通過Filter實現UrlWriter

每個網頁或請求都是一個url地址，一般，這個地址可能是.do .action .php之類的並加上'?'號、'&'號查詢串等構成的一個長長的的url。不利於閱讀，同時將實際的地址暴露給用戶，容易遭到攻擊。

一般的url----------------------------------------------------------較好的 url
http://www.xxx.net/user/profile.do?id=20001 ====> http://www.xxx.net/user/20001
http://www.xxx.net/forum/board.do?name=java ====> http://www.xxx.net/forum/java
http://www.xxx.net/forum/thread.do?id=29923 ====> http://www.xxx.net/thread/29923

後者明顯較爲直觀和漂亮。

使用url rewrite可以很好的改善這個狀況。網站url rewrite應用是非常廣泛的，良好的url設計給用戶帶來的非常好的體驗，同時也能吸引搜索引擎的注意。

一種rewrite稱爲一個規則，上面的例子就是3個規則。Apache HTTP Server 2.x的mod_rewrite提供了不錯的Rewrite處理方法，java中有 http://tuckey.org/urlrewrite/。
下面我們用一個Filter來模擬實現。

public class RewriteFilter implements Filter {
  private ServletContext ctx;
  private List<RewriteEntry> entries = new ArrayList<RewriteEntry>();
  public void destroy() {
  }
  public void doFilter(ServletRequest request, ServletResponse response,
      FilterChain nextFilter) throws IOException, ServletException {
    HttpServletRequest req = (HttpServletRequest) request;
    HttpServletResponse resp = (HttpServletResponse) response;
    String url = req.getRequestURI();
    for (RewriteEntry entry : entries) {
      Matcher matcher = entry.getPattern().matcher(url);
      if (!matcher.find(0))
        continue;
      String replacement = replace(matcher, entry.getTarget());
      String query = req.getQueryString();
      if (query != null) {
        if (replacement.indexOf("?") > 0)
          replacement = replacement + "&" + query;
        else
          replacement = replacement + "?" + query;
      }
      if (replacement.startsWith("/")) {
        RequestDispatcher dispatcher = ctx.getRequestDispatcher(replacement);
        dispatcher.forward(request, response);
        return;
      } else {
        resp.sendRedirect(resp.encodeRedirectURL(replacement));
        return;
      }
    }
    // TODO 只有不匹配的情況下，程序才能運行到這裏
    // 這樣必須把RewriteFilter配置到Filter順序的最後
    nextFilter.doFilter(request, response);
  }
  private String replace(Matcher matcher, String target) {
    StringBuffer cb = new StringBuffer(512);
    int length = target.length();
    for (int i = 0; i < length; i++) {
      char ch = target.charAt(i);
      // target : /index.jsp?id=$1
      if (ch != '$' || i == length - 1)
        cb.append(ch);
      else {
        ch = target.charAt(i + 1);
        if (ch >= '0' && ch <= '9') {
          int group = ch - '0';
          cb.append(matcher.group(group));
          i++;
        } else if (ch == '$') {
          cb.append('$');
          i++;
        } else
          cb.append('$');
      }
    }
    return cb.toString();
  }
  /**
   * 解析rewrite.properties文件，生辰需要rewrite的Bean
   */
  public void init(FilterConfig filterConfig) throws ServletException {
    ctx = filterConfig.getServletContext();
    File cfgFile = new File(ctx.getRealPath("/WEB-INF/rewrite.properties"));
    BufferedReader reader = null;
    try {
      reader = new BufferedReader(new FileReader(cfgFile));
      String line = null;
      while ((line = reader.readLine()) != null) {
        if (line.trim().equals("") || line.trim().startsWith("#"))
          continue;
        String[] temp = line.split(" ", 2);
        entries.add(new RewriteEntry(temp[0], temp[1]));
      }
    } catch (IOException e) {
      throw new RuntimeException(e);
    } finally {
      if (reader != null) {
        try {
          reader.close();
        } catch (IOException e) {
        }
      }
    }
  }
  public static class RewriteEntry {
    private Pattern pattern;
    private String target;
    public RewriteEntry(String pattern, String target) {
      this.pattern = Pattern.compile(pattern);
      this.target = target;
    }
    public Pattern getPattern() {
      return pattern;
    }
    public String getTarget() {
      return target;
    }
  }
}

web.xml文件

<web-app>
  <filter>
    <filter-name>rewriteFilter</filter-name>
    <filter-class>demo.RewriteFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>rewriteFilter</filter-name>
    <url-pattern>*.html</url-pattern>
  </filter-mapping>
</web-app>

/WEB-INF/rewrite.properties文件
[quote]^\/index\/(.*)\.html$ /index.jsp?id=$1
^\/hello\/(.*)\.html$ /hello.jsp?name=$1[/quote]

hello.jsp
<h3>Hello, <%=request.getParameter("name")%>!</h3>

index.jsp
<h3>URL: <%=request.getRequestURI()%></h3>
<h3>Query: <%=request.getQueryString()%></h3>

我們運行頁面，輸入
http://localhost:8888/index/xace.html
輸出：
URL: /index.jsp
Query: id=xace

http://localhost:8888/hello/xace.html
輸出：
Hello, xace!