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.ch4.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.ch4.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.ch4.conditional;public interface ListService {  public String showListCmd();}

2.2 Window下创建的Bean类

package com.wisely.highlight_spring4.ch4.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.ch4.conditional;public class LinuxListService implements ListService{  @Override  public String showListCmd() {   return "ls";  }}

3 配置类

package com.wisely.highlight_spring4.ch4.conditional;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Conditional;import org.springframework.context.annotation.Configuration;@Configurationpublic 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.ch4.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.ch4.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.ch4.conditional.Main.main(Main.java:11)

报错信息很明显：

[com.wisely.highlight_spring4.ch4.conditional.ListService] is defined: expected single matching bean but found 2: linuxListService,windowsListService