filter:过滤器

一、什么是过滤器及它的作用

1、过滤器的作用好比一个保安，是Servlet规范中的技术

2、用户在访问应用的资源之前或之后，可以对请求和响应做出一定的处理



二、编写过滤器的步骤

1、编写一个类，实现javax.servlet.Filter接口

public class FilterDemo1 implements Filter {
public FilterDemo1(){
System.out.println("调用了FIlterDemo1的默认构造方法");
}
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("调用了FIlterDemo1的init方法");
}
//每次访问都会经过该方法
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("FilterDemo1过滤器了");
chain.doFilter(request, response);//让目标资源执行。放行
}

public void destroy() {
System.out.println("调用了FIlterDemo1的销毁方法");
}

}

2、配置过滤哪些资源

修改web.xml,配置要过滤器的资源的映射地址

<servlet>
<servlet-name>ServletDemo1</servlet-name>
<servlet-class>com.itheima.servlet.ServletDemo1</servlet-class>
</servlet>

<filter>
<!-- 定义过滤器 -->
<filter-name>FilterDemo1</filter-name>
<filter-class>com.itheima.filter.FilterDemo1</filter-class>
</filter>

<filter-mapping>
<filter-name>FilterDemo1</filter-name>
<!-- 过滤本应用中的所有资源 -->
<url-pattern>/*</url-pattern>
</filter-mapping>

3、让目标资源执行，放行

//每次访问都会经过该方法
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("FilterDemo1过滤器了");
chain.doFilter(request, response);//让目标资源执行。放行
}

三、过滤器的生命周期

1、过滤器的实例由服务器创建的，其中的方法也是由服务器调用的

2、过滤器在应用被加载时实例化并初始化（调用init方法），只有一次；

用户每次访问过滤范围之内的资源，过滤器的doFIlter方法都会执行；应用被卸载或服务器停止时会调用销毁方法（destroy）。

3、特点：过滤器中的实例变量，注意线程安全问题

四、过滤器的过滤顺序



总结：多个过滤器过滤相同的资源，过滤顺序按照web.xml中filter-mapping的出现顺序依次进行过滤的。

Servlet VS. Filter

1、Servlet能做的过滤器都能做；过滤器比Servlet多了一个是否放行的功能。

2、Servlet在开发中是作为控制器来用的；Filter也可以作为控制器来用。

3、Struts1框架：Servlet作为控制器

Struts2框架：FIlter作为控制器（Struts2框架）

过滤器的简单案例

<filter>
<filter-name>SetAllCharacterEncodingFilter</filter-name>
<filter-class>com.itheima.filter.SetAllCharacterEncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>UTF-8</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>SetAllCharacterEncodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

//解决GET和POST请求参数和响应输出的编码过滤器
public class SetAllCharacterEncodingFilter implements Filter {
private FilterConfig filterConfig;
public void init(FilterConfig filterConfig) throws ServletException {
this.filterConfig = filterConfig;
}

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {
HttpServletRequest request;
HttpServletResponse response;
try{
request = (HttpServletRequest)req;
response = (HttpServletResponse)resp;
}catch(Exception e){
throw new RuntimeException("non-http request");
}
String encoding = "UTF-8";//默认参数
String value = filterConfig.getInitParameter("encoding");
if(value!=null){
encoding = value;
}

//请求时的编码
request.setCharacterEncoding(encoding);//POST请求方式
//响应时的编码
response.setCharacterEncoding(encoding);
response.setContentType("text/html;charset="+encoding);

//对request 请求进行包装，处理get提交的乱码
MyHttpServletRequest mrequest = new MyHttpServletRequest(request);
chain.doFilter(mrequest, response);
}

public void destroy() {

}

}

//装饰设计模式应用：
class MyHttpServletRequest extends HttpServletRequestWrapper{
private HttpServletRequest request;
public MyHttpServletRequest(HttpServletRequest request){
super(request);
this.request = request;
}
public String getParameter(String name) {
String value = request.getParameter(name);
if(value==null)
return value;
try {
if("get".equalsIgnoreCase(request.getMethod()))
//utf-8 进行编码
value = new String(value.getBytes("ISO-8859-1"),request.getCharacterEncoding());
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
return value;
}

}

2、动态资源不要缓存过滤器

<filter>
<filter-name>DynamicResourceNoCacheFilter</filter-name>
<filter-class>com.itheima.filter.DynamicResourceNoCacheFilter</filter-class>
</filter>

<filter-mapping>
<filter-name>DynamicResourceNoCacheFilter</filter-name>
<url-pattern>*.jsp</url-pattern>
</filter-mapping>

<filter-mapping>
<filter-name>DynamicResourceNoCacheFilter</filter-name>
<url-pattern>/servlet/*</url-pattern>
</filter-mapping>

//动态资源不要缓存
public class DynamicResourceNoCacheFilter implements Filter {

public void init(FilterConfig filterConfig) throws ServletException {

}

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

HttpServletRequest request;
HttpServletResponse response;
try{
request = (HttpServletRequest)req;
response = (HttpServletResponse)resp;
}catch(Exception e){
throw new RuntimeException("non-http request");
}

//设置响应头,不缓存
response.setHeader("Expires", "-1");
response.setHeader("Cache-Control", "no-cache");
response.setHeader("Pragma", "no-cache");
chain.doFilter(request, response);
}

public void destroy() {

}

}

3、控制静态资源的缓存时间

<filter>
<filter-name>StaticResourceNeedCache</filter-name>
<filter-class>com.itheima.filter.StaticResourceNeedCache</filter-class>
<init-param>
<param-name>html</param-name>
<param-value>1</param-value>
</init-param>
<init-param>
<param-name>css</param-name>
<param-value>2</param-value>
</init-param>
<init-param>
<param-name>js</param-name>
<param-value>3</param-value>
</init-param>
</filter>

<filter-mapping>
<filter-name>StaticResourceNeedCache</filter-name>
<url-pattern>*.html</url-pattern>
</filter-mapping>

<filter-mapping>
<filter-name>StaticResourceNeedCache</filter-name>
<url-pattern>*.css</url-pattern>
</filter-mapping>

<filter-mapping>
<filter-name>StaticResourceNeedCache</filter-name>
<url-pattern>*.js</url-pattern>
</filter-mapping>

//控制静态资源的缓存时间
public class StaticResourceNeedCache implements Filter {
private FilterConfig filterConfig;
public void init(FilterConfig filterConfig) throws ServletException {
this.filterConfig = filterConfig;
}

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

HttpServletRequest request;
HttpServletResponse response;
try{
request = (HttpServletRequest)req;
response = (HttpServletResponse)resp;
}catch(Exception e){
throw new RuntimeException("non-http request");
}
long time = 0;//缓存的时间

//针对不同的资源，获取参数，并设置缓存的时间
String uri = request.getRequestURI();//  request.getRequestURI():/day19_01_simpleFilterDemo/1.html

//截取要访问的资源的扩展名
String type = uri.substring(uri.lastIndexOf(".")+1);//   html

//获取参数，根据访问的资源类型获取时间
String htmlTime = filterConfig.getInitParameter("html");
String cssTime = filterConfig.getInitParameter("css");
String jsTime = filterConfig.getInitParameter("js");

//根据请求的不同后缀，来设置不同文件的缓存时间
if("html".equals(type)){
//访问的是html资源
time = Long.parseLong(htmlTime)*60*60*1000;
}
if("css".equals(type)){
//访问的是html资源
time = Long.parseLong(cssTime)*60*60*1000;
}
if("js".equals(type)){
//访问的是html资源
time = Long.parseLong(jsTime)*60*60*1000;
}

response.setDateHeader("Expires", System.currentTimeMillis()+time);
chain.doFilter(request, response);
}

public void destroy() {

}

}

Base64编码（很重要）

1、原理：3字节变为4字节（计算机中存储的都是字节）

2、编码原理：

1010 0101 1110 0100 1001 1111 3个字节

0010 1001 0001 1110 0001 0010 0001 1111 4个字节

每个字节表示的范围：00000000~0011111

表示的整数范围：0~63。每一个整数都对应着键盘上的一个可见字符

3、Base64是一个码表

可以把任何的二进制转变为可见字符串（A~Za~z0-9+/）

public class Base64Demo {
@Test
public void test1(){
String s = "王斐";
BASE64Encoder base64 = new BASE64Encoder();
String s1 = base64.encode(s.getBytes());//GBK
System.out.println(s1); //编码
}

//
@Test
public void test2() throws IOException{
String s = "zfXssw==";
BASE64Decoder base64 = new BASE64Decoder();
byte b[] = base64.decodeBuffer(s);
System.out.println(new String(b));//GBK //解码
}
}

public class MD5Demo {
@Test
public void test1() throws NoSuchAlgorithmException{
MessageDigest md = MessageDigest.getInstance("MD5");
byte b[] = md.digest("123".getBytes());//返回值是二进制，不一定对应着可见字符

//数据指纹
String s = new BASE64Encoder().encode(b);
System.out.println(s);
}
}

4、自动登录过滤器

//实现自动登录的过滤器
public class AutoLoginFilter implements Filter {
public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws IOException, ServletException {

HttpServletRequest request;
HttpServletResponse response;
try{
request = (HttpServletRequest)req;
response = (HttpServletResponse)resp;
}catch(Exception e){
throw new RuntimeException("non-http request");
}
//判断用户是否登录
HttpSession session = request.getSession();
User user = (User)session.getAttribute("user");
//没有登录：自动登录
if(user==null){
//从cookie中取数据：用户名和密码
Cookie cs[] = request.getCookies();
for(int i=0;cs!=null&&i<cs.length;i++){
Cookie c = cs[i];
//查询保存密码的cookie
if("loginInfo".equals(c.getName())){
//再次验证用户名和密码的正确性
String usernameBase64 = c.getValue().split("_")[0];
String passwordMD5 = c.getValue().split("_")[1];

//将cookie中的用户名解析为正常的字符
String username = Base64Util.base64decode(usernameBase64);
//从数据库查询密码
User dbUser = UserDB.findUser(username);
if(passwordMD5.equals(MD5Util.md5(dbUser.getPassword()))){
//用户名和密码都正确
session.setAttribute("user", dbUser);
}

}
}
}
//放行
chain.doFilter(request, response);
}
public void init(FilterConfig filterConfig) throws ServletException {

}

public void destroy() {

}

}

向浏览器发送自定义cookie:记录登录用户密码的cookie

public class LoginServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
String remember = request.getParameter("remember");

//查询用户
User user = UserDB.findUser(username, password);
if(user!=null){
request.getSession().setAttribute("user", user);
username = Base64Util.base64encode(username);
password = MD5Util.md5(password);

//保存cookie
Cookie cookie = new Cookie("loginInfo", username+"_"+password);//用户名_密码
cookie.setPath(request.getContextPath());//当前应用都能获取该Cookie

System.err.println("request.getContextPath()"+request.getContextPath());
//判断用户是否选择了下次自动登录功能
if(remember!=null){
//cookie的保存时间
cookie.setMaxAge(Integer.MAX_VALUE);
}else{
cookie.setMaxAge(0);
}

//将浏览器写cookie
response.addCookie(cookie);

//刷新跳转
response.getWriter().write("登录成功！2秒后转向主页");
response.setHeader("Refresh", "2;URL="+request.getContextPath());
}else{
response.getWriter().write("错误的用户名或密码");
}
}

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

}

过滤器的高级配置

<filter>
<filter-name>FilterDemo1</filter-name>
<filter-class>com.itheima.filter.FilterDemo1</filter-class>
</filter>
<filter-mapping>
<filter-name>FilterDemo1</filter-name>
<url-pattern>/*</url-pattern>

//过滤其它类型的请求配置
<dispatcher>REQUEST</dispatcher> //默认
<dispatcher>FORWARD</dispatcher>
<dispatcher>INCLUDE</dispatcher>
<dispatcher>ERROR</dispatcher> //jsp error指令的配置请求过滤
</filter-mapping>

<error-page>
<exception-type>java.lang.Exception</exception-type>
<location>/error.jsp</location>
</error-page>