Spring核心接口之Ordered

一、Ordered接口介绍
Spring中提供了一个Ordered接口。从单词意思就知道Ordered接口的作用就是用来排序的。
Spring框架是一个大量使用策略设计模式的框架，这意味着有很多相同接口的实现类，那么必定会有优先级的问题。于是Spring就提供了Ordered这个接口，来处理相同接口实现类的优先级问题。

二、Ordered接口分析
1、Ordered接口的定义：

 public interface Ordered { /*** Useful constant for the highest precedence value. * @see java.lang.Integer#MIN_VALUE */ int HIGHEST_PRECEDENCE = Integer.MIN_VALUE; /** * Useful constant for the lowest precedence value. * @see java.lang.Integer#MAX_VALUE */ int LOWEST_PRECEDENCE = Integer.MAX_VALUE; /** * Get the order value of this object. * <p>Higher values are interpreted as lower priority. As a consequence, * the object with the lowest value has the highest priority (somewhat * analogous to Servlet {@code load-on-startup} values). * <p>Same order values will result in arbitrary sort positions for the * affected objects. * @return the order value * @see #HIGHEST_PRECEDENCE * @see #LOWEST_PRECEDENCE */ int getOrder();

}

该接口卡只有1个方法getOrder()及 2个变量HIGHEST_PRECEDENCE最高级(数值最小)和LOWEST_PRECEDENCE最低级(数值最大)。

2、OrderComparator类：实现了Comparator接口的一个比较器。

public class OrderComparator implements Comparator<Object> { /** * Shared default instance of OrderComparator. */ public static final OrderComparator INSTANCE = new OrderComparator(); public int compare(Object o1, Object o2) { boolean p1 = (o1 instanceof PriorityOrdered); boolean p2 = (o2 instanceof PriorityOrdered); if (p1 && !p2) { return -1; } else if (p2 && !p1) { return 1; } // Direct evaluation instead of Integer.compareTo to avoid unnecessary object creation. int i1 = getOrder(o1); int i2 = getOrder(o2); return (i1 < i2) ? -1 : (i1 > i2) ? 1 : 0; } /** * Determine the order value for the given object. * <p>The default implementation checks against the {@link Ordered} * interface. Can be overridden in subclasses. * @param obj the object to check * @return the order value, or {@code Ordered.LOWEST_PRECEDENCE} as fallback */ protected int getOrder(Object obj) { return (obj instanceof Ordered ? ((Ordered) obj).getOrder() : Ordered.LOWEST_PRECEDENCE); } /** * Sort the given List with a default OrderComparator. * <p>Optimized to skip sorting for lists with size 0 or 1, * in order to avoid unnecessary array extraction. * @param list the List to sort * @see java.util.Collections#sort(java.util.List, java.util.Comparator) */ public static void sort(List<?> list) { if (list.size() > 1) { Collections.sort(list, INSTANCE); } } /** * Sort the given array with a default OrderComparator. * <p>Optimized to skip sorting for lists with size 0 or 1, * in order to avoid unnecessary array extraction. * @param array the array to sort * @see java.util.Arrays#sort(Object[], java.util.Comparator) */ public static void sort(Object[] array) { if (array.length > 1) { Arrays.sort(array, INSTANCE); } } }

提供了2个静态排序方法：sort(List<?> list)用来排序list集合、sort(Object[] array)用来排序Object数组
可以下OrderComparator类的public int compare(Object o1, Object o2)方法，可以看到另外一个类PriorityOrdered，这个方法的逻辑解析如下：

1. 若对象o1是Ordered接口类型，o2是PriorityOrdered接口类型，那么o2的优先级高于o1
2. 若对象o1是PriorityOrdered接口类型，o2是Ordered接口类型，那么o1的优先级高于o2
3. 其他情况，若两者都是Ordered接口类型或两者都是PriorityOrdered接口类型，调用Ordered接口的getOrder方法得到order值，order值越大，优先级越小
   简单来说就是：
   OrderComparator比较器进行排序的时候，若2个对象中有一个对象实现了PriorityOrdered接口，那么这个对象的优先级更高。若2个对象都是PriorityOrdered或Ordered接口的实现类，那么比较Ordered接口的getOrder方法得到order值，值越低，优先级越高。

三、Spring中使用Ordered接口在的例子
在spring配置文件中添加：<mvc:annotation-driven/>，那么SpringMVC默认会注入RequestMappingHandlerAdapter和RequestMappingHandlerMapping这两个类。 既然SpringMVC已经默认为我们注入了RequestMappingHandlerAdapter和RequestMappingHandlerMapping这两个类，如果再次配置这两个类，将会出现什么效果呢？
当我们配置了annotation-driven以及这两个bean的时候。Spring容器就有了2个RequestMappingHandlerAdapter和2个RequestMappingHandlerMapping。
DispatcherServlet内部有HandlerMapping(RequestMappingHandlerMapping是其实现类)集合和HandlerAdapter(RequestMappingHandlerAdapter是其实现类)集合。
//RequestMappingHandlerMapping集合
private List<HandlerMapping> handlerMappings;
//HandlerAdapter集合
private List<HandlerAdapter> handlerAdapters;
在仔细看下DispatcherServlet类的private void initHandlerMappings(ApplicationContext context)方法可以看到如下代码：
//detectAllHandlerMappings默认为true
if (this.detectAllHandlerMappings) {
// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
Map<String, HandlerMapping> matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
// We keep HandlerMappings in sorted order.
//进行排序
AnnotationAwareOrderComparator.sort(this.handlerMappings);
}
}
AnnotationAwareOrderComparator继承了OrderComparator类

再看下<mvc:annotation-driven/>配置的RequestMappingHandlerMapping和RequestMappingHandlerAdapter@Bean
br/>@Bean
br/>@Bean

进入RequestMappingHandlerMapping和RequestMappingHandlerAdapter代码里面看看它们的order属性是如何定义的。 RequestMappingHandlerMapping // Ordered.LOWEST_PRECEDENCE只为Integer.MAX_VALUE public abstract class AbstractHandlerMapping extends WebApplicationObjectSupport implements HandlerMapping, Ordered { private int order = Integer.MAX_VALUE; AbstractHandlerMapping是RequestMappingHandlerMapping的父类。 RequestMappingHandlerAdapter public abstract class AbstractHandlerMethodAdapter extends WebContentGenerator implements HandlerAdapter, Ordered { // Ordered.LOWEST_PRECEDENCE只为Integer.MAX_VALUE private int order = Ordered.LOWEST_PRECEDENCE; AbstractHandlerMethodAdapter是RequestMappingHandlerAdapter的父类。 可以看到RequestMappingHandlerMapping和RequestMappingHandlerAdapter没有设置order属性的时候，order属性的默认值都是Integer.MAX_VALUE，即优先级最低。 

总结： 如果配置了<mvc:annotation-driven/>，又配置了自定义的RequestMappingHandlerAdapter，并且没有设置RequestMappingHandlerAdapter的order值，那么这2个RequestMappingHandlerAdapter的order值都是Integer.MAX_VALUE。那么谁先定义的，谁优先级高。 <mvc:annotation-driven/>配置在自定义的RequestMappingHandlerAdapter配置之前，那么<mvc:annotation-driven/>配置的RequestMappingHandlerAdapter优先级高，反之自定义的RequestMappingHandlerAdapter优先级高。

如果配置了<mvc:annotation-driven/>，又配置了自定义的RequestMappingHandlerMapping，并且没有设置RequestMappingHandlerMapping的order值。那么<mvc:annotation-driven/>配置的RequestMappingHandlerMapping优先级高，因为<mvc:annotation-driven />内部会设置RequestMappingHandlerMapping的order为0。

四、应用
1、定义接口
import java.util.Map;
import org.springframework.core.Ordered;

public interface Filter extends Ordered{ public void doFiler(Map<String, String> prams); }

2、实现接口

import java.util.Map; @Component public class LogFilter implements Filter { private int order =1; public int getOrder() { return order; } public void setOrder(int order) { this.order = order; } public void doFiler(Map<String, String> prams) { System.out.println("打印日志"); } } import java.util.Map; @Component public class PowerLogFilter implements Filter { private int order =2; public int getOrder() { return order; } public void setOrder(int order) { this.order = order; } public void doFiler(Map<String, String> prams) { System.out.println("权限控制"); } } 3、测试进行排序 public static void main(String[] args) throws Exception { String config = Test.class.getPackage().getName().replace('.', '/') + "/bean.xml"; ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(config); context.start(); Map<String, Filter> filters = context.getBeansOfType(Filter.class); System.out.println(filters.size()); List<Filter> f= new ArrayList<Filter>(filters.values()); OrderComparator.sort(f); for(int i=0; i<f.size(); i++){ Map<String, String> params = new HashMap<String, String>(); f.get(i).doFiler(params); } }

4、配置文件

<beans xmlns="http://www.springframework.org/schema/beans" xmlns:context="http://www.springframework.org/schema/context" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:dubbo="http://code.alibabatech.com/schema/dubbo" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd http://code.alibabatech.com/schema/dubbo http://code.alibabatech.com/schema/dubbo/dubbo.xsd"> <context:component-scan base-package="com" /> </beans>