一、接口Condition、Conditional（原理）

主要提供一下方法

boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

true：表示装配

false：表示不装配

1.1、Conditional

　　在Spring4中引入，其主要作用就是判断条件是否满足，从而决定是否初始化并向容器注册Bean

注解：Conditional() 参数是数组，数组内的都是true才装配

@Target({ElementType.TYPE, ElementType.METHOD})

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface Conditional {

    /**

     * All {@link Condition}s that must {@linkplain Condition#matches match}

     * in order for the component to be registered.

     */

    Class<? extends Condition>[] value();

}

通常配合使用。

示例：

接口：EncodingConvert

package com.lhx.spring.springboot_auto_config;

public interface EncodingConvert {

}

接口实现一：UTF8EncodingConvert

package com.lhx.spring.springboot_auto_config;

public class UTF8EncodingConvert implements EncodingConvert {

}

接口实现二：GBKEncodingConvert

package com.lhx.spring.springboot_auto_config;

public class GBKEncodingConvert implements EncodingConvert {

}

配置类：

package com.lhx.spring.springboot_auto_config;

import org.springframework.boot.SpringBootConfiguration;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Conditional;

@SpringBootConfiguration

public class EncodingConvertConfiguration {

    @Bean

    public EncodingConvert createGBKEncodingConvert() {

        return new GBKEncodingConvert();

    }

    @Bean

    public EncodingConvert createUTF8EncodingConvert() {

        return new UTF8EncodingConvert();

    }

}

App：

package com.lhx.spring.springboot_auto_config;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

import org.springframework.context.ConfigurableApplicationContext;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Condition;

import org.springframework.context.annotation.Conditional;

@SpringBootApplication

public class App {

    @Bean

    public Runnable createRunnable() {

        return () -> {

            System.out.println("spring boot is running");

        };

    }

    public static void main(String[] args) {

        ConfigurableApplicationContext context = SpringApplication.run(App.class, args);

        context.getBean(Runnable.class).run();

        //可以通过启动参数修改-Dfile.encoding=GBK

        System.out.println(System.getProperty("file.encoding"));

        System.out.println(context.getBeansOfType(EncodingConvert.class));

        context.close();

    }

}

此时，会发现连个接口实现都会被装配进来。

UTF8Condition实现Condition接口

package com.lhx.spring.springboot_auto_config;

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) {

        String encoding = System.getProperty("file.encoding");

        if (encoding != null) {

            return "utf-8".equalsIgnoreCase(encoding);

        }

        return false;

    }

}

GBKCondition实现Condition接口

package com.lhx.spring.springboot_auto_config;

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) {

        String encoding = System.getProperty("file.encoding");

        if (encoding != null) {

            return "gbk".equalsIgnoreCase(encoding);

        }

        return false;

    }

}

修改配置类

package com.lhx.spring.springboot_auto_config;

import org.springframework.boot.SpringBootConfiguration;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Conditional;

@SpringBootConfiguration

public class EncodingConvertConfiguration {

    @Bean

    @Conditional(GBKCondition.class)

    public EncodingConvert createGBKEncodingConvert() {

        return new GBKEncodingConvert();

    }

    @Bean

    @Conditional(UTF8Condition.class)

    public EncodingConvert createUTF8EncodingConvert() {

        return new UTF8EncodingConvert();

    }

}

此时，会发现会根据file.encoding值来装配接口类。

可在启动参数增加

　　-Dfile.encoding=GBK　　

然后调试，发现装配类也变了

注意：@Conditional也可以作用在类上

二、Spring提供的Conditional自动配置

jar：spring-boot-autoconfigure中，org.springframework.boot.autoconfigure.condition;即spring-boot提供

ConditionalOnBean：当存在某个bean时候装配

ConditionalOnMissingBean：当不存在某个bean时候装配注解的bean

ConditionalOnClass：当classpath有才装配

ConditionalOnExpression：

ConditionalOnJava：JDK版本符合时候才装配

ConditionalOnNotWebApplication：不是web环境才装配

ConditionalOnWebApplication：是web环境才装配

ConditionalOnResource：资源存在才装配

ConditionalOnProperty：配置存在才装配

2.1、ConditionalOnProperty 配置存在匹配时候才配置

增加配置类

package com.lhx.spring.springboot_auto_config;

import org.springframework.boot.SpringBootConfiguration;

import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;

import org.springframework.context.annotation.Bean;

@SpringBootConfiguration

public class UserConfiguration {

    @Bean

    @ConditionalOnProperty(name = "runnable.enable", havingValue = "true")

    public Runnable createRunnable() {

        return () -> {

        };

    }

}

App2编写

package com.lhx.spring.springboot_auto_config;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

import org.springframework.context.ConfigurableApplicationContext;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Condition;

import org.springframework.context.annotation.Conditional;

@SpringBootApplication

public class App2 {

    public static void main(String[] args) {

        ConfigurableApplicationContext context = SpringApplication.run(App2.class, args);

        System.out.println(context.getBeansOfType(Runnable.class));

        context.close();

    }

}

默认是不能装配的

可以再application.properties中添加runnable.enable=true即可装配

或者@ConditionalOnProperty(name = "runnable.enable", havingValue = "true")增加

matchIfMissing=true，表示配置没有的时候也生效

2.2、ConditionalOnClass classpath 有某个类才装配

增加或删除maven，查看效果

        <dependency>

            <groupId>com.google.code.gson</groupId>

            <artifactId>gson</artifactId>

            <version>2.8.2</version>

        </dependency>

使用代码

    @Bean

    @ConditionalOnClass(name="com.google.gson.Gson")

    public Runnable createGsonRunnable() {

        return () -> {

        };

    }

2.3、ConditionalOnBean：根据容器中是否存在某个Bean进行装配

    @Bean

    @ConditionalOnBean(name="user")

    public Runnable createOnBeanRunnable() {

        return () -> {

        };

    }

示例一、根据配置选择不同的实现类注入

需求：根据配置选择不同的实现类注入，有个业务逻辑，有两套模式，大部分功能流程一致，但涉及某些实现不一致，标准的模板模式:004-行为型-02-模板方法模式（Template Method）

实际使用中会有，默认是一种实现，如果有某个配置，则生效。可以参看springboot自动装配实现，如Cache等

实现：https://github.com/bjlhx15/common.git spring-framework-core/spring-aop/testconditional

基础业务接口

public interface ITestConditionalService {

    String getBefore();

    String get();

}

如果新增业务需要新增配置

public class ConstBean {

    public final static String atest="atest";

    public final static String btest="btest";

}

抽象类实现公共方案步骤

public abstract class AbstractTestConditionalService implements ITestConditionalService {

    @Override

    public abstract String getBefore();

    @Override

    public String get() {

        String before = this.getBefore();

        // do something

        return "before:" + before;

    }

}

第一种实现方案：也就是默认方案

@Service("aTestConditionalServiceImpl")

@ConditionalOnProperty(name = "set.test",havingValue = ConstBean.atest,matchIfMissing = true)

public class ATestConditionalServiceImpl extends AbstractTestConditionalService {

    @Override

    public String getBefore() {

        return "atest test";

    }

}

第二种方案

@Service("bTestConditionalServiceImpl")

@ConditionalOnProperty(name = "set.test",havingValue = ConstBean.btest)

public class BTestConditionalServiceImpl extends AbstractTestConditionalService {

    @Override

    public String getBefore() {

        return "btest test";

    }

}

配置如果是a逻辑可默认不写

# 配置实现 atest、btest

set.test=atest

巴特西

006-Spring Boot自动配置-Condition、Conditional、Spring提供的Conditional自动配置