Spring核心接口之Ordered

一、Ordered接口介紹
Spring中提供了一個Ordered接口。從單詞意思就知道Ordered接口的作用就是用來排序的。
Spring框架是一個大量使用策略設計模式的框架，這意味著有很多相同接口的實現類，那么必定會有優先級的問題。于是Spring就提供了Ordered這個接口，來處理相同接口實現類的優先級問題。

二、Ordered接口分析
1、Ordered接口的定義：

 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();

}

該接口卡只有1個方法getOrder()及 2個變量HIGHEST_PRECEDENCE最高級(數值最小)和LOWEST_PRECEDENCE最低級(數值最大)。

2、OrderComparator類：實現了Comparator接口的一個比較器。

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個靜態排序方法：sort(List<?> list)用來排序list集合、sort(Object[] array)用來排序Object數組
可以下OrderComparator類的public int compare(Object o1, Object o2)方法，可以看到另外一個類PriorityOrdered，這個方法的邏輯解析如下：

1. 若對象o1是Ordered接口類型，o2是PriorityOrdered接口類型，那么o2的優先級高于o1
2. 若對象o1是PriorityOrdered接口類型，o2是Ordered接口類型，那么o1的優先級高于o2
3. 其他情況，若兩者都是Ordered接口類型或兩者都是PriorityOrdered接口類型，調用Ordered接口的getOrder方法得到order值，order值越大，優先級越小
   簡單來說就是：
   OrderComparator比較器進行排序的時候，若2個對象中有一個對象實現了PriorityOrdered接口，那么這個對象的優先級更高。若2個對象都是PriorityOrdered或Ordered接口的實現類，那么比較Ordered接口的getOrder方法得到order值，值越低，優先級越高。

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默認會注入RequestMappingHandlerAdapter和RequestMappingHandlerMapping這兩個類。 既然SpringMVC已經默認為我們注入了RequestMappingHandlerAdapter和RequestMappingHandlerMapping這兩個類，如果再次配置這兩個類，將會出現什么效果呢？
當我們配置了annotation-driven以及這兩個bean的時候。Spring容器就有了2個RequestMappingHandlerAdapter和2個RequestMappingHandlerMapping。
DispatcherServlet內部有HandlerMapping(RequestMappingHandlerMapping是其實現類)集合和HandlerAdapter(RequestMappingHandlerAdapter是其實現類)集合。
//RequestMappingHandlerMapping集合
private List<HandlerMapping> handlerMappings;
//HandlerAdapter集合
private List<HandlerAdapter> handlerAdapters;
在仔細看下DispatcherServlet類的private void initHandlerMappings(ApplicationContext context)方法可以看到如下代碼：
//detectAllHandlerMappings默認為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.
//進行排序
AnnotationAwareOrderComparator.sort(this.handlerMappings);
}
}
AnnotationAwareOrderComparator繼承了OrderComparator類

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

進入RequestMappingHandlerMapping和RequestMappingHandlerAdapter代碼里面看看它們的order屬性是如何定義的。
RequestMappingHandlerMapping
// Ordered.LOWEST_PRECEDENCE只為Integer.MAX_VALUE
public abstract class AbstractHandlerMapping extends WebApplicationObjectSupport
    implements HandlerMapping, Ordered {
    private int order = Integer.MAX_VALUE; 

AbstractHandlerMapping是RequestMappingHandlerMapping的父類。

RequestMappingHandlerAdapter
public abstract class AbstractHandlerMethodAdapter extends WebContentGenerator implements HandlerAdapter, Ordered {
    // Ordered.LOWEST_PRECEDENCE只為Integer.MAX_VALUE
    private int order = Ordered.LOWEST_PRECEDENCE;

AbstractHandlerMethodAdapter是RequestMappingHandlerAdapter的父類。   可以看到RequestMappingHandlerMapping和RequestMappingHandlerAdapter沒有設置order屬性的時候，order屬性的默認值都是Integer.MAX_VALUE，即優先級最低。 

總結： 如果配置了<mvc:annotation-driven/>，又配置了自定義的RequestMappingHandlerAdapter，并且沒有設置RequestMappingHandlerAdapter的order值，那么這2個RequestMappingHandlerAdapter的order值都是Integer.MAX_VALUE。那么誰先定義的，誰優先級高。 <mvc:annotation-driven/>配置在自定義的RequestMappingHandlerAdapter配置之前，那么<mvc:annotation-driven/>配置的RequestMappingHandlerAdapter優先級高，反之自定義的RequestMappingHandlerAdapter優先級高。

如果配置了<mvc:annotation-driven/>，又配置了自定義的RequestMappingHandlerMapping，并且沒有設置RequestMappingHandlerMapping的order值。那么<mvc:annotation-driven/>配置的RequestMappingHandlerMapping優先級高，因為<mvc:annotation-driven />內部會設置RequestMappingHandlerMapping的order為0。

四、應用
1、定義接口
import java.util.Map;
import org.springframework.core.Ordered;

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

}

2、實現接口

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("權限控制");
    }
}

3、測試進行排序
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>