Servlet Filter

1. Filter 是什麼 

①. JavaWEB 的一個重要組件, 可以對發送到 Servlet 的請求進行攔截, 並對響應也進行攔截. 
②. Filter 是實現了 Filter 接口的 Java 類.
③. Filter 需要在 web.xml 文件中進行配置和映射.

創建一個 Filter

①. 創建一個 Filter 類: 實現 Filter 接口: public class HelloFilter implements Filter
②. 在 web.xml 文件中配置並映射該 Filter. 其中 url-pattern 指定該 Filter 可以攔截哪些資源, 即可以通過哪些 url 訪問到該 Filter

public class HelloFilter implements Filter {

	public void init(FilterConfig fConfig) throws ServletException {
		System.out.println("init ....");
	}

	public void destroy() {
		System.out.println("destroy .....");
	}

	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("doFilter");
		chain.doFilter(request, response);
	}

}

web.xml配置

 <!-- 註冊Filter -->
	<filter>
		<filter-name>helloFilter</filter-name>
		<filter-class>com.filter.HelloFilter</filter-class>
	</filter>
	<!-- 映射Filter -->
	<filter-mapping>
		<filter-name>helloFilter</filter-name>
		<url-pattern>/test.jsp</url-pattern>
	</filter-mapping>

2.Filter生命週期

服務器啓動，控制檯打印：init ....

每次訪問/test.jsp ,都打印：doFilter

服務器停止， 打印：destroy .....

所以

過濾器的生命週期：（一定要實現javax.servlet包的Filter接口的三個方法init()、doFilter()、destroy()，空實現也行） 
(1)、啓動服務器時加載過濾器的實例，並調用init()方法來初始化實例； 
(2)、每一次請求時都只調用方法doFilter()進行處理； 
(3)、停止服務器時調用destroy()方法，銷燬實例。

3.Filter 相關的 API

public void init(FilterConfig filterConfig): 類似於 Servlet 的 init 方法. 在創建 Filter 對象(Filter 對象在 Servlet 容器加載當前 WEB 應用時即被創建)後, 
立即被調用, 且只被調用一次. 該方法用於對當前的 Filter 進行初始化操作. Filter 實例是單例的.

*  FilterConfig 類似於 ServletConfig

* 可以在 web.xml 文件中配置當前 Filter 的初始化參數. 配置方式也和 Servlet 類似。

	<filter>
		<filter-name>helloFilter</filter-name>
		<filter-class>com.filter.HelloFilter</filter-class>
		<init-param>
		   <param-name>name</param-name>
		   <param-value>root</param-value>
		</init-param>
	</filter>

public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain):

 真正 Filter 的邏輯代碼需要編寫在該方法中. 每次攔截都會調用該方法.

* FilterChain: Filter 鏈. 多個 Filter 可以構成一個 Filter 鏈.
      - doFilter(ServletRequest request, ServletResponse response): 把請求傳給 Filter 鏈的下一個 Filter,
若當前 Filter 是 Filter 鏈的最後一個 Filter, 將把請求給到目標 Serlvet(或 JSP)  

                        - 多個 Filter 攔截的順序和 <filter-mapping> 配置的順序有關, 靠前的先被調用.

public class HelloFilter implements Filter {

	public void init(FilterConfig fConfig) throws ServletException {
	}

	public void destroy() {
	}

	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("1.Before HelloFilter's chain.doFilter");
		chain.doFilter(request, response);
		System.out.println("2.After HelloFilter's chain.doFilter");
	}

}

public class SecondFilter implements Filter {

	public void init(FilterConfig fConfig) throws ServletException {
	}

	public void destroy() {
	}


	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
		System.out.println("3.Before HelloFilter's chain.doFilter");
		chain.doFilter(request, response);
		System.out.println("4.After HelloFilter's chain.doFilter");
	}
}

  <filter>
    <filter-name>helloFilter</filter-name>
    <filter-class>com.filter.HelloFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>helloFilter</filter-name>
    <url-pattern>/test.jsp</url-pattern>
  </filter-mapping>
  
  <filter>
     <filter-name>secondFilter</filter-name>
     <filter-class>com.filter.SecondFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>secondFilter</filter-name>
    <url-pattern>/test.jsp</url-pattern>
  </filter-mapping>

訪問/test.jsp 打印結果 執行順序：

1.Before HelloFilter's chain.doFilter
3.Before SecondFilter's chain.doFilter
5 .Test JSP
4.After SecondFilter's chain.doFilter
2.After HelloFilter's chain.doFilter

public void destroy(): 釋放當前 Filter 所佔用的資源的方法. 在 Filter 被銷燬之前被調用, 且只被調用一次.

4. <dispatcher> 元素: 指定過濾器所攔截的資源被 Servlet 容器調用的方式，
可以是REQUEST,INCLUDE,FORWARD和ERROR之一，默認REQUEST. 
可以設置多個<dispatcher> 子元素用來指定 Filter 對資源的多種調用方式進行攔截

①. REQUEST：當用戶直接訪問頁面時，Web容器將會調用過濾器。

如果目標資源是通過RequestDispatcher的include()或forward()方法訪問時，那麼該過濾器就不會被調用。
通過 GET 或 POST 請求直接訪問。

request.getRequestDispatcher("/test.jsp").forward(request, response);

<jsp:forward page="/test.jsp"></jsp:forward>

<jsp:include page="/test.jsp"></jsp:include>

上面方式訪問/test.jsp Filter 不會攔截

②. FORWARD：如果目標資源是通過RequestDispatcher的forward()方法訪問時，那麼該過濾器將被調用，除此之外，該過濾器不會被調用。

或 <jsp:forward page="/..." /> 或 通過 page 指令的 errorPage 轉發頁面. <%@ page errorPage="test.jsp" %>

<filter-mapping>
    <filter-name>secondFilter</filter-name>
    <url-pattern>/test.jsp</url-pattern>
    <dispatcher>FORWARD</dispatcher>
  </filter-mapping>

②. INCLUDE：如果目標資源是通過RequestDispatcher的include()方法訪問時，那麼該過濾器將被調用。除此之外，該過濾器不會被調用。

或 <jsp:include file="/..." />

<filter-mapping>
    <filter-name>secondFilter</filter-name>
    <url-pattern>/test.jsp</url-pattern>
    <dispatcher>INCLUDE</dispatcher>
  </filter-mapping>

④. ERROR：如果目標資源是通過聲明式異常處理機制調用時，那麼該過濾器將被調用。除此之外，過濾器不會被調用。

在 web.xml 文件中通過 error-page 節點進行聲明:

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

<filter-mapping>
    <filter-name>secondFilter</filter-name>
    <url-pattern>/test.jsp</url-pattern>
    <dispatcher>INCLUDE</dispatcher>
  </filter-mapping>

四種方式都攔截可以配置

<filter-mapping>
	<filter-name>secondFilter</filter-name>
	<url-pattern>/test.jsp</url-pattern>
	<dispatcher>REQUEST</dispatcher>
	<dispatcher>FORWARD</dispatcher>
	<dispatcher>INCLUDE</dispatcher>
	<dispatcher>ERROR</dispatcher>
</filter-mapping>
	

5.Fitler應用

編碼過濾 

public class EncodingFilter implements Filter {
  
	private String encoding;
	
    public void init(FilterConfig fConfig) throws ServletException {
    	encoding = fConfig.getServletContext().getInitParameter("encoding");
	}
   
	public void destroy() {
		// TODO Auto-generated method stub
	}


	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println(encoding);
        request.setCharacterEncoding(encoding);//統一編碼
		chain.doFilter(request, response);
	}
	

}

<context-param>
     <param-name>encoding</param-name>
     <param-value>UTF-8</param-value>
  </context-param>
  
   <filter>
    <filter-name>encodingFilter</filter-name>
    <filter-class>com.filter.EncodingFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>encodingFilter</filter-name>
    <url-pattern>/encoding/*</url-pattern>
  </filter-mapping>

<%--
		//編寫一個 EncodingFilter
		//1. 讀取 web.xml 文件中配置的當前 WEB 應用的初始化參數 encoding
		//2. 指定請求的字符編碼爲 1 讀取到的編碼
		//3. 調用 chain.doFilter() 方法 "放行" 請求
		request.setCharacterEncoding("UTF-8");
	--%>
	
	Hello: ${param.name }

過濾器的應用---登錄攔截

public class LoginFilter implements Filter{
	
	//1.從web.xml 文件中獲取userSessionKey，rediretPage，uncheckedUrls
	private String sessionKey;
	private String redirectUrl;
	private String uncheckedUrls;

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		ServletContext servletContext = filterConfig.getServletContext();
		sessionKey = servletContext.getInitParameter("userSessionKey");
		redirectUrl = servletContext.getInitParameter("rediretPage");
		uncheckedUrls = servletContext.getInitParameter("uncheckedUrls");
	}
	@Override
	public void destroy() {	
	}

	@Override
	public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterChain)
			throws IOException, ServletException {
		HttpServletRequest request = (HttpServletRequest) req;
		HttpServletResponse response = (HttpServletResponse) resp;
		
		//1.獲取請求的 servletPath
		String servletPath = request.getServletPath();
		
		//2.檢查 1 獲取的servletPath 不需要檢查URL中的其中一個，若是，放行，方法結束
		List<String> urls = Arrays.asList(uncheckedUrls.split(","));
		if(urls.contains(servletPath)){
			filterChain.doFilter(req, resp);
			return;
		}
		
		//3. 從 session 中獲取 sessionKey 對應的值, 若值不存在, 則重定向到 redirectUrl
		Object user = request.getSession().getAttribute(sessionKey);
		if(user==null){
			response.sendRedirect(request.getContextPath()+redirectUrl);
			return;
		}
		
		//4.若存在，放行
		filterChain.doFilter(req, resp);
		
		
	}

	

}

<!-- 用戶信息放入到 session 中的鍵的名字 -->
  <context-param>
  	<param-name>userSessionKey</param-name>
  	<param-value>USERSESSIONKEY</param-value>
  </context-param>
  
  <!-- 若未登錄, 需重定向的頁面 -->
  <context-param>
  	<param-name>rediretPage</param-name>
  	<param-value>/login/login.jsp</param-value>
  </context-param>
  
  <!-- 不需要攔截(或檢查)的 URL 列表 -->
  <context-param>
  	<param-name>uncheckedUrls</param-name>
  	<param-value>/login/a.jsp,/login/list.jsp,/login/login.jsp,/login/doLogin.jsp,/login/b.jsp</param-value>
  </context-param>
  
  
   <filter>
    <filter-name>LoginFilter</filter-name>
    <filter-class>com.filter.LoginFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>LoginFilter</filter-name>
    <url-pattern>/login/*</url-pattern>
  </filter-mapping>

設置沒有緩存的過濾器

public class NoCacheFilter extends HttpFilter {

	@Override
	public void doFilter(HttpServletRequest request,
			HttpServletResponse response, FilterChain filterChain)
			throws IOException, ServletException {
		
		System.out.println("cacheFilter's doFilter..");
		
		response.setDateHeader("Expires",-1);
		response.setHeader("Cache-Control","no-cache");
		response.setHeader("Pragma","no-cache");
		
		filterChain.doFilter(request, response);
	}

    

}