spring boot: 条件注解@Condition

@Conditional根据满足某一个特定的条件创建一个特定的Bean（基于条件的Bean的创建，即使用@Conditional注解）。

比方说，当一个jar包在一个类的路径下的时候，自动配置一个或多个Bean，或者只有某个Bean被创建才会去创建另外一个Bean。

通过实现Condition接口，并重写期matches方法来构造判断条件。若在windows系统洗运行程序，则输出列表命令dir,若在linux操作系统下运行程序，则输出命令为：ls.

1.判断条件定义

判断windows的条件

 package ch2.conditional2;

 //条件注解,并复写此类
 import org.springframework.context.annotation.Condition;
 //条件注解容器
 import org.springframework.context.annotation.ConditionContext;
 import org.springframework.core.type.AnnotatedTypeMetadata;

 public class LinuxCondition implements Condition {

     @Override
     public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
         // TODO Auto-generated method stub
         return context.getEnvironment().getProperty("os.name").contains("Linux");
     }

 }

判定linux的条件

 package ch2.conditional2;

 //条件注解接口类，复写metches
 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 context, AnnotatedTypeMetadata metadata) {
         // TODO Auto-generated method stub
         return context.getEnvironment().getProperty("os.name").contains("Windows");
     }

 }

2不同系统下的Bean类

1接口

 package ch2.conditional2;

 public interface ListService {

     public String showListCmd();
 }

2windows下要创建的Bean类

 package ch2.conditional2;

 import ch2.conditional2.ListService;

 public class WindowsListService implements ListService {

     @Override
     public String showListCmd() {
         // TODO Auto-generated method stub
         return "dir";
     }

 }

3linux下要创建的Bean类

 package ch2.conditional2;

 import ch2.conditional2.ListService;

 public class LinuxListService implements ListService {

     @Override
     public String showListCmd() {
         // TODO Auto-generated method stub
         return "ls";
     }

 }

3配置类

package ch2.conditional2;

//配置类声明
import org.springframework.context.annotation.Configuration;
//bean声明
import org.springframework.context.annotation.Bean;
//条件限制
import org.springframework.context.annotation.Conditional;

//声明为配置类
@Configuration
public class ConditionConfig {

    //注解为Bean
    @Bean
    //注解条件判断
    @Conditional(WindowsCondition.class)
    public ListService windowsListService()
    {
        return new WindowsListService();
    }

    //注解为Bean
    @Bean
    //注解条件判断
    @Conditional(LinuxCondition.class)
    public ListService linuxListService()
    {
        return new LinuxListService();
    }
}

4运行

package ch2.conditional2;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Main {

    public static void main(String[] args)
    {

        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(ConditionConfig.class);
        ListService listService = context.getBean(ListService.class);
        System.out.println(
                context.getEnvironment().getProperty("os.name") +
                "系统的查看目录的命令是： " +
                listService.showListCmd()
                );
        context.close();
    }
}

运行结果：Windows 10系统的查看目录的命令是： dir