HttpServletRequestWrapper 学习

使用过滤器不能改变HttpServletRequest对象的参数确实大大缩减了filter的应用范围，幸运的是，可以通过使用装饰模式来改变其状态
下面就是一个HttpServletRequest装饰类的样例：
一个删除空白字符的Filter
通过实现一个删除空白字符的filter，来演示如何使用javax.servlet.http.HttpServletRequestWrapper类来装饰HttpServletRequest对象，在本例中，这个filter将删除所传来的参数中多余的空白字符，这在许多servlet/JSP应用中是很有用的，包括Struts及JavaServer Faces等应用。例如，Struts通过调用HttpServletRequest对象的getParameterValues()对象来处理action表单。通过覆盖装饰类中此方法，你可以改变当前HttpServletRequest对象的状态。要创建HttpServletRequest的装饰类，需要继承HttpServletRequestWrapper并且覆盖希望改变的方法。

package trimmer.filter; 

import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletRequestWrapper; 

public final class MyRequestWrapper extends HttpServletRequestWrapper { 
public MyRequestWrapper(HttpServletRequest servletRequest) { 
super(servletRequest); 
} 

public String[] getParameterValues(String parameter) { 
String[] results = super.getParameterValues(parameter); 
if (results == null) 
return null; 
int count = results.length; 
String[] trimResults = new String[count]; 
for (int i = 0; i < count; i++) { 
trimResults[i] = results[i].trim(); 
} 
return trimResults; 
} 
} 

[b]filter[/b]

package trimmer.filter; 

import java.io.IOException; 
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.http.HttpServletRequest; 

public class MyFilter implements Filter { 
private FilterConfig filterConfig; 

public void init(FilterConfig filterConfig) throws ServletException { 
System.out.println("Filter initialized"); 
this.filterConfig = filterConfig; 
} 

public void destroy() { 
System.out.println("Filter destroyed"); 
this.filterConfig = null; 
} 

public void doFilter(ServletRequest request, ServletResponse response, 
FilterChain chain) throws IOException, ServletException { 
chain.doFilter(new MyRequestWrapper((HttpServletRequest) request), 
response); 
} 
} 

[b]web.xml[/b]

<filter> 
       <filter-name>TrimmerFilter</filter-name> 
       <filter-class>trimmer.filter.MyFilter</filter-class> 
   </filter> 
   <filter-mapping> 
       <filter-name>TrimmerFilter</filter-name> 
       <url-pattern>*.do</url-pattern> 
   </filter-mapping> 

启动这个应用后，在表单中输入一些值，提交表单，看看这个filter是如何修整输入数值的。这是一个实用的装饰模式的应用。