1. 簡介
@Conditional注解在Spring4.0中引入,其主要作用就是判斷條件是否滿足,從而決定是否初始化並向容器注冊Bean。
2. 定義
2.1 @Conditional
@Conditional
注解定義如下:其內部只有一個參數為Class對象數組,且必須繼承自Condition
接口,通過重寫Condition
接口的matches
方法來判斷是否需要加載Bean
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
Class<? extends Condition>[] value();
}
2.2 Condition
Condition
接口定義如下:該接口為一個函數式接口,只有一個matches
接口,形參為ConditionContext context, AnnotatedTypeMetadata metadata
。ConditionContext
定義如2.2.1
,AnnotatedTypeMetadata
見名知意,就是用來獲取注解的元信息的
@FunctionalInterface
public interface Condition {
boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}
2.2.1 ConditionContext
ConditionContext
接口定義如下:通過查看源碼可以知道,從這個類中可以獲取很多有用的信息
public interface ConditionContext {
/**
* 返回Bean定義信息
* 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工廠
* 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();
/**
* 返回環境變量 比如在application.yaml中定義的信息
* Return the {@link Environment} for which the current application is running.
*/
Environment getEnvironment();
/**
* 返回資源加載器
* Return the {@link ResourceLoader} currently being used.
*/
ResourceLoader getResourceLoader();
/**
* 返回類加載器
* 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. 使用說明
通過一個簡單的小例子測試一下@Conditional
是不是真的能實現Bean的條件化注入。
3.1 創建項目
首先我們創建一個SpringBoot項目
3.1.1 導入依賴
這里我們除了springboot依賴,再添加個lombok依賴
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.5.3</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
<groupId>com.ldx</groupId>
<artifactId>condition</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>condition</name>
<description>Demo project for Spring Boot</description>
<properties>
<java.version>1.8</java.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.12</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
</project>
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 創建User類
package com.ldx.condition;
import lombok.AllArgsConstructor;
import lombok.Data;
/**
* 用戶信息
* @author ludangxin
* @date 2021/8/1
*/
@Data
@AllArgsConstructor
public class User {
private String name;
private Integer age;
}
3.1.4 創建條件實現類
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;
/**
* 用戶bean條件判斷
* @author ludangxin
* @date 2021/8/1
*/
public class UserCondition implements Condition {
@Override
public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
Environment environment = conditionContext.getEnvironment();
// 獲取property user.enable
String property = environment.getProperty("user.enable");
// 如果user.enable的值等於true 那么返回值為true,反之為false
return "true".equals(property);
}
}
3.1.5 修改啟動類
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);
// 獲取類型為User類的Bean
User user = applicationContext.getBean(User.class);
log.info("user bean === {}", user);
}
/**
* 注入User類型的Bean
*/
@Bean
@Conditional(UserCondition.class)
public User getUser(){
return new User("張三",18);
}
}
3.2 測試
3.2.1 當user.enable=false
報錯找不到可用的User類型的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 當user.enable=true
正常輸出UserBean實例信息
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: 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=張三, age=18)
3.3 小結
上面的例子通過使用@Conditional
和Condition
接口,實現了spring bean的條件化注入。
好處:
- 可以實現某些配置的開關功能,如上面的例子,我們可以將UserBean換成開啟緩存的配置,當property的值為true時,我們才開啟緩存的配置。
- 當有多個同名的bean時,如何抉擇的問題。
- 實現自動化的裝載。如判斷當前classpath中有mysql的驅動類時(說明我們當前的系統需要使用mysql),我們就自動的讀取application.yaml中的mysql配置,實現自動裝載;當沒有驅動時,就不加載。
4. 改進
從上面的使用說明中我們了解到了條件注解的大概使用方法,但是代碼中還是有很多硬編碼的問題。比如:UserCondition
中的property的key包括value都是硬編碼,其實我們可以通過再擴展一個注解來實現動態的判斷和綁定。
4.1 創建注解
import org.springframework.context.annotation.Conditional;
import java.lang.annotation.*;
/**
* 自定義條件屬性注解
* <p>
* 當配置的property name對應的值 與設置的 value值相等時,則注入bean
* @author ludangxin
* @date 2021/8/1
*/
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
// 指定condition的實現類
@Conditional({UserCondition.class})
public @interface MyConditionOnProperty {
// 配置信息的key
String name();
// 配置信息key對應的值
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;
/**
* 用戶bean條件判斷
* @author ludangxin
* @date 2021/8/1
*/
public class UserCondition implements Condition {
@Override
public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
Environment environment = conditionContext.getEnvironment();
// 獲取自定義的注解
Map<String, Object> annotationAttributes = annotatedTypeMetadata.getAnnotationAttributes("com.ldx.condition.MyConditionOnProperty");
// 獲取在注解中指定的name的property的值 如:user.enable的值
String property = environment.getProperty(annotationAttributes.get("name").toString());
// 獲取預期的值
String value = annotationAttributes.get("value").toString();
return value.equals(property);
}
}
測試后,結果符合預期。
其實在spring中已經內置了許多常用的條件注解,其中我們剛實現的就在內置的注解中已經實現了,如下。
5. Spring內置條件注解
注解 | 說明 |
---|---|
@ConditionalOnSingleCandidate |
當給定類型的bean存在並且指定為Primary的給定類型存在時,返回true |
@ConditionalOnMissingBean |
當給定的類型、類名、注解、昵稱在beanFactory中不存在時返回true.各類型間是or的關系 |
@ConditionalOnBean |
與上面相反,要求bean存在 |
@ConditionalOnMissingClass |
當給定的類名在類路徑上不存在時返回true,各類型間是and的關系 |
@ConditionalOnClass |
與上面相反,要求類存在 |
@ConditionalOnCloudPlatform |
當所配置的CloudPlatform為激活時返回true |
@ConditionalOnExpression |
spel表達式執行為true |
@ConditionalOnJava |
運行時的java版本號是否包含給定的版本號.如果包含,返回匹配,否則,返回不匹配 |
@ConditionalOnProperty |
要求配置屬性匹配條件 |
@ConditionalOnJndi |
給定的jndi的Location 必須存在一個.否則,返回不匹配 |
@ConditionalOnNotWebApplication |
web環境不存在時 |
@ConditionalOnWebApplication |
web環境存在時 |
@ConditionalOnResource |
要求制定的資源存在 |