<filter>
    <filter-name>myFilter</filter-name>
    <filter-class>com.MyFilter</filter-class>
    <init-param>
        <param-name>name</param-name>
        <param-value>我的Filter</param-value>
    </init-param>
    <init-param>
        <param-name>URL</param-name>
        <param-value>www.MyFilter.net</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>myFilter</filter-name>
    <url-pattern>/login</url-pattern>
    <dispatcher>REQUEST</dispatcher>
    <dispatcher>FORWARD</dispatcher>
</filter-mapping>

@WebFilter(filterName="FirstFilter",urlPatterns={"/first","*.do","*.jsp"})

@WebFilter(filterName = "ManagerFilter", urlPatterns = {"/pages/manager/*", "/manager/*", "/BookServlet"})
public class ManagerFilter implements Filter {
    public void init(FilterConfig config) throws ServletException {
    }

    public void destroy() {
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException
            , IOException {
        HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        User user = (User) httpServletRequest.getSession().getAttribute("user");
        if (user == null) {
            httpServletRequest.getRequestDispatcher("/pages/user/login.jsp").forward(request, response);
        } else if (user.getId() <= 3) {
            // 判斷為管理員ID
            System.out.println("有管理員來操作");
            chain.doFilter(request, response);
        } else {
            System.out.println("雖然登入但不是管理員");
            request.getRequestDispatcher("/pages/user/login.jsp").forward(request, response);
        }
    }
}

private static ThreadLocal<資料的類> 資料 = new ThreadLocal<>();

public class JdbcUtils {
    private static DruidDataSource dataSource;
    // 造一個ThreadLocal，裡面存的是Connection
    // 我預期讓同一個線程都用同一個conn來完成資料庫的事務操作
    // 因為事務要全部提交或全部回滾所以需要是同一個conn
    private static ThreadLocal<Connection> threadLocalConn = new ThreadLocal<>();

    // 讀取設定
    static {
        Properties properties = new Properties();
        try {
            properties.load(JdbcUtils.class.getClassLoader().getResourceAsStream("jdbc.properties"));
            dataSource = (DruidDataSource) DruidDataSourceFactory.createDataSource(properties);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    // 獲取連接，返回null說明連接失敗
    public static Connection getConnection() {
        // 從threadLocal拿連接
        Connection conn = threadLocalConn.get();
        if (conn == null) {
            // 裡面還沒存東西呢
            try {
                // 就從連接池拿一個新的連線
                conn = dataSource.getConnection();
                // 並且存到線程域中，以後都調用它
                threadLocalConn.set(conn);
                // 設置手動提交準備搞事務
                conn.setAutoCommit(false);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return conn;
    }

    /**
     * 提交事務並且關閉
     */
    public static void commitAndClose() {
        // 現在不需要把conn當作參數傳進來了，因為同一個線程我get就能取到它
        Connection conn = threadLocalConn.get();
        if (conn != null) {
            // 表示這個線程中有某dao取了連接並做了某些操作
            try {
                // 提交
                conn.commit();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            // 銷毀線程中的conn防止記憶體洩漏，
            // 雖然GC內建會做這事，但手動可以增加效率
            // 且Tomcat使用了線程池，不手動remove會直接報錯
            threadLocalConn.remove();
        }
    }

    /**
     * 回滾事務並且關閉
     */
    public static void rollbackAndClose() {
        Connection conn = threadLocalConn.get();
        if (conn != null) {
            try {
                // 回滾
                conn.rollback();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
            threadLocalConn.remove();
        }
    }

    // // 關閉連接
    // public static void close(Connection conn) {
    //     if (conn != null) {
    //         try {
    //             conn.close();
    //         } catch (SQLException e) {
    //             e.printStackTrace();
    //         }
    //     }
    //
    // }
}

public int update(String sql, Object... args) {
    Connection conn = JdbcUtils.getConnection();
    try {
        return queryRunner.update(conn, sql, args);
    } catch (SQLException e) {
        e.printStackTrace();
        // 遇到異常手動往外拋，否則事務不知道要回滾
        throw new RuntimeException(e);
    }
}

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException,
        IOException {
    request.setCharacterEncoding("UTF-8");
    // 判斷請求的隱藏標籤
    String action = request.getParameter("action");
    try {
        // 用反射找到方法
        Method declaredMethod = this.getClass().getDeclaredMethod(action, HttpServletRequest.class,
                HttpServletResponse.class);
        // 調用方法
        declaredMethod.invoke(this, request, response);
    } catch (Exception e) {
        e.printStackTrace();
        // 手動拋出異常給過濾器
        throw new RuntimeException(e);
    }
}

@WebFilter(filterName = "TransactionFilter", value = "/*")
public class TransactionFilter implements Filter {
    public void init(FilterConfig config) throws ServletException {
    }

    public void destroy() {
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException
            , IOException {
        // 因為篩選條件是/*所有人都要來這關，當然也包含了所有事務
        try {
            chain.doFilter(request, response);
            // 在此提交事務
            JdbcUtils.commitAndClose();
        } catch (Exception e) {
            // 出錯就回滾，剛剛讓baseServlet手動拋出就是為了在這邊能接到
            JdbcUtils.rollbackAndClose();
            e.printStackTrace();
        }
    }
}

catch (Exception e) {
// 出錯就回滾，剛剛讓baseServlet手動拋出就是為了在這邊能接到
JdbcUtils.rollbackAndClose();
e.printStackTrace();
// 手動拋出異常給Tomcat
throw new RuntimeException(e);
}

<error-page>
	<location>/pages/error/error.html</location>
</error-page>

也可以指定
<error-page>
    <error-code>
    <exception-type>
        把不同類型的error引導到對應的頁面