spring @Order标记

@Order标记定义了组件的加载顺序。

@Order标记从spring 2.0出现，但是在spring 4.0之前，@Order标记只支持AspectJ的切面排序。spring 4.0对@Order做了增强，它开始支持对装载在诸如Lists和Arrays容器中的自动包装（auto-wired）组件的排序。

在spring内部，对基于spring xml的应用，spring使用OrderComparator类来实现排序。对基于注解的应用，spring采用AnnotationAwareOrderComparator来实现排序。

@Order 标记定义如下：

@Retention(value=RUNTIME)
@Target(value={TYPE,METHOD,FIELD})
@Documented
public @interface Order

这个标记包含一个value属性。属性接受整形值。如：1,2 等等。值越小拥有越高的优先级。
下面让我们来看一个使用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);
	}

}

输出结果：

[RankOne, RankThree, RankTwo]

从结果可以看出，结果是没有顺序的。因为spring 3.x不支持对自动包装组件的排序。

接下来，我升级spring的版本至4.x， 并对RanOne,RankTwo和RankThree加上order标记，值做相应的调整。
@Component
@Order(1)
public class RankOne implements Ranks{
private String rank = "RankOne";
    
    public String toString(){
        return this.rank;
    }
}

输出结果如下：

[RankOne, RankTwo, RankThree]

参考文章： http://javapapers.com/spring/spring-order-annotation/