Detailed explanation of the usage of annotations in Java's Spring framework

1. Use Spring annotations to inject properties

1.1. How did we inject properties before using annotations
Implementation of class:

class UserManagerImpl implements UserManager {
  private UserDao userDao;
  public void setUserDao(UserDao userDao) {
    this.userDao = userDao;
  }
  ...
}

Configuration file :

<bean id="userManagerImpl" class="com.kedacom.spring.annotation.service.UserManagerImpl">
  <property name="userDao" ref="userDao" />
</bean>
<bean id="userDao" class="com.kedacom.spring.annotation.persistence.UserDaoImpl">
  <property name="sessionFactory" ref="mySessionFactory" />
</bean>

1.2. Introduction of @Autowired annotation (not recommended, it is recommended to use @Resource)
Implementation of class (annotation of member variables)

public class UserManagerImpl implements UserManager {
  @Autowired
  private UserDao userDao;
  ...
}

Or (mark the method)

UserManagerImpl implements UserManager {
  private UserDao userDao;
  @Autowired
  public void setUserDao(UserDao userDao) {
    this.userDao = userDao;
  }
  ...
}

Configuration file

<bean id="userManagerImpl" class="com.kedacom.spring.annotation.service.UserManagerImpl" />
<bean id="userDao" class="com.kedacom.spring.annotation.persistence.UserDaoImpl">
  <property name="sessionFactory" ref="mySessionFactory" />
</bean>

@Autowired can mark member variables, methods and The constructor is annotated to complete the automatic assembly work. In the above two different implementations, the @Autowired annotation position is different. They will automatically assemble the userDao attribute when Spring initializes the userManagerImpl bean. The difference is: in the first implementation, Spring will directly assemble the only one of the UserDao type. The bean is assigned to the userDao member variable; in the second implementation, Spring will call the setUserDao method to assemble the only bean of the UserDao type to the userDao attribute.

1.3. Let @Autowired work
To make @Autowired work, you need to add the following code to the configuration file

class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor" />

1.4. @Qualifier
@Autowired is automatically assembled based on type. In the above example, if there is more than one bean of type UserDao in the Spring context, a BeanCreationException will be thrown; if there is no bean of type UserDao in the Spring context, a BeanCreationException will also be thrown. We can use @Qualifier with @Autowired to solve these problems.
1. There may be multiple UserDao instances

@Autowired
public void setUserDao(@Qualifier("userDao") UserDao userDao) {
  this.userDao = userDao;
}

In this way, Spring will find the bean with the id userDao for assembly.
2. UserDao instance may not exist

   
@Autowired(required = false)
public void setUserDao(UserDao userDao) {
  this.userDao = userDao;
}

1.5. @Resource (JSR-250 standard annotation, it is recommended to use it instead of Spring’s proprietary @Autowired annotation)
Spring not only supports its own @Autowired annotation, but also supports several annotations defined by the JSR-250 specification, which are @Resource, @PostConstruct and @PreDestroy.
The function of @Resource is equivalent to @Autowired, except that @Autowired is automatically injected by byType, while @Resource is automatically injected by byName by default. @Resource has two important attributes, namely name and type. Spring resolves the name attribute of the @Resource annotation to the name of the bean, and the type attribute resolves to the type of the bean. Therefore, if the name attribute is used, the byName automatic injection strategy is used, and when the type attribute is used, the byType automatic injection strategy is used. If neither name nor type attribute is specified, the byName automatic injection strategy will be used through the reflection mechanism.
@Resource assembly order

If name and type are specified at the same time, the only matching bean will be found from the Spring context for assembly. If it is not found, an exception will be thrown.
If name is specified, then Find the bean with matching name (id) from the context for assembly. If no bean is found, an exception will be thrown.
If type is specified, the only bean with matching type will be found from the context for assembly. If no bean is found or multiple bean is found, Will throw an exception
If neither name nor type is specified, assembly will be automatically performed in byName mode (see 2); if there is no match, it will fall back to a primitive type (UserDao) for matching. If there is a match, then Automatic assembly;
1.6. @PostConstruct (JSR-250)
Add the annotation @PostConstruct to the method, and this method will be executed by the Spring container after the Bean is initialized (Note: Bean initialization includes, instantiating the Bean, and assemble the Bean's properties (dependency injection)).
A typical application scenario is when you need to inject a property defined in its parent class into a Bean, and you cannot override the parent class's property or property's setter method, such as:

public class UserDaoImpl extends HibernateDaoSupport implements UserDao {
  private SessionFactory mySessionFacotry;
  @Resource
  public void setMySessionFacotry(SessionFactory sessionFacotry) {
    this.mySessionFacotry = sessionFacotry;
  }
  @PostConstruct
  public void injectSessionFactory() {
    super.setSessionFactory(mySessionFacotry);
  }
  ...
}

Here, through @PostConstruct, we inject our own defined sessionFactory into a sessionFactory private property defined in the parent class of UserDaoImpl (the setSessionFactory method of the parent class is final and cannot be overridden), and then we This property can be accessed by calling super.getSessionFactory().

1.7. @PreDestroy (JSR-250)
Add the annotation @PreDestroy to the method, and this method will be executed by the Spring container after the Bean is initialized. Since we currently have no scenarios where we need to use it, we will not demonstrate it here. Its usage is the same as @PostConstruct.

1.8. Use bfe8fab089282832fed14fcb6f680c1e to simplify configuration
Spring2.1 adds a new context Schema namespace, which is useful for annotation drivers and property file introduction and loading. Features such as period weaving provide convenient configuration. We know that the annotation itself does not do anything, it only provides metadata information. For metadata information to be truly useful, the processor responsible for processing the metadata must be put to work.
AutowiredAnnotationBeanPostProcessor and CommonAnnotationBeanPostProcessor are processors that process these annotation metadata. But it is clumsy to define these beans directly in the Spring configuration file. Spring provides us with a convenient way to register these BeanPostProcessors, which is bfe8fab089282832fed14fcb6f680c1e:

<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:annotation-config />
</beans>

02b5e3170f8825be11991279b556a682将隐式地向Spring容器注册AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、 PersistenceAnnotationBeanPostProcessor以及RequiredAnnotationBeanPostProcessor这4个BeanPostProcessor。

2. 使用Spring注解完成Bean的定义 
以上我们介绍了通过@Autowired或@Resource来实现在Bean中自动注入的功能，下面我们将介绍如何注解Bean，从而从XML配置文件中完全移除Bean定义的配置。

2.1. @Component（不推荐使用）、@Repository、@Service、@Controller 
只需要在对应的类上加上一个@Component注解，就将该类定义为一个Bean了：

@Component
public class UserDaoImpl extends HibernateDaoSupport implements UserDao {
  ...
}

使用@Component注解定义的Bean，默认的名称（id）是小写开头的非限定类名。如这里定义的Bean名称就是userDaoImpl。你也可以指定Bean的名称： 
@Component("userDao") 
@Component是所有受Spring管理组件的通用形式，Spring还提供了更加细化的注解形式：@Repository、@Service、@Controller，它们分别对应存储层Bean，业务层Bean，和展示层Bean。目前版本（2.5）中，这些注解与@Component的语义是一样的，完全通用，在Spring以后的版本中可能会给它们追加更多的语义。所以，我们推荐使用@Repository、@Service、@Controller来替代@Component。

2.2. 使用5fac6a8af37bdf6785fe01c166cb3779让Bean定义注解工作起来

<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.kedacom.ksoa" />
</beans>

这里，所有通过60e23eb984d18edbb092da6b8f295aba元素定义Bean的配置内容已经被移除，仅需要添加一行5fac6a8af37bdf6785fe01c166cb3779配置就解决所有问题了——Spring XML配置文件得到了极致的简化（当然配置元数据还是需要的，只不过以注释形式存在罢了）。5fac6a8af37bdf6785fe01c166cb3779的base-package属性指定了需要扫描的类包，类包及其递归子包中所有的类都会被处理。 
5fac6a8af37bdf6785fe01c166cb3779还允许定义过滤器将基包下的某些类纳入或排除。Spring支持以下4种类型的过滤方式：

过滤器类型 表达式范例 说明
注解 org.example.SomeAnnotation 将所有使用SomeAnnotation注解的类过滤出来
类名指定 org.example.SomeClass 过滤指定的类
正则表达式 com\.kedacom\.spring\.annotation\.web\..* 通过正则表达式过滤一些类
AspectJ表达式 org.example..*Service+ 通过AspectJ表达式过滤一些类

以正则表达式为例，我列举一个应用实例：

<context:component-scan base-package="com.casheen.spring.annotation">
  <context:exclude-filter type="regex" expression="com\.casheen\.spring\.annotation\.web\..*" />
</context:component-scan>

值得注意的是5fac6a8af37bdf6785fe01c166cb3779配置项不但启用了对类包进行扫描以实施注释驱动Bean定义的功能，同时还启用了注释驱动自动注入的功能（即还隐式地在内部注册了AutowiredAnnotationBeanPostProcessor和CommonAnnotationBeanPostProcessor），因此当使用5fac6a8af37bdf6785fe01c166cb3779后，就可以将bfe8fab089282832fed14fcb6f680c1e移除了。

2.3. 使用@Scope来定义Bean的作用范围 
在使用XML定义Bean时，我们可能还需要通过bean的scope属性来定义一个Bean的作用范围，我们同样可以通过@Scope注解来完成这项工作：

@Scope("session")
@Component()
public class UserSessionBean implements Serializable {
  ...
}

3.在使用annotation之前定义三个bean之间的关系是这样的

package com.baobaotao;
public class Office {
  private String officeNo =”001”;
  
  //省略 get/setter
  
  @Override
  public String toString() {
    return "officeNo:" + officeNo;
  }
}
 
ackage com.baobaotao;
  
public class Car {
  private String brand;
  private double price;
  
  // 省略 get/setter
  
  @Override
  public String toString() {
    return "brand:" + brand + "," + "price:" + price;
  }
}
 
package com.baobaotao;
  
public class Boss {
  private Car car;
  private Office office;
  
  // 省略 get/setter
  
  @Override
  public String toString() {
    return "car:" + car + "\n" + "office:" + office;
  }
}

配置文件如下：

   
<?xml version="1.0" encoding="UTF-8" ?>
<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="boss" class="com.baobaotao.Boss">
    <property name="car" ref="car"/>
    <property name="office" ref="office" />
  </bean>
  <bean id="office" class="com.baobaotao.Office">
    <property name="officeNo" value="002"/>
  </bean>
  <bean id="car" class="com.baobaotao.Car" scope="singleton">
    <property name="brand" value=" 红旗 CA72"/>
    <property name="price" value="2000"/>
  </bean>
</beans>

测试文件如下：

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class AnnoIoCTest {
  
  public static void main(String[] args) {
    String[] locations = {"beans.xml"};
    ApplicationContext ctx =
      new ClassPathXmlApplicationContext(locations);
    Boss boss = (Boss) ctx.getBean("boss");
    System.out.println(boss);
  }
}

 

4.接下来我们可以使用autowired来注解，他可以对成员变量，方法及构造函数进行标准，完成自动装配的工作。

autoware来注解成员变量的用法

package com.baobaotao;
import org.springframework.beans.factory.annotation.Autowired;
  
public class Boss {
  
  @Autowired
  private Car car;
  
  @Autowired
  private Office office;
  
  …
}

相应的配置文件如下：

<?xml version="1.0" encoding="UTF-8" ?>
<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">
  
  <!-- 该 BeanPostProcessor 将自动起作用，对标注 @Autowired 的 Bean 进行自动注入 -->
  <bean class="org.springframework.beans.factory.annotation.
    AutowiredAnnotationBeanPostProcessor"/>
  
  <!-- 移除 boss Bean 的属性注入配置的信息 -->
  <bean id="boss" class="com.baobaotao.Boss"/>
  
  <bean id="office" class="com.baobaotao.Office">
    <property name="officeNo" value="001"/>
  </bean>
  <bean id="car" class="com.baobaotao.Car" scope="singleton">
    <property name="brand" value=" 红旗 CA72"/>
    <property name="price" value="2000"/>
  </bean>
</beans>

autoware也可以用在setter方法及构造函数上

package com.baobaotao;
  
public class Boss {
  private Car car;
  private Office office;
  
   @Autowired
  public void setCar(Car car) {
    this.car = car;
  }
  
  @Autowired
  public void setOffice(Office office) {
    this.office = office;
  }
  …
}
 
package com.baobaotao;
  
public class Boss {
  private Car car;
  private Office office;
  
  @Autowired
  public Boss(Car car ,Office office){
    this.car = car;
    this.office = office ;
  }
  
  …
}

当候选bean的数目为0时，我们可以使用@Autowired(required = false)来防止spring找不到bean时报错。

当有多个候选bean的时候，我们可以通过@Qualifier 注释指定注入 Bean 的名称。

@Autowired 和 @Qualifier 结合使用时，自动注入的策略就从 byType 转变成 byName 了。@Autowired 可以对成员变量、方法以及构造函数进行注释，而 @Qualifier 的标注对象是成员变量、方法入参、构造函数入参。正是由于注释对象的不同，所以 Spring 不将 @Autowired 和 @Qualifier 统一成一个注释类。

5.@resorce是按照名字来进行反射，他有两个参数，name和type，使用name即按照byname来映射，使用type即按照bytype来进行映射。

package com.baobaotao;
  
import javax.annotation.Resource;
  
public class Boss {
  // 自动注入类型为 Car 的 Bean
  @Resource
  private Car car;
  
  // 自动注入 bean 名称为 office 的 Bean
  @Resource(name = "office")
  private Office office;
}
@postconstructor和preDestory是用来注解类初始化后和销毁前的方法。
 
 
package com.baobaotao;
  
import javax.annotation.Resource;
import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
  
public class Boss {
  @Resource
  private Car car;
  
  @Resource(name = "office")
  private Office office;
  
  @PostConstruct
  public void postConstruct1(){
    System.out.println("postConstruct1");
  }
  
  @PreDestroy
  public void preDestroy1(){
    System.out.println("preDestroy1");
  }
  …
}

6.@compent可以直接定义bean，这样xml配置文件中就不需要配置bean了

package com.baobaotao;
  
import org.springframework.stereotype.Component;
  
@Component
public class Car {
  …
}
 
package com.baobaotao;
  
import org.springframework.stereotype.Component;
  
@Component
public class Office {
  private String officeNo = "001";
  …
}
 
package com.baobaotao;
  
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Required;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;
  
@Component("boss")
public class Boss {
  @Autowired
  private Car car;
  
  @Autowired
  private Office office;
  …
}

@Component 有一个可选的入参，用于指定 Bean 的名称，在 Boss 中，我们就将 Bean 名称定义为“boss”。一般情况下，Bean 都是 singleton 的，需要注入 Bean 的地方仅需要通过 byType 策略就可以自动注入了，所以大可不必指定 Bean 的名称。

<?xml version="1.0" encoding="UTF-8" ?>
<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.baobaotao"/>
</beans>

7.@scope可以用来指定其目标

package com.baobaotao;
import org.springframework.context.annotation.Scope;
…
@Scope("prototype")
@Component("boss")
public class Boss {

Spring 2.5 中除了提供 @Component 注释外，还定义了几个拥有特殊语义的注释，它们分别是：@Repository、@Service 和 @Controller。在目前的 Spring 版本中，这 3 个注释和 @Component 是等效的，但是从注释类的命名上，很容易看出这 3 个注释分别和持久层、业务层和控制层（Web 层）相对应。虽然目前这 3 个注释和 @Component 相比没有什么新意，但 Spring 将在以后的版本中为它们添加特殊的功能。所以，如果 Web 应用程序采用了经典的三层分层结构的话，最好在持久层、业务层和控制层分别采用 @Repository、@Service 和 @Controller 对分层中的类进行注释，而用 @Component 对那些比较中立的类进行注释。

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