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SpringBoot自动装配之Condition深入讲解

不死鸟.亚历山大.狼崽子 人气:0

Condition是在Spring 4.0 增加的条件判断功能,通过这个可以功能可以实现选择性的创建 Bean操作。

思考:

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

看一个例子:

当我们没导入redis-start时,会报错

引出问题

Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'redisTemplate' available
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanDefinition(DefaultListableBeanFactory.java:874)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getMergedLocalBeanDefinition(AbstractBeanFactory.java:1358)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:309)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1154)
	at com.example.condition.SpringbootDemo01Application.main(SpringbootDemo01Application.java:13)

当导入redis起步依赖后

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-redis</artifactId>
</dependency>
org.springframework.data.redis.core.RedisTemplate@5a6d5a8f

问题:

SpringBoot是怎么知道我们导入redis坐标的呢?

案例一

需求:

在 Spring 的 IOC 容器中有一个 User 的 Bean,现要求:

通过condition设置加载或者不加载。

新建实体类:

package com.example.condition.entity;
public class User {
}

新建配置文件:

package com.example.condition.config;
import com.example.condition.condition.ClassCondition;
import com.example.condition.entity.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:

如果为true则加载,如果为false则不加载

package com.example.condition.condition;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
public class ClassCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return true;
    }
}

测试:

package com.example.condition;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication
public class SpringbootDemo01Application {
    public static void main(String[] args) {
        //启动SpringBoot应用,返回Spring的IOC容器
        ConfigurableApplicationContext context = SpringApplication.run(SpringbootDemo01Application.class, args);
        Object user = context.getBean("user");
        System.out.println(user);
    }
}

案例二

需求:

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User实体类:

package com.example.condition.entity;
public class User {
}

新建配置文件:

package com.example.condition.config;
import com.example.condition.condition.ClassCondition;
import com.example.condition.entity.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();
    }
}

condition通过反射判断jedis是否已经加载完毕

package com.example.condition.condition;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
public class ClassCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        boolean flag =true;
        try {
            Class<?> cls = Class.forName("redis.clients.jedis.Jedis");
        }catch (ClassNotFoundException e){
            flag =false;
        }
        return flag;
    }
}

测试类:

package com.example.condition;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication
public class SpringbootDemo01Application {
    public static void main(String[] args) {
        //启动SpringBoot应用,返回Spring的IOC容器
        ConfigurableApplicationContext context = SpringApplication.run(SpringbootDemo01Application.class, args);
        Object user = context.getBean("user");
        System.out.println(user);
    }
}

引入jedis进行测试判断:

        <dependency>
            <groupId>redis.clients</groupId>
            <artifactId>jedis</artifactId>
            <version>3.6.0</version>
        </dependency>

案例三

需求:

在 Spring 的 IOC 容器中有一个 User 的 Bean,现要求:

实体类:

package com.example.condition.entity;
public class User {
}

自定义注解:

package com.example.condition.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();}

Condition类:

package com.example.condition.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();
}

配置类:

package com.example.condition.condition;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.env.Environment;
import org.springframework.core.type.AnnotatedTypeMetadata;
import java.util.Map;
public class ClassCondition implements Condition {
    /**
     * @param context  上下文对象,用于获取环境,ClassLoader对象
     * @param metadata 注解的元对象,可以用于注解定义的属性值
     * @return
     */
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        Environment environment = context.getEnvironment();
        //1.需求:导入指定坐标后创建Bean
        //思路:判断指定坐标文件是否存在
        //获取注解属性值 value
        Map<String, Object> map = metadata.getAnnotationAttributes(ConditionOnClass.class.getName());
        String[] value = (String[]) map.get("value");
        boolean flag = true;
        try {
            for (String className : value) {
                Class<?> cls = Class.forName(className);
            }
        } catch (ClassNotFoundException e) {
            flag = false;
        }
        return flag;
    }
}

测试类:

package com.example.condition;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication
public class SpringbootDemo01Application {
    public static void main(String[] args) {
        //启动SpringBoot应用,返回Spring的IOC容器
        ConfigurableApplicationContext context = SpringApplication.run(SpringbootDemo01Application.class, args);
        Object user = context.getBean("user");
        System.out.println(user);
    }
}

总结

自定义条件:

① 定义条件类:自定义类实现Condition接口,重写 matches 方法,在 matches 方法中进行逻辑判断,返回 boolean值 。 matches 方法两个参数:

② 判断条件:在初始化Bean时,使用 @Conditional(条件类.class)注解

SpringBoot 提供的常用条件注解:

@Bean
@ConditionalOnProperty(name = "com",havingValue = "example")
public User user1(){
    return new User();
}

配置文件添加一下属性:

com = example

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