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)
@Documented
public @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
@FunctionalInterface
public 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}).
*/
@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 isn't accessible).
* @see org.springframework.util.ClassUtils#forName(String, ClassLoader)
*/
@Nullable
ClassLoader 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
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
4.0.0 org.springframework.boot spring-boot-starter-parent 2.5.3 com.ldx condition 0.0.1-SNAPSHOT condition Demo project for Spring Boot 1.8 org.springframework.boot spring-boot-starter org.projectlombok lombok 1.18.12 org.springframework.boot spring-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
@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.enable=false
报错找不到可用的User类型的Bean
. ____ _ __ _ _ /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot :: (v2.5.3) 2021-08-01 17:07:51.994 INFO 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 17:07:51.997 INFO 47036 --- [ main] com.ldx.condition.ConditionApplication : No active profile set, falling back to default profiles: default 2021-08-01 17:07:52.461 INFO 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 17:13:38.022 INFO 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 17:13:38.024 INFO 47129 --- [ main] com.ldx.condition.ConditionApplication : No active profile set, falling back to default profiles: default 2021-08-01 17:13:38.434 INFO 47129 --- [ main] com.ldx.condition.ConditionApplication : Started ConditionApplication in 0.711 seconds (JVM running for 1.166) 2021-08-01 17:13:38.438 INFO 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key String 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 {
@Override
public 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 = 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 |
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