一、条件注解@Conditional，组合注解，元注解
1.
@Conditional：满足特定条件创建一个Bean，SpringBoot就是利用这个特性进行自动配置的。
例子:
首先，两个Condition，判断当前系统是否是Windows或者Linux（True False）
然后，2个ListService实现类，表明不同系统下的ListService实现。
主要，ConditionConfig使用了Java配置与@Conditional注解，根据LinuxCondition，或者WindowsCondition作为判断条件
产生相应与系统匹配的实现类。
最后，App.java 测试成功。
package com.springboot.springboot_test2_1; import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata; public class LinuxCondition implements Condition { @Override
public boolean matches(ConditionContext arg0, AnnotatedTypeMetadata arg1) {
return arg0.getEnvironment().getProperty("os.name").contains("Linux");
} }
LinuxCondition.java
package com.springboot.springboot_test2_1; import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata; public class WindowsCondition implements Condition { @Override
public boolean matches(ConditionContext arg0, AnnotatedTypeMetadata arg1) {
return arg0.getEnvironment().getProperty("os.name").contains("Windows");
} }
WindowsCondition.java
package com.springboot.springboot_test2_1;
public interface ListService {
public String showListCmd();
}
ListService.java
package com.springboot.springboot_test2_1;
public class LinuxListService implements ListService{
@Override
public String showListCmd() {
return "ls";
}
}
LinuxListService.java
package com.springboot.springboot_test2_1;
public class WindowsListService implements ListService{
@Override
public String showListCmd() {
return "dir";
}
}
WindowsListService.java
package com.springboot.springboot_test2_1; import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration; @Configuration
public class ConditionConfig { @Bean
@Conditional(WindowsCondition.class)
public ListService windowsListService() {
return new WindowsListService();
} @Bean
@Conditional(LinuxCondition.class)
public ListService linuxListService() {
return new LinuxListService();
} }
ConditionConfig.java
package com.springboot.springboot_test2_1; import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext; /**
* Hello world!
*
*/
public class App {
public static void main(String[] args) { ApplicationContext context = new AnnotationConfigApplicationContext(
ConditionConfig.class); ListService ls = context.getBean(ListService.class);
System.out.println(context.getEnvironment().getProperty("os.name")
+ "系统下的列表命令为:" + ls.showListCmd());
}
}
App.java
2.
元注解：可以注解到别的注解上的注解。
组合注解：元注解+被注解=组合注解。
例子：
package com.springboot.springboot_test2_1; 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; import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration; @Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@ComponentScan
public @interface testConfig {
String[] value() default{};
}
testConfig.java
二、通过条件注解@Conditional，组合注解，元注解理解SpringBoot的自动配置
@SpringBootApplication，源码：
@Target({java.lang.annotation.ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters={@org.springframework.context.annotation.ComponentScan.Filter(type=org.springframework.context.annotation.FilterType.CUSTOM, classes={org.springframework.boot.context.TypeExcludeFilter.class}), @org.springframework.context.annotation.ComponentScan.Filter(type=org.springframework.context.annotation.FilterType.CUSTOM, classes={AutoConfigurationExcludeFilter.class})})
public @interface SpringBootApplication
{
@AliasFor(annotation=EnableAutoConfiguration.class, attribute="exclude")
Class<?>[] exclude() default {};
@AliasFor(annotation=EnableAutoConfiguration.class, attribute="excludeName")
String[] excludeName() default {};
@AliasFor(annotation=ComponentScan.class, attribute="basePackages")
String[] scanBasePackages() default {};
@AliasFor(annotation=ComponentScan.class, attribute="basePackageClasses")
Class<?>[] scanBasePackageClasses() default {};
}
SpringBootApplication源码
它的核心功能是由@EnableAutoConfiguration注解提供的，
@EnableAutoConfiguration，源码:
@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};
}
EnableAutoConfiguration源码
@EnableAutoConfiguration其中的@AutoConfigurationPackage，会扫描/META-INF/spring.factories文件中的jar包，
spring.factories文件如下：
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
……
2、核心注解
spring.factories文件里每一个xxxAutoConfiguration文件一般都会有下面的条件注解:
@ConditionalOnBean：当容器里有指定Bean的条件下
@ConditionalOnClass：当类路径下有指定类的条件下
@ConditionalOnExpression：基于SpEL表达式作为判断条件
@ConditionalOnJava：基于JV版本作为判断条件
@ConditionalOnJndi：在JNDI存在的条件下差在指定的位置
@ConditionalOnMissingBean：当容器里没有指定Bean的情况下
@ConditionalOnMissingClass：当类路径下没有指定类的条件下
@ConditionalOnNotWebApplication：当前项目不是Web项目的条件下
@ConditionalOnProperty：指定的属性是否有指定的值
@ConditionalOnResource：类路径是否有指定的值
@ConditionalOnSingleCandidate：当指定Bean在容器中只有一个，或者虽然有多个但是指定首选Bean
@ConditionalOnWebApplication：当前项目是Web项目的条件下。
上面@ConditionalOnXXX都是组合@Conditional元注解，使用了不同的条件Condition