12.1、Decorator模式

即使没有某一个对象的类的源代码，甚至即便这个类是声明为final的，Decorator模式和Wrapper模式都允许装饰或包装这个对象。

Decorator模式适用于无法使用继承的情况（比如，所指对象的类为final），或者你不想亲自创建对象，而是想从另一个子系统中获取。例如，Servlet容器创建了一个ServletRequest和一个ServletResponse，并将他们传给Servlet的service方法。改变ServletRequest和ServletResponse行为的唯一方法是将他们包在其他对象中。唯一必须满足的条件是，被装饰对象的类要实现一个接口，并且要包装的方法必须从这个接口处继承。

12.2、Servlet Wrapper类

Servlet API中提供了4个类，他们很少用到，但是功能非常强大，分别是：ServletRequestWrapper、ServletResponseWrapper，以及HttpServletRequestWrapper和HttpServletResponseWrapper。

ServletRequestWrapper使用起来非常方便，由于它为调用被包装ServletRequest中的对等方法的每一个方法都提供了默认实现。通过继承则只好直接实现ServletRequest，并为接口中的每一个方法都提供实现。

12.3、范例：AutoCorrect过滤器

在web应用程序中，用户经常会在输入值时，在其前面或者后面添加一些空格，甚至在词与词之间也会有多余空格。你又不想到应用程序的逐个Servlet中进行检查并删除多余的空格。那么本届介绍的AutoCorrect过滤器的特性就可以帮你完成这些工作。这个过滤器中包含一个HttpServletRequestWrapper的子类，命名为AutoCorrectHttpServletRequestWrapper，并覆盖返回一个或多个参数值的下列方法：getParameter、getParameterValues和getParameterMap。

AutoCorrectFilter.java

package filter;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
@WebFilter(filterName = "AutoCorrectFilter", urlPatterns = {"/*"})
public class AutoCorrectFilter implements Filter{

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		
	}
	
	@Override
	public void init(FilterConfig arg0) throws ServletException {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain filterChain) throws IOException, ServletException {
		HttpServletRequest httpServletRequest = (HttpServletRequest)request ;
		AutoCorrectHttpServletRequestWrapper wrapper = new 
				AutoCorrectHttpServletRequestWrapper(httpServletRequest) ;
		filterChain.doFilter(wrapper, response);
	}
	
	class AutoCorrectHttpServletRequestWrapper extends HttpServletRequestWrapper{
		private HttpServletRequest httpServletRequest ;
		public AutoCorrectHttpServletRequestWrapper(HttpServletRequest httpServletRequest) {
			super(httpServletRequest);
			this.httpServletRequest = httpServletRequest ;
		}
		@Override
		public String getParameter(String name) {
			return autoCorrect(httpServletRequest.getParameter(name)) ;
		}
		@Override
		public String[] getParameterValues(String name) {
			// TODO Auto-generated method stub
			return autoCorrect(httpServletRequest.getParameterValues(name));
		}
		@Override
		public Map<String, String[]> getParameterMap() {
			// TODO Auto-generated method stub
			final Map<String, String[]> parameterMap = httpServletRequest.getParameterMap() ;
			Map<String, String[]> newMap = new Map<String, String[]>(){
				@Override
				public int size() {
					return parameterMap.size();
				}

				@Override
				public boolean isEmpty() {
					return parameterMap.isEmpty();
				}

				@Override
				public boolean containsKey(Object key) {
					return parameterMap.containsKey(key);
				}

				@Override
				public boolean containsValue(Object value) {
					return parameterMap.containsValue(value);
				}

				@Override
				public String[] get(Object key) {
					return autoCorrect(parameterMap.get(key));
				}

				@Override
				public String[] put(String key, String[] value) {
					return parameterMap.put(key, value);
				}

				@Override
				public String[] remove(Object key) {
					return parameterMap.remove(key);
				}

				@Override
				public void putAll(Map<? extends String, ? extends String[]> m) {
					parameterMap.putAll(m);
				}

				@Override
				public void clear() {
					parameterMap.clear();
				}

				@Override
				public Set<String> keySet() {
					return parameterMap.keySet();
				}

				@Override
				public Collection<String[]> values() {
					return autoCorrect(parameterMap.values());
				}

				@Override
				public Set<java.util.Map.Entry<String, String[]>> entrySet() {
					return autoCorrect(parameterMap.entrySet());
				}
				
			} ;
			return newMap ;
		}
		
	} 
	private String autoCorrect(String value){
		if(value =http://www.mamicode.com/= null){>
test1.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href=http://www.mamicode.com/"">>test2.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href=http://www.mamicode.com/"">>
运行结果：


第十二章_请求和响应的装饰