JavaEE基礎(05)：過濾器、監聽器、攔截器，應用詳解

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一、Listener監聽器

1、概念簡介

JavaWeb三大元件：Servlet，Listener，Filter。監聽器就是指在應用程式中監聽相關對象狀态變化的元件。

2、事件源對象

指被監聽對象。

• ServletContext

ServletContextListener

生命周期監聽，它有兩個方法，出生時調用

contextInitialized()

，銷毀時調用

contextDestroyed()

;

ServletContextAttributeListener

屬性監聽，它有三個方法，添加屬性

attributeAdded()

，替換屬性

attributeReplaced()

，移除屬性時

attributeRemoved()

。

• HttpSession

HttpSessionListener

生命周期監聽：它有兩個方法，出生時調用

sessionCreated()

sessionDestroyed()

；

HttpSessioniAttributeListener

屬性監聽：它有三個方法，添加屬性

attributeAdded()

attributeReplaced()

，移除屬性

attributeRemoved()

• ServletRequest

ServletRequestListener

requestInitialized()

requestDestroyed()

ServletRequestAttributeListener

attributeAdded()

attributeReplaced()

attributeRemoved()

3、編碼案例

• 相關監聽器

TheContextListener

public class TheContextListener implements ServletContextListener {
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {
        System.out.println("初始化：TheContextListener");
        ServletContext servletContext = servletContextEvent.getServletContext() ;
        servletContext.setAttribute("author","cicada");
    }
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
        System.out.println("銷毀：TheContextListener");
    }
}           

TheRequestListener

public class TheRequestListener implements ServletRequestListener {
    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
        System.out.println("初始化：TheRequestListener");
    }
    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent) {
        System.out.println("銷毀：TheRequestListener");
    }
}           

TheSessionListener

public class TheSessionListener implements HttpSessionListener {
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        System.out.println("初始化：TheSessionListener");
    }
    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        System.out.println("銷毀：TheSessionListener");
    }
}           

RequestAttributeListener

public class RequestAttributeListener implements ServletRequestAttributeListener {
    @Override
    public void attributeAdded(ServletRequestAttributeEvent evt) {
        System.out.println("Request添加屬性："+evt.getName()+";"+evt.getValue());
    }
    @Override
    public void attributeRemoved(ServletRequestAttributeEvent evt) {
        System.out.println("Request移除屬性："+evt.getName()+";"+evt.getValue());
    }
    @Override
    public void attributeReplaced(ServletRequestAttributeEvent evt) {
        System.out.println("Request替換屬性："+evt.getName()+";"+evt.getValue());
    }
}           

• web.xml配置檔案

<!-- 監聽器相關配置 -->
<listener>
    <listener-class>com.node05.servlet.listener.TheContextListener</listener-class>
</listener>
<listener>
    <listener-class>com.node05.servlet.listener.TheSessionListener</listener-class>
</listener>
<listener>
    <listener-class>com.node05.servlet.listener.TheRequestListener</listener-class>
</listener>
<listener>
    <listener-class>com.node05.servlet.listener.RequestAttributeListener</listener-class>
</listener>
<session-config>
    <!-- 設定session失效時間為1分鐘 -->
    <session-timeout>1</session-timeout>
</session-config>           

• 測試接口

public class ListenerServletImpl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        // 1、擷取TheContextListener初始化資料
        ServletContext servletContext = this.getServletContext() ;
        String author = String.valueOf(servletContext.getAttribute("author")) ;
        System.out.println("TheContextListener Author:"+author);
        // 2、Request屬性設定
        request.setAttribute("mood","smile");
        request.setAttribute("mood","agitated");
        // 3、Session建立，1分鐘失效，調用銷毀
        HttpSession session = request.getSession(true) ;
        session.setAttribute("casually","casually");
        response.getWriter().print("Hello:Listener");
    }
}           

二、Filter過濾器

1、過濾器簡介

用戶端請求Servlet時，先執行相關Filter，如果Filter通過，則繼承執行請求的Servlet；如果Filter不通過，則不會執行使用者請求的Servlet。過濾器可以動态地攔截請求和響應。

2、Filter接口

Filter接口定義了三個核心方法。

• init()

應用程式啟動時，伺服器執行個體化Filter對象，并調用其init方法，讀取web.xml配置，完成對象的初始化加載。

• doFilter()

實際的過濾操作，請求達到伺服器時，Servlet容器将先調用過濾器的doFilter方法。

• destroy()

容器在銷毀過濾器前調用該方法，釋放過濾器占用的資源。

• 編寫過濾器

public class ThePrintLogFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String myName = filterConfig.getInitParameter("myName") ;
        System.out.println("myName："+myName);
    }
    @Override
    public void doFilter(ServletRequest servletRequest,
                         ServletResponse servletResponse,
                         FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest)servletRequest ;
        HttpServletResponse response = (HttpServletResponse)servletResponse ;
        String name = request.getParameter("name") ;
        if (!name.equals("cicada")){
            response.getWriter().print("User Error !");
            return ;
        }
        chain.doFilter(servletRequest,servletResponse);
    }
    @Override
    public void destroy() {
        System.out.println("ThePrintLogFilter destroy()");
    }
}           

<!-- 過濾器相關配置 -->
<filter>
    <filter-name>thePrintLogFilter</filter-name>
    <filter-class>com.node05.servlet.filter.ThePrintLogFilter</filter-class>
    <init-param>
        <param-name>myName</param-name>
        <param-value>cicada</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>thePrintLogFilter</filter-name>
    <url-pattern>/filterServletImpl</url-pattern>
</filter-mapping>           

public class FilterServletImpl extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().print("Hello:Filter");
    }
}           

三、Interceptor攔截器

Spring架構中的攔截器Interceptor類似于Servlet中的過濾器Filter，主要用于攔截使用者請求并作相應的處理。例如通過攔截器可以進行權限驗證、記錄請求資訊的日志、判斷使用者是否登入等。請求轉發不執行攔截、過濾；重定向執行攔截和過濾。

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