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Servlet Filter过滤器 Servlet Filter过滤器执行顺序

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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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