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CIS 5930-04 – Spring 2001 Part 7: JDBC Tutorial http://aspen.csit.fsu.edu/it1spring01 Instructors: Geoffrey Fox , Bryan Carpenter Computational Science and Information Technology Florida State University Acknowledgements: Nancy McCracken Syracuse University [email protected] 1 Introduction JDBC—usually interpreted as an acronym for Java Database Connectivity—was introduced in 1996, and revamped in 1998 for Java 2. It is an API definition intended for implementation as a Java interface to an SQL database. SQL (“sequel”) is the Structured Query Language, originally defined by IBM in the 70’s, standardized by ANSI/ISO in 1992. SQL is in turn based on the relational database model. It is implemented (with much variation) by many vendors of RDBMS (Relational Database Management System) software. First commercial implementation of SQL: Oracle, 1979. [email protected] 2 SQL [email protected] 3 Relations Mathematically, a relation is a subset of a product space. Equivalently, it is a set of “tuples” with entries from some fixed domains. In mathematics, the most important relations tend to be binary relations between entities of the same type: LessThan, on {0, 1, 2, 3} {0, 1, 2, 3}: { (0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3) } Contains, on {sunder, sun, sundry, dry} {sunder, sun, sundry, dry}: { (sunder, sunder), (sunder, sun), (sun, sun), (sundry, sundry), (sundry, sun), (sundry, dry), (dry, dry) } SameColor, on {tomato, cherry, banana} {tomato, cherry, banana}: { (tomato, tomato), (tomato, cherry), (cherry, tomato), (cherry, cherry), (banana, banana) } Examples respectively [email protected] of total order, partial order and 4 General Relations For databases, we are nearly always interested in n-ary relations between distinct domains, e.g. assume: Login = {wcao, Flora, Fulay, zyr98210, jin0328} LastName = {Cao, Flora, Fulay, Zhang} Dept = {CSIT, EE, CS} then we may define the relation: Students, on Login LastName Dept : { (wcao, Cao, CSIT), (Flora, Flora, EE ), (Fulay, Fulay, CS ), (zyr98210, Zhang, CS ), (jin0328, Zhang, CS ) } [email protected] 5 SQL Tables SQL is inspired by this mathematical model. Names are changed, and for pragmatic reasons the SQL model does not try to be mathematically pure. A relation is replaced by a table. The individual tuples in the relation are called rows. The domain sets of the relation are replaced by columns or attributes. Typing is not as strict as the mathematical model suggests. We define the allowed values of a column in terms of a limited set of predefined types, rather than actual sets. [email protected] 6 Creating a Table A possible SQL command to create a table for our Students relation is: CREATE TABLE students ( login VARCHAR(10), lastname VARCHAR(20), dept VARCHAR(5) ) Things to note: – Case is not significant here. By convention we use upper case for SQL keywords. – White-space, including line-breaks, is not significant. – The CREATE TABLE statement is syntactically like a class definition, defining columns (c.f. fields) and their types. – In this analogy, rows of the table would be like class instances. [email protected] 7 Column Types There are a limited set of types for column data. Unfortunately (although there is supposed to be a standard) in practice the available types are completely dependent on the vendor. In JDBC the types are standardized and include: INTEGER FLOAT(N) CHARACTER(N) VARCHAR(N) BLOB Typically 32-bit integer. Floating point with N bits of precision. Fixed length string, N characters. Variable length string, maximum N. A large binary object. [email protected] 8 SQL*Plus To create a table using the Oracle SQL*Plus interpreter, type the sqlplus command and enter your Oracle account name and password, then: SQL> create table students ( 2 login varchar(10), 3 lastname varchar(20), 4 dept varchar(5) 5 ); Table created In this interpreter commands are terminated by a semicolon. [email protected] 9 Inserting a Row The SQL command for adding a row to a table is, e.g.: INSERT INTO students VALUES (‘wcao’, ‘Cao’, ‘CSIT’) In sqlplus I enter: SQL> insert into students values ( 2 ‘wcao’, 3 ‘Cao’, 4 ‘CSIT’) ; 1 row created. The following examples assume the other tuples from the Student relation are entered in the same way. [email protected] 10 The SELECT command To view the Students table I can use the SELECT command: SELECT * FROM students The asterisk is a wildcard that causes all columns to be display. In sqlplus: SQL> select * from students ; LOGIN LASTNAME DEPT ----------------- ----------------------------- -----------wcao Cao CSIT Flora Flora EE Fulay Fulay CS zyr98210 Zhang CS jin0328 Zhang CS 5 rows selected [email protected] 11 Displaying Chosen Columns To limit the set of columns printed, specify them in the SELECT command, e.g.: SELECT login, lastname FROM students In sqlplus: SQL> select login, lastname from students ; LOGIN LASTNAME ----------------- ----------------------------wcao Cao Flora Flora Fulay Fulay zyr98210 Zhang jin0328 Zhang 5 rows selected [email protected] 12 Displaying Chosen Rows To limit the set of rows printed, I add a WHERE clause: SELECT * FROM students WHERE dept=‘CS’ Other kinds of tests that can appear in the WHERE clause will be described later. In sqlplus: SQL> select * from students where dept=‘CS’ ; LOGIN LASTNAME DEPT ----------------- ----------------------------- -----------Fulay Fulay CS zyr98210 Zhang CS jin0328 Zhang CS 3 rows selected [email protected] 13 A Second Table The following examples assume I define a second table by: CREATE TABLE departments ( abbrev VARCHAR(5), name VARCHAR(50) ) and add the rows: { (‘CSIT’, ‘Computational Science and Information Technology’), (‘EE’, ‘Electrical Engineering’), (‘CS’, ‘Computer Science’) } [email protected] 14 Selecting from Multiple Tables A SELECT command can display data from more than one tables: SELECT * FROM students, departments By itself this just produces a mess. In sqlplus: SQL> select * from students, departments ; LOGIN LASTNAME DEPT ABBRE ----------------- ----------------------------- ------------ ---------NAME -------------------------------------------------------------------wcao Cao CSIT CSIT Computational Science and Information Technology Flora Flora EE CSIT Computational Science and Information Technology Fulay Fulay CS CSIT Computational Science and Information Technology ... 15 rows selected [email protected] 15 Joins The previous query returned 15 rows. It simply yielded a “Cartesian product” of the two relations, with every row of students being combined with every row of departments. In itself this is not a useful result. But is a basis from which to add a WHERE clause to select out some meaningful combinations of values from the two tables. If two tables appearing in the same statement share some identical column names, can disambiguate by using qualified names, e.g.: students.login, students.lastname, students.dept, etc. [email protected] 16 References Between Tables Here is a meaningful query involving the two tables: SELECT login, name FROM students, departments WHERE dept=abbrev In sqlplus: SQL> select login, name from students, departments 2 where dept=abbrev ; LOGIN NAME ----------------- -------------------------------------------------------------------------Fulay Computer Science zyr98210 Computer Science jin0328 Computer Science wcao Computational Science and Information Technology Flora Electrical Engineering 5 rows selected [email protected] 17 Primary Keys and Foreign Keys This kind of cross-reference is so important that SQL provides syntax to allow automatic “integrity checks” and allow optimizations. We can change the abbrev column to be a primary key of the departments table by the following SQL command: ALTER TABLE departments ADD PRIMARY KEY (abbrev) We add a constraint that any allowed value in the dept column of students must be a valid primary key in the departments table by the SQL command: ALTER TABLE students ADD FOREIGN KEY (dept) REFERENCES departments (abbrev) [email protected] 18 Integrity Checks The system will now forbid addition of values to the dept column of students that do not correspond to values in the abbrev column of departments. For example, I can try to change the dept column of the row describing wcao by the SQL UPDATE command: UPDATE students SET dept=‘IE’ WHERE login=‘wcao’ Because of the constraints, Oracle will refuse to make this update. In sqlplus: SQL> update students set dept=‘IE’ where login=‘wcao’ ; ERROR at line 1 : ORA-02291: integrity constraint violated - parent key not found. Incidentally this example illustrates the use of UPDATE command, which changes attributes of existing rows. [email protected] 19 Other Useful Commands . . . in addition to CREATE, SELECT and UPDATE, which were illustrated earlier: DELETE FROM table_name WHERE condition Deletes selected rows. DROP TABLE table_name Removes a table. DESCRIBE table_name Describes columns of a table. COMMIT Save changes made in this transaction. ROLLBACK Undo changes made in this transaction. [email protected] 20 Conditions Conditions in WHERE clauses are Boolean expressions built from: – Comparision operators =, <>, <, <=, >, >= – Boolean operators AND, OR, NOT – The LIKE operator. The LIKE operator compares a column value to a pattern. – In the pattern the wildcard % represents zero or more characters. – The wildcard _ represents a single character. [email protected] 21 JDBC [email protected] 22 A Simple Example import java.sql.* ; public class ShowStudents { public static void main(String args[]) throws Exception { System.setProperty(“jdbc.drivers”, “oracle.jdbc.driver.OracleDriver”) ; String url = “jdbc:oracle:thin:@sirah.csit.fsu.edu:1521:oralin” ; Connection conn = DriverManager.getConnection(url, “dbc”, “ . . . ”) ; Statement stat = conn.createStatement() ; ResultSet rs = stat.executeQuery(“SELECT * FROM students”) ; while(rs.next()) System.out.println(rs.getString(1) + “ ” + rs.getString(2) + “ ” + rs.getString(3)) ; conn.close() ; } } [email protected] 23 Remarks To compile and run this example you must have the JDBC driver code on your class path. On the course hosts, add /usr/oracle/jdbc/lib/classes111.zip to your CLASSPATH. For example you might add the line export CLASSPATH=$CLASSPATH:\ $ORACLE_HOME/jdbc/lib/classes111.zip to the end of you .bashrc file. [email protected] 24 Running ShowStudents If we run ShowStudents we may see something like: sirah$ java ShowStudents wcao Cao CSIT Flora Flora EE Fulay Fulay CS zyr98210 Zhang CS jin0328 Zhang CS Effect is essentially like typing the command select * from students ; directly into the SQL*Plus interpreter. [email protected] 25 Classes in the Example The example introduces the following classes and interfaces from java.sql: DriverManager Manages a set of JDBC drivers. Connection A connection or session with a specific database. Context in which SQL statements are executed and results returned. Statement Object used for executing an SQL statement. ResultSet Provides access to a table of data generated by executing a statement. [email protected] 26 The Driver Manager The driver manager sits between the JDBC application and one or more JDBC drivers. A JDBC driver contains vendor-specific code to interface to a particular back-end database. On initialization, the class DriverManager will try to load driver classes referenced in the jdbc.drivers property. One can also load JDBC drivers explicitly, e.g.: Class.forName(“oracle.jdbc.driver.OracleDriver”) ; [email protected] 27 Making a Connection There are several getConnection() methods on DriverManager with different argument lists. The one we will use is: static Connection getConnection(String url, String username, String password) Before you can use this method, you will, of course, need an account on the database concerned. – The username and password are associated with the database (not your UNIX account!) [email protected] 28 Database URLs Ideally the syntax of the URL would follow the normal conventions for Internet URLs: protocol // host : port / name – protocol will be a sub-protocol of jdbc:, e.g. jdbc:oracle:thin: – host and port are self-explanatory. – name is the name of the database on the host. Oracle JDBC URLs follow this general pattern, but they use different separators. – @ and : in place of // and /. [email protected] 29 The Connection Interface The Connection interface includes many methods. Our first example only uses two simple ones: Statement createStatement() Creates a Statement object for sending SQL statements to the database. void close() Releases database connection and associated JDBC resources. [email protected] 30 The Statement Interface Object used for executing an SQL statement. The most important methods for us will be: ResultSet executeQuery(String sql) throws SQLException Executes an SQL statement that returns a single result set. Typically sql will be a SELECT statement. int executeUpdate(String sql) throws SQLException Executes an SQL statement that returns nothing. Typically sql will be an INSERT, UPDATE or DELETE statement. Result is number rows modified. Note only one ResultSet can exist per Statement. If you need to interleave queries, use multiple Statement objects. Other useful methods: – addBatch(), executeBatch() to batch several SQL commands. [email protected] 31 The ResultSet Interface A result set behaves something like an Enumeration or Iterator. This allows to iterate over the set of rows returned by a query. The next() method is used to move to the first row, then all subsequent rows. There is a large number of methods of the form: XXX getXXX(int columnIndex) that extract the contents of a column in the current row. The one we will use most is getString(), which interprets the column as a String. Note columnIndex starts at 1, not zero. ResultSet has many other methods. Many were added in JDBC 2.0. [email protected] 32 Example: Web Front End to SQL Will now go through a fairly substantial example that combines JDBC with servlets. This Web application will allow a user to connect to an Oracle account on the server host, and submit arbitrary SQL commands and queries. Besides introducing a couple of new JDBC features, it will illustrate (in more depth than the vending machine examples) the structure of a Web application involving multiple servlets. [email protected] 33 A First Screen The introductory screen to our application is a static HTML form, sqlconnect.html: <html> <head> . . . </head> <body> <h1>Connect to Oracle database</h1> <form method=post action=“/dbc/servlet/SQLConnect”> Oracle user name: <input type=text name=username size=20> <p> Oracle password: <input type=password name=password size=20> <p> <input type=submit value=“Connect to database”> </form> </body> </html> [email protected] 34 Remarks The form prompts for Oracle user name and password. These are sent using the POST method (for privacy) to the servlet SQLConnect. Note the form of the action URL. This is a URL to a the servlet context /dbc in the same Web server. [email protected] 35 The SQLConnect servlet public class SQLConnect extends HttpServlet { ... public void doPost(HttpServletRequest req, HttpServletResponse resp) throws . . . { try { String username = req.getParameter(“username”) ; String password = req.getParameter(“password”) ; HttpSession session = req.getSession(true) ; Connection conn = DriverManager.getConnection(url, username, password) ; session.setAttribute(“connection”, conn) ; resp.sendRedirect( resp.encodeRedirectURL(“/dbc/servlet/SQLCommand”)) ; } catch (SQLException e) { . . . make suitable sendError() call . . . } [email protected] 36 } Remarks The essential code creates a servlet session, connects to the database, and stores the Connection object in the servlet session. This done, the browser is redirected to the SQLCommand servlet. encodeRedirectURL() is similar to encodeURL(), but specifically intended for use with a sendRedirect() . We assume that the driver is loaded by a suitable init() method, e.g.: public void init() { Class.forName(“oracle.jdbc.driver.OracleDriver”) ; } and the URL for the database is predefined in a static variable url. [email protected] 37 The SQLCommand servlet public class SQLCommand extends HttpServlet { public void doGet(HttpServletRequest req, HttpServletResponse resp) throws . . . { HttpSession session = req.getSession(true) ; Connection conn = (Connection) session.getAttribute(“connection”) ; if(conn == null) . . . session probably timed out: call sendError() . . . else { resp.setContentType(“text/html”) ; PrintWriter out = resp.getWriter() ; out.println(“<html><head></head><body>”) ; . . . print forms . . . out.println(“</body></html>”) ; } } } [email protected] 38 Remarks The connection is retrieved from the current session, if it exists. Otherwise the servlet just prints three forms, for SQL queries, SQL action commands, and disconnecting, respectively. . . [email protected] 39 Printing the Forms out.println("<form action=" + resp.encodeURL("/dbc/servlet/SQLQuery") + ">") ; out.println("<h1>SQL <em>query</em></h1>") ; out.println("<textarea name=query cols=60 rows=3></textarea><p>") ; out.println("<input type=submit value=\"Submit Query\">") ; out.println("</form>"); out.println("<form action=" + resp.encodeURL("/dbc/servlet/SQLUpdate") + ">") ; out.println("<h1>SQL <em>update</em></h1>") ; out.println("<textarea name=update cols=60 rows=3></textarea><p>") ; out.println("<input type=submit value=\"Submit Update\">") ; out.println("</form>"); out.println("<form action=" + resp.encodeURL("/dbc/servlet/SQLClose") + ">") ; out.println("Close Oracle Connection:") ; out.println("<input type=submit value=\"Close\">") ; out.println("</form>"); [email protected] 40 Handling Queries The most complicated servlet in our application is SQLQuery. This will execute an arbitrary SELECT command, and print out the results as an HTML table. This presents some special problems because we don’t know in advance how many columns the result table will need, or what is the meaning of the entries. We can cope with this kind of situation using result set metadata. [email protected] 41 Result Set Metadata The ResultSet class has a method: ResultSetMetaData getMetaData() that returns a metadata object describing the results. The only methods on ResultSetMetaData we will use here are: int getColumnCount() returns the number of columns in the result set. String getColumnLabel(int columnIndex) returns a suggested label for use in printouts. [email protected] 42 The SQLQuery servlet public class SQLQuery extends HttpServlet { public void doGet(HttpServletRequest req, HttpServletResponse resp) throws . . . { HttpSession session = req.getSession(true) ; Connection conn = (Connection) session.getAttribute(“connection”) ; if(conn == null) . . . session probably timed out: call sendError() . . . else { String query = req.getParameter(“query”) ; try { Statement stat = conn.createStatement() ; ResultSet rs = stat.executeQuery(query) ; . . . generate response . . . } catch (SQLException e) { . . . make suitable sendError() call . . . } } [email protected] } 43 Remarks This servlet gets the SQL query command from the form data, creates a Statement, and executes the query. The non-trivial work is in generating the HTML response. . . [email protected] 44 Generating the Response ResultSetMetaData rsmd = rs.getMetaData() ; int cols = rsmd.getColumnCount() ; . . . set response content type, print HTML headers . . . out.println("<table border cellspacing=0 cellpadding=5>") ; out.println("<tr bgcolor=lightblue>") ; // print column headers for(int i = 1 ; i <= cols ; i++) out.println("<td>" + rsmd.getColumnLabel(i) + "</td>") ; out.println("</tr>") ; while(rs.next()) { // print the rows out.println("<tr>") ; for(int i = 1 ; i <= cols ; i++) out.println("<td>" + rs.getString(i) + "</td>") ; out.println("</tr>") ; } out.println("</table>") ; . . . print HTML footers . . . [email protected] 45 Remarks The complete code generates a couple of form buttons at the end of the HTML page, giving the option to submit another command (go back to the SQLCommand servlet) or close the connection (go to the SQLClose servlet). [email protected] 46 Action Statements The last two servlets are relatively simple. SQLUpdate has a similar structure to SQLQuery but it reads the update parameter, and executes the command by: String update = req.getParameter(“update”) ; try { Statement stat = conn.createStatement() ; int rows = stat.executeUpdate(update) ; . . . generate response . . . } catch (. . .) { . . .} SQLClose calls conn.close() then redirects the browser back to the sqlconnect.html page. [email protected] 47 Submitting a Command [email protected] 48 The Response [email protected] 49 An Elementary “3-tier” Application [email protected] 50 A Student Database The application is highly simplified, but it illustrates a form-based interface to an Oracle database on a server, with a servlet “middle-tier”. It is more developed than the preceding SQL frontend—it doesn’t require the client to actually type SQL commands and interpret the results! It uses session tracking to implement an “undo” feature for updates. It also illustrates a new feature of the servlet sessiontracking API—session binding listeners. Otherwise it is quite limited: – It doesn’t include features to add or delete records, only to modify existing records. – The user interface is not very pretty. [email protected] 51 Creating the Table As a preliminary, we need to create the initial state for database table. This is a management task for our Web application—not part of the online functionality. Nevertheless it is quite convenient to write a Java program to create the table. [email protected] 52 Java “Script” to Create Table class MakeTable implements DBConstants { public static void main(String args[]) throws Exception { . . . Load driver . . . Connection conn = DriverManager.getConnection(url, username, password); Statement stat = conn.createStatement(); stat.executeUpdate( "CREATE TABLE " + table + " (" + "login VARCHAR(10)," + "lastname VARCHAR(20)," + "firstnames VARCHAR(20)," + "email VARCHAR(50)," + "dept VARCHAR(5)" + ")" ); . . . Add entries to table . . . conn.close(); } } [email protected] 53 Adding the Entries BufferedReader in = new BufferedReader(new FileReader(table)) ; String login, lastname, firstnames, email, dept ; login = in.readLine() ; while(login != null) { lastname = in.readLine() ; firstnames = in.readLine() ; email = in.readLine() ; dept = in.readLine() ; stat.executeUpdate( "INSERT INTO " + table + " VALUES ('" + login + "', '" + lastname + "', '" + firstnames + "', " + "'" + email + "', '" + dept + "')" ); in.readLine() ; // skip blank line ; login = in.readLine() ; } [email protected] 54 Remarks This assumes the initial data is stored in a file having the same name as the table to be created. Various constants associated with the database account and table are stored in the DBConstants interface, e.g.: public interface DBConstants { static final String url = “jdbc:oracle:thin:@sirah.csit.fsu.edu:1521:oralin” ; static final String username = “dbc” ; static final String password = “ . . . ” ; static final String table = “it1fall00” ; } [email protected] 55 The Servlets This particular application consists of several servlets and associated classes. We will collect them together into a new context in our Web server, under the path /students. In conf/server.xml we add the lines: <Context path=“/students” docBase=“webapps/students” debug=“0” reloadable=“true”> </Context> and create the associated document directories (see the lecture on configuring the Tomcat server). [email protected] 56 Selection Page The “main” page of our application offers a list of “keys” to select from—in our case student login IDs. This page is generated by a servlet called Select. This servlet checks session data is in order and retrieves or creates an object of auxiliary class DBSession, which we use to manage the database connection. It extracts to column of login IDs from the table. Finally it generates the HTML for the displayed page. [email protected] 57 Overview of Select servlet public class Select extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws . . . { try { DBSession dbs ; . . . Initialize session, and extract dbs. ResultSet rs = dbs.stat.executeQuery( "SELECT login FROM " + DBConstants.table ); . . . Generate response . . . } catch (SQLException e) { response.sendError(HttpServletResponse.SC_OK, "SQL error: " + e.getMessage()) ; } } [email protected] 58 } Initializing the Session HttpSession session = request.getSession(true) ; dbs = (DBSession) session.getAttribute("dbs") ; if(dbs == null) { // Session new or timed out dbs = new DBSession() ; session.setAttribute("dbs", dbs) ; session.setMaxInactiveInterval(300) ; // 5 minutes. } We don’t want to accumulate many idle database connections, so we explicitly set a relatively short timeout interval for the session. The DBSession constructor creates a database connection. The object then stores the Connection object and a reusable Statement object. [email protected] 59 The DBSession class public class DBSession implements HttpSessionBindingListener, DBConstants { static { Class.forName("oracle.jdbc.driver.OracleDriver") ; } DBSession() throws SQLException { conn = DriverManager.getConnection(url, username, password); stat = conn.createStatement(); } public void valueBound(HttpSessionBindingEvent evt) {} public void valueUnbound(HttpSessionBindingEvent evt) { try { conn.close() ; } catch (SQLException e) {} // Not much to be done about it. } public Connection conn ; public Statement stat ; } [email protected] 60 Remarks The most interesting feature of this class is that it implements HttpSessionBindingListener. The event-handling method valueBound() is invoked whenever this object is bound to a session using setAttribute(). The method valueUnbound() is invoked whenever this object is unbound from a session, either by removeAttribute() or (more importantly) when the session is invalidated (e.g. when it times out). This ensures that the database connection is closed promptly (and not left to the garbage collector). The class also illustrates use of a static initialization clause. [email protected] 61 Generating the Response response.setContentType("text/html"); PrintWriter out = response.getWriter(); . . . print HTML headers . . . out.println("<form action=" + response.encodeURL("/students/servlet/View") + ">") ; out.println("Select by ID:<br>") ; out.println("<select name=login size=15>") ; while(rs.next()) out.println("<option> " + rs.getString(1)) ; out.println("</select>") ; out.println("View selected record: " + "<input type=submit value=\"View\">") ; out.println("</form>"); . . . print HTML footers . . . [email protected] 62 Remarks This mainly just prints a form containing big select element holding all the key values from the result set. [email protected] 63 Viewing and Modifying Record The viewing page displays all the fields in the selected record in a form that allows them to be modified, if desired. This page is generated by a servlet called View. The session handling is similar to Select (at this point if the session has timed out we simply create a new one). The servlet extracts the row associated with the selected login ID. For future reference, it saves the current value of the fields in the session object. It generates the HTML for the displayed page. [email protected] 64 Overview of the View servlet public class View extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws . . . { try { DBSession dbs ; . . . Initialize session, and extract dbs. String login, lastname, firstnames, email, dept ; login = request.getParameter("login") ; // Get the key string if(login == null) { . . . Call sendError() . . . } . . . Query the database, and extract values lastname, etc. . . . session.setAttribute("curr", new Student(login, lastname, firstnames, email, dept)) ; . . . Generate updateable display of current record . . . } catch (SQLException e) { . . . Call sendError() . [email protected] .. 65 } Remarks The Student class defines a simple object that just holds 5 public fields. [email protected] 66 Querying the Database ResultSet rs = dbs.stat.executeQuery( "SELECT * FROM " + DBConstants.table + " " + "WHERE login=’ " + login + “ ’ ” ); if(rs.next()) { lastname = rs.getString(2) ; firstnames = rs.getString(3) ; email = rs.getString(4) ; dept = rs.getString(5) ; } else { response.sendError(HttpServletResponse.SC_OK, "Record not found") ; return ; } [email protected] 67 Generating the Response response.setContentType("text/html"); PrintWriter out = response.getWriter(); . . . Print HTML headers . . . out.println("<form method=post action=" + response.encodeURL("/students/servlet/Update") + ">") ; out.println("Login ID: " + "<input type=text size=10 name=login value=\"" + login + "\"><br>") ; out.println("Last Name: " + "<input type=text size=20 name=lastname value=\"" + lastname +"\"><br>") ; . . . etc... Input elements for the other three fields . . . out.println("<br>Update this record: " + "<input type=submit value=Update>") ; out.println("</form>"); . . . Print small alternate form to return to selection page . . . [email protected] 68 . . . Print HTML footers . . . Updating the Database This is handled by a servlet that normally generates no output. When it is finished it just redirects the browser back to the selection page. The session handling is slightly different to Select. In this case if the servlet cannot find a current session it will report an error. The servlet updates the row associated with the selected login ID. [email protected] 69 Overview of the Update Servlet public class Update extends HttpServlet { public void doPost(HttpServletRequest request, HttpServletResponse response) throws . . . { try { DBSession dbs ; Student curr ; . . . Extract dbs, curr from the session object . . . . . . Update row in database . . . . . . Build a command to undo the update, and save it . . . response.sendRedirect( response.encodeRedirectURL("/students/servlet/Select")); } catch (SQLException e) { . . . Call sendError() . . . } } } [email protected] 70 Extracting Session Data HttpSession session = request.getSession(true) ; DBSession dbs = (DBSession) session.getAttribute("dbs") ; Student curr = (Student) session.getAttribute("curr") ; if(dbs == null || curr == null) { response.sendError(HttpServletResponse.SC_OK, "Session timed out. Please resubmit form.") ; return ; } In this case it is difficult to proceed without the session data. [email protected] 71 Updating the Row String login = request.getParameter("login") ; String lastname = request.getParameter("lastname") ; String firstnames = request.getParameter("firstnames") ; String email = request.getParameter("email") ; String dept = request.getParameter("dept") ; dbs.stat.executeUpdate( "UPDATE " + DBConstants.table + " SET " + "login=’ " + login + “', " + "lastname=’ " + lastname + “', " + "firstnames=’ " + firstnames + “', " + "email=’ " + email + “', " + "dept=’ " + dept + “' " + "WHERE login=’ " + curr.login + “'” ); [email protected] 72 Building an “Undo” Command String sql = "UPDATE " + DBConstants.table + " SET " + "login='" + curr.login + "', " + "lastname='" + curr.lastname + "', " + "firstnames='" + curr.firstnames + "', " + ”email='" + curr.email + "', " + "dept='" + curr.dept + "' " + "WHERE login='" + login + "'" ; Vector undoList = (Vector) session.getAttribute("undoList") ; if(undoList == null) { undoList = new Vector() ; session.setAttribute("undoList", undoList) ; } undoList.addElement(sql) ; [email protected] 73 Remarks The “undo” command will reverse the effect of the update. It is not executed here: it is added to a stack held in the session. [email protected] 74 An Undo Button To complete the application we generate the following extra form in the Select servlet: Vector undoList = (Vector) session.getAttribute("undoList") ; if (undoList != null && undoList.size() > 0) { out.println("<form action=" + response.encodeURL("/students/servlet/Undo") + ">") ; out.println("Undo most recent change: " + "<input type=submit value=\"Undo\">") ; out.println("</form>"); } The Undo servlet just retrieves the undoList from the session and does: int last = undoList.size() - 1 ; [email protected] undoList.remove(last)) ; 75 dbs.stat.executeUpdate((String)