（007）Spring Boot之自动装配接口Condition、对应注解Conditional及其一些默认实现

　　1、通过实现Condition接口和添加Conditional注解实现springboot的自动装配

　　有时候我们需要根据条件自动装配一些bean，比如根据当前运行的文件是utf-8还是gbk编码来绝定装配哪个bean，请看下面示例，装配了utf8的bean

　　pom.xml

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.edu.spring</groupId>
    <artifactId>springboot</artifactId>
    <version>1.0.0</version>
    <packaging>jar</packaging>

    <name>springboot</name>
    <!-- FIXME change it to the project's website -->
    <url>http://www.example.com</url>
    
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-dependencies</artifactId>
                <version>2.1.6.RELEASE</version>
                <scope>import</scope>
                <type>pom</type>
            </dependency>
        </dependencies>
    </dependencyManagement>
    
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
    </dependencies>

</project>

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　　EncodingConvert.java，接口

package com.edu.spring.springboot;

public interface EncodingConvert {

}

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　　GBKEncodingConvert.java，实现类1

package com.edu.spring.springboot;

public class GBKEncodingConvert implements  EncodingConvert {

}

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　　UTF8EncodingConvert.java，实现类2

package com.edu.spring.springboot;

public class UTF8EncodingConvert implements EncodingConvert {

}

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　　GBKCondition.java，实现Condition接口

package com.edu.spring.springboot;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;

public class GBKCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
        // TODO Auto-generated method stub
        String encoding=System.getProperty("file.encoding");
        if(encoding!=null){
            return "gbk".equals(encoding.toLowerCase());
        }
        return false;
    }

}

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　　UTF8Condition.java，实现Condition接口

package com.edu.spring.springboot;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;

public class UTF8Condition implements Condition {

    @Override
    public boolean matches(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
        // TODO Auto-generated method stub
        String encoding=System.getProperty("file.encoding");
        if(encoding!=null){
            return "utf-8".equals(encoding.toLowerCase());
        }
        return false;
    }

}

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　　EncodingConvertConfig.java，创建bean

package com.edu.spring.springboot;

import org.springframework.boot.SpringBootConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;

@SpringBootConfiguration
public class EncodingConvertConfig {

    @Bean
    @Conditional(UTF8Condition.class)
    public  EncodingConvert createUTF8EncodingConvert(){
        return new UTF8EncodingConvert();
    }
    
    @Bean
    @Conditional(GBKCondition.class)
    public  EncodingConvert createGBKEncodingConvert(){
        return new GBKEncodingConvert();
    }
}

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　　App.java

package com.edu.spring.springboot;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class App 
{
    public static void main( String[] args )
    {
        ConfigurableApplicationContext context=SpringApplication.run(App.class,args);
        System.out.println(context.getBeansOfType(EncodingConvert.class));
        context.close();
    }
}

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　　运行结果如下：

 　　总结：@Conditional注解与Condition接口要一块使用，Condition接口实现类中的matches方法返回true时，@Conditional标记的bean才会装配。

　　　　    @Conditional注解可以加在配置类上，表明该配置类中装配的bean都需要满足返回true的条件。

                  @Conditional注解可以添加多个类，表明要同时满足多个条件才会装配，如@Conditional({UTF8Condition.class,GBKCondition.class})。