通常情况下，声明所有的Bean类或组件的XML bean配置文件，这样Spring容器可以检测并注册Bean类或组件。 其实，Spring是能够自动扫描，检测和预定义的项目包并实例化bean，不再有繁琐的Bean类声明在XML文件中。
下面是一个简单的Spring项目，包括客户服务和DAO层。让我们来探讨手动申明组件和自动扫描组件之间的不同。
1、手动声明组件
看到在 Spring 的一个正常方式来声明一个 bean。
一个正常的 bean.
package com.gp6.scanning.manual;
public class CustomerDao {
@Override
public String toString() {
return "Hello , This is CustomerDao";
}
}
package com.gp6.scanning.manual;
public class CustomerService {
CustomerDao customerDao;
public CustomerDao getCustomerDao() {
return customerDao;
}
public void setCustomerDao(CustomerDao customerDao) {
this.customerDao = customerDao;
}
@Override
public String toString() {
return "CustomerService [customerDAO=" + customerDao + "]";
}
}
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
<bean id="customerService" class="com.gp6.scanning.manual.CustomerService">
<property name="customerDao" ref="customerDao" />
</bean>
<bean id="customerDao" class="com.gp6.scanning.manual.CustomerDao" />
</beans>
package com.gp6.scanning.manual;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Test {
public static void main( String[] args ) {
ApplicationContext context = new ClassPathXmlApplicationContext("com/gp6/scanning/manual/applicationContext.xml");
CustomerService cust = (CustomerService)context.getBean("customerService");
System.out.println(cust);
}
}
输出
CustomerService [customerDAO=Hello , This is CustomerDao]
2. 自动组件扫描
现在，启用Spring组件扫描功能。
使用@Component注释来表示这是类是一个自动扫描组件。
package com.gp6.scanning.auto;
import org.springframework.stereotype.Component;
@Component
public class CustomerDao {
@Override
public String toString() {
return "Hello , This is CustomerDao";
}
}
package com.gp6.scanning.auto;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class CustomerService {
@Autowired
CustomerDao customerDao;
public CustomerDao getCustomerDao() {
return customerDao;
}
public void setCustomerDao(CustomerDao customerDao) {
this.customerDao = customerDao;
}
@Override
public String toString() {
return "CustomerService [customerDAO=" + customerDao + "]";
}
}
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-2.5.xsd">
<context:component-scan base-package="com.gp6.scanning.auto" />
</beans>
package com.gp6.scanning.auto;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Test {
public static void main( String[] args ) {
ApplicationContext context = new ClassPathXmlApplicationContext("com/gp6/scanning/auto/applicationContext.xml");
CustomerService cust = (CustomerService)context.getBean("customerService");
System.out.println(cust);
}
}
输出结果
CustomerService [customerDAO=Hello , This is CustomerDao]
自定义自动扫描组件名称
默认情况下，Spring 将小写部件的第一字符- 从'CustomerService'到'CustomerService'。可以检索该组件名称为“CustomerService”。
CustomerService cust = (CustomerService)context.getBean("customerService");
要创建组件的自定义名称，你可以这样自定义名称：
@Service("AAA")
public class CustomerService
...
现在，可以用'AAA'这个名称进行检索。
CustomerService cust = (CustomerService)context.getBean("AAA");
自动组件扫描注释类型
在Spring2.5中，有4种类型的组件自动扫描注释类型
1 @Component – 指示自动扫描组件。
2 @Repository – 表示在持久层DAO组件。
3 @Service – 表示在业务层服务组件。
4 @Controller – 表示在表示层控制器组件。
因此，使用哪一个？其实并不那么重要。参见 @Repository，@Service 或 @Controller 源代码。
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Repository {
String value() default "";
}
你可能会发现，所有的 @Repository, @Service 或 @Controller 被注解为 @Component。因此，我们可以只使用 @Component 对所有组件进行自动扫描？是的，Spring会自动扫描所有组件的 @Component 注解。
它工作正常，但不是一个好的做法，为便于阅读，应该始终声明@Repository，@ Service 或 @Controller 在指定的层，使你的代码更易于阅读，如下：
DAO 层
package com.gp6.scanning.auto;
import org.springframework.stereotype.Repository;
@Repository
public class CustomerDAO
{
@Override
public String toString() {
return "Hello , This is CustomerDAO";
}
}
Service 层
package com.gp6.scanning.auto;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class CustomerService
{
@Autowired
CustomerDAO customerDAO;
@Override
public String toString() {
return "CustomerService [customerDAO=" + customerDAO + "]";
}
}
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