@Conditional

• 作用范围
  类和方法上
  作用在类上 主要加在@Component或者@Configuration 组件上面

• <li>as a type-level annotation on any class directly or indirectly annotated with
• {@code @Component}, including {@link Configuration @Configuration} classes</li>
• <li>as a meta-annotation, for the purpose of composing custom stereotype

作用在方法 用于@Bean 的method上面

• <li>as a method-level annotation on any {@link Bean @Bean} method</li>

• 属性
  Class 是实现了Condition 接口的类

/**

• All {@link Condition Conditions} that must {@linkplain Condition#matches match}
• in order for the component to be registered.
  */
  Class<? extends Condition>[] value();

/**
 * Indicates that a component is only eligible for registration when all
 * {@linkplain #value specified conditions} match.
 *
 * <p>A <em>condition</em> is any state that can be determined programmatically
 * before the bean definition is due to be registered (see {@link Condition} for details).
 *
 * <p>The {@code @Conditional} annotation may be used in any of the following ways:
 * <ul>
 * <li>as a type-level annotation on any class directly or indirectly annotated with
 * {@code @Component}, including {@link Configuration @Configuration} classes</li>
 * <li>as a meta-annotation, for the purpose of composing custom stereotype
 * annotations</li>
 * <li>as a method-level annotation on any {@link Bean @Bean} method</li>
 * </ul>
 *
 * <p>If a {@code @Configuration} class is marked with {@code @Conditional},
 * all of the {@code @Bean} methods, {@link Import @Import} annotations, and
 * {@link ComponentScan @ComponentScan} annotations associated with that
 * class will be subject to the conditions.
 *
 * <p><strong>NOTE</strong>: Inheritance of {@code @Conditional} annotations
 * is not supported; any conditions from superclasses or from overridden
 * methods will not be considered. In order to enforce these semantics,
 * {@code @Conditional} itself is not declared as
 * {@link java.lang.annotation.Inherited @Inherited}; furthermore, any
 * custom <em>composed annotation</em> that is meta-annotated with
 * {@code @Conditional} must not be declared as {@code @Inherited}.
 *
 * @author Phillip Webb
 * @author Sam Brannen
 * @since 4.0
 * @see Condition
 */

Condition

• @FunctionalInterface
• ConditionContext

       ConditionContext  可以获取以下类
       ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
        ClassLoader classLoader = context.getClassLoader();
        Environment environment = context.getEnvironment();
        BeanDefinitionRegistry registry = context.getRegistry();
        ResourceLoader resourceLoader = context.getResourceLoader();

• AnnotatedTypeMetadata 获取注解上的信息

@FunctionalInterface
public interface Condition {

    /**
     * Determine if the condition matches.
     * @param context the condition context
     * @param metadata the metadata of the {@link org.springframework.core.type.AnnotationMetadata class}
     * or {@link org.springframework.core.type.MethodMetadata method} being checked
     * @return {@code true} if the condition matches and the component can be registered,
     * or {@code false} to veto the annotated component's registration
     */
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

}

ConditionDemo

public class ConditionDemo {

    @Conditional(LinuxEnvironment.class)
    @Bean(value = "lilei")
    public Person person(){
        return  new Person("李磊");
    }

    @Conditional(WindowsEnvironment.class)
    @Bean(value = "xiaohong")
    public Person person2(){
        return  new Person("小红");
    }

    public static void main(String[] args) {
        AnnotationConfigApplicationContext app = new AnnotationConfigApplicationContext(ConditionDemo.class);

        String[] beanDefinitionNames = app.getBeanDefinitionNames();
        for (String beanDefinitionName : beanDefinitionNames) {

            System.out.println(beanDefinitionName);
        }
        Person bean = app.getBean(Person.class);
        System.out.println(bean);
    }
}

class Person{
    String name;

    public Person(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                '}';
    }
}

class  LinuxEnvironment implements Condition{


    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return Utils.isMathOsName(context.getEnvironment(),"linux");
    }

}

class  WindowsEnvironment implements Condition{
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
         return  Utils.isMathOsName(context.getEnvironment(),"Windows 10");
    }
}
class Utils{

    public static boolean isMathOsName(Environment environment,String osName) {
        String os_name = environment.getProperty("os.name");
        if(os_name.contains(osName)){
            return true;
        }
        return false;
    }
}

实现Condition的另一种方式

使用配置文件配置
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