Spring @Qualifier注解教程 · ZetCode 中文系列教程

# Spring `@Qualifier`注解教程 > 原文： [http://zetcode.com/spring/qualifier/](http://zetcode.com/spring/qualifier/) Spring `@Qualifier`注解教程显示了如何使用`@Qualifier`来区分 Spring 应用中的 bean。 Spring 是用于创建企业应用的流行 Java 应用框架。 ## Spring `@Qualifier`注解如果 Spring 不能这样做，则`@Qualifier`注解有助于消除 Bean 引用的歧义。 ## Spring `@Qualifier`示例该应用具有不同类型的消息 bean。我们用`@Qualifier`区分它们。 ```java src ├───main │ ├───java │ │ └───com │ │ └───zetcode │ │ │ Application.java │ │ ├───bean │ │ │ IMessage.java │ │ │ Info.java │ │ │ Warning.java │ │ └───service │ │ MessageProducer.java │ └───resources │ logback.xml └───test └───java ``` 这是项目结构。 `pom.xml` ```java <?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 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.zetcode</groupId> <artifactId>qualifierannotation</artifactId> <version>1.0-SNAPSHOT</version> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <maven.compiler.source>11</maven.compiler.source> <maven.compiler.target>11</maven.compiler.target> <spring-version>5.1.3.RELEASE</spring-version> </properties> <dependencies> <dependency> <groupId>ch.qos.logback</groupId> <artifactId>logback-classic</artifactId> <version>1.2.3</version> </dependency> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>${spring-version}</version> </dependency> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>${spring-version}</version> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.codehaus.mojo</groupId> <artifactId>exec-maven-plugin</artifactId> <version>1.6.0</version> <configuration> <mainClass>com.zetcode.Application</mainClass> </configuration> </plugin> </plugins> </build> </project> ``` 在`pom.xml`文件中，我们具有基本的 Spring 依赖项`spring-core`和`spring-context`和日志记录`logback-classic`依赖项。 `exec-maven-plugin`用于在命令行上从 Maven 执行 Spring 应用。 `resources/logback.xml` ```java <?xml version="1.0" encoding="UTF-8"?> <configuration> <logger name="org.springframework" level="ERROR"/> <logger name="com.zetcode" level="INFO"/> <appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender"> <encoder> <Pattern>%d{HH:mm:ss.SSS} [%thread] %blue(%-5level) %magenta(%logger{36}) - %msg %n </Pattern> </encoder> </appender> <root> <level value="INFO" /> <appender-ref ref="consoleAppender" /> </root> </configuration> ``` `logback.xml`是 Logback 日志库的配置文件。 `com/zetcode/bean/IMessage.java` ```java package com.zetcode.bean; public interface IMessage { String getMessage(); } ``` `IMessage`接口具有一个方法声明。 `com/zetcode/bean/Info.java` ```java package com.zetcode.bean; import org.springframework.beans.factory.annotation.Qualifier; import org.springframework.stereotype.Component; @Component @Qualifier("info") public class Info implements IMessage { @Override public String getMessage() { return "This is an information message"; } } ``` 接口的第一个实现提供了一条信息消息。 `@Qualifier`用于标识 bean。 `com/zetcode/bean/Warning.java` ```java package com.zetcode.bean; import org.springframework.beans.factory.annotation.Qualifier; import org.springframework.stereotype.Component; @Component @Qualifier("warning") public class Warning implements IMessage { public String getMessage() { return "This is a warning message"; } } ``` 第二种实现给出警告消息。它也以`@Qualifier`命名。 `com/zetcode/service/MessageProducer.java` ```java package com.zetcode.service; import com.zetcode.bean.IMessage; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.beans.factory.annotation.Qualifier; import org.springframework.stereotype.Service; @Service public class MessageProducer { private static final Logger logger = LoggerFactory.getLogger(MessageProducer.class); @Autowired @Qualifier("info") private IMessage infoMessage; @Autowired @Qualifier("warning") private IMessage warningMessage; public void produce() { logger.info("{}", infoMessage.getMessage()); logger.warn("{}", warningMessage.getMessage()); } } ``` `MessageProducer`注入两个`IMessage` bean。为了区分它们，我们使用`@Qualifier`注解。 `com/zetcode/Application.java` ```java package com.zetcode; import com.zetcode.service.MessageProducer; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.context.annotation.AnnotationConfigApplicationContext; import org.springframework.context.annotation.ComponentScan; @ComponentScan(basePackages = "com.zetcode.bean;com.zetcode.service") public class Application { private static final Logger logger = LoggerFactory.getLogger(Application.class); public static void main(String[] args) { logger.info("Application starting"); try (var ctx = new AnnotationConfigApplicationContext(Application.class)) { var messageProducer = (MessageProducer) ctx.getBean("messageProducer"); messageProducer.produce(); } } } ``` 这是主要的应用类。它检索`messageProducer` bean 并调用其`produce()`方法。 ```java $ mvn -q exec:java 10:50:03.309 [com.zetcode.Application.main()] INFO com.zetcode.Application - Application starting 10:50:03.574 [com.zetcode.Application.main()] INFO com.zetcode.service.MessageProducer - This is an information message 10:50:03.574 [com.zetcode.Application.main()] WARN com.zetcode.service.MessageProducer - This is a warning message ``` 我们运行该应用。在本教程中，我们使用了 Spring 的`@Qualifier`注解。您可能也对这些相关教程感兴趣： [Spring `@Bean`教程](/spring/bean/)， [Spring Boot `@Qualifier`注解](/articles/springbootqualifier/)， [Spring `@ComponentScan`教程](/spring/componentscan/)， [Java 教程](/lang/java/)或列出[所有 Spring 教程](/all/#spring)。