Filter

過濾器

• 過濾器屬於Servlet規範，從2.3版本就開始有了。

• 過濾器就是對訪問的內容進行篩選（攔截）。利用過濾器對請求和響應進行過濾
  

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過濾器執行過程

生命週期

誕生：過濾器的實例是在應用被加載時就完成的實例化，並初始化的。
存活：和應用的生命週期一致的。在內存中是單例的。針對攔截範圍內的資源訪問，每次訪問都會調用void doFIlter(request,response.chain)進行攔截。
死亡：應用被卸載。

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過濾器簡單例子

public class FilterDemo1 implements Filter {

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
    
        //對請求進行攔截，代碼寫在這裏
        System.out.println("過濾前");
        
        chain.doFilter(request, response);//放行，訪問過濾文件
        
        //對響應進行攔截，代碼寫在這裏
        System.out.println("過濾後");
    }

    public void init(FilterConfig arg0) throws ServletException {
        System.out.println("初始化方法");
    }
    
    public void destroy() {
    }

}

串聯過濾器

解決亂碼的過濾器

public class SetCharacterEncodingFilter implements Filter {
    private FilterConfig filterConfig;
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
    }

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        
        String encoding = filterConfig.getInitParameter("encoding");//用戶可能忘記了配置該參數
        if(encoding==null){
            encoding = "UTF-8";//默認編碼
        }
        
        request.setCharacterEncoding(encoding);//只能解決（表單提交）POST請求參數的中文問題
        response.setCharacterEncoding(encoding);//輸出流編碼
        response.setContentType("text/html;charset="+encoding);//輸出流編碼，通知了客戶端應該使用的編碼
        chain.doFilter(request, response);
    }

    public void destroy() {

    }

}

控制動態資源不要緩存過濾器

//控制servlet，jsp不要緩存過濾器
public class NoCacheFilter implements Filter {

    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
        
//        HttpServletRequest request = (HttpServletRequest)req;
//        HttpServletResponse response = (HttpServletResponse)resp;
        
        HttpServletRequest request = null;
        HttpServletResponse response = null;
        try{
            request  = (HttpServletRequest)req;
            response = (HttpServletResponse)resp;
        }catch(Exception e){
            throw new RuntimeException("not-http request or response");
        }
        
        
        response.setHeader("Expires", "-1");
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma", "no-cache");
        chain.doFilter(request, response);
    }

    public void destroy() {

    }

}