本文实例讲述了Spring条件注解用法。分享给大家供大家参考，具体如下：
一 点睛
Spring 4 提供了一个更通用的基于条件的Bean的创建，即使用@Conditional注解。
@Conditional根据满足仅一个特定条件创建一个特定的Bean。也就是根据特定的条件来控制Bean的创建行为，这样就可以利用这个特性进行一些自动的配置。
二 项目说明
以不同的操作系统为条件，通过实现@Condition接口，并重写matches方法来构造条件。若在windows系统下运行，则输出列表命令为dir；若在Linux操作系统下运行程序，则输出列表命令为ls。
三 实战
1 判断条件定义
1.1 windows的判定条件
package com.wisely.highlight_spring4.ch3.conditional;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
public class WindowsCondition implements Condition {
public boolean matches(ConditionContext context,
AnnotatedTypeMetadata metadata) {
return context.getEnvironment().getProperty("os.name").contains("Windows");
}
}
1.2 Linux的判定条件
package com.wisely.highlight_spring4.ch3.conditional;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
public class LinuxCondition implements Condition {
public boolean matches(ConditionContext context,
AnnotatedTypeMetadata metadata) {
return context.getEnvironment().getProperty("os.name").contains("Linux");
}
}
2 不同系统下的Bean类
2.1 接口
package com.wisely.highlight_spring4.ch3.conditional;
public interface ListService {
public String showListCmd();
}
2.2 Window下创建的Bean类
package com.wisely.highlight_spring4.ch3.conditional;
public class WindowsListService implements ListService {
@Override
public String showListCmd() {
return "dir";
}
}
2.3 Linux下所创建的Bean类
package com.wisely.highlight_spring4.ch3.conditional;
public class LinuxListService implements ListService{
@Override
public String showListCmd() {
return "ls";
}
}
3 配置类
package com.wisely.highlight_spring4.ch3.conditional;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
@Configuration
public class ConditionConifg {
@Bean
@Conditional(WindowsCondition.class) //符合window条件，则实例化WindowsListService
public ListService windowsListService() {
return new WindowsListService();
}
@Bean
@Conditional(LinuxCondition.class) //符合Linux条件，则实例化LinuxListService
public ListService linuxListService() {
return new LinuxListService();
}
}
4 主类
package com.wisely.highlight_spring4.ch3.conditional;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
public class Main {
public static void main(String[] args) {
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(ConditionConifg.class);
ListService listService = context.getBean(ListService.class);
System.out.println(context.getEnvironment().getProperty("os.name")
+ "系统下的列表命令为: "
+ listService.showListCmd());
context.close();
}
}
四 运行
windows下运行结果如下：
Windows 10系统下的列表命令为: dir
五 扩展
如果把LinuxCondition条件改成和WindowsCondition一样的条件会怎样呢？即有两个条件都匹配会怎样呢？
修改后的代码如下：
public class LinuxCondition implements Condition {
public boolean matches(ConditionContext context,
AnnotatedTypeMetadata metadata) {
return context.getEnvironment().getProperty("os.name").contains("Windows");
}
}
修改后再运行，报错了：
Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.wisely.highlight_spring4.ch3.conditional.ListService] is defined: expected single matching bean but found 2: linuxListService,windowsListService
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:365)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:331)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:968)
at com.wisely.highlight_spring4.ch3.conditional.Main.main(Main.java:11)
报错信息很明显：
[com.wisely.highlight_spring4.ch3.conditional.ListService] is defined: expected single matching bean but found 2: linuxListService,windowsListService
匹配到了两条，所以针对条件判断，设计程序时，只能匹配上一条，否则会抛异常。
希望本文所述对大家java程序设计有所帮助。
