Die Anleitung zu Java Servlet Filter

package org.o7planning.tutorial.servletfilter;

import java.io.IOException;
import java.util.Date;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

public class LogFilter implements Filter {

	public LogFilter() {
	}

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

	@Override
	public void destroy() {
		System.out.println("LogFilter destroy!");
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {

		HttpServletRequest req = (HttpServletRequest) request;

		String servletPath = req.getServletPath();

		System.out.println("#INFO " + new Date() + " - ServletPath :" + servletPath //
				+ ", URL =" + req.getRequestURL());

		// Genehmigen, die Request weiter zu gehen. (durch den Filter).
		chain.doFilter(request, response);
	}

}

// Genehmigen, die Request weiter vorwärts zu gehen. 
// Sie kann zum nächsten Filter oder dem Ziel gehen
chain.doFilter(request, response);

package org.o7planning.tutorial.servletfilter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class Log2Filter implements Filter {

	private String logFile;

	public Log2Filter() {
	}

	@Override
	public void init(FilterConfig fConfig) throws ServletException {
		this.logFile = fConfig.getInitParameter("logFile");

		System.out.println("Log File " + logFile);
	}

	@Override
	public void destroy() {
		System.out.println("Log2Filter destroy!");
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {

		if (this.logFile != null) {
			// Die Log Information in die File aufschreiben.
			this.logToFile(this.logFile);
		}

		// Die Request weiter gehen. (durch den Filter).
		chain.doFilter(request, response);
	}

	private void logToFile(String fileName) {
		// Die Log in die File aufschreiben
		System.out.println("Write log to file " + fileName);
	}

}

package org.o7planning.tutorial.servletfilter;

import java.io.File;
import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebFilter(urlPatterns = { "*.png", "*.jpg", "*.gif" }, initParams = {
		@WebInitParam(name = "notFoundImage", value = "/images/image-not-found.png") })
public class ImageFilter implements Filter {

	private String notFoundImage;

	public ImageFilter() {
	}

	@Override
	public void init(FilterConfig fConfig) throws ServletException {

		// ==> /images/image-not-found.png
		notFoundImage = fConfig.getInitParameter("notFoundImage");
	}

	@Override
	public void destroy() {
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {

		HttpServletRequest req = (HttpServletRequest) request;

		// ==> /images/path/my-image.png
		// ==> /path1/path2/image.pngs
		String servletPath = req.getServletPath();

		// Der absolute Pfad des Hauptverzeichnis vom WebApp (WebContent).
		String realRootPath = request.getServletContext().getRealPath("");

		// Der absolute Pfad zur Foto-File.
		String imageRealPath = realRootPath + servletPath;

		System.out.println("imageRealPath = " + imageRealPath);

		File file = new File(imageRealPath);

		// Prüfen, ob das Foto existiert oder nicht.
		if (file.exists()) {

			// Genehmigen, die Request weiter zu gehen (durch den Filter).
			// (um zur angeforderten Fotofile zu gehen).
			chain.doFilter(request, response);

		} else if (!servletPath.equals(this.notFoundImage)) {

			// Zur die Foto-File 'image not found' umleiten (redirect).
			HttpServletResponse resp = (HttpServletResponse) response;

			// ==> /ServletFilterTutorial + /images/image-not-found.png
			resp.sendRedirect(req.getContextPath() + this.notFoundImage);

		}

	}

}

// Redirect:

// ==> /ServletFilterTutorial + /images/image-not-found.png
response.sendRedirect(request.getContextPath()+ this.notFoundImage);

// Forward:

request.getServletContext().getRequestDispatcher(this.notFoundImage).forward(request, response);

package org.o7planning.tutorial.servletfilter.conn;

import java.sql.Connection;

public class ConnectionUtils {

	public static Connection getConnection() {

		// Eine Verbindung (connection) zur Database erstellen
		Connection conn = null;

		// .....
		return conn;
	}

	public static void closeQuietly(Connection conn) {
		try {
			conn.close();
		} catch (Exception e) {
		}
	}

	public static void rollbackQuietly(Connection conn) {
		try {
			conn.rollback();
		} catch (Exception e) {
		}
	}
}

package org.o7planning.tutorial.servletfilter.conn;

import java.sql.Connection;

import javax.servlet.ServletRequest;

public class MyUtils {

	public static final String ATT_NAME = "MY_CONNECTION_ATTRIBUTE";

	// Das Objekt Connection in einer Attribute von der Request 
	// Die Information zur Archivierung existiert in der Zeitraum von Request zum 
	// wenn die Daten nach den Benutzersbrowser zurückgegeben werden
	public static void storeConnection(ServletRequest request, Connection conn) {
		request.setAttribute(ATT_NAME, conn);
	}

	// Das Objekt Connection, das in einer Attribute der Request archiviert wird, entnehmen 
	public static Connection getStoredConnection(ServletRequest request) {
		Connection conn = (Connection) request.getAttribute(ATT_NAME);
		return conn;
	}
}

package org.o7planning.tutorial.servletfilter.conn;

import java.io.IOException;
import java.sql.Connection;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;

@WebFilter(urlPatterns = { "/*" })
public class JDBCFilter implements Filter {

	public JDBCFilter() {
	}

	@Override
	public void init(FilterConfig fConfig) throws ServletException {

	}

	@Override
	public void destroy() {

	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {

		HttpServletRequest req = (HttpServletRequest) request;

		// 
		String servletPath = req.getServletPath();

		// Die Verbindung (connection) für die Request mit den besonderen Pfad nur öffnen 
		// ( Z.B den Pfad zum servlet, jsp ...)
		// vermeinden, die Connection für die normalen Anforderungen zu öffnen
		// (Z. B image, css, javascript,... )
		if (servletPath.contains("/specialPath1") || servletPath.contains("/specialPath2")) {
			Connection conn = null;
			try {
				// Ein Objekt Connection mit Database erstellen.
				conn = ConnectionUtils.getConnection();
				// Automatisch commit = false einstellen um aktiv zu kontrollieren.
				conn.setAutoCommit(false);

				// Die Attribute von Request archivieren.
				MyUtils.storeConnection(request, conn);

				// Genehmigen, die Request weiter zu gehen (durch den Filter).
				chain.doFilter(request, response);

				// Die Aufruf auf commit() um die Transaction mit DB zu erledigen.
				conn.commit();
			} catch (Exception e) {
				ConnectionUtils.rollbackQuietly(conn);
				throw new ServletException();
			} finally {
				ConnectionUtils.closeQuietly(conn);
			}
		}
		// Für die normalen Anforderungen.
		else {
			// Genehmigen, die Request weiter zu gehen (durch den Filter).
			chain.doFilter(request, response);
		}

	}

}

Connection conn = (Connection) request.getAttribute(ATT_NAME);

// Oder
Connection conn = MyUtils.getStoredConnection();