@Order標記定義了組件的加載順序。
@Order標記從spring 2.0出現，但是在spring 4.0之前，@Order標記只支持AspectJ的切面排序。spring 4.0對@Order做了增強，它開始支持對裝載在諸如Lists和Arrays容器中的自動包裝（auto-wired）組件的排序。
在spring內部，對基於spring xml的應用，spring使用OrderComparator類來實現排序。對基於注解的應用，spring采用AnnotationAwareOrderComparator來實現排序。
@Order 標記定義如下：
@Retention(value=RUNTIME) @Target(value={TYPE,METHOD,FIELD}) @Documented public @interface Order
這個標記包含一個value屬性。屬性接受整形值。如：1,2 等等。值越小擁有越高的優先級。
下面讓我們來看一個使用spring 3.x 和spring 4.x 的例子：
Ranks.java
package com.javapapers.spring3.autowire.collection; public interface Ranks { }
RankOne.java
package com.javapapers.spring3.autowire.collection; import org.springframework.core.annotation.Order; import org.springframework.stereotype.Component; @Component public class RankOne implements Ranks{ private String rank = "RankOne"; public String toString(){ return this.rank; } }
RankTwo.java
package com.javapapers.spring3.autowire.collection; import org.springframework.core.annotation.Order; import org.springframework.stereotype.Component; @Component public class RankTwo implements Ranks{ private String rank = "RankTwo"; public String toString(){ return this.rank; } }
RankThree.java
package com.javapapers.spring3.autowire.collection; import org.springframework.core.annotation.Order; import org.springframework.stereotype.Component; @Component public class RankThree implements Ranks{ private String rank = "RankThree"; public String toString(){ return this.rank; } }
Results.java
Component to hold student ranks in a collection as shown below.
package com.javapapers.spring3.autowire.collection; import java.util.List; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Component; @Component public class Results { @Autowired private List ranks ; @Override public String toString(){ return ranks.toString(); } }
beans.xml
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config /> <context:component-scan base-package="com.javapapers.spring3"/> </beans>
RanksClient.java
package com.javapapers.spring3.autowire.collection; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class RanksClient { public static void main(String[] args) { ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml"); Results results = (Results)context.getBean("results"); System.out.println(results); } }
輸出結果：
[RankOne, RankThree, RankTwo]
從結果可以看出，結果是沒有順序的。因為spring 3.x不支持對自動包裝組件的排序。
接下來，我升級spring的版本至4.x， 並對RanOne,RankTwo和RankThree加上order標記，值做相應的調整。
@Component
@Order(1)
public class RankOne implements Ranks{
private String rank = "RankOne";
public String toString(){
return this.rank;
}
}
輸出結果如下：
[RankOne, RankTwo, RankThree]
參考文章： http://javapapers.com/spring/spring-order-annotation/