（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>

　　EncodingConvert.java，接口

package com.edu.spring.springboot;

public interface EncodingConvert {

}

　　GBKEncodingConvert.java，实现类1

package com.edu.spring.springboot;

public class GBKEncodingConvert implements  EncodingConvert {

}

　　UTF8EncodingConvert.java，实现类2

package com.edu.spring.springboot;

public class UTF8EncodingConvert implements EncodingConvert {

}

　　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;
    }

}

　　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;
    }

}

　　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();
    }
}

　　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();
    }
}

　　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})。

2、Conditional的一些默认实现注解

　　（1）@ConditionalOnProperty：根据配置文件中的属性装配bean。如下，配置文件中存在runnable.enble，且值为true时装配

　　application.properties

runnable.enble=true

　　UserConfig.java

package com.edu.spring.springboot;

import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.context.annotation.Bean;

@SpringBootConfiguration
public class UserConfig {
    
    @Bean
    @ConditionalOnProperty(name="runnable.enble",havingValue="true")
    public Runnable createRunnable (){
        return () -> {};
    }
}

　　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(Runnable.class));
        context.close();
    }
}

　　运行结果如下：

 　　说明：如果改为runnable.enble=false，或者删掉该配置，再次运行则不会创建bean。添加matchIfMissing=true，即便是属性不存在，运行也会创建bean

　　（2）@ConditionalOnClass：classpath路径中存在指定的类则创建bean，如下：

　　pom.xml，添加gson的依赖

<?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>
        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
        </dependency>
    </dependencies>

</project>

　　UserConfig.java

package com.edu.spring.springboot;

import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.context.annotation.Bean;

@SpringBootConfiguration
public class UserConfig {
    
    
    @Bean
    @ConditionalOnClass(name="com.google.gson.Gson")
    public Runnable createRunnable(){
        return () -> {};
    }
}

　　运行结果如下：

 　　创建成功，如果删掉依赖则不会创建。

　　（3）@ConditionalOnBean：如果某个bean存在时，则创建bean，如下：

　　UserConfig.java

package com.edu.spring.springboot;

import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.Bean;

@SpringBootConfiguration
public class UserConfig {
    
    @Bean
    @ConditionalOnBean(name="user")
    public Runnable createRunnable3(){
        return () -> {};
    }
}

　　运行如下：不会创建bean

 　　User.java，创建user

package com.edu.spring.springboot;

import org.springframework.stereotype.Component;

@Component
public class User {

}

　　再次运行，创建成，结果如下：

 　　（4）@ConditionalOnMissingBean：当bean不存在时创建，与ConditionalOnBean相反。