Servlet3.1学习(三)

Filter

过滤器(Filter)能够修改HTTP请求的内容、响应、Header等信息，过滤器能够包装请求、响应，好比防止XSS攻击等，过滤器一样也能够拦截不安全的请求，好比防止CSRF攻击等等。

生命周期

Filter生命周期与Servlet生命周期相似，init()初始化Filter、destory()在销毁时调用、doFilter()负责处理过滤响应和请求。

包装响应、请求 Filter最核心的概念就是包装请求或响应，以便它能够执行新的行为。Servlet提供HttpServletRequestWrapper、HttpServletResponseWrapper对象进行包装请求和响应，使用时直接继承便可。

下面是防止XSS攻击，进行请求包装

private static class XssHttpServletRequestWrapper extends HttpServletRequestWrapper {

    XssHttpServletRequestWrapper(HttpServletRequest servletRequest) {
      super(servletRequest);
    }

    @Override
    public String getHeader(String name) {
      return HtmlUtils.htmlEscape(super.getHeader(name));
    }

    @Override
    public String getQueryString() {
      return HtmlUtils.htmlEscape(super.getQueryString());
    }

    @Override
    public String getParameter(String parameter) {
      return HtmlUtils.htmlEscape(super.getParameter(parameter));
    }

    @Override
    public String[] getParameterValues(String parameter) {
      String[] values = super.getParameterValues(parameter);
      if (values == null) {
        return null;
      }
      for (int i = 0; i < values.length; i++) {
        values[i] = HtmlUtils.htmlEscape(values[i]);
      }
      return values;
    }

    @Override
    public Map<String, String[]> getParameterMap() {
      Map<String, String[]> paramMap = new HashMap<>(super.getParameterMap());
      for (Map.Entry<String, String[]> entry : paramMap.entrySet()) {
        String[] values = entry.getValue();
        String[] after = new String[values.length];
        int index = 0;
        for (String value : values) {
          after[index++] = HtmlUtils.htmlEscape(value);
        }
        entry.setValue(after);
      }
      return paramMap;
    }
  }
}

Spring Session就是使用Wrapper把获取Session的API进行包装，部分代码以下：

public class SessionRepositoryFilter<S extends ExpiringSession> extends OncePerRequestFilter {
	@Override
	protected void doFilterInternal(HttpServletRequest request,
			HttpServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {
		request.setAttribute(SESSION_REPOSITORY_ATTR, this.sessionRepository);

		SessionRepositoryRequestWrapper wrappedRequest = new SessionRepositoryRequestWrapper(
				request, response, this.servletContext);
		SessionRepositoryResponseWrapper wrappedResponse = new SessionRepositoryResponseWrapper(
				wrappedRequest, response);

		HttpServletRequest strategyRequest = this.httpSessionStrategy
				.wrapRequest(wrappedRequest, wrappedResponse);
		HttpServletResponse strategyResponse = this.httpSessionStrategy
				.wrapResponse(wrappedRequest, wrappedResponse);

		try {
			filterChain.doFilter(strategyRequest, strategyResponse);
		}
		finally {
			wrappedRequest.commitSession();
		}
	}
}

Filter和RequestDispatcher

从Servlet2.4以后咱们可用使用forward()和include()进行请求分派，一样Filter一样能够拦截分派的请求。

在配置Filter-Mapping时有<dispatcher>元素，指定该Filter拦截那种请求

• REQUEST：拦截客户端请求，Filter-Mapping默认就是该类型
• FORWARD：拦截forward()分派请求
• INCLUDE：拦截include()分派请求
• ASYNC：拦截异步请求
• ERROR：拦截错误请求

配置以下

<filter>
    <filter-name>corsFilter</filter-name>
    <filter-class>com.kanyuxia.servlet.chapter.filter.CrossOriginFilter</filer-class>
</filter>
<filter-mapping>
    <filter-name>corsFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>FORWARD</dispatcher>
</filter-mapping>

Listener

事件监听器可以控制ServletContext、HttpSession和ServletRequest的生命周期相关的活动

监听器接口	监听器事件
ServletContextListener	ServletContextEvent
ServletContextAttributeListener	ServletContextAttributeEvent
HttpSessionListener	HttpSessionEvent
HttpSessionAttributeListener	HttpSessionBindingEvent
HttpSessionIdListener	HttpSessionEvent
HttpSessionActivationListener	HttpSessionEvent
HttpSessionBindingListener	HttpSessionBindingEvent
ServletRequestListener	ServletRequestEvent
ServletRequestAttributeListener	ServletRequestAttributeEvent
AsyncListener	AsyncEvent

监听器的常见应用于其控制的相关对象的生命周期，咱们能够基于此让全部请求入库

@WebListener
public class AccessManager implements ServletRequestListener {
  @Override
    public void requestInitialized(ServletRequestEvent requestEvent) {
        ServletContext context = requestEvent.getServletContext();
        ConnectionPool connectionPool = (ConnectionPool) context.getAttribute(ConnectionManager.CONNECTION_POOL_NAME);
        HttpServletRequest request = (HttpServletRequest)requestEvent.getServletRequest();
        recordAccessLog(connectionPool, request);
    }

    @Override
    public void requestDestroyed(ServletRequestEvent requestEvent) {}
  
    private void recordAccessLog(ConnectionPool connectionPool, HttpServletRequest request) {
        // 省略部分代码逻辑
    }
}

Cookie和Session

因为HTTP是无状态的基于请求/响应模式的协议。在构建有效的Web应用，必须与来自特定客户端的请求彼此相互关联，就是会话跟踪机制。会话跟踪机制有cookie-session、无状态的JWT、token-session机制，这里主要说的是cookie-session会话机制。

Cookie和Session在Servlet中如何使用就不说了，这里主要说一下本身在应用过程当中遇到的问题

• Cookie的domain：Cookie中的domain指的是该cookie在该domain(域名或IP地址)下有效，在浏览器中只能看到domian下的cookie。
• Cookie的http-only：Cookie中的Http-Only选项指的是该Cookie是否只能Http请求使用，主要是防止CSRF攻击。
• 分布式下的Session：因为Session表明用户，因此每一个用户应该有惟一的Session。通常状况下，分布式环境下集中存放Session，例如使用Redis集中存放Session，因此就出现了Spring Session进行集中式存放Session。

映射到Servlet

Servlet容器在接受到HTTP请求后，须要选择合适的Servlet处理该请求。选择的Servlet根据URL匹配==最长==上下文路径的Servlet，其中"*"表明匹配任意字符串，最后咱们发现若是"/"会匹配任意的请求。"/"表明"default"的Servlet，Servlet容器会自动注入一个匹配路径为"/"的默认的Servlet，处理静态文件获取、404错误等等。

这是Tomcat9.0.2注入的默认Servlet部分代码

public class DefaultServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request,
        HttpServletResponse response) throws IOException, ServletException {
        // Serve the requested resource, including the data content
        serveResource(request, response, true, fileEncoding);
    }
    
    /**
     * Serve the specified resource, optionally including the data content.
     */
    protected void serveResource(HttpServletRequest request,
        HttpServletResponse response, boolean content,
        String inputEncoding) throws IOException, ServletException {
        // 省略代码
    }
}