Servlet过滤器

Servlet过滤器与自来水杯过滤的原理相似，它主要用于读客户端（浏览器）的请求进行过滤处理，再讲过滤后的请求转交给下一个资源。对于程序开发人员而言，过滤器实质就是在Web服务器上的一个Web应用组件，用于拦截客户端（浏览器）对目标资源的请求，并对这些请求进行一定的过滤处理再发送给目标资源。

如图 过滤器的处理方式

在Web容器中部署了过滤器以后，不仅客户端发送的请求会经过过滤器的处理，而且请求在发送到目标资源处理以后，请求的回应信息也同样要经过过滤器。

@WebFilter(filterName = "TestFilter", urlPatterns = {"/test"})
public class TestFilter implements Filter {
    
    private static final boolean debug = true;
    // The filter configuration object we are associated with.  If
    // this value is null, this filter instance is not currently
    // configured. 
    private FilterConfig filterConfig = null;
    
    public TestFilter() {
    }    
    
    private void doBeforeProcessing(ServletRequest request, ServletResponse response)
            throws IOException, ServletException {
        if (debug) {
            log("TestFilter:DoBeforeProcessing");
        }

        // Write code here to process the request and/or response before
        // the rest of the filter chain is invoked.

        // For example, a logging filter might log items on the request object,
        // such as the parameters.
	/*
         for (Enumeration en = request.getParameterNames(); en.hasMoreElements(); ) {
         String name = (String)en.nextElement();
         String values[] = request.getParameterValues(name);
         int n = values.length;
         StringBuffer buf = new StringBuffer();
         buf.append(name);
         buf.append("=");
         for(int i=0; i < n; i++) {
         buf.append(values[i]);
         if (i < n-1)
         buf.append(",");
         }
         log(buf.toString());
         }
         */
    }    
    
    private void doAfterProcessing(ServletRequest request, ServletResponse response)
            throws IOException, ServletException {
        if (debug) {
            log("TestFilter:DoAfterProcessing");
        }

        // Write code here to process the request and/or response after
        // the rest of the filter chain is invoked.

        // For example, a logging filter might log the attributes on the
        // request object after the request has been processed. 
	/*
         for (Enumeration en = request.getAttributeNames(); en.hasMoreElements(); ) {
         String name = (String)en.nextElement();
         Object value = request.getAttribute(name);
         log("attribute: " + name + "=" + value.toString());

         }
         */

        // For example, a filter might append something to the response.
	/*
         PrintWriter respOut = new PrintWriter(response.getWriter());
         respOut.println("<P><B>This has been appended by an intrusive filter.</B>");
         */
    }

    /**
     *
     * @param request The servlet request we are processing
     * @param response The servlet response we are creating
     * @param chain The filter chain we are processing
     *
     * @exception IOException if an input/output error occurs
     * @exception ServletException if a servlet error occurs
     */
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain)
            throws IOException, ServletException {
        
        if (debug) {
            log("TestFilter:doFilter()");
        }
        
        doBeforeProcessing(request, response);
        
        Throwable problem = null;
        try {
            chain.doFilter(request, response);
        } catch (Throwable t) {
            // If an exception is thrown somewhere down the filter chain,
            // we still want to execute our after processing, and then
            // rethrow the problem after that.
            problem = t;
            t.printStackTrace();
        }
        
        doAfterProcessing(request, response);

        // If there was a problem, we want to rethrow it if it is
        // a known type, otherwise log it.
        if (problem != null) {
            if (problem instanceof ServletException) {
                throw (ServletException) problem;
            }
            if (problem instanceof IOException) {
                throw (IOException) problem;
            }
            sendProcessingError(problem, response);
        }
    }

    /**
     * Return the filter configuration object for this filter.
     */
    public FilterConfig getFilterConfig() {
        return (this.filterConfig);
    }

    /**
     * Set the filter configuration object for this filter.
     *
     * @param filterConfig The filter configuration object
     */
    public void setFilterConfig(FilterConfig filterConfig) {
        this.filterConfig = filterConfig;
    }

    /**
     * Destroy method for this filter
     */
    public void destroy() {        
    }

    /**
     * Init method for this filter
     */
    public void init(FilterConfig filterConfig) {        
        this.filterConfig = filterConfig;
        if (filterConfig != null) {
            if (debug) {                
                log("TestFilter:Initializing filter");
            }
        }
    }

    /**
     * Return a String representation of this object.
     */
    @Override
    public String toString() {
        if (filterConfig == null) {
            return ("TestFilter()");
        }
        StringBuffer sb = new StringBuffer("TestFilter(");
        sb.append(filterConfig);
        sb.append(")");
        return (sb.toString());
    }
    
    private void sendProcessingError(Throwable t, ServletResponse response) {
        String stackTrace = getStackTrace(t);        
        
        if (stackTrace != null && !stackTrace.equals("")) {
            try {
                response.setContentType("text/html");
                PrintStream ps = new PrintStream(response.getOutputStream());
                PrintWriter pw = new PrintWriter(ps);                
                pw.print("<html>\n<head>\n<title>Error</title>\n</head>\n<body>\n"); //NOI18N

                // PENDING! Localize this for next official release
                pw.print("<h1>The resource did not process correctly</h1>\n<pre>\n");                
                pw.print(stackTrace);                
                pw.print("</pre></body>\n</html>"); //NOI18N
                pw.close();
                ps.close();
                response.getOutputStream().close();
            } catch (Exception ex) {
            }
        } else {
            try {
                PrintStream ps = new PrintStream(response.getOutputStream());
                t.printStackTrace(ps);
                ps.close();
                response.getOutputStream().close();
            } catch (Exception ex) {
            }
        }
    }
    
    public static String getStackTrace(Throwable t) {
        String stackTrace = null;
        try {
            StringWriter sw = new StringWriter();
            PrintWriter pw = new PrintWriter(sw);
            t.printStackTrace(pw);
            pw.close();
            sw.close();
            stackTrace = sw.getBuffer().toString();
        } catch (Exception ex) {
        }
        return stackTrace;
    }
    
    public void log(String msg) {
        filterConfig.getServletContext().log(msg);        
    }
}