HttpFilter

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;
import javax.servlet.http.HttpServletResponse;

/**
 * 自定义的 HttpFilter, 实现自 Filter 接口
 *
 */
public abstract class HttpFilter implements Filter {

    /**
     * 用于保存 FilterConfig 对象. 
     */
    private FilterConfig filterConfig;
    
    /**
     * 不建议子类直接覆盖. 若直接覆盖, 将可能会导致 filterConfig 成员变量初始化失败
     */
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
        init();
    }

    /**
     * 供子类继承的初始化方法. 可以通过 getFilterConfig() 获取 FilterConfig 对象. 
     */
    protected void init() {}

    /**
     * 直接返回 init(ServletConfig) 的 FilterConfig 对象
     */
    public FilterConfig getFilterConfig() {
        return filterConfig;
    }
    
    /**
     * 原生的 doFilter 方法, 在方法内部把 ServletRequest 和 ServletResponse 
     * 转为了 HttpServletRequest 和 HttpServletResponse, 并调用了 
     * doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
     * 
     * 若编写 Filter 的过滤方法不建议直接继承该方法. 而建议继承
     * doFilter(HttpServletRequest request, HttpServletResponse response, 
     *        FilterChain filterChain) 方法
     */
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp,
            FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;
        
        doFilter(request, response, chain);
    }
    
    /**
     * 抽象方法, 为 Http 请求定制. 必须实现的方法. 
     * @param request
     * @param response
     * @param filterChain
     * @throws IOException
     * @throws ServletException
     */
    public abstract void doFilter(HttpServletRequest request, HttpServletResponse response, 
            FilterChain filterChain) throws IOException, ServletException;

    /**
     * 空的 destroy 方法。 
     */
    @Override
    public void destroy() {}

}