java之过滤器Filter （应用场景）

filter在开发中的常见应用：
* 1.filter可以目标资源执行之前，进行权限检查，检查用户有无权限，如有权限则放行，如没有，则拒绝访问
* 2.filter可以放行之前，对request和response进行预处理，从而实现一些全局性的设置。
* 3.filter在放行之后，可以捕获到目标资源的输出，从而对输出作出类似于压缩这样的设置

一、解决全网站的乱码：

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;

request.setCharacterEncoding("UTF-8");  //这个编码只能解决Post提交，不能解决get方式的提交，所以需要对request中getParameter
//方法增强
response.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");

chain.doFilter(new MyRequest(request), response);   //request.getparameter("password");
}

/*
1.写一个类，实现与被增强对象相同的接口
2.定义一个变量，记住被增强对象
3.定义一个构造方法，接收被增强对象
4.覆盖想增强的方法
5.对于不想增强的方法，直接调用被增强对象（目标对象）的方法
*/

class MyRequest extends HttpServletRequestWrapper{

private HttpServletRequest request;
public MyRequest(HttpServletRequest request) {
super(request);
this.request = request;
}
@Override
public String getParameter(String name) {

String value = this.request.getParameter(name);
if(!request.getMethod().equalsIgnoreCase("get")){
return value;
}

if(value==null){
return null;
}

try {
return value = new String(value.getBytes("iso8859-1"),request.getCharacterEncoding());
} catch (UnsupportedEncodingException e) {
throw new RuntimeException(e);
}

}

｝

　　二，实现自动登录

实现用户自动登陆的过滤器
在用户登陆成功后，发送一个名称为user的cookie给客户端，cookie的值为用户名和md5加密后的密码。编写一个AutoLoginFilter，这个filter检查用户是否带有名称为user的cookie来，如果有，则调用dao查询cookie的用户名和密码是否和数据库匹配，匹配则向session中存入user对象（即用户登陆标记），以实现程序完成自动登陆。

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;

//1.先检查用户是否已登陆，没登陆才自动登陆
User user = (User) request.getSession().getAttribute("user");

if(user!=null){
chain.doFilter(request, response);
return;
}

//2.没登陆，再执行自动登陆逻辑

//看用户有没有带自动登陆的cookie
Cookie autoLoginCookie = null;
Cookie cookies[] = request.getCookies();

for(int i=0;cookies!=null && i<cookies.length;i++){
if(cookies[i].getName().equals("autologin")){
autoLoginCookie = cookies[i];
}
}
if(autoLoginCookie==null){
chain.doFilter(request, response);
return;
}

//用户带了自动登陆的cookie，则先检查cookie的有效期
String values[] = autoLoginCookie.getValue().split("\\:");
if(values.length!=3){
chain.doFilter(request, response);
return;
}
long expirestime = Long.parseLong(values[1]);
if(System.currentTimeMillis()>expirestime){
chain.doFilter(request, response);
return;
}

//代表cookie时间有效，再检查cookie的有效性
String username = values[0];
String client_md5 = values[2];

BusinessService service = new BusinessService();
user = service.findUser(username);
if(user==null){
chain.doFilter(request, response);
return;
}
////autologin=username:expirestime:md5(password:expirestime:username)
String server_md5 = md5(user.getUsername(),user.getPassword(),expirestime);
if(!server_md5.equals(client_md5)){
chain.doFilter(request, response);
return;
}

//执行登陆
request.getSession().setAttribute("user", user);
chain.doFilter(request, response);

}

private String md5(String username,String password,long expirestime){

try{
String value = password + ":" + expirestime + ":" + username;
MessageDigest md = MessageDigest.getInstance("md5");
byte md5[] = md.digest(value.getBytes());
BASE64Encoder encode = new BASE64Encoder();
return encode.encode(md5);
}catch (Exception e) {
throw new RuntimeException(e);
}
}

　　

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

String username = request.getParameter("username");
String password = request.getParameter("password");

BusinessService service = new BusinessService();
User user = service.login(username, password);
if(user==null){
request.setAttribute("message", "用户名或密码错误！！");
request.getRequestDispatcher("/message.jsp").forward(request, response);
return;
}

request.getSession().setAttribute("user", user);
int expirestime = Integer.parseInt(request.getParameter("time"));
//给客户机发送自动登陆的 cookie
//autologin=username:expirestime:md5(password:expirestime:username)
Cookie cookie = makeCookie(user, expirestime);
response.addCookie(cookie);
response.sendRedirect("/login/index.jsp");
}

public Cookie makeCookie(User user,int expirestime){
long currenttime = System.currentTimeMillis();
String cookieValue = user.getUsername() + ":" + (currenttime+expirestime*1000) + ":" + md5(user.getUsername(), user.getPassword(), (currenttime+expirestime*1000));
Cookie cookie = new Cookie("autologin",cookieValue);
cookie.setMaxAge(expirestime);
cookie.setPath("/login");
return cookie;
}

private String md5(String username,String password,long expirestime){

try{
String value = password + ":" + expirestime + ":" + username;
MessageDigest md = MessageDigest.getInstance("md5");
byte md5[] = md.digest(value.getBytes());
BASE64Encoder encode = new BASE64Encoder();
return encode.encode(md5);
}catch (Exception e) {
throw new RuntimeException(e);
}
}

　　三，禁止浏览器缓存所有动态页面的过滤器：
有 3 个 HTTP 响应头字段都可以禁止浏览器缓存当前页面，它们在 Servlet 中的示例代码如下：
response.setDateHeader("Expires",-1);
response.setHeader("Cache-Control","no-cache");
response.setHeader("Pragma","no-cache");
并不是所有的浏览器都能完全支持上面的三个响应头，因此最好是同时使用上面的三个响应头。
Expires数据头：值为GMT时间值，为-1指浏览器不要缓存页面
Cache-Control响应头有两个常用值：
no-cache指浏览器不要缓存当前页面。
max-age:xxx指浏览器缓存页面xxx秒。

四，控制浏览器缓存页面中的静态资源的过滤器：
有些动态页面中引用了一些图片或css文件以修饰页面效果，这些图片和css文件经常是不变化的，所以为减轻服务器的压力，可以使用filter控制浏览器缓存这些文件，以提升服务器的性能。

private Map<String,byte[]> map = new HashMap();

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;

//1.得到用户想访问的资源（uri）
String uri = request.getRequestURI();

//2.看map集合中是否保存了该资源的数据
byte b[] = map.get(uri);

//3.如果保存了，则直接取数据打给浏览器
if(b!=null){
response.getOutputStream().write(b);
return;
}

//4.如果没有保存数据，则放行让目标资源执行，这时还需写一个response的包装类，捕获目标资源的输出
MyResponse my = new MyResponse(response);
chain.doFilter(request, my);
byte data[] = my.getBuffer();

//5.以资源uri为关键字，打资源的数据保存map集合中，以备于下次访问
map.put(uri, data);

//6.输出数据给浏览器
response.getOutputStream().write(data);

}

class MyResponse extends HttpServletResponseWrapper{
private ByteArrayOutputStream bout = new ByteArrayOutputStream();
private PrintWriter pw;

private HttpServletResponse response;
public MyResponse(HttpServletResponse response) {
super(response);
this.response = response;
}
@Override
public ServletOutputStream getOutputStream() throws IOException {
return new MyServletOutputStream(bout);    //myresponse.getOutputStream().write("hahah");
}

@Override
public PrintWriter getWriter() throws IOException {
pw = new PrintWriter(new OutputStreamWriter(bout,response.getCharacterEncoding()));
return pw;
}
public byte[] getBuffer(){
if(pw!=null){
pw.close();
}
return bout.toByteArray();
}
}

class MyServletOutputStream extends ServletOutputStream{

private ByteArrayOutputStream bout;
public MyServletOutputStream(ByteArrayOutputStream bout){
this.bout = bout;
}
@Override
public void write(int b) throws IOException {
bout.write(b);
}

}

　　五、使用Decorator模式包装request对象，实现html标签转义功能（Tomcat服务器中提供了转义html标签的工具类）

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;

chain.doFilter(new MyRequest(request), response);  //request.getParameter("resume");  //<script>

}

class MyRequest extends HttpServletRequestWrapper{
private HttpServletRequest request;
public MyRequest(HttpServletRequest request) {
super(request);
this.request = request;
}
@Override
public String getParameter(String name) {

String value = this.request.getParameter(name);
if(value==null){
return null;
}
return filter(value);
}

public String filter(String message) {

if (message == null)
return (null);

char content[] = new char[message.length()];
message.getChars(0, message.length(), content, 0);
StringBuffer result = new StringBuffer(content.length + 50);
for (int i = 0; i < content.length; i++) {
switch (content[i]) {
case '<':
result.append("<");
break;
case '>':
result.append(">");
break;
case '&':
result.append("&");
break;
case '"':
result.append(""");
break;
default:
result.append(content[i]);
}
}
return (result.toString());

}
｝

　　六、压缩响应信息

服务器发给浏览器的数据时，先进行压缩，然后在发出，（注意filter要拦截的对象，都会压缩，所以要在web.xml中配置好想要拦截的对象）。

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;
MyResponse myresponse = new MyResponse(response);

chain.doFilter(request, myresponse);  //response.getwriter  response.getOutputStream

//取出缓冲的数据压缩后输出
byte out[] = myresponse.getBuffer();  //得到目标资源的输出
byte gzipout[] = gzip(out);
response.setHeader("content-encoding", "gzip");
response.setHeader("content-length", gzipout.length + "");
response.getOutputStream().write(gzipout);
}

public byte[] gzip(byte b[]) throws IOException{

ByteArrayOutputStream bout = new ByteArrayOutputStream();
GZIPOutputStream gout = new GZIPOutputStream(bout);
gout.write(b);
gout.close();
return bout.toByteArray();
}

class MyResponse extends HttpServletResponseWrapper{
private ByteArrayOutputStream bout = new ByteArrayOutputStream();
private PrintWriter pw;

private HttpServletResponse response;
public MyResponse(HttpServletResponse response) {
super(response);
this.response = response;
}
@Override
public ServletOutputStream getOutputStream() throws IOException {
return new MyServletOutputStream(bout);    //myresponse.getOutputStream().write("hahah");
}

@Override
public PrintWriter getWriter() throws IOException {
pw = new PrintWriter(new OutputStreamWriter(bout,response.getCharacterEncoding()));
return pw;
}
public byte[] getBuffer(){
if(pw!=null){
pw.close();
}
return bout.toByteArray();
}
}

class MyServletOutputStream extends ServletOutputStream{

private ByteArrayOutputStream bout;
public MyServletOutputStream(ByteArrayOutputStream bout){
this.bout = bout;
}
@Override
public void write(int b) throws IOException {
bout.write(b);
}

}

　　七、敏感词过滤(z注意词库中词语的格式：例如：傻逼|1 后面的1表示识别词库中的等级 ，禁用词为1，审查词为2，替换词为3)

private List<String> banWords = new ArrayList();//禁用词，例如：傻逼|1
private List<String> auditWords = new ArrayList();//审查词   例如： 中共|2
private List<String> replaceWords = new ArrayList();//替换词  例如： 色情|3

public void init(FilterConfig filterConfig) throws ServletException {
try{
String path = WordsFilter.class.getClassLoader().getResource("cn/xxx/xxx").getPath();//地址表词库存放在那个包下面
File files[] =  new File(path).listFiles();
for(File file : files){
if(!file.getName().endsWith(".txt")){//表示词库的后缀名文件
continue;
}
BufferedReader br = new BufferedReader(new FileReader(file));
String line = null;
while((line=br.readLine())!=null){
String s[] = line.split("\\|");
if(s.length!=2){
continue;
}
if(s[1].trim().equals("1")){
banWords.add(s[0].trim());
}

if(s[1].trim().equals("2")){
auditWords.add(s[0].trim());
}

if(s[1].trim().equals("3")){
replaceWords.add(s[0].trim());
}
}
}
System.out.println("haha");
}catch (Exception e) {
throw new RuntimeException(e);
}

}

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) resp;

//检查提交数据是否包含禁用词
Enumeration e = request.getParameterNames();
while(e.hasMoreElements()){
String name = (String) e.nextElement();
String data = request.getParameter(name);
for(String regex : banWords){
Pattern pattern = Pattern.compile(regex);
Matcher m = pattern.matcher(data);
if(m.find()){
request.setAttribute("message", "文章中包括非法词汇，请检查后再提交！！");
request.getRequestDispatcher("/message.jsp").forward(request, response);
return;
}
}
}

//检查审核词

//检查替换词
chain.doFilter(new MyRequest(request), response);

}

class MyRequest extends HttpServletRequestWrapper{
private HttpServletRequest request;
public MyRequest(HttpServletRequest request) {
super(request);
this.request = request;
}
@Override
public String getParameter(String name) {

String data = this.request.getParameter(name);
if(data==null){
return null;
}
for(String regex : auditWords){//检查审核词
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(data);
if(m.find()){
String value = m.group();  //找出客户机提交的数据中和正则表达式相匹配的数据
data = data.replaceAll(regex, "<font color='red'>" + value + "</font>");
}
}

for(String regex : replaceWords){//检查替换词
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(data);
if(m.find()){
data = data.replaceAll(regex, "*******");
}
}

return data;
}
}