JavaWeb学习笔记（二十二）—— 过滤器filter
一、什么是过滤器
过滤器filter是JavaWeb三大组件之一，它与Servlet很相似！不过过滤器是用来拦截请求的，而不是处理请求的。WEB开发人员通过Filter技术，对web服务器管理的所有web资源：例如Jsp, Servlet, 静态图片文件或静态 html 文件等进行拦截，从而实现一些特殊的功能。例如实现URL级别的权限访问控制、过滤敏感词汇、压缩响应信息等一些高级功能。
Servlet API中提供了一个Filter接口，开发web应用时，如果编写的Java类实现了这个接口，则把这个java类称之为过滤器Filter。通过Filter技术，开发人员可以实现用户在访问某个目标资源之前，对访问的请求和响应进行拦截，如下所示：
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" alt="" />
二、Filter是如何实现拦截的？
Filter接口中有一个doFilter方法，当我们编写好Filter，并配置对哪个web资源进行拦截后，WEB服务器每次在调用web资源的service方法之前，都会先调用一下filter的doFilter方法，因此，在该方法内编写代码可达到如下目的：
- 调用目标资源之前，让一段代码执行。
- 是否调用目标资源（即是否让用户访问web资源）。
- 调用目标资源之后，让一段代码执行。
web服务器在调用doFilter方法时，会传递一个filterChain对象进来，filterChain对象是filter接口中最重要的一个对 象，它也提供了一个doFilter方法，开发人员可以根据需求决定是否调用此方法，调用该方法，则web服务器就会调用web资源的service方 法，即web资源就会被访问，否则web资源不会被访问。
三、Filter开发入门
第一步：写一个类实现Filter接口
public class HelloFilter implements Filter {
/**
* 创建之后马上执行,用来做初始化
*/
@Override
public void init(FilterConfig filterConfig) throws ServletException {
}
/**
* 每次过滤时都会执行
*/
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
System.out.println("AFilter#start");
chain.doFilter(request, response);
System.out.println("AFilter#end");
}
/**
* 销毁之前执行,用来对非内存资源进行释放
*/
@Override
public void destroy() {
}
}
第二步:在web.xml中进行配置
<filter>
<filter-name>helloFilter</filter-name>
<filter-class>cn.itcast.filter.HelloFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>helloFilter</filter-name>
<!--指定要拦截的路径,当用户访问index.jsp页面时,HelloFilter就会被执行-->
<url-pattern>/index.jsp</url-pattern>
</filter-mapping>
四、Filter的生命周期
【Filter的创建】——init(FilterConfig)：
Filter的创建和销毁由WEB服务器负责。 web 应用程序启动时，web 服务器将创建Filter 的实例对象，并调用其init方法，完成对象的初始化功能，从而为后续的用户请求作好拦截的准备工作，filter对象只会创建一次，init方法也只会执行一次。通过init方法的参数，可获得代表当前filter配置信息的FilterConfig对象。
【Filter的销毁】——destroy()
服务器会在创建Filter对象之后，把Filter放到缓存中一直使用，通常不会销毁它。一般会在服务器关闭时销毁Filter对象，在销毁Filter对象之前，服务器会调用Filter对象的destory()方法。
【doFilter方法】
doFilter(ServletRequest req,ServletResponse res,FilterChain chain)：这个方法会在用户每次访问“目标资源（<url->pattern>index.jsp</url-pattern>）”时执行，如果需要“放行”，那么需要调用FilterChain的doFilter(ServletRequest,ServletResponse)方法，如果不调用FilterChain的doFilter()方法，那么目标资源将无法执行；
五、FilterConfig
Filter接口中的init()方法的参数类型为FilterConfig类型。它的功能与ServletConfig相似，与web.xml文件中的配置信息对应。下面是FilterConfig的功能介绍：
- ServletContext getServletContext()：获取ServletContext的方法；
- String getFilterName()：获取Filter的配置名称；与<filter-name>元素对应；
- String getInitParameter(String name)：获取Filter的初始化配置，与<init-param>元素对应；
- Enumeration getInitParameterNames()：获取所有初始化参数的名称。
六、FilterChain
doFilter()方法的参数中有一个类型为FilterChain的参数，它只有一个方法：doFilter(ServletRequest,ServletResponse)。
前面我们说doFilter()方法的放行，让请求流访问目标资源！但这么说不严密，其实调用该方法的意思是，“我（当前Filter）”放行了，但不代表其他人（其他过滤器）也放行。
也就是说，一个目标资源上，可能部署了多个过滤器，就好比在你去北京的路上有多个打劫的匪人（过滤器），而其中第一伙匪人放行了，但不代表第二伙匪人也放行了，所以调用FilterChain类的doFilter()方法表示的是执行下一个过滤器的doFilter()方法，或者是执行目标资源！
如果当前过滤器是最后一个过滤器，那么调用chain.doFilter()方法表示执行目标资源；如果不是最后一个过滤器，那么chain.doFilter()表示执行下一个过滤器的doFilter()方法。
七、多个过滤器的执行顺序
一个目标资源可以指定多个过滤器，过滤器的执行顺序是在web.xml文件中的部署顺序。
MyFilter1：
public class MyFilter1 extends HttpFilter {
public void doFilter(HttpServletRequest request, HttpServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("filter1 start...");
chain.doFilter(request, response);//放行，执行MyFilter2的doFilter()方法
System.out.println("filter1 end...");
}
}
MyFilter2：
public class MyFilter2 extends HttpFilter {
public void doFilter(HttpServletRequest request, HttpServletResponse response,
FilterChain chain) throws IOException, ServletException {
System.out.println("filter2 start...");
chain.doFilter(request, response);//放行，执行目标资源
System.out.println("filter2 end...");
}
}
web.xml
<filter>
<!--因为MyFilter1配置在前面，所以先执行MyFilter1的doFilter()方法。-->
<filter-name>myFilter1 </filter-name>
<filter-class>cn.itcast.filter.MyFilter1</filter-class>
</filter>
<filter-mapping>
<filter-name>myFilter1</filter-name>
<url-pattern>/index.jsp</url-pattern>
</filter-mapping>
<filter>
<filter-name>myFilter2</filter-name>
<filter-class>cn.itcast.filter.MyFilter2</filter-class>
</filter>
<filter-mapping>
<filter-name>myFilter2</filter-name>
<url-pattern>/index.jsp</url-pattern>
</filter-mapping>
index.jsp
<body>
This is my JSP page. <br>
<h1>index.jsp</h1>
<%System.out.println("index.jsp"); %>
</body>
当有用户访问index.jsp页面时，输出结果如下：
filter1 start...
filter2 start...
index.jsp
filter2 end...
filter1 end...
八、四种拦截方式
我们来做个测试，写一个过滤器，指定过滤的资源为b.jsp，然后我们在浏览器中直接访问b.jsp，你会发现过滤器执行了！
但是，当我们在a.jsp中request.getRequestDispathcer(“/b.jsp”).forward(request,response)时，就不会再执行过滤器了！也就是说，默认情况下，只能直接访问目标资源才会执行过滤器，而forward执行目标资源，不会执行过滤器！
其实过滤器有四种拦截方式！分别是：REQUEST、FORWARD、INCLUDE、ERROR。
- REQUEST（默认）：直接访问目标资源时执行过滤器。包括：在地址栏中直接访问、表单提交、超链接、重定向，只要在地址栏中可以看到目标资源的路径，就是REQUEST；
- FORWARD：转发访问执行过滤器。包括RequestDispatcher#forward()方法、<jsp:forward>标签都是转发访问；
- INCLUDE：包含访问执行过滤器。包括RequestDispatcher#include()方法、<jsp:include>标签都是包含访问；
- ERROR：当目标资源在web.xml中配置为<error-page>中时，并且真的出现了异常，转发到目标资源时，会执行过滤器。
可以在<filter-mapping>中添加0~n个<dispatcher>子元素，来说明当前访问的拦截方式。
<filter-mapping>
<filter-name>myfilter</filter-name>
<url-pattern>/b.jsp</url-pattern>
<!--b.jsp为目标资源，当直接请求b.jsp时，会执行过滤器-->
<dispatcher>REQUEST</dispatcher>
<!--当转发到b.jsp页面时，会执行过滤器-->
<dispatcher>FORWARD</dispatcher>
</filter-mapping> <!--当没有给出拦截方式时，那么默认为REQUEST-->
<filter-mapping>
<filter-name>myfilter</filter-name>
<url-pattern>/b.jsp</url-pattern>
</filter-mapping> <filter-mapping>
<filter-name>myfilter</filter-name>
<url-pattern>/b.jsp</url-pattern>
<!--当转发到b.jsp页面时，会执行过滤器！
因为已经给出了<dispatcher>FORWARD</dispatcher>了，
那么就没有默认的REQUEST了！所以只有在转发到b.jsp时才会执行过滤，
而转发到b.jsp时，不会执行b.jsp-->
<dispatcher>FORWARD</dispatcher>
</filter-mapping>
九、过滤器的应用场景
过滤器的应用场景：
- 执行目标资源之前做预处理工作，例如设置编码，这种试通常都会放行，只是在目标资源执行之前做一些准备工作；（
几乎是的Sevlet中都需要写request.setCharacterEndoing() 可以把它入到一个Filter中）
- 通过条件判断是否放行，例如校验当前用户是否已经登录，或者用户IP是否已经被禁用；
- 在目标资源执行后，做一些后续的特殊处理工作，例如把目标资源输出的数据进行处理；
下面通过几个具体的案例来说明过滤器的应用:
9.1 统一全站字符编码
乱码问题：
- 获取请求参数中的乱码问题；
- POST请求：request.setCharacterEncoding(“utf-8”)；
- GET请求：new String(request.getParameter(“xxx”).getBytes(“iso-8859-1”), “utf-8”)；
- 响应的乱码问题：response.setContextType(“text/html;charset=utf-8”)。
基本上在每个Servlet中都要处理乱码问题，所以应该把这个工作放到过滤器中来完成。
参考：https://www.cnblogs.com/xdp-gacl/p/3948353.html
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