BAEL-325: Added intercepting filters.

patterns/pom.xml

@@ -4,7 +4,13 @@
          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
 
-    <groupId>com.baeldung.enterprise.patterns</groupId>
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <groupId>com.baeldung.patterns</groupId>
     <artifactId>patterns</artifactId>
     <packaging>war</packaging>
 
@@ -15,14 +21,14 @@
             <version>3.1.0</version>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-api</artifactId>
+            <version>1.7.21</version>
+            <scope>provided</scope>
+        </dependency>
     </dependencies>
 
-    <parent>
-        <groupId>com.baeldung</groupId>
-        <artifactId>parent-modules</artifactId>
-        <version>1.0.0-SNAPSHOT</version>
-    </parent>
-
     <build>
         <plugins>
             <plugin>
@@ -34,13 +40,21 @@
                     <target>1.8</target>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-war-plugin</artifactId>
+                <version>3.0.0</version>
+                <configuration>
+                    <failOnMissingWebXml>false</failOnMissingWebXml>
+                </configuration>
+            </plugin>
             <plugin>
                 <groupId>org.eclipse.jetty</groupId>
                 <artifactId>jetty-maven-plugin</artifactId>
-                <version>9.4.0.M1</version>
+                <version>9.3.11.v20160721</version>
                 <configuration>
                     <webApp>
-                        <contextPath>/front-controller</contextPath>
+                        <contextPath>/</contextPath>
                     </webApp>
                 </configuration>
             </plugin>

patterns/src/main/java/com/baeldung/enterprise/patterns/front/controller/FrontControllerServlet.java

@@ -1,38 +0,0 @@
-package com.baeldung.enterprise.patterns.front.controller;
-
-import com.baeldung.enterprise.patterns.front.controller.commands.FrontCommand;
-import com.baeldung.enterprise.patterns.front.controller.commands.UnknownCommand;
-
-import javax.servlet.ServletException;
-import javax.servlet.http.HttpServlet;
-import javax.servlet.http.HttpServletRequest;
-import javax.servlet.http.HttpServletResponse;
-import java.io.IOException;
-
-public class FrontControllerServlet extends HttpServlet {
-    @Override
-    protected void doGet(
-      HttpServletRequest request,
-      HttpServletResponse response
-    ) throws ServletException, IOException {
-        FrontCommand command = getCommand(request);
-        command.init(getServletContext(), request, response);
-        command.process();
-    }
-
-    private FrontCommand getCommand(HttpServletRequest request) {
-        try {
-            Class type = Class.forName(
-              String.format(
-                "com.baeldung.enterprise.patterns.front.controller.commands.%sCommand",
-                request.getParameter("command")
-              )
-            );
-            return (FrontCommand) type
-              .asSubclass(FrontCommand.class)
-              .newInstance();
-        } catch (Exception e) {
-            return new UnknownCommand();
-        }
-    }
-}

patterns/src/main/java/com/baeldung/enterprise/patterns/front/controller/commands/FrontCommand.java

@@ -1,32 +0,0 @@
-package com.baeldung.enterprise.patterns.front.controller.commands;
-
-import javax.servlet.RequestDispatcher;
-import javax.servlet.ServletContext;
-import javax.servlet.ServletException;
-import javax.servlet.http.HttpServletRequest;
-import javax.servlet.http.HttpServletResponse;
-import java.io.IOException;
-
-public abstract class FrontCommand {
-    protected ServletContext context;
-    protected HttpServletRequest request;
-    protected HttpServletResponse response;
-
-    public void init(
-      ServletContext servletContext,
-      HttpServletRequest servletRequest,
-      HttpServletResponse servletResponse
-    ) {
-        this.context = servletContext;
-        this.request = servletRequest;
-        this.response = servletResponse;
-    }
-
-    public abstract void process() throws ServletException, IOException;
-
-    protected void forward(String target) throws ServletException, IOException {
-        target = String.format("/WEB-INF/jsp/%s.jsp", target);
-        RequestDispatcher dispatcher = context.getRequestDispatcher(target);
-        dispatcher.forward(request, response);
-    }
-}

patterns/src/main/java/com/baeldung/enterprise/patterns/front/controller/commands/SearchCommand.java

@@ -1,21 +0,0 @@
-package com.baeldung.enterprise.patterns.front.controller.commands;
-
-import com.baeldung.enterprise.patterns.front.controller.data.Book;
-import com.baeldung.enterprise.patterns.front.controller.data.BookshelfImpl;
-
-import javax.servlet.ServletException;
-import java.io.IOException;
-
-public class SearchCommand extends FrontCommand {
-    @Override
-    public void process() throws ServletException, IOException {
-        Book book = new BookshelfImpl().getInstance()
-          .findByTitle(request.getParameter("title"));
-        if (book != null) {
-            request.setAttribute("book", book);
-            forward("book-found");
-        } else {
-            forward("book-notfound");
-        }
-    }
-}

patterns/src/main/java/com/baeldung/enterprise/patterns/front/controller/data/Bookshelf.java

@@ -1,15 +0,0 @@
-package com.baeldung.enterprise.patterns.front.controller.data;
-
-public interface Bookshelf {
-
-    default void init() {
-        add(new Book("Wilson, Robert Anton & Shea, Robert", "Illuminati", 9.99));
-        add(new Book("Fowler, Martin", "Patterns of Enterprise Application Architecture", 27.88));
-    }
-
-    Bookshelf getInstance();
-
-    <E extends Book> boolean add(E book);
-
-    Book findByTitle(String title);
-}

patterns/src/main/java/com/baeldung/enterprise/patterns/front/controller/data/BookshelfImpl.java

@@ -1,24 +0,0 @@
-package com.baeldung.enterprise.patterns.front.controller.data;
-
-import java.util.ArrayList;
-
-public class BookshelfImpl extends ArrayList<Book> implements Bookshelf {
-    private static Bookshelf INSTANCE;
-
-    @Override
-    public Bookshelf getInstance() {
-        if (INSTANCE == null) {
-            INSTANCE = new BookshelfImpl();
-            INSTANCE.init();
-        }
-        return INSTANCE;
-    }
-
-    @Override
-    public Book findByTitle(String title) {
-        return this.stream()
-          .filter(book -> book.getTitle().toLowerCase().contains(title.toLowerCase()))
-          .findFirst()
-          .orElse(null);
-    }
-}

patterns/src/main/java/com/baeldung/patterns/FrontControllerServlet.java

@@ -0,0 +1,66 @@
+package com.baeldung.patterns;
+
+import com.baeldung.patterns.commands.FrontCommand;
+import com.baeldung.patterns.commands.UnknownCommand;
+import com.baeldung.patterns.data.Bookshelf;
+import com.baeldung.patterns.data.BookshelfImpl;
+
+import javax.servlet.ServletConfig;
+import javax.servlet.ServletException;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+@WebServlet(name = "front-controller", urlPatterns = "/index")
+public class FrontControllerServlet extends HttpServlet {
+    @Override
+    public void init(ServletConfig config) throws ServletException {
+        super.init(config);
+        Bookshelf bookshelf = new BookshelfImpl();
+        bookshelf.init();
+        getServletContext().setAttribute("bookshelf", bookshelf);
+    }
+
+    @Override
+    protected void doGet(
+      HttpServletRequest request,
+      HttpServletResponse response
+    ) throws ServletException, IOException {
+        doCommand(request, response);
+    }
+
+    @Override
+    protected void doPost(
+      HttpServletRequest request,
+      HttpServletResponse response
+    ) throws ServletException, IOException {
+          doCommand(request, response);
+    }
+
+    private void doCommand(
+      HttpServletRequest request,
+      HttpServletResponse response
+    ) throws ServletException, IOException {
+        FrontCommand command = getCommand(request);
+        command.init(request, response);
+        command.process();
+    }
+
+    private FrontCommand getCommand(HttpServletRequest request) {
+        try {
+            Class type = Class.forName(
+              String.format(
+                "com.baeldung.patterns.commands.%sCommand",
+                request.getParameter("command")
+              )
+            );
+            return (FrontCommand) type
+              .asSubclass(FrontCommand.class)
+              .newInstance();
+        } catch (Exception e) {
+            return new UnknownCommand();
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/CheckoutCommand.java

@@ -0,0 +1,27 @@
+package com.baeldung.patterns.commands;
+
+import com.baeldung.patterns.data.Order;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpSession;
+import java.io.IOException;
+
+public class CheckoutCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        super.process();
+        HttpSession session = request.getSession(false);
+        if (request.getMethod().equals("POST")) {
+            session.removeAttribute("order");
+            response.sendRedirect("index?command=Home&message=Thank you for buying!");
+        } else {
+            Order order = (Order) session.getAttribute("order");
+            Double total = order.getItems().entrySet().stream()
+              .map(entry -> entry.getKey().getPrice() * entry.getValue())
+              .reduce((p1, p2) -> p1 + p2)
+              .orElse(0.00);
+            request.setAttribute("total", total);
+            forward("shopping-cart");
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/FrontCommand.java

@@ -0,0 +1,43 @@
+package com.baeldung.patterns.commands;
+
+import com.baeldung.patterns.filters.FilterManager;
+import com.baeldung.patterns.filters.OnIntercept;
+
+import javax.servlet.RequestDispatcher;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+public abstract class FrontCommand implements OnIntercept {
+    protected HttpServletRequest request;
+    protected HttpServletResponse response;
+    private boolean intercept;
+
+    public FrontCommand() {
+    }
+
+    public void init(HttpServletRequest request, HttpServletResponse response) {
+        this.request = request;
+        this.response = response;
+    }
+
+    public void process() throws ServletException, IOException {
+        FilterManager.process(request, response, this);
+    }
+
+    public void forward(String target) throws ServletException, IOException {
+        if (intercept) {
+            return;
+        }
+        String path = String.format("/WEB-INF/jsp/%s.jsp", target);
+        RequestDispatcher dispatcher = request.getServletContext()
+          .getRequestDispatcher(path);
+        dispatcher.forward(request, response);
+    }
+
+    @Override
+    public void intercept() {
+        intercept = true;
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/HomeCommand.java

@@ -0,0 +1,17 @@
+package com.baeldung.patterns.commands;
+
+import com.baeldung.patterns.data.Bookshelf;
+
+import javax.servlet.ServletException;
+import java.io.IOException;
+
+public class HomeCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        super.process();
+        Bookshelf bookshelf = (Bookshelf) request.getServletContext()
+          .getAttribute("bookshelf");
+        request.setAttribute("books", bookshelf);
+        forward("home");
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/LoginCommand.java

@@ -0,0 +1,24 @@
+package com.baeldung.patterns.commands;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpSession;
+import java.io.IOException;
+
+public class LoginCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        if (request.getMethod().equals("POST")) {
+            HttpSession session = request.getSession(true);
+            session.setAttribute("username", request.getParameter("username"));
+            response.sendRedirect(request.getParameter("redirect"));
+        } else {
+            String queryString = request.getQueryString();
+            if (queryString == null) {
+                queryString = "command=Home";
+            }
+            request.setAttribute("redirect", request.getRequestURL()
+              .append("?").append(queryString).toString());
+            forward("login");
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/LogoutCommand.java

@@ -0,0 +1,16 @@
+package com.baeldung.patterns.commands;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpSession;
+import java.io.IOException;
+
+public class LogoutCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        super.process();
+        HttpSession session = request.getSession(false);
+        session.removeAttribute("username");
+        session.removeAttribute("order");
+        response.sendRedirect("index?command=Home");
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/OrderCommand.java

@@ -0,0 +1,33 @@
+package com.baeldung.patterns.commands;
+
+import com.baeldung.patterns.data.Book;
+import com.baeldung.patterns.data.Bookshelf;
+import com.baeldung.patterns.data.Order;
+import com.baeldung.patterns.data.OrderImpl;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpSession;
+import java.io.IOException;
+
+public class OrderCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        super.process();
+        if (request.getMethod().equals("POST")) {
+            HttpSession session = request.getSession(false);
+            Order order = (Order) session.getAttribute("order");
+            if (order == null) {
+                String username = (String) session.getAttribute("username");
+                order = new OrderImpl(username);
+            }
+            Bookshelf bookshelf = (Bookshelf) request.getServletContext()
+              .getAttribute("bookshelf");
+            String isbn = request.getParameter("isbn");
+            Integer quantity = Integer.parseInt(request.getParameter("quantity"));
+            Book book = bookshelf.get(isbn);
+            order.add(book, quantity);
+            session.setAttribute("order", order);
+            response.sendRedirect(String.format("index?command=Show&isbn=%s", isbn));
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/SearchCommand.java

@@ -0,0 +1,25 @@
+package com.baeldung.patterns.commands;
+
+import com.baeldung.patterns.data.Book;
+import com.baeldung.patterns.data.Bookshelf;
+
+import javax.servlet.ServletException;
+import java.io.IOException;
+import java.util.List;
+
+public class SearchCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        super.process();
+        Bookshelf bookshelf = (Bookshelf) request.getServletContext()
+          .getAttribute("bookshelf");
+        String q = request.getParameter("q");
+        List<Book> books = bookshelf.find(q);
+        if (books.size() > 0) {
+            request.setAttribute("books", books);
+            forward("book-found");
+        } else {
+            forward("book-notfound");
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/ShowCommand.java

@@ -0,0 +1,21 @@
+package com.baeldung.patterns.commands;
+
+import com.baeldung.patterns.data.Book;
+import com.baeldung.patterns.data.Bookshelf;
+
+import javax.servlet.ServletException;
+import java.io.IOException;
+import java.util.Collections;
+
+public class ShowCommand extends FrontCommand {
+    @Override
+    public void process() throws ServletException, IOException {
+        super.process();
+        Bookshelf bookshelf = (Bookshelf) request.getServletContext()
+          .getAttribute("bookshelf");
+        String title = request.getParameter("isbn");
+        Book book = bookshelf.get(title);
+        request.setAttribute("books", Collections.singletonList(book));
+        forward("book-found");
+    }
+}

patterns/src/main/java/com/baeldung/patterns/commands/UnknownCommand.java

@@ -1,4 +1,4 @@
-package com.baeldung.enterprise.patterns.front.controller.commands;
+package com.baeldung.patterns.commands;
 
 import javax.servlet.ServletException;
 import java.io.IOException;
@@ -6,6 +6,7 @@ import java.io.IOException;
 public class UnknownCommand extends FrontCommand {
     @Override
     public void process() throws ServletException, IOException {
+        super.process();
         forward("unknown");
     }
 }

patterns/src/main/java/com/baeldung/patterns/data/Book.java

@@ -0,0 +1,19 @@
+package com.baeldung.patterns.data;
+
+public interface Book {
+    String getIsbn();
+
+    void setIsbn(String isbn);
+
+    String getAuthor();
+
+    void setAuthor(String author);
+
+    String getTitle();
+
+    void setTitle(String title);
+
+    Double getPrice();
+
+    void setPrice(Double price);
+}

patterns/src/main/java/com/baeldung/patterns/data/BookImpl.java

@@ -1,39 +1,57 @@
-package com.baeldung.enterprise.patterns.front.controller.data;
+package com.baeldung.patterns.data;
 
-public class Book {
+public class BookImpl implements Book {
+    private String isbn;
     private String author;
     private String title;
     private Double price;
 
-    public Book() {
+    public BookImpl() {
     }
 
-    public Book(String author, String title, Double price) {
+    public BookImpl(String isbn, String author, String title, Double price) {
+        this.isbn = isbn;
         this.author = author;
         this.title = title;
         this.price = price;
     }
 
+    @Override
+    public String getIsbn() {
+        return isbn;
+    }
+
+    @Override
+    public void setIsbn(String isbn) {
+        this.isbn = isbn;
+    }
+
+    @Override
     public String getAuthor() {
         return author;
     }
 
+    @Override
     public void setAuthor(String author) {
         this.author = author;
     }
 
+    @Override
     public String getTitle() {
         return title;
     }
 
+    @Override
     public void setTitle(String title) {
         this.title = title;
     }
 
+    @Override
     public Double getPrice() {
         return price;
     }
 
+    @Override
     public void setPrice(Double price) {
         this.price = price;
     }

patterns/src/main/java/com/baeldung/patterns/data/Bookshelf.java

@@ -0,0 +1,18 @@
+package com.baeldung.patterns.data;
+
+import java.util.List;
+
+public interface Bookshelf {
+
+    default void init() {
+        add(new BookImpl("001", "Wilson, Robert Anton & Shea, Robert", "Illuminati", 9.99));
+        add(new BookImpl("002", "Fowler, Martin", "Patterns of Enterprise Application Architecture", 27.88));
+        add(new BookImpl("003", "Unknown", "Something about German Umlauts (äüö) and ß", 5.49));
+    }
+
+    <E extends Book> boolean add(E book);
+
+    Book get(String isbn);
+
+    List<Book> find(String q);
+}

patterns/src/main/java/com/baeldung/patterns/data/BookshelfImpl.java

@@ -0,0 +1,23 @@
+package com.baeldung.patterns.data;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class BookshelfImpl extends ArrayList<Book> implements Bookshelf {
+    @Override
+    public Book get(String isbn) {
+        return this.stream()
+          .filter(book -> book.getIsbn().equals(isbn))
+          .findFirst()
+          .orElse(null);
+    }
+
+    @Override
+    public List<Book> find(String q) {
+        return this.stream()
+          .filter(book -> book.getTitle().toLowerCase().contains(q.toLowerCase())
+            || book.getAuthor().toLowerCase().contains(q.toLowerCase()))
+          .collect(Collectors.toList());
+    }
+}

patterns/src/main/java/com/baeldung/patterns/data/Order.java

@@ -0,0 +1,11 @@
+package com.baeldung.patterns.data;
+
+import java.util.Map;
+
+public interface Order {
+    String getUsername();
+
+    Map<Book, Integer> getItems();
+
+    void add(Book item, Integer quantity);
+}

patterns/src/main/java/com/baeldung/patterns/data/OrderImpl.java

@@ -0,0 +1,40 @@
+package com.baeldung.patterns.data;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class OrderImpl implements Order {
+    private String username;
+    private Map<Book, Integer> items = new HashMap<>();
+
+    public OrderImpl(String username) {
+        this.username = username;
+    }
+
+    @Override
+    public String getUsername() {
+        return username;
+    }
+
+    public void setUsername(String username) {
+        this.username = username;
+    }
+
+    @Override
+    public Map<Book, Integer> getItems() {
+        return items;
+    }
+
+    public void setItems(Map<Book, Integer> items) {
+        this.items = items;
+    }
+
+    @Override
+    public void add(Book item, Integer quantity) {
+        Integer q = 0;
+        if (this.items.containsKey(item)) {
+            q = this.items.get(item);
+        }
+        this.items.put(item, quantity + q);
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/AuthenticationFilter.java

@@ -0,0 +1,40 @@
+package com.baeldung.patterns.filters;
+
+import com.baeldung.patterns.commands.FrontCommand;
+import com.baeldung.patterns.commands.LoginCommand;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import javax.servlet.http.HttpSession;
+import java.io.IOException;
+
+public class AuthenticationFilter extends BaseFilter {
+    private OnIntercept callback;
+
+    public AuthenticationFilter(OnIntercept callback) {
+        this.callback = callback;
+    }
+
+    @Override
+    public void doFilter(
+      ServletRequest request,
+      ServletResponse response,
+      FilterChain chain
+    ) throws IOException, ServletException {
+        HttpServletRequest httpServletRequest = (HttpServletRequest) request;
+        HttpServletResponse httpServletResponse = (HttpServletResponse) response;
+        HttpSession session = httpServletRequest.getSession(false);
+        if (session == null || session.getAttribute("username") == null) {
+            callback.intercept();
+            FrontCommand command = new LoginCommand();
+            command.init(httpServletRequest, httpServletResponse);
+            command.process();
+        } else {
+            chain.doFilter(request, response);
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/BaseFilter.java

@@ -0,0 +1,25 @@
+package com.baeldung.patterns.filters;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.servlet.Filter;
+import javax.servlet.FilterConfig;
+import javax.servlet.ServletException;
+
+public abstract class BaseFilter implements Filter {
+    private static final Logger log = LoggerFactory.getLogger(BaseFilter.class);
+
+    protected FilterConfig filterConfig;
+
+    @Override
+    public void init(FilterConfig filterConfig) throws ServletException {
+        log.info("Initialize filter: {}", getClass().getSimpleName());
+        this.filterConfig = filterConfig;
+    }
+
+    @Override
+    public void destroy() {
+        log.info("Destroy filter: {}", getClass().getSimpleName());
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/EncodingFilter.java

@@ -0,0 +1,30 @@
+package com.baeldung.patterns.filters;
+
+import javax.servlet.*;
+import javax.servlet.annotation.WebFilter;
+import java.io.IOException;
+
+@WebFilter(servletNames = "front-controller")
+public class EncodingFilter extends BaseFilter {
+    private String encoding;
+
+    @Override
+    public void init(FilterConfig filterConfig) throws ServletException {
+        super.init(filterConfig);
+        this.encoding = filterConfig.getInitParameter("encoding");
+    }
+
+    @Override
+    public void doFilter(
+      ServletRequest request,
+      ServletResponse response,
+      FilterChain chain
+    ) throws IOException, ServletException {
+        String encoding = request.getParameter("encoding");
+        if (encoding == null) {
+            encoding = this.encoding;
+        }
+        response.setCharacterEncoding(encoding);
+        chain.doFilter(request, response);
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/FilterChainImpl.java

@@ -0,0 +1,28 @@
+package com.baeldung.patterns.filters;
+
+import javax.servlet.Filter;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Iterator;
+
+public class FilterChainImpl implements javax.servlet.FilterChain {
+    private Iterator<Filter> filters;
+
+    public FilterChainImpl(Filter... filters) {
+        this.filters = Arrays.asList(filters).iterator();
+    }
+
+    @Override
+    public void doFilter(
+      ServletRequest request,
+      ServletResponse response
+    ) throws IOException, ServletException {
+        if (filters.hasNext()) {
+            Filter filter = filters.next();
+            filter.doFilter(request, response, this);
+        }
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/FilterManager.java

@@ -0,0 +1,21 @@
+package com.baeldung.patterns.filters;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+public class FilterManager {
+    public static void process(
+      HttpServletRequest request,
+      HttpServletResponse response,
+      OnIntercept callback
+    ) throws ServletException, IOException {
+        FilterChain filterChain = new FilterChainImpl(
+          new AuthenticationFilter(callback),
+          new VisitorCounterFilter()
+        );
+        filterChain.doFilter(request, response);
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/LoggingFilter.java

@@ -0,0 +1,33 @@
+package com.baeldung.patterns.filters;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.annotation.WebFilter;
+import javax.servlet.http.HttpServletRequest;
+import java.io.IOException;
+
+@WebFilter(servletNames = "front-controller")
+public class LoggingFilter extends BaseFilter {
+    private static final Logger log = LoggerFactory.getLogger(LoggingFilter.class);
+
+    @Override
+    public void doFilter(
+      ServletRequest request,
+      ServletResponse response,
+      FilterChain chain
+    ) throws IOException, ServletException {
+        chain.doFilter(request, response);
+        HttpServletRequest httpServletRequest = (HttpServletRequest) request;
+        String username = (String) httpServletRequest.getAttribute("username");
+        if (username == null) {
+            username = "guest";
+        }
+        log.info("Request from '{}@{}': {}?{}", username, request.getRemoteAddr(),
+          httpServletRequest.getRequestURI(), request.getParameterMap());
+    }
+}

patterns/src/main/java/com/baeldung/patterns/filters/OnIntercept.java

@@ -0,0 +1,5 @@
+package com.baeldung.patterns.filters;
+
+public interface OnIntercept {
+    void intercept();
+}

patterns/src/main/java/com/baeldung/patterns/filters/VisitorCounterFilter.java

@@ -0,0 +1,30 @@
+package com.baeldung.patterns.filters;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpSession;
+import java.io.IOException;
+import java.util.HashSet;
+import java.util.Set;
+
+public class VisitorCounterFilter extends BaseFilter {
+    private static Set<String> users = new HashSet<>();
+
+    @Override
+    public void doFilter(
+      ServletRequest request,
+      ServletResponse response,
+      FilterChain chain
+    ) throws IOException, ServletException {
+        HttpSession session = ((HttpServletRequest) request).getSession(false);
+        String username = (String) session.getAttribute("username");
+        if (!users.contains(username)) {
+            users.add(username);
+        }
+        request.setAttribute("counter", users.size());
+        chain.doFilter(request, response);
+    }
+}

patterns/src/main/webapp/WEB-INF/jsp/book-found.jsp

@@ -1,12 +1,24 @@
+<%@ page import="com.baeldung.patterns.data.Book" %>
+<%@ page import="java.util.List" %>
 <!DOCTYPE html>
 <html>
     <head>
         <title>Bookshelf: Title found</title>
     </head>
     <body>
-        <p>Our Bookshelf contains this title:</p>
-        <h2>${book.getTitle()}</h2>
-        <p>Author: ${book.getAuthor()}</p>
-        <input type="submit" value="Buy it: ${book.getPrice()}$">
+        <p>Our Bookshelf contains following titles:</p>
+        <% for (Book book : (List<Book>) request.getAttribute("books")) { %>
+            <h2><%= book.getTitle() %></h2>
+            <p>Author: <%= book.getAuthor() %></p>
+            <form action="index?command=Order" method="POST">
+                <label for="quantity">Quantity:</label>
+                <input type="text" id="quantity" name="quantity" value="1" required>
+                <input type="hidden" name="isbn" value="<%= book.getIsbn() %>">
+                <input type="submit" value="Buy it: <%= book.getPrice() %>$">
+                <a href="index?command=Home">Go back...</a>
+            </form>
+        <% } %>
+        <%@ include file="shopping-cart-hint.jsp"%>
+        <%@ include file="visitor-counter.jsp"%>
     </body>
 </html>

patterns/src/main/webapp/WEB-INF/jsp/book-notfound.jsp

@@ -1,10 +1,11 @@
 <!DOCTYPE html>
 <html>
-<head>
-    <title>Bookshelf: Title not found</title>
-</head>
+    <head>
+        <title>Bookshelf: Title not found</title>
+    </head>
     <body>
         <p>Our Bookshelf doesn't contains this title:</p>
-        <h2>${param.get("title")}</h2>
+        <h2><%= request.getParameter("q") %></h2>
+        <%@ include file="visitor-counter.jsp"%>
     </body>
 </html>

patterns/src/main/webapp/WEB-INF/jsp/home.jsp

@@ -0,0 +1,27 @@
+<%@ page import="com.baeldung.patterns.data.Book" %>
+<%@ page import="java.util.List" %>
+<!DOCTYPE>
+<html>
+    <head>
+        <title>Bookshelf: Home</title>
+    </head>
+    <body>
+        <form action="index?command=Search" method="POST">
+            <input type="text" id="search" name="q">
+            <input type="submit" value="Go!">
+        </form>
+        <% if (request.getParameter("message") != null) { %>
+            <h2><%= request.getParameter("message") %></h2>
+        <% } else { %>
+            <h2>Welcome to the Bookshelf!</h2>
+        <% } %>
+        <% for (Book book : (List<Book>) request.getAttribute("books")) { %>
+            <hr/>
+            <p><b><%= book.getAuthor() %>:</b></p>
+            <h3><%= book.getTitle() %></h3>
+            <a href="index?command=Show&isbn=<%= book.getIsbn() %>">More...</a>
+        <% } %>
+        <%@ include file="shopping-cart-hint.jsp"%>
+        <%@ include file="visitor-counter.jsp"%>
+    </body>
+</html>

patterns/src/main/webapp/WEB-INF/jsp/login.jsp

@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>Bookshelf: Login</title>
+    </head>
+    <body>
+        <p>Please input a username:</p>
+        <h2>Login</h2>
+        <form action="index?command=Login" method="POST">
+            <input type="text" name="username" placeholder="Username">
+            <input type="hidden" name="redirect" value="<%= (String) request.getAttribute("redirect") %>">
+            <input type="submit" value="Proceed">
+        </form>
+        <%@ include file="visitor-counter.jsp" %>
+    </body>
+</html>

patterns/src/main/webapp/WEB-INF/jsp/shopping-cart-hint.jsp

@@ -0,0 +1,16 @@
+<%@ page import="com.baeldung.patterns.data.Order" %>
+<% if (session != null && session.getAttribute("order") != null) { %>
+    <% Order order = ((Order) session.getAttribute("order")); %>
+    <% if (order != null && order.getItems().size() > 0) { %>
+        <hr/>
+        <p>
+            Your shopping cart is holding
+            <% if (order.getItems().size() == 1) { %>
+                1 item.
+            <% } else { %>
+                <%= (order.getItems().size()) %> items.
+            <% } %>
+            <a href="index?command=Checkout">Checkout</a>
+        </p>
+    <% } %>
+<% } %>

patterns/src/main/webapp/WEB-INF/jsp/shopping-cart.jsp

@@ -0,0 +1,29 @@
+<%@ page import="com.baeldung.patterns.data.Book" %>
+<%@ page import="com.baeldung.patterns.data.Order" %>
+<%@ page import="java.util.Map" %>
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>Bookshelf: Checkout</title>
+    </head>
+    <body>
+        <p>You are about to buy the following books:</p>
+        <h2>Shopping Cart</h2>
+        <% Order order = (Order) session.getAttribute("order"); %>
+        <ul>
+            <% for (Map.Entry<Book, Integer> entry : order.getItems().entrySet()) { %>
+                <li>
+                    <b><%= entry.getValue() %> x <%= entry.getKey().getPrice() %></b>
+                    <h3><%= entry.getKey().getTitle() %></h3>
+                    <i> by <%= entry.getKey().getAuthor()%></i>
+                </li>
+            <% } %>
+        </ul>
+        <p>
+            <b>Total: <%= request.getAttribute("total") %></b>
+        </p>
+        <form action="index?command=Checkout" method="POST">
+            <input type="submit" value="Donate :)">
+        </form>
+    </body>
+</html>

patterns/src/main/webapp/WEB-INF/jsp/visitor-counter.jsp

@@ -0,0 +1,5 @@
+<% Integer counter = (Integer) request.getAttribute("counter"); %>
+<% if (counter != null && counter > 0) { %>
+    <hr/>
+    <p>You are visitor #<%= counter %>. <a href="index?command=Logout">Logout</a></p>
+<% } %>

patterns/src/main/webapp/WEB-INF/web.xml

@@ -1,15 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
-         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
-		   http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
-         version="3.1">
-    <servlet>
-        <servlet-name>front-controller</servlet-name>
-        <servlet-class>com.baeldung.enterprise.patterns.front.controller.FrontControllerServlet</servlet-class>
-    </servlet>
-    <servlet-mapping>
-        <servlet-name>front-controller</servlet-name>
-        <url-pattern>/</url-pattern>
-    </servlet-mapping>
-</web-app>