【Spring Boot】 SpringBoot自动装配-Condition

目录一、前言二、 定义2.1 Conditional2.2 Condition2.2.1 ConditionContext三、 使用说明3.1 创建项目3.1.1 导入依赖3.1.2 添加配置信息3.1.3 创建User类3.1.4 创建条件实现类3.1.5 修改启动类3.2 测试3.2.1 当user.enablefalse3.2.2 当user.enabletrue3.3 小结四、改进4.1 创建注解4.2 修改UserCondition五、 Spring内置条件注解一、前言Conditional注解在Spring4.0中引入其主要作用就是判断条件是否满足从而决定是否初始化并向容器注册Bean。二、 定义2.1 ConditionalConditional注解定义如下其内部只有一个参数为Class对象数组且必须继承自Condition接口通过重写Condition接口的matches方法来判断是否需要加载BeanTarget({ElementType.TYPE, ElementType.METHOD}) Retention(RetentionPolicy.RUNTIME) Documented public interface Conditional { Class? extends Condition[] value(); }2.2 ConditionCondition接口定义如下该接口为一个函数式接口只有一个matches接口形参为ConditionContext context, AnnotatedTypeMetadata metadata。ConditionContext定义如2.2.1,AnnotatedTypeMetadata见名知意就是用来获取注解的元信息的FunctionalInterface public interface Condition { boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata); }2.2.1 ConditionContextConditionContext接口定义如下通过查看源码可以知道从这个类中可以获取很多有用的信息public interface ConditionContext { /** * 返回Bean定义信息 * Return the {link BeanDefinitionRegistry} that will hold the bean definition * should the condition match. * throws IllegalStateException if no registry is available (which is unusual: * only the case with a plain {link ClassPathScanningCandidateComponentProvider}) */ BeanDefinitionRegistry getRegistry(); /** * 返回Bean工厂 * Return the {link ConfigurableListableBeanFactory} that will hold the bean * definition should the condition match, or {code null} if the bean factory is * not available (or not downcastable to {code ConfigurableListableBeanFactory}). */ Nullable ConfigurableListableBeanFactory getBeanFactory(); /** * 返回环境变量 比如在application.yaml中定义的信息 * Return the {link Environment} for which the current application is running. */ Environment getEnvironment(); /** * 返回资源加载器 * Return the {link ResourceLoader} currently being used. */ ResourceLoader getResourceLoader(); /** * 返回类加载器 * Return the {link ClassLoader} that should be used to load additional classes * (only {code null} if even the system ClassLoader isnt accessible). * see org.springframework.util.ClassUtils#forName(String, ClassLoader) */ Nullable ClassLoader getClassLoader(); }三、 使用说明通过一个简单的小例子测试一下Conditional是不是真的能实现Bean的条件化注入。3.1 创建项目首先我们创建一个SpringBoot项目3.1.1 导入依赖这里我们除了springboot依赖再添加个lombok依赖?xml version1.0 encodingUTF-8? project xmlnshttp://maven.apache.org/POM/4.0.0 xmlns:xsihttp://www.w3.org/2001/XMLSchema-instance xsi:schemaLocationhttp://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd modelVersion4.0.0/modelVersion parent groupIdorg.springframework.boot/groupId artifactIdspring-boot-starter-parent/artifactId version2.5.3/version relativePath/ !-- lookup parent from repository -- /parent groupIdcom.ldx/groupId artifactIdcondition/artifactId version0.0.1-SNAPSHOT/version namecondition/name descriptionDemo project for Spring Boot/description properties java.version1.8/java.version /properties dependencies dependency groupIdorg.springframework.boot/groupId artifactIdspring-boot-starter/artifactId /dependency dependency groupIdorg.projectlombok/groupId artifactIdlombok/artifactId version1.18.12/version /dependency /dependencies build plugins plugin groupIdorg.springframework.boot/groupId artifactIdspring-boot-maven-plugin/artifactId /plugin /plugins /build /project3.1.2 添加配置信息在application.yaml 中加入配置信息user: enable: false3.1.3 创建User类package com.ldx.condition; import lombok.AllArgsConstructor; import lombok.Data; /** * 用户信息 * author ludangxin * date 2021/8/1 */ Data AllArgsConstructor public class User { private String name; private Integer age; }3.1.4 创建条件实现类package com.ldx.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; /** * 用户bean条件判断 * author ludangxin * date 2021/8/1 */ public class UserCondition implements Condition { Override public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) { Environment environment conditionContext.getEnvironment(); // 获取property user.enable String property environment.getProperty(user.enable); // 如果user.enable的值等于true 那么返回值为true反之为false return true.equals(property); } }3.1.5 修改启动类package com.ldx.condition; import lombok.extern.slf4j.Slf4j; 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.Conditional; Slf4j SpringBootApplication public class ConditionApplication { public static void main(String[] args) { ConfigurableApplicationContext applicationContext SpringApplication.run(ConditionApplication.class, args); // 获取类型为User类的Bean User user applicationContext.getBean(User.class); log.info(user bean {}, user); } /** * 注入User类型的Bean */ Bean Conditional(UserCondition.class) public User getUser(){ return new User(张三,18); } }3.2 测试3.2.1 当user.enablefalse报错找不到可用的User类型的Bean. ____ _ __ _ _ /\ / ____ __ _ _(_)_ __ __ _( ( )___ | _ | | | ’/ |/)| |)| | | | | || (| | ) ) ) )’ || .__|| ||| |, | / / / /|||////_/:: Spring Boot :: (v2.5.3)Exception in thread main org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type com.ldx.condition.User available at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:351) at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:342) at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1172) at com.ldx.condition.ConditionApplication.main(ConditionApplication.java:16) Process finished with exit code 13.2.2 当user.enabletrue正常输出UserBean实例信息. ____ _ __ _ _ /\ / ____ __ _ _(_)_ __ __ _( ( )___ | _ | | | ’/ |/)| |)| | | | | || (| | ) ) ) )’ || .__|| ||| |, | / / / /|||////_/:: Spring Boot :: (v2.5.3)com.ldx.condition.ConditionApplication : user bean User(name张三, age18)3.3 小结上面的例子通过使用Conditional和Condition接口实现了spring bean的条件化注入。好处可以实现某些配置的开关功能如上面的例子我们可以将UserBean换成开启缓存的配置当property的值为true时我们才开启缓存的配置当有多个同名的bean时如何抉择的问题。实现自动化的装载。如判断当前classpath中有mysql的驱动类时说明我们当前的系统需要使用mysql我们就自动的读取application.yaml中的mysql配置实现自动装载当没有驱动时就不加载。四、改进从上面的使用说明中我们了解到了条件注解的大概使用方法但是代码中还是有很多硬编码的问题。比如UserCondition中的property的key包括value都是硬编码其实我们可以通过再扩展一个注解来实现动态的判断和绑定。4.1 创建注解import org.springframework.context.annotation.Conditional; import java.lang.annotation.*; /** * 自定义条件属性注解 * p * 当配置的property name对应的值 与设置的 value值相等时则注入bean * author ludangxin * date 2021/8/1 */ Target({ElementType.TYPE, ElementType.METHOD}) Retention(RetentionPolicy.RUNTIME) Documented // 指定condition的实现类 Conditional({UserCondition.class}) public interface MyConditionOnProperty { // 配置信息的key String name(); // 配置信息key对应的值 String value(); }4.2 修改UserConditionpackage com.ldx.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; /** * 用户bean条件判断 * author ludangxin * date 2021/8/1 */ public class UserCondition implements Condition { Override public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) { Environment environment conditionContext.getEnvironment(); // 获取自定义的注解 MapString, Object annotationAttributes annotatedTypeMetadata.getAnnotationAttributes(com.ldx.condition.MyConditionOnProperty); // 获取在注解中指定的name的property的值 如user.enable的值 String property environment.getProperty(annotationAttributes.get(name).toString()); // 获取预期的值 String value annotationAttributes.get(value).toString(); return value.equals(property); } }测试后结果符合预期。其实在spring中已经内置了许多常用的条件注解其中我们刚实现的就在内置的注解中已经实现了如下。五、 Spring内置条件注解注解 说明ConditionalOnSingleCandidate 当给定类型的bean存在并且指定为Primary的给定类型存在时,返回trueConditionalOnMissingBean 当给定的类型、类名、注解、昵称在beanFactory中不存在时返回true.各类型间是or的关系ConditionalOnBean 与上面相反要求bean存ConditionalOnMissingClass 当给定的类名在类路径上不存在时返回true,各类型间是and的关系ConditionalOnClass 与上面相反要求类存在ConditionalOnCloudPlatform 当所配置的CloudPlatform为激活时返回trueConditionalOnExpression spel表达式执行为trueConditionalOnJava 运行时的java版本号是否包含给定的版本号.如果包含,返回匹配,否则,返回不匹配ConditionalOnProperty 要求配置属性匹配条件 ConditionalOnJndi 给定的jndi的Location必须存在一个.否则,返回不匹配ConditionalOnNotWebApplication web环境不存在时ConditionalOnWebApplication web环境存在时ConditionalOnResource 要求制定的资源存在