Spring条件注解用法案例分析

时间:2021-04-02 人气:0

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。

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