SpringBoot自动装配Condition的实现方式 SpringBoot自动装配Condition的实现

1. 简介
2. 定义

2.1 @Conditional
2.2 Condition

3. 使用说明

3.1 创建项目
3.2 测试
3.3 小结

4. 改进

4.1 创建注解
4.2 修改UserCondition

5. Spring内置条件注解

1. 简介 @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。

2. 定义
2.1 @Conditional
@Conditional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Bean

@Target({ElementType.TYPE, ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface Conditional {Class[] value(); }

2.2 Condition
Condition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

@FunctionalInterfacepublic interface Condition {boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata); }

2.2.1 ConditionContext
ConditionContext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}).*/@NullableConfigurableListableBeanFactory 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 isn't accessible).* @see org.springframework.util.ClassUtils#forName(String, ClassLoader)*/@NullableClassLoader getClassLoader(); }

3. 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

文章图片

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

4.0.0org.springframework.bootspring-boot-starter-parent2.5.3 com.ldxcondition0.0.1-SNAPSHOTconditionDemo project for Spring Boot1.8org.springframework.bootspring-boot-starterorg.projectlomboklombok1.18.12org.springframework.bootspring-boot-maven-plugin

3.1.2 添加配置信息
在application.yaml 中加入配置信息

user:enable: false

3.1.3 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@AllArgsConstructorpublic 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 {@Overridepublic boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {Environment environment = conditionContext.getEnvironment(); // 获取property user.enableString property = environment.getProperty("user.enable"); // 如果user.enable的值等于true 那么返回值为true，反之为falsereturn "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@SpringBootApplicationpublic class ConditionApplication {public static void main(String[] args) {ConfigurableApplicationContext applicationContext = SpringApplication.run(ConditionApplication.class, args); // 获取类型为User类的BeanUser 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.enable=false
报错找不到可用的User类型的Bean

._______ _ _ /\\ / ___'_ __ _ _(_)_ ____ _ \ \ \ \( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/___)| |_)| | | | | || (_| |) ) ) )'|____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::(v2.5.3)2021-08-01 17:07:51.994INFO 47036 --- [main] com.ldx.condition.ConditionApplication: Starting ConditionApplication using Java 1.8.0_261 on ludangxindeMacBook-Pro.local with PID 47036 (/Users/ludangxin/workspace/idea/condition/target/classes started by ludangxin in /Users/ludangxin/workspace/idea/condition)2021-08-01 17:07:51.997INFO 47036 --- [main] com.ldx.condition.ConditionApplication: No active profile set, falling back to default profiles: default2021-08-01 17:07:52.461INFO 47036 --- [main] com.ldx.condition.ConditionApplication: Started ConditionApplication in 0.791 seconds (JVM running for 1.371)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 1

3.2.2 当user.enable=true
正常输出UserBean实例信息

._______ _ _ /\\ / ___'_ __ _ _(_)_ ____ _ \ \ \ \( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/___)| |_)| | | | | || (_| |) ) ) )'|____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::(v2.5.3)2021-08-01 17:13:38.022INFO 47129 --- [main] com.ldx.condition.ConditionApplication: Starting ConditionApplication using Java 1.8.0_261 on ludangxindeMacBook-Pro.local with PID 47129 (/Users/ludangxin/workspace/idea/condition/target/classes started by ludangxin in /Users/ludangxin/workspace/idea/condition)2021-08-01 17:13:38.024INFO 47129 --- [main] com.ldx.condition.ConditionApplication: No active profile set, falling back to default profiles: default2021-08-01 17:13:38.434INFO 47129 --- [main] com.ldx.condition.ConditionApplication: Started ConditionApplication in 0.711 seconds (JVM running for 1.166)2021-08-01 17:13:38.438INFO 47129 --- [main] com.ldx.condition.ConditionApplication: user bean === User(name=张三, age=18)

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。

4. 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 * *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keyString name(); // 配置信息key对应的值String value(); }

4.2 修改UserCondition

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; import java.util.Map; /** * 用户bean条件判断 * @author ludangxin * @date 2021/8/1 */public class UserCondition implements Condition {@Overridepublic boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {Environment environment = conditionContext.getEnvironment(); // 获取自定义的注解Map 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 = https://www.it610.com/article/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。

5. 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true
@ConditionalOnMissingBean	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存在
@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true
@ConditionalOnExpression	spel表达式执行为true
@ConditionalOnJava	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