Filter

Filter 是浏览器访问Servlet前要执行的部分。

添加 Filter

实现Filter接口，并添加@WebFilter注解即可：

@WebFilter("/*")   //路径的匹配规则和Servlet一致，这里表示匹配所有请求
public class TestFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
System.out.println(request.getRequestURL());
		filterChain.doFilter(servletRequest, servletResponse);
    }
}

现在我们发起的所有请求，一律需要经过此过滤器。

filterChain.doFilter(servletRequest, servletResponse);

通过在最后添加该语句，将请求继续传输给下一个过滤器，只有当执行该代码后没有过滤器可以传输时，才会传输给Servlet。 实际上，当doFilter方法调用时，就会一直向下直到Servlet，在Servlet处理完成之后，又依次返回到最前面的Filter，类似于递归的结构。

HttpFilter 实现类

同Servlet一样，Filter也有对应的HttpFilter专用类，它针对HTTP请求进行了专门处理，因此我们可以直接使用HttpFilter来编写：

public abstract class HttpFilter extends GenericFilter {
    private static final long serialVersionUID = 7478463438252262094L;
 
    public HttpFilter() {
    }
 
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        if (req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
            this.doFilter((HttpServletRequest)req, (HttpServletResponse)res, chain);
        } else {
            throw new ServletException("non-HTTP request or response");
        }
    }
 
    protected void doFilter(HttpServletRequest req, HttpServletResponse res, FilterChain chain) throws IOException, ServletException {
        chain.doFilter(req, res);
    }
}

实现未登录禁止访问

用户在未登录情况下，只允许静态资源和登陆页面请求通过，登陆之后畅行无阻：

@WebFilter("/*")
public class MainFilter extends HttpFilter {
    @Override
    protected void doFilter(HttpServletRequest req, HttpServletResponse res, FilterChain chain) throws IOException, ServletException {
        String url = req.getRequestURL().toString();
        //判断是否为静态资源
        if(!url.endsWith(".js") && !url.endsWith(".css") && !url.endsWith(".png")){
            HttpSession session = req.getSession();
            User user = (User) session.getAttribute("user");
            //判断是否未登陆
            if(user == null && !url.endsWith("login")){
                res.sendRedirect("login");
                return;
            }
        }
        //交给过滤链处理
        chain.doFilter(req, res);
    }
}