概述

假设在某些特定的场景下，希望根据特定的条件去加载某个或某些bean，我们可以使用@Condtional注解， Spring 4.0的时候加入的这个注解。

例子

假设，我们在配置了数据库的几个属性时，才去实例化bean，否则不初始化这个bean。这个例子可能不太合适，权当理解这个注解的用法了

Step1 实现Condition接口，重写matches方法

package com.artisan.springbootmaster.conditional;

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

/**
 *
 */
public class DatabaseCondtional implements Condition {

    /**
     * 数据库bean的装配条件
     *
     * @param context  上下文
     * @param metadata 注释类型的元数据
     * @return  true 装配Bean ，否则不装配
     */
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        // 根据context取出对应的Env信息
        Environment environment = context.getEnvironment();
        // 判断属性文件是否存在对应的数据库配置
        return environment.containsProperty("datasource.driverName")
                && environment.containsProperty("datasource.url")
                && environment.containsProperty("datasource.username")
                && environment.containsProperty("datasource.password");
    }
}

Step2 在对应的@Bean上使用@Conditional注解

package com.artisan.springbootmaster.conditional;

import org.apache.commons.dbcp2.BasicDataSourceFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.*;

import javax.sql.DataSource;
import java.util.Properties;

@Configuration
@PropertySource(value = "classpath:jdbc.properties",ignoreResourceNotFound = true)
public class Config {


    @Bean(name = "datasource" ,destroyMethod = "close")
    @Conditional(DatabaseCondtional.class)
    public DataSource initDataSource(
            @Value("${datasource.driverName}") String driver,
            @Value("${datasource.url}") String url,
            @Value("${datasource.username}") String username,
            @Value("${datasource.password}") String password){


        System.out.println("driver:" + driver + "\n url:" + url+ "\n username:" + username
        + "\n password:" + password) ;

        Properties properties = new Properties();
        properties.setProperty("driver",driver);
        properties.setProperty("url",url);
        properties.setProperty("username",username);
        properties.setProperty("password",password);

        DataSource dataSource = null;
        try {
            dataSource = BasicDataSourceFactory.createDataSource(properties);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return dataSource;
    }
}

测试

package com.artisan.springbootmaster.conditional;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import javax.sql.DataSource;

public class Test {

    public static void main(String[] args){

        ApplicationContext ctx = new AnnotationConfigApplicationContext(Config.class);

       //DataSource dataSource = (DataSource) ctx.getBean("datasource");
        DataSource dataSource = ctx.getBean(DataSource.class);
        System.out.println("datasource:" + dataSource);

    }
}