<context:annotation-config> vs <context:component-scan>

<context:annotation-config> 用来注入已经在上下文注册的bean，无论bean是定义在XML中还是被 package scanning。

<context:component-scan>仅scans packages 去注册应用上线文中的Bean。

example：

Lets start with a basic setup of three beans of type A, B and C, with B and C being injected into A.

package com.xxx;
public class B {
  public B() {
    System.out.println("creating bean B: " + this);
  }
}

package com.xxx;
public class C {
  public C() {
    System.out.println("creating bean C: " + this);
  }
}

package com.yyy;
import com.xxx.B;
import com.xxx.C;
public class A { 
  private B bbb;
  private C ccc;
  public A() {
    System.out.println("creating bean A: " + this);
  }
  public void setBbb(B bbb) {
    System.out.println("setting A.bbb with " + bbb);
    this.bbb = bbb;
  }
  public void setCcc(C ccc) {
    System.out.println("setting A.ccc with " + ccc);
    this.ccc = ccc; 
  }
}

XML配置：

<bean id="bBean" class="com.xxx.B" />
<bean id="cBean" class="com.xxx.C" />
<bean id="aBean" class="com.yyy.A">
  <property name="bbb" ref="bBean" />
  <property name="ccc" ref="cBean" />
</bean>

加载环境产生输出：

creating bean B: com.xxx.B@c2ff5
creating bean C: com.xxx.C@1e8a1f6
creating bean A: com.yyy.A@1e152c5
setting A.bbb with com.xxx.B@c2ff5
setting A.ccc with com.xxx.C@1e8a1f6

这是正常的输出，但是这是老的spring 风格.，现在我们使用注解。

First, lets autowire the bbb and ccc properties on bean A like so:

package com.yyy;
import org.springframework.beans.factory.annotation.Autowired;
import com.xxx.B;
import com.xxx.C;
public class A { 
  private B bbb;
  private C ccc;
  public A() {
    System.out.println("creating bean A: " + this);
  }
  @Autowired
  public void setBbb(B bbb) {
    System.out.println("setting A.bbb with " + bbb);
    this.bbb = bbb;
  }
  @Autowired
  public void setCcc(C ccc) {
    System.out.println("setting A.ccc with " + ccc);
    this.ccc = ccc;
  }
}

XML配置：

<bean id="bBean" class="com.xxx.B" />
<bean id="cBean" class="com.xxx.C" />
<bean id="aBean" class="com.yyy.A" />

加载结果：

creating bean B: com.xxx.B@5e5a50
creating bean C: com.xxx.C@54a328
creating bean A: com.yyy.A@a3d4cf

上面的结果并没有完成注入，这是因为@Autowired 只是一个注解，如果让它完成注入工作需要<context:annotation-config />

改变一下XML的配置：

<context:annotation-config />
<bean id="bBean" class="com.xxx.B" />
<bean id="cBean" class="com.xxx.C" />
<bean id="aBean" class="com.yyy.A" />

输出结果：

creating bean B: com.xxx.B@15663a2
creating bean C: com.xxx.C@cd5f8b
creating bean A: com.yyy.A@157aa53
setting A.bbb with com.xxx.B@15663a2
setting A.ccc with com.xxx.C@cd5f8b

现在，我们使用注解注释Bean 删除砸xml中的定义：

package com.xxx;
import org.springframework.stereotype.Component;
@Component
public class B {
  public B() {
    System.out.println("creating bean B: " + this);
  }
}

package com.xxx;
import org.springframework.stereotype.Component;
@Component
public class C {
  public C() {
    System.out.println("creating bean C: " + this);
  }
}

package com.yyy;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import com.xxx.B;
import com.xxx.C;
@Component
public class A { 
  private B bbb;
  private C ccc;
  public A() {
    System.out.println("creating bean A: " + this);
  }
  @Autowired
  public void setBbb(B bbb) {
    System.out.println("setting A.bbb with " + bbb);
    this.bbb = bbb;
  }
  @Autowired
  public void setCcc(C ccc) {
    System.out.println("setting A.ccc with " + ccc);
    this.ccc = ccc;
  }
}

xml中的配置：

<context:annotation-config />

加载后什么都没有输出。

<context:component-scan base-package="com.xxx" />

<context:annotation-config />
<context:component-scan base-package="com.xxx" />
<bean id="aBean" class="com.yyy.A" />

OK，得到了我们想要的结果。

 

总结：

The difference between the two is really simple!.

<context:annotation-config />

Enables you to use annotations that are restricted to wiring up properties and constructors only of beans!.

Where as

<context:component-scan base-package="org.package"/>

Enables everything that <context:annotation-config /> can do, with addition of using stereotypes eg.. @Component, @Service , @Repository. So you can wire entire beans and not just restricted to constructors or properties!.

 

<context:annotation-config>: Scanning and activating annotations for already registered beans in spring config xml.

<context:component-scan>: Bean registration + <context:annotation-config>

---

@Autowired and @Required are targets property level so bean should register in spring IOC before use these annotations. To enable these annotations either have to register respective beans or include <context:annotation-config />. i.e. <context:annotation-config /> works with registered beans only.

@Required enables RequiredAnnotationBeanPostProcessor processing tool
@Autowired enables AutowiredAnnotationBeanPostProcessor processing tool

Note: Annotation itself nothing to do, we need a Processing Tool, which is a class underneath, responsible for the core process.

---

@Repository, @Service and @Controller are @Component, and they targets class level.

<context:component-scan> it scans the package and find and register the beans, and it includes the work done by <context:annotation-config />.