @Conditional(GBKCondition.class) public EncodingConvert createGBKEncodingConvert() { return new GBKEncodingConvert(); } @Bean @Conditional(UTF8Condition.class) public EncodingConvert createUTF8EncodingConvert() { return new UTF8EncodingConvert(); }} 此时，会发现会根据file.encoding值来装配接口类。
可在启动参数增加
-Dfile.encoding=GBK
然后调试，发现装配类也变了
注意：@Conditional也可以作用在类上
二、Spring提供的Conditional自动配置
jar：spring-boot-autoconfigure中，org.springframework.boot.autoconfigure.condition;即spring-boot提供
ConditionalOnBean：当存在某个bean时候装配
ConditionalOnMissingBean：当不存在某个bean时候装配
ConditionalOnClass：当classpath有才装配
ConditionalOnExpression：
ConditionalOnJava：JDK版本符合时候才装配
ConditionalOnNotWebApplication：不是web环境才装配
ConditionalOnWebApplication：是web环境才装配ConditionalOnResource：资源存在才装配
ConditionalOnProperty：配置存在才装配
示例一：ConditionalOnProperty 配置存在匹配时候才配置
增加配置类
package com.lhx.spring.springboot_auto_config;import org.springframework.boot.SpringBootConfiguration;import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;import org.springframework.context.annotation.Bean;@SpringBootConfigurationpublic class UserConfiguration { @Bean @ConditionalOnProperty(name = "runnable.enable", havingValue = "true") public Runnable createRunnable() { return () -> { }; }} App2编写
package com.lhx.spring.springboot_auto_config;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;import org.springframework.context.ConfigurableApplicationContext;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Condition;import org.springframework.context.annotation.Conditional;@SpringBootApplicationpublic class App2 { public static void main(String[] args) { ConfigurableApplicationContext context = SpringApplication.run(App2.class, args); System.out.println(context.getBeansOfType(Runnable.class)); context.close(); }} 默认是不能装配的
可以再application.properties中添加runnable.enable=true即可装配
或者@ConditionalOnProperty(name = "runnable.enable", havingValue = "true")增加
matchIfMissing=true，表示配置没有的时候也生效
示例二：ConditionalOnClass classpath有某个类才装配
增加或删除maven，查看效果
com.google.code.gson gson 2.8.2
使用代码
@Bean @ConditionalOnClass(name="com.google.gson.Gson") public Runnable createGsonRunnable() { return () -> { }; } 示例三、ConditionalOnBean：根据容器中是否存在某个Bean进行装配
@Bean @ConditionalOnBean(name="user") public Runnable createOnBeanRunnable() { return () -> { }; }