一、接口Condition、Conditional（原理）

主要提供一下方法

boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

true：表示裝配

false：表示不裝配

註解：Conditional() 參數是數組，數組內的都是true才裝配

@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 {

}

接口實現一：UTF8EncodingConvert

package com.lhx.spring.springboot_auto_config;

public class UTF8EncodingConvert implements EncodingConvert {

}

接口實現二：GBKEncodingConvert

package com.lhx.spring.springboot_auto_config;

public
 
 class GBKEncodingConvert implements EncodingConvert {

}

配置類：

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();
        //可以通過啟動參數修改-Dfile.encoding=GBK
        System.out.println(System.getProperty("file.encoding"));
        System.out.println(context.getBeansOfType(EncodingConvert.class));
        context.close();
    }
}

此時，會發現連個接口實現都會被裝配進來。

UTF8Condition實現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實現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;
    }

}

修改配置類

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();
    }
}

此時，會發現會根據file.encoding值來裝配接口類。

可在啟動參數增加

　　-Dfile.encoding=GBK　　

然後調試，發現裝配類也變了

註意：@Conditional也可以作用在類上

二、Spring提供的Conditional自動配置

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

ConditionalOnBean：當存在某個bean時候裝配

ConditionalOnMissingBean：當不存在某個bean時候裝配

ConditionalOnClass：當classpath有才裝配

ConditionalOnExpression：

ConditionalOnJava：JDK版本符合時候才裝配

ConditionalOnNotWebApplication：不是web環境才裝配
ConditionalOnWebApplication：是web環境才裝配

ConditionalOnResource：資源存在才裝配

ConditionalOnProperty：配置存在才裝配

示例一：ConditionalOnProperty 配置存在匹配時候才配置

增加配置類

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編寫

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();
    }
}

默認是不能裝配的

可以再application.properties中添加runnable.enable=true即可裝配

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

matchIfMissing=true，表示配置沒有的時候也生效

示例二：ConditionalOnClass classpath有某個類才裝配

增加或刪除maven，查看效果

        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
            <version>2.8.2</version>
        </dependency>

使用代碼

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

示例三、ConditionalOnBean：根據容器中是否存在某個Bean進行裝配

    @Bean
    @ConditionalOnBean(name="user")
    public Runnable createOnBeanRunnable() {
        return () -> {
        };
    }

006-Spring Boot自動配置