SpringBoot自动装配之Condition深入讲解
不死鸟.亚历山大.狼崽子 人气:0Condition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:
- 导入Jedis坐标后,加载该Bean,没导入,则不加载。
- 将类的判断定义为动态的。判断哪个字节码文件存在可以动态指定。
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:
- context:上下文对象,可以获取属性值,获取类加载器,获取BeanFactory等。
- metadata:元数据对象,用于获取注解属性。
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:
- ConditionalOnProperty:判断配置文件中是否有对应属性和值才初始化Bean
@Bean @ConditionalOnProperty(name = "com",havingValue = "example") public User user1(){ return new User(); }
配置文件添加一下属性:
com = example
- ConditionalOnClass:判断环境中是否有对应字节码文件才初始化Bean
- ConditionalOnMissingBean:判断环境中没有对应Bean才初始化Bean
加载全部内容