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17点睛Spring4.1-@Conditional.md

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17.1 @Conditional

  • @Conditional为按照条件配置spring的bean提供了支持,即满足某种条件下,怎么配置对应的bean;
  • 应用场景
    • 当某一个jar包在classpath中的时候,配置某几个bean;
    • 当某一个bean配置好后,会自动配置一个特定的bean;
    • 当某种环境变量被设置后,创建某个bean;
  • @Conditional为敏捷开发所提倡的原则"习惯优于配置"提供了支持;
  • @Conditional是Spring Boot快速开发框架实现"习惯优于配置"的核心技术;

17.2 示例

演示在windows和linux系统下,初始化不同的bean,windows和linux作为判断条件;

17.2.1 构造判断条件

条件的构造需要类实现Condition接口,并实现matches方法

  • WindowsCondition
package com.wisely.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");
	}

}
  • LinuxCondition
package com.wisely.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");
	}

}

17.2.2 编写不同条件下需要的bean

  • 接口
package com.wisely.conditional;


public interface CommandService {

	public String showListCommand();
}
  • WindowsCommnadService
package com.wisely.conditional;

public class WindowsCommnadService implements CommandService {

	public String showListCommand() {
		return "dir";
	}

}
  • LinuxCommandService
package com.wisely.conditional;

public class LinuxCommandService implements CommandService {

	public String showListCommand() {
		return "ls";
	}

}

17.2.3 编写配置类

package com.wisely.conditional;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;

@Configuration
public class DemoConfig {
	@Bean
	@Conditional(WindowsCondition.class)
	public CommandService commandService() {
		return new WindowsCommnadService();
	}

	@Bean
	@Conditional(LinuxCondition.class)
	public CommandService linuxEmailerService() {
		return new LinuxCommandService();
	}

}

17.2.4 测试

  • windows下
package com.wisely.conditional;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Main {

	public static void main(String[] args) {
		AnnotationConfigApplicationContext context =
        		new AnnotationConfigApplicationContext("com.wisely.conditional");
		CommandService cs = context.getBean(CommandService.class);
		System.out.println(cs.showListCommand());
		context.close();
	}
}

输出结果

dir
  • Linux下(本例没有切换到linux,直接修改os.name为Linux)
package com.wisely.conditional;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Main {

	public static void main(String[] args) {
		System.setProperty("os.name", "Linux");
		AnnotationConfigApplicationContext context =  new AnnotationConfigApplicationContext("com.wisely.conditional");
		CommandService cs = context.getBean(CommandService.class);
		System.out.println(cs.showListCommand());
		context.close();

	}
}

输出结果

ls