Servlet中的过滤器相当于守护后台资源的一道关卡,我们可以在过滤器中进行身份校验、权限认证、请求过滤等。
过滤器本身并不难,我们只需要知道他的定义方法、作用范围、执行顺序即可。
网上对于过滤器执行顺序的描述可能会让人产生误解。
图片来源于网络
客户端请求到达的时候,经过一次过滤器。
服务器处理完请求的时候,经过一次过滤器。
虽然经过两次过滤器,但不代表同样的代码执行了两次。
下面做了个简单的测试,看下执行结果就应该知道真正的执行流程了。
测试环境
tomcat9(servlet4.0)
jdk1.8
新版servlet可以通过注解注册servlet组件以及过滤器,无需再到web.xml下注册了。
测试过程
测试之间要先知道filterChain(过滤链)是干嘛的。
一个过滤器处理完后,会把request和response对象通过filterchain传递给下一个过滤器,如果没有下一个过滤器,则会直接开始执行业务代码,
单个过滤器
定义一个过滤器A
@WebFilter(value = "/*", filterName="A") public class FilterA implements Filter { @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(format.format(new Date())); System.out.println("A:拦截1"); chain.doFilter(request, response); System.out.println(format.format(new Date())); System.out.println("A:拦截2"); } @Override public void init(FilterConfig filterConfig) throws ServletException { } @Override public void destroy() { } }
定义一个servlet,sleep5秒
@WebServlet("/mainUrl") public class MainController extends HttpServlet { private static final long serialVersionUID = 1L; /** * @see HttpServlet#HttpServlet() */ public MainController() { super(); // TODO Auto-generated constructor stub } /** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */ protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub try { Thread.sleep(5000); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } /** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */ protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub doGet(request, response); } }
运行结果
2020-12-01 10:46:50 A:拦截1 2020-12-01 10:46:55 A:拦截2
执行顺序:
filterChain之前的代码 ——>业务处理——>filterChain之后的代码。
多个过滤器
servlet的注解在多个过滤器的情况下,是按照过滤器的名称来排序的,例如A开头的过滤器,在B开头的后面。
A过滤器
@WebFilter(value = "/*", filterName="A") public class FilterA implements Filter { @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(format.format(new Date())); System.out.println("A:拦截1"); chain.doFilter(request, response); System.out.println(format.format(new Date())); System.out.println("A:拦截2"); } @Override public void init(FilterConfig filterConfig) throws ServletException { } @Override public void destroy() { } }
B过滤器
@WebFilter(value = "/*", filterName="B") public class FilterB implements Filter{ @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // TODO Auto-generated method stub SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(format.format(new Date())); System.out.println("B:拦截1"); chain.doFilter(request, response); System.out.println(format.format(new Date())); response.setContentType("normal content"); System.out.println("B:拦截2"); } @Override public void init(FilterConfig filterConfig) throws ServletException { } @Override public void destroy() { } }
运行结果:
2020-12-01 10:53:00 A:拦截1 2020-12-01 10:53:00 B:拦截1 2020-12-01 10:53:05 B:拦截2 2020-12-01 10:53:05 A:拦截2
执行顺序:
B:拦截1和B:拦截2之间,停顿了5秒处理业务。所以先执行了 chain.doFilter前的 A、B过滤器代码,处理完业务返回的时候正好相反,先返回执行B的代码,再执行的A的代码。
总结
再来看这个图,可以略微改一下了。
分界线是filterChain过滤链,请求进来的时候,执行filterchain之前的代码,返回response的时候,执行filterchain之后的代码。
多个过滤器之间的执行顺序,满足“先进后出” (栈结构)的原则。
其他
如果在测试过程中,发现过滤器执行了很多次,那么也可能是因为测试环境中包含了某些静态资源。
过滤器A
@WebFilter(value = "/*", filterName="A") public class FilterA implements Filter { @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(format.format(new Date())); System.out.println("A:拦截1"); chain.doFilter(request, response); System.out.println(format.format(new Date())); System.out.println("A:拦截2"); } @Override public void init(FilterConfig filterConfig) throws ServletException { } @Override public void destroy() { } }
过滤器B
@WebFilter(value = "/*", filterName="B") public class FilterB implements Filter{ @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // TODO Auto-generated method stub HttpServletRequest req = (HttpServletRequest) request; SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); System.out.println(req.getRequestURL()); System.out.println(format.format(new Date())); System.out.println("B:拦截1"); chain.doFilter(request, response); System.out.println(format.format(new Date())); response.setContentType("normal content"); System.out.println("B:拦截2"); } @Override public void init(FilterConfig filterConfig) throws ServletException { } @Override public void destroy() { } }
主程序
@WebServlet("/mainUrl") public class MainController extends HttpServlet { private static final long serialVersionUID = 1L; /** * @see HttpServlet#HttpServlet() */ public MainController() { super(); // TODO Auto-generated constructor stub } /** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */ protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub try { Thread.sleep(5000); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } request.getRequestDispatcher("/WEB-INF/pages/main.jsp").forward(request, response); // response.sendRedirect("/WEB-INF/pages/main.jsp"); } /** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */ protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub doGet(request, response); } }
执行结果:
2020-12-01 11:09:38 A:拦截1 http://localhost:8080/StudentManage/mainUrl 2020-12-01 11:09:38 B:拦截1 2020-12-01 11:09:43 B:拦截2 2020-12-01 11:09:43 A:拦截2 2020-12-01 11:09:44 A:拦截1 http://localhost:8080/StudentManage/css/bootstrap.css.map 2020-12-01 11:09:44 B:拦截1 2020-12-01 11:09:44 B:拦截2 2020-12-01 11:09:44 A:拦截2
转发(forward)的页面中需要请求静态资源,再次触发了过滤器。
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