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
public RequestMappingHandlerAdapter requestMappingHandlerAdapter()方法
@Bean
public RequestMappingHandlerMapping requestMappingHandlerMapping()方法 分析代码可以知道：RequestMappingHandlerMapping默认会设置order属性为0，RequestMappingHandlerAdapter没有设置order属性。

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>


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