分析

1. 使用IO操作获取txt文本的敏感词汇
   一定要右击txt文本，点击Package File
   
   加载文本资源最适合写在Filter的init方法中，这样只会被加载一次
2. 对request对象进行增强，增强获取参数相关方法
3. 放行，传递代理对象
   
   注意要传递proxy_req，而不是req

实现

SensitiveWordsFilter.java：

/**
 * 敏感词汇过滤器
 */
@WebFilter("/*")
public class SensitiveWordsFilter implements Filter {
    @Override
    public void destroy() {
    }

    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        //1.创建代理对象，增强getParameter方法
        ServletRequest proxy_req = (ServletRequest) Proxy.newProxyInstance(req.getClass().getClassLoader(),
                req.getClass().getInterfaces(), new InvocationHandler() {
                    @Override
                    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                        //增强getParameter方法
                        //判断是否是getParameter方法
                        if (method.getName().equals("getParameter")) {
                            //增强返回值
                            //获取返回值
                            String value = (String) method.invoke(req, args);
                            if (value != null) {
                                for (String str : list) {
                                    if (value.contains(str)) {
                                        value = value.replaceAll(str, "***");
                                    }
                                }
                            }
                            return value;
                        } else {
                            return method.invoke(req, args);
                        }
                    }
                });
        //2.放行
        chain.doFilter(proxy_req, resp);
    }

    private List<String> list = new ArrayList<String>();//敏感词汇集合

    @Override
    public void init(FilterConfig config) throws ServletException {
        try {
            //1.获取文件真实路径
            ServletContext servletContext = config.getServletContext();
            String realPath = servletContext.getRealPath("/WEB-INF/classes/敏感词汇.txt");
            //2.读取文件
            BufferedReader br = new BufferedReader(new FileReader(realPath));
            //3.将文件的每一行数据添加到list中
            String line = null;
            while ((line = br.readLine()) != null) {
                list.add(line);
            }
            br.close();
            System.out.println(list);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

TestServlet.java：

@WebServlet("/testServlet")
public class TestServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String name = request.getParameter("name");
        String msg = request.getParameter("msg");
        System.out.println(name + ":" + msg);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

访问：localhost/Filter/testServlet?name=张三&msg=笨蛋
效果如下：