springboot之三：原理分析之自动配置condition

导入：

SpringBoot是如何知道要创建哪个Bean的？比如Spring Boot是如何知道要创建RedisTemplate的？

Condition：

Condition是在Spring4.0增加的条件判断功能，通过这个可以实现选择性的创建Bean操作。

• 自定义条件：

• SpringBoot 提供的常用条件注解：

创建模块：

需求一：

在Spring的IOC容器中有一个User的Bean,现要求：

导入Jedis坐标后，加载该Bean,没导入，则不加载。

新建domain软件包，User 类：

package com.itheima.springbootcondition.domain;
 
public class User {
}

新建config软件包，UserConfig 类：

package com.itheima.springbootcondition.config;
 
 
import com.itheima.springbootcondition.condition.ClassCondition;
import com.itheima.springbootcondition.domain.User;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
 
@Configuration
public class UserConfig {
    @Bean
    @Conditional(ClassCondition.class)
    public User user(){
        return new User();
    }
}

新建condition软件包：ClassCondition类：

package com.itheima.springbootcondition.condition;
 
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
 
 
public class ClassCondition implements Condition {
    /**
     *
     * @param context  上下文对象。用于获取环境，ioc容器，classloader对象
     * @param metadata 注解元对象。用于获取注解定义的属性值。
     * @return
     */
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
 
        //需求1： 导入Jedis坐标后，加载该Bean，没导入，则不加载。
        try {
            Class cls = Class.forName("redis.clients.jedis.Jedis");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }
}

SpringbootConditionApplication类：

package com.itheima.springbootcondition;
 
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
 
 
@SpringBootApplication
public class SpringbootConditionApplication {
    public static void main(String[] args) {
        //启动springboot的应用，返回spring的ioc容器
        ConfigurableApplicationContext context = SpringApplication.run(SpringbootConditionApplication.class, args);
        
        Object user = context.getBean("user");
        System.out.println(user);
    }
 
}

在pom文件里增删jedis坐标测试：

<dependency>
   <groupId>redis.clientsgroupId>
   <artifactId>jedisartifactId>
dependency>

需求二：

package com.itheima.springbootcondition.condition;
 
import org.springframework.context.annotation.Conditional;
 
import java.lang.annotation.*;
 
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(ClassCondition.class)
public @interface ConditionOnClass {
    String[] value();
}

package com.itheima.springbootcondition.condition;
 
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
 
import java.util.Iterator;
import java.util.Map;
 
public class ClassCondition implements Condition {
    /**
     *
     * @param context  上下文对象。用于获取环境，ioc容器，classloader对象
     * @param metadata 注解元对象。用于获取注解定义的属性值。
     * @return
     */
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
 
 
        //需求2：导入通过注解属性值value指定坐标后创建bean
 
        //获取注解属性值 value
        Map map = metadata.getAnnotationAttributes(ConditionOnClass.class.getName());
//        System.out.println(map);
        String[] value = (String[]) map.get("value");
        try {
            for (String classname : value) {
                Class cls = Class.forName(classname);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }
}

package com.itheima.springbootcondition.config;
 
 
import com.itheima.springbootcondition.condition.ClassCondition;
import com.itheima.springbootcondition.condition.ConditionOnClass;
import com.itheima.springbootcondition.domain.User;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
 
@Configuration
public class UserConfig {
    @Bean
//    @Conditional(ClassCondition.class)
    @ConditionOnClass("redis.clients.jedis.Jedis")
    public User user(){
        return new User();
    }
}