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自动依赖注入大大简化了我们的工作量，但是也有缺陷，如果一个接口有多个实现类，我们该注入哪一个呢？一种方法是给bean设置一个限定符(qualifier)

package shangbo.spring.example36;public interface MessageService {	String getMessage();}

package shangbo.spring.example36;public class MessageServiceDBImpl implements MessageService {	public String getMessage() {		return "This message from database";	}}

package shangbo.spring.example36;public class MessageServiceFileImpl implements MessageService {	public String getMessage() {		return "This message from file";	}}

package shangbo.spring.example36;import org.springframework.beans.factory.annotation.Qualifier;public class MessagePrinter {    private MessageService service;    // 使用限定符    @Qualifier("DB")    public void setService(MessageService service) {		this.service = service;	}	public void printMessage() {        System.out.println(service.getMessage());    }}

package shangbo.spring.example36;import org.springframework.context.ApplicationContext;import org.springframework.context.support.ClassPathXmlApplicationContext;public class App {	public static void main(String[] args) {		// 实例化 Spring IoC 容器		ApplicationContext context = new ClassPathXmlApplicationContext("example.xml", MessagePrinter.class);				// 从容器中获得 MessagePrinter 的实例		MessagePrinter printer = context.getBean(MessagePrinter.class);				// 使用对象		printer.printMessage();	}}

下面是使用 Java 配置的例子。

package shangbo.spring.example37;public interface MessageService {	String getMessage();}

package shangbo.spring.example37;public class MessageServiceDBImpl implements MessageService {	public String getMessage() {		return "This message from database";	}}

package shangbo.spring.example37;public class MessageServiceFileImpl implements MessageService {	public String getMessage() {		return "This message from file";	}}

package shangbo.spring.example37;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.beans.factory.annotation.Qualifier;public class MessagePrinter {    private MessageService service;    // 限定符    @Autowired    @Qualifier("DB")    public void setService(MessageService service) {		this.service = service;	}	public void printMessage() {        System.out.println(service.getMessage());    }}

package shangbo.spring.example37;import org.springframework.beans.factory.annotation.Qualifier;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;@Configurationpublic class AppConfig {	@Bean	@Qualifier("DB")	public MessageService dbMessageService() {		return new MessageServiceDBImpl();	}		@Bean	@Qualifier("FILE")	public MessageService fileMessageService() {		return new MessageServiceDBImpl();	}		@Bean	public MessagePrinter messagePrinter() {		return new MessagePrinter();	}}

package shangbo.spring.example37;import org.springframework.context.ApplicationContext;import org.springframework.context.annotation.AnnotationConfigApplicationContext;public class App {    public static void main( String[] args )    {		// 实例化 Spring IoC 容器		ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);				// 从容器中获得 MessagePrinter 的实例		MessagePrinter printer = context.getBean(MessagePrinter.class);				// 使用对象		printer.printMessage();    }}

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