Java Web Programming [3/9] : Servlet Advanced

Module 3: Servlet AdvancedThanisa Kruawaisayawan Thanachart Numnonda www.imcinstitute.com

Objectives Including, forwarding to, and redirecting to other web resources Servlet & JDBC Session Tracking API Scope Objects Servlet Listener Servlet Filter 2

When to Include another Web resource? When it is useful to add static or dynamic contents already created by another web resource  Adding a banner content or copyright information in the response returned from a Web component 3

How to Include another Web resource? Get RequestDispatcher object from ServletContext object RequestDispatcher dispatcher = getServletContext().getRequestDispatcher(“/banner"); Then, invoke the include() method of the RequestDispatcher object passing request and response objects  dispatcher.include(request, response); 4

When to use “Forwarding” to another Web resource? When you want to have one Web component do preliminary processing of a request and have another component generate the response You might want to partially process a request and then transfer to another component depending on the nature of the request 5

How to do “Forwarding” to another Web resource? Get RequestDispatcher object from HttpServletRequest object  Set “request URL” to the path of the forwarded page RequestDispatcher dispatcher = request.getRequestDispatcher(“/template.jsp"); If the original URL is required for any processing, you can save it as a request attribute Invoke the forward() method of the RequestDispatcher object  dispatcher.forward(request, response); 6

Redirecting a Request Two programming models for directing a request Method 1: response.setStatus(response.SC_MOVED_PERMANTLY); response.setHeader("Location", "http://..."); Method 2: response.sendRedirect("http://..."); 7

Servlet & JDBC 8

What is JDBC?Standard Java API for accessingrelational database Hides database specific details from applicationPart of Java SE (J2SE) Java SE 7 has JDBC 4.1 9

JDBC API Defines a set of Java Interfaces, which are implemented by vendor-specific JDBC Drivers  Applications use this set of Java interfaces for performing database operations - portability Majority of JDBC API is located in java.sql package  DriverManager, Connection, ResultSet, DatabaseMetaData, ResultSetMetaData, PreparedStatement, CallableStatement and Types Other advanced functionality exists in the javax.sql package  DataSource 10

JDBC Driver Database specific implemention of JDBC interfaces  Everydatabase server has corresponding JDBC driver(s) You can see the list of available drivers from  http://industry.java.sun.com/products/jdbc/ drivers 11

Database URL Used to make a connection to the database – Can contain server, port, protocol etc… jdbc:subprotocol_name:driver_dependant_databasename – Oracle thin driver jdbc:oracle:thin:@machinename:1521:dbname – MySQL jdbc:mysql://localhost:3306/test 12

MySQL Tools mysql-installer-5.5.22.1.msi (Database) mysql-workbench-gpi-5.2-40-win32.msi (GUI) mysql-connector-java-5.1.19-bin.jar (Driver)  Copy to C:Program FilesApache Software FoundationApache Tomcat 7.0.27lib 13

SQL Commands CREATE - Builds a new table with the specified field name and types INSERT - Adds a new record to the named table SELECT - Retrieves records from the named table DELETE - Removes one or more record(s) from the table UPDATE - Modified one or more field(s) of particular record(s) DROP - Completely removes a table from the database 14

CREATE Statement The formal syntax CREATE TABLE table_name(column [ , column] …) For example CREATE TABLE Books(ISBN varchar(5) not null PRIMATY KEY, title varchar(50) not null, author varchar(50), price float); 15

INSERT Statement The formal syntax INSERT INTO table_name (column [ , column] …) VALUES (value [, value] …) For example INSERT INTO Books VALUES(11111, J2ME on Symbian OS, Sam, 40); 16

SELECT Statement The formal syntax SELECT [table.]column [ , [table].column] … FROM table [ , table] … [WHERE [table.]column OPERATOR VALUE [AND] OR [table.]column OPERATOR VALUE … [ORDER BY [table.]column [DESC] [ ,[table.]column [DESC}] For example SELECT * FROM Books WHERE ISBN = 11111; 17

DELETE Statement The formal syntax DELETE FROM table_name WHERE column OPERATOR value [AND | OR column OPERATOR value ] … For example DELETE FROM Book WHERE ISBN = 11111; 18

Steps of Using JDBC1. Load DB-specific JDBC driver2. Get a Connection object3. Get a Statement object4. Execute queries and/or updates5. Read results6. Read Meta-data (optional step)7. Close Statement and Connection objects 19

1. Load DB-Specific Database Driver To manually load the database driver and register it with the DriverManager, load its class file – Class.forName(<database-driver>) try { // This loads an instance of the Pointbase DB Driver. // The driver has to be in the classpath. Class.forName("com.mysql.jdbc.Driver"); }catch (ClassNotFoundException cnfe){ System.out.println("" + cnfe); } 20

2. Get a Connection Object DriverManager class is responsible for selecting the database and and creating the database connection  Using DataSource is a prefered means of getting a conection object Create the database connection as follows:try { Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "root");} catch(SQLException sqle) { System.out.println("" + sqle);} 21

DriverManager & Connection java.sql.DriverManager • getConnection(String url, String user, String password) throws SQLException java.sql.Connection • Statement createStatement() throws SQLException • void close() throws SQLException • void setAutoCommit(boolean b) throws SQLException • void commit() throws SQLException • void rollback() throws SQLException 22

Sub-Topics DataSource interface and DataSource object Properties of a DataSource object JNDI registration of a DataSource object DataSource object that implements Connection pooling 23

javax.sql.DataSource Interface and DataSource Object Driver vendor implements the interface DataSource object is the factory for creating database connections 24

Properties of DataSource Object A DataSource object has properties that can be modified when necessary – these are defined in a containers configuration file  location of the database server  name of the database  network protocol to use to communicate with the server The benefit is that because the data sources properties can be changed, any code accessing that data source does not need to be changed In the GlassFish Application Server, a data source is called a JDBC resource 25

JNDI Registration of a DataSource (JDBC Resource) Object The JNDI name of a JDBC resource is expected in the java:comp/env/jdbc subcontext  For example, the JNDI name for the resource of a BookDB database could be java:comp/env/jdbc/BookDB Because all resource JNDI names are in the java:comp/env subcontext, when you specify the JNDI name of a JDBC resource enter only jdbc/name. For example, for a payroll database, specify jdbc/BookDB 26

Why Connection Pooling? Database connection is an expensive and limited resource  Using connection pooling, a smaller number of connections are shared by a larger number of clients Creating and destroying database connections are expensive operations  Using connection pooling, a set of connections are pre- created and are available as needed basis cutting down on the overhead of creating and destroying database connections 27

Connection Pooling & DataSource DataSource objects that implement connection pooling also produce a connection to the particular data source that the DataSource class represents The connection object that the getConnection method returns is a handle to a PooledConnection object rather than being a physical connection  The application code works the same way 28

3. Get a Statement Object Create a Statement Object from Connection object • java.sql.Statement – ResultSet executeQuery(string sql) – int executeUpdate(String sql) • Example: Statement statement = connection.createStatement(); The same Statement object can be used for many, unrelated queries 29

4. Executing Query or Update From the Statement object, the 2 most used commands are – (a) QUERY (SELECT) • ResultSet rs = statement.executeQuery("select * from customer_tbl"); – (b) ACTION COMMAND (UPDATE/DELETE) • int iReturnValue = statement.executeUpdate("update manufacture_tbl set name = ‘IBM where mfr_num = 19985678"); 30

5. Reading Results Loop through ResultSet retrieving information  java.sql.ResultSet  boolean next()  xxx getXxx(int columnNumber)  xxx getXxx(String columnName)  void close() The iterator is initialized to a position before the first row – You must call next() once to move it to the first row 31

5. Reading Results (cont.) Once you have the ResultSet, you can easily retrieve the data by looping through it while (rs.next()){ // Wrong this will generate an error String value0 = rs.getString(0); // Correct! String value1 = rs.getString(1); int value2 = rs.getInt(2); int value3 = rs.getInt(“ADDR_LN1"); } 32

5. Reading Results (cont.) When retrieving data from the ResultSet, use the appropriate getXXX() method • getString() • getInt() • getDouble() • getObject() Thereis an appropriate getXXX method of each java.sql.Types datatype 33

6. Read ResultSet MetaData and DatabaseMetaData (Optional) Once you have the ResultSet or Connection objects, you can obtain the Meta Data about the database or the query This gives valuable information about the data that you are retrieving or the database that you are using – DatabaseMetaData dbmd = connection.getMetaData(); – ResultSetMetaData rsMeta = rs.getMetaData(); • There are approximately 150 methods in the DatabaseMetaData class. 34

Examplepublic void showDBInfo() {{ public void showDBInfo() try {{ try DatabaseMetaData dbmd == connection.getMetaData(); DatabaseMetaData dbmd connection.getMetaData(); System.out.println("Product: "" ++ dbmd.getDatabaseProductName()); System.out.println("Product: dbmd.getDatabaseProductName()); System.out.println("Version: "" ++ dbmd.getDatabaseProductVersion()); System.out.println("Version: dbmd.getDatabaseProductVersion()); System.out.println ("Stored Procedures: "" ++ System.out.println ("Stored Procedures: dbmd.supportsStoredProcedures()); dbmd.supportsStoredProcedures()); System.out.println("Transactions: "" ++ System.out.println("Transactions: dbmd.supportsTransactions()); dbmd.supportsTransactions()); }} catch (SQLException ex) {{ catch (SQLException ex) ex.getMessage(); ex.getMessage(); }}}} 35

ResultSetMetaData ExampleResultSetMetaData meta = rs.getMetaData();//Return the column countint iColumnCount = meta.getColumnCount();for (int i =1 ; i <= iColumnCount ; i++){ System.out.println(“Column Name: " + meta.getColumnName(i)); System.out.println(“Column Type" + meta.getColumnType(i)); System.out.println("Display Size: " + meta.getColumnDisplaySize(i) ); System.out.println("Precision: " + meta.getPrecision(i)); System.out.println(“Scale: " + meta.getScale(i) );} 36

What Are They? PreparedStatement • SQL is sent to the database and compiled or prepared beforehand CallableStatement • Executes SQL Stored Procedures 37

PreparedStatement Interface To call the same SQL statement multiple times, use a PrepareStatement object. Extend PreparedStatement from Statement class.  public interface PreparedStatement extends Statement Advantage  Using Statement needs to compile every times.  PreparedStatement will compile only once. 38

Example public void insertData() {{ public void insertData() PreparedStatement pstmt; PreparedStatement pstmt; String insertCmd == "INSERT INTO Book VALUES(?,?,?,?)"; String insertCmd "INSERT INTO Book VALUES(?,?,?,?)"; try {{ try pstmt == connection.prepareStatement(insertCmd); pstmt connection.prepareStatement(insertCmd); pstmt.setString(1,"11111"); pstmt.setString(1,"11111"); pstmt.setString(2,"J2ME on Symbian OS"); pstmt.setString(2,"J2ME on Symbian OS"); pstmt.setString(3,"Sam"); pstmt.setString(3,"Sam"); pstmt.setDouble(4,40); pstmt.setDouble(4,40); pstmt.executeUpdate(); pstmt.executeUpdate(); }} catch (SQLException ex) {{ catch (SQLException ex) ex.getMessage(); ex.getMessage(); }}}} 39

CallableStatement The interface used to execute SQL stored procedures A stored procedure is a group of SQL statements that form a logical unit and perform a particular task A CallableStatement object contains a call to a stored procedure; it does not contain the stored procedure itself. The part that is enclosed in curly braces is the escape syntax for stored procedures. CallableStatement cs = connection.prepareCall("{call SHOW_SUPPLIERS}"); ResultSet rs = cs.executeQuery(); 40

JDBC Core API 41

Servlet & JDBC 42

Servlet & JDBC (cont.) 43

Why Session Tracking? Need a mechanism to maintain state across a series of requests from the same user (or originating from the same browser) over some period of time  Example: Online shopping cart Yet, HTTP is stateless protocol  Each time, a client talks to a web server, it opens a new connection  Server does not automatically maintains “conversational state” of a user 44

Session Tracking Use Cases When clients at an online store add an item to their shopping cart, how does the server know what’s already in the cart? When clients decide to proceed to checkout, how can the server determine which previously created shopping cart is theirs? 45

A Session Maintains Client Identity and State across multiple HTTP requests Client 1 server Session 1 Session ID 1 Client 2 Session ID 2 Session 2 46

Three “underlying” Session Tracking Mechanisms Cookies URL rewriting Hidden form fields Note that these are just underlying mechanisms of passing “session id”  do not provide high-level programming APIs  do not provide a framework for managing sessions  This is what Servlet Session Tracking feature provides 47

What is HTTP Cookie? Cookie is a small amount of information sent by a servlet to a Web browser Saved by the browser, and later sent back to the server in subsequent requests  A cookie has a name, a single value, and optional attributes  A cookies value can uniquely identify a client Server uses cookies value to extract information about the session from some location on the server 48

Class Cookie 49

Cookies as Session Tracking Mechanism Advantages:  Very easy to implement  Highly customizable  Persist across browser shut-downs Disadvantages:  Often: users turn off cookies for privacy or security reason  Not quite universal browser support 50

URL Rewriting URLs can be rewritten or encoded to include session information. URL rewriting usually includes a session id Example:http://localhost:8080/bookstore1/cashier;jsessionid =c0o7fszeb1 51

URL Rewriting as Session Tracking Mechanism Advantages:  Letuser remain anonymous  They are universally supported(most styles) Disadvantages:  Tediousto rewrite all URLs  Only works for dynamically created documents 52

If Cookie is turned off.. If your application makes use of session objects  you must ensure that session tracking is enabled by having the application rewrite URLs whenever the client turns off cookies  by calling the responses encodeURL(URL) method on all URLs returned by a servlet  This method includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged 53

Example: response.encodeURL()out.println("<strong><a href="" + response.encodeURL(request.getContextPath() + "/cashier") + ""></a></strong>"); If cookies are turned off  http://localhost:8080/bookstore1/cashier;jsessionid=c0o7fszeb1 If cookies are turned on  http://localhost:8080/bookstore1/cashier 54

Hidden Form Fields Hidden form fields do not display in the browser, but can be sent back to the server by submit<INPUT TYPE=”HIDDEN” NAME=”session” VALUE=”...”> Fields can have identification (session id) or just some thing to remember (occupation) Servlet reads the fields using req.getParameter() 55

Hidden Form Fields as Session Tracking Mechanism Advantages:  Universally supported.  Allow anonymous users Disadvantages:  Only works for a sequence of dynamically generated forms.  Breaks down with static documents, emailed documents, bookmarked documents.  No browser shutdowns. 56

HttpSession Toget a users existing or new session object:  HttpSession session = request.getSession(true);  "true" means the server should create a new session object if necessary  HttpSession is Java interface  Container creates a object of HttpSession type 57

Example: Getting HttpSession Objectpublic class CatalogServlet extends HttpServlet { public void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Get the users session and shopping cart HttpSession session = request.getSession(true); ... out = response.getWriter(); ... } } ... 58

HttpSession Java Interface Contains Methods to  View and manipulate information about a session, such as the session identifier, creation time, and last accessed time  Bind objects to sessions, allowing user information to persist across multiple user connections 59

HttpSession Methods getId()  Returns the unique identifier isNew()  Determines if session is new to client (not page) getCreationTime()  Returns time at which session was first created getLastAccessedTime()  Returns time at which the session was last sent from the client invalidate()  Invalidate the session and unbind all objects associated with it 60

Store and Retrieve of Attribute To stores values:  session.setAttribute("cart", cart); To retrieves values:  session.getAttribute("cart"); 61

Session Timeout Used when an end-user can leave the browser without actively closing a session Sessions usually timeout after 30 minutes of inactivity  Product specific  A different timeout may be set by server admin getMaxInactiveInterval(), setMaxInactiveInterval() methods of HttpSession interface  Gets or sets the amount of time, session should go without access before being invalidated 62

Issues with “Stale” Session Objects The number of “stale” session objects that are in “to be timed out” could be rather large Example  1000 users with average 2 minutes session time, thus 15000 users during the 30 minutes period  4K bytes of data per session  15000 sessions * 4K -= 60M bytes of session data  This is just for one Web application Could have an performance impact  Use the data space in Session object with care 63

Session Invalidation Can be used by servlet programmer to end a session proactively  when a user at the browser clicks on “log out” button  when a business logic ends a session (“checkout” page in the example code in the following slide) public void invalidate()  Expire the session and unbinds all objects with it Caution  Remember that a session object is shared by multiple servlets/JSP- pages and invalidating it could destroy data that other servlet/JSP- pages are using 64

Example: Invalidate a Session public class ReceiptServlet extends HttpServlet { public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... scart = (ShoppingCart) session.getAttribute("examples.bookstore.cart"); ... // Clear out shopping cart by invalidating the session session.invalidate(); // set content type header before accessing the Writer response.setContentType("text/html"); out = response.getWriter(); ... }} 65

Scope Objects Enables sharing information among collaborating web components via attributes maintained in Scope objects  Attributes are name/object pairs Attributes maintained in the Scope objects are accessed with  getAttribute() & setAttribute() 4 Scope objects are defined  Web context, session, request, page 66

Four Scope Objects: Accessibility Web context (ServletContext)  Accessible from Web components within a Web context Session  Accessible from Web components handling a request that belongs to the session Request  Accessible from Web components handling the request Page  Accessible from JSP page that creates the object 67

Four Scope Objects: Class Web context  javax.servlet.ServletContext Session  javax.servlet.http.HttpSession Request  javax.servlet.http.HttpServletRequest Page  javax.servlet.jsp.PageContext 68

Session, Application ScopeClient 1 server Session 1 Session ID 1 Client 2 Session ID 2 Session 2 Client 1 server application Client 2

Session, Request, Page Scope client request response request response Page 1 forward Page 2 Page 3 Page 4 forwardPage scope Page scope Page scope Page scope request scope request scope Session scope

Scope of Objects 71

What is ServletContext For? Used by servlets to  Set and get context-wide (application-wide) object- valued attributes  Get request dispatcher  To forward to or include web component  Access Web context-wide initialization parameters set in the web.xml file  Access Web resources associated with the Web context  Log  Access other misc. information 72

Scope of ServletContext Context-wide scope  Shared by all servlets and JSP pages within a "web application"  Why it is called “web application scope”  A "web application" is a collection of servlets and content installed under a specific subset of the servers URL namespace and possibly installed via a *.war file  All servlets in BookStore web application share same ServletContext object  There is one ServletContext object per "web application" per Java Virtual Machine 73

Example: Getting Attribute Value from ServletContextpublic class CatalogServlet extends HttpServlet { private BookDB bookDB; public void init() throws ServletException { // Get context-wide attribute value from // ServletContext object bookDB = (BookDB)getServletContext(). getAttribute("bookDB"); if (bookDB == null) throw new UnavailableException("Couldnt get database."); }} 74

What is Servlet Request? Contains data passed from client to servlet All servlet requests implement ServletRequest interface which defines methods for accessing  Client sent parameters  Object-valued attributes  Locales  Client and server  Input stream  Protocol information  Content type  If request is made over secure channel (HTTPS) 75

Requestsdata,client, server, headerservlet itself Request Servlet 1 Servlet 2 Response Servlet 3 Web Server 76

Servlet Lifecycle Events Support event notifications for state changes in  ServletContext  Startup/shutdown  Attribute changes  HttpSession  Creationand invalidation  Changes in attributes 77

Listener Interfaces ServletContextListener  contextInitialized/Destroyed(ServletContextEvent) ServletContextAttributeListener  attributeAdded/Removed/Replaced(ServletContextAttributeEvent) HttpSessionListener  sessionCreated/Destroyed(HttpSessionEvent) HttpSessionAttributeListener  attributedAdded/Removed/Replaced(HttpSessionBindingEvent) HttpSessionActivationListener  Handles sessions migrate from one server to another  sessionWillPassivate(HttpSessionEvent)  sessionDidActivate(HttpSessionEvent) 78

Listener Registration Web container  creates an instance of each listener class  registers it for event notifications before processing first request by the application  Registers the listener instances according to  the interfaces they implement  the order in which they appear in the deployment descriptor web.xml Listeners are invoked in the order of their registration during execution 79

Steps for Implementing Servlet Lifecycle Event1. Decide which scope object you want to receive an event notification2. Implement appropriate interface3. Override methods that need to respond to the events of interest4. Obtain access to important Web application objects and use them5. Configure web.xml accordingly6. Provide any needed initialization parameters 80

Example: Context Listenerpublic final class ContextListener implements ServletContextListener { private ServletContext context = null; public void contextInitialized(ServletContextEvent event) { context = event.getServletContext(); try { BookDB bookDB = new BookDB(); context.setAttribute("bookDB", bookDB); } catch (Exception ex) { context.log("Couldnt create bookstore database bean: " + ex.getMessage()); } Counter counter = new Counter(); context.setAttribute("hitCounter", counter); counter = new Counter(); context.setAttribute("orderCounter", counter); } 81

Example: Context Listener public void contextDestroyed(ServletContextEvent event) { context = event.getServletContext(); BookDB bookDB =(BookDB)context.getAttribute("bookDB"); bookDB.remove(); context.removeAttribute("bookDB"); context.removeAttribute("hitCounter"); context.removeAttribute("orderCounter"); }} 82

Listener Configuration<web-app> <display-name>Bookstore1</display-name> <description>no description</description> <filter>..</filter> <filter-mapping>..</filter-mapping> <listener> <listener-class>listeners.ContextListener</listener-class> </listener> <servlet>..</servlet> <servlet-mapping>..</servlet-mapping> <session-config>..</session-config> <error-page>..</error-page> ...</web-app> 83

What are Java Servlet Filters? New component framework for intercepting and modifying requests and responses  Filters can be chained and plugged in to the system during deployment time Allows range of custom activities:  Marking access, blocking access  Caching, compression, logging  Authentication, access control, encryption Introduced in Servlet 2.3 (Tomcat 4.0)  Content transformations 84

What Can a Filter Do? Examine the request headers Customize the request object if it wishes to modify request headers or data Customize the response object if it wishes to modify response headers or data Invoke the next entity in the filter chain Examine response headers after it has invoked the next filter in the chain Throw an exception to indicate an error in processing 85

How Servlet Filter Work? Servlet ServletContainer Filter Chain Filter 1 Filter 2 Filter N doFilter( User Servlet service( ServletRequest, implemented container ServletRequest, filters filter ServletResponse, FilterChain) ServletResponse) 86

javax.servlet.Filter Interface init(FilterConfig)  called only once when the filter is first initialized  get ServletContext object from FilterConfig object and save it somewhere so that doFilter() method can access it  read filter initialization parameters from FilterConfig object through getInitParameter() method destroy()  called only once when container removes filter object  close files or database connections 87

How Filter Chain Works Multiple filters can be chained  order is dictated by the order of <filter> elements in the web.xml deployment descriptor The first filter of the filter chain is invoked by the container  via doFilter(ServletRequest req, ServletResponse res, FilterChain chain)  the filter then perform whatever filter logic and then call the next filter in the chain by calling chain.doFilter(..) method The last filters call to chain.doFilter() ends up calling service() method of the Servlet 88

Steps for Building a Servlet Filter Decide what custom filtering behavior you want to implement for a web resource Create a class that implements Filter interface  Implement filtering logic in the doFilter() method  Call the doFilter() method of FilterChain object Configure the filter with Target servlets and JSP pages  use <filter> and <filter-mapping> elements 89

Example: HitCounterFilterpublic final class HitCounterFilter implements Filter { private FilterConfig filterConfig = null; public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; } public void destroy() { this.filterConfig = null; } // Continued in the next page... 90

Example: HitCounterFilter public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { if (filterConfig == null) return; StringWriter sw = new StringWriter(); PrintWriter writer = new PrintWriter(sw); Counter counter = (Counter) filterConfig.getServletContext().getAttribute("hitCounter"); writer.println("The number of hits is: " + counter.incCounter()); // Log the resulting string writer.flush(); filterConfig.getServletContext().log(sw.getBuffer().toString()); ... chain.doFilter(request, wrapper); ... }} 91

HitCounterFilter Configuration<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE web-app PUBLIC -//Sun Microsystems, Inc.//DTD Web Application 2.3//EN http://java.sun.com/dtd/web-app_2_3.dtd><web-app> <display-name>Bookstore1</display-name> <description>no description</description> <filter> <filter-name>HitCounterFilter</filter-name> <filter-class>filters.HitCounterFilter</filter-class> </filter> <filter-mapping> <filter-name>HitCounterFilter</filter-name> <url-pattern>/enter</url-pattern> </filter-mapping> ... 92

Configuration in web.xml <filter>  <filter-name>: assigns a name of your choosing to the filter  <filter-class>: used by the container to identify the filter class </filter> <filter-mapping>  <filter-name>: assigns a name of your choosing to the filter  <url-pattern>: declares a pattern URLs (Web resources) to which the filter applies </filter-mapping> 93

Concurrency Issues on a Servlet The service() method of a servlet instance can be invoked by multiple clients (multiple threads) Servlet programmer has to deal with concurrency issue  shared data needs to be protected  this is called “servlet synchronization” use of synchronized block 94

Many Threads, One Servlet Instance Web Serverrequest threadrequest threadrequest thread Servlet threadrequest threadrequest Servlet container 95

Use of synchronized block Synchronized blocks are used to guarantee only one thread at a time can execute within a section of code synchronized(this) { myNumber = counter + 1; counter = myNumber; } ... synchronized(this) { counter = counter + 1 ; } 96

Thank you [email protected]/imcinstitute www.imcinstitute.com 97

Java Web Programming [3/9] : Servlet Advanced

by IMC Institute on Jan 06, 2013

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Java Web Programming [3/9] : Servlet Advanced Presentation Transcript