1 Technologies for an Information Age:.opennet JDBC Tutorial Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana) Time Bryan Carpenter and Geoffrey.

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Technologies for an Information Age: .opennet JDBC Tutorial

Fall Semester 2001 MW 5:00 pm - 6:20 pm CENTRAL (not Indiana) Time

Bryan Carpenter and Geoffrey FoxPTLIU Laboratory for Community

Grids Computer Science, Informatics, PhysicsIndiana UniversityBloomington IN [email protected]

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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.

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These Lectures Start with a naïve introduction to SQL. Followed by an introduction to basics of JDBC. Continue with an example invoking SQL

commands through a JSP-based Web application. To run the examples exactly as presented here

you will need to have MySQL installed.– An earlier version of these lectures was based on

Oracle instead, and that will show through—responses of the MySQL interpreter may not always be formatted exactly as shown in these slides.

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Installing MySQL (circa 9/5/01) Go to www.mysql.com (not mysql.org!) Download the Zip file, MySQL 3.23.44 for Windows. File

is mysql-3.23.44-win.zip.– If you don’t have Winzip archiving program or equivalent, you will

need to download that as well, e.g. from download.com, keyword Winzip.

From Windows Explorer, double-click the zip-file to unpack it.

Double-click the command \mysql\bin\winmysqladmin.exe to start the MySQL administration tool.

Click on the traffic-light icon to start the database server itself (“Win NT” → “Start the server standalone”).

Execute the command \mysql\bin\mysql to start the SQL interpreter.

Before issuing SQL commands, you will have to create and select a named database in this server (c.f. end of SQL section in these lectures).

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SQL

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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 of total order, partial order and equivalence relation.

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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 ) }

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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 called a table. The individual tuples in the relation are called

rows. The domain sets of the relation are called 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.

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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.

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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 Typically 32-bit integer. FLOAT(N) Floating point with N bits of

precision. CHARACTER(N) Fixed length string, N characters. VARCHAR(N) Variable length string, maximum

N. BLOB A large binary object.

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MySQL To create a table using the MySQL interpreter,

type then: mysql> 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.

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Inserting a Row The SQL command for adding a row to a table is,

e.g.:INSERT INTO students VALUES (‘wcao’, ‘Cao’,

‘CSIT’) In mysql I enter:

mysql> 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.

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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 mysql: mysql> select * from students ; LOGIN LASTNAME DEPT ----------------- ----------------------------- ------------ wcao Cao CSIT Flora Flora EE Fulay Fulay CS zyr98210 Zhang CS jin0328 Zhang CS 5 rows selected

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Displaying Chosen Columns To limit the set of columns printed, specify them

in the SELECT command, e.g.: SELECT login, lastname FROM students

In mysql: mysql> select login, lastname from

students ; LOGIN LASTNAME ----------------- ----------------------------- wcao Cao Flora Flora Fulay Fulay zyr98210 Zhang jin0328 Zhang 5 rows selected

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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 mysql: mysql> select * from students where dept=‘CS’ ; LOGIN LASTNAME DEPT ----------------- ----------------------------- ------------ Fulay Fulay CS zyr98210 Zhang CS jin0328 Zhang CS 3 rows selected

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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’) }

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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 mysql:mysql> select * from students, departments ;LOGIN LASTNAME DEPT ABBRE----------------- ----------------------------- ------------ ---------- NAME-------------------------------------------------------------------- wcao Cao CSIT CSITComputational Science and Information TechnologyFlora Flora EE CSITComputational Science and Information TechnologyFulay Fulay CS CSITComputational Science and Information Technology. . .15 rows selected

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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.

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References Between Tables Here is a meaningful query involving the two tables:

SELECT login, name FROM students, departments WHERE dept=abbrev

In mysql:mysql> select login, name from students, departments 2 where dept=abbrev ;

LOGIN NAME-----------------

--------------------------------------------------------------------------Fulay Computer Sciencezyr98210 Computer Sciencejin0328 Computer Sciencewcao Computational Science and Information

TechnologyFlora Electrical Engineering5 rows selected

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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 can also 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)

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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 (for example) 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.

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MySQL Idiosynchrasies Last two slides will work as described for Oracle. MySQL seems to be more pernickety about adding a

primary key to an existing table. I first had to specify: alter table departments modify abbrev varchar(5) not null

; to redundantly assure the system that the primary key

would never go null.– Note SQL also allows you to specify these column constraints in

the CREATE TABLE command. Also note, MySQL does not currently implement the

integrity checks for foreign keys, described in the preceding slide.– The system accepts the FOREIGN KEY constraint, but ignores

it.– The documentation says such checks will be implemented in the

version 4.1

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Other Useful Commands . . . in addition to CREATE, SELECT and

UPDATE, which were illustrated earlier:DELETE FROM table_name WHERE conditionDeletes selected rows.DROP TABLE table_nameRemoves a table.DESCRIBE table_nameDescribes columns of a table.COMMITSave changes made in this transaction.ROLLBACKUndo changes made in this transaction.

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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.

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MySQL Administrivia To create a database called dbname:

mysql> create database dbname ; To start using that database:

mysql> use dbname ; To get a list of all current databases:

mysql> show databases ; To get a list of all current tables:

mysql> show tables ; To show the column characteristics of a table

called tablename:mysql> describe tablename ;

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JDBC

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A Simple Exampleimport java.sql.* ;public class ShowStudents { public static void main(String args[]) throws Exception { String url = “jdbc:mysql://localhost/opennet_fall01” ; System.setProperty(“jdbc.drivers”, “org.gjt.mm.mysql.Driver”) ; Connection conn = DriverManager.getConnection(url) ; 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() ; }}

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Remarks To compile and run this example you must have a JDBC

driver installed for the database, and it must be on your class path.

For MySQL, go to http://mmmysql.sourceforge.net/ and download the file mm.mysql-2.0.7-you-must-unjar-me.jar.

On Windows, save it in the folder C:\, for example. In this folder, run the command

jar xvf mm.mysql-2.0.7-you-must-unjar-me.jar You should find the class file for

org.gjt.mm.mysql.Driver inside the C:\mm.mysql-2.0.7\ folder.

Create a CLASSPATH environment variable with value: .;c:\mm.mysql-2.0.7

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Running ShowStudents If we run ShowStudents we may see something

like: C:\ptliuegs> java ShowStudents wcao Cao CSIT Flora Flora EE Fulay Fulay CS zyr98210 Zhang CS jin0328 Zhang CS

Effect is essentially like typing the commandselect * from students ;

directly into the MySQL interpreter.

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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.

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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(“org.gjt.mm.mysql.Driver”) ;

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Making a Connection There are several getConnection() methods on

DriverManager with different argument lists. The one we used is: static Connection getConnection(String url)

If you are using a non-default DB account and password, use instead: static Connection getConnection(String url, String

username, String

password)

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Database URLs Generally speaking the syntax of the URL should

follow the normal conventions for Internet URLs: protocol // host : port / name– protocol will be a sub-protocol of jdbc:, e.g. jdbc:mysql:– host and port are self-explanatory.– name is the name of the database on the host.

The precise format depends on the driver. Oracle JDBC URLs follow the general pattern, but they use different separators.– @ and : in place of // and /.

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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.

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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.

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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.

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A Web Front-end to SQL

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Example: Web Front End to SQL We will go through a fairly substantial example

that combines JDBC with JSP. This Web application will allow a user to connect

to a MySQL database 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 JSP pages.

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Preliminaries Before making your database accessible through a Web

server, you should set up some password protection. First, use the mysqladmin command in mysql\bin\ to set

the database administrator password, e.g.:C:\mysql\bin> mysqladmin -u root password admin-password– Can still connect to the database from the local host without

specifying a user name or password, but other computers can no longer connect to your database with dangerous privileges.

Example on following slides assumes a special account is created with free access to one specified database on the server. The account is created by issuing the command:mysql> GRANT ALL ON opennet_fall01.* TO

tomcat@localhost IDENTIFIED BY “password” ;– The account name is tomcat, database is opennet_fall01.– Issue this command from within the mysql interpreter.

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A First Screen The introductory screen to our application is a static

HTML form, sqllogin.html:<html> <head></head> <body> <h1>Connect to SQL database</h1> <form method=“post” action=“sqlconnect.jsp”> SQL password: <input type=“password”

name=“password” size=“20”/> <p/> <input type=“submit” value=“Connect to

database”/> </form> </body></html>

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Browser displays:

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Remarks The form prompts for a password

– We will see that this is the password associated with a database account for a user called tomcat.

This is sent using the POST method (for limited privacy) to the JSP page sqlconnect.jsp.

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The sqlconnect.jsp JSP page<%@ page errorPage=“sqlerror.jsp” %><jsp:useBean id=“sql” class=“mybeans.SQLBean” scope=“session”/> <jsp:setProperty name=“sql” property=“password”/><% sql.connect() ; %><html><head></head> <body> <jsp:include page="sqlcommand.jsp"/> </body></html>

This page establishes a session bean, sql, class mybeans.SQLBean.

We assume this bean has properties and methods:– A write-only property password, set from the form parameter of the same

name. – A void, no-argument method called connect(), which attempts to establish

the database connection. Further properties and methods will be introduced as needed. After connection, bulk of document is generated by

sqlcommand.jsp.

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Aside: An Unfortunate Feature If a form field is left empty, the parameter value sent to

the server is the empty string, “”. Unfortunately the jsp:setProperty action does nothing in

this case. In our example this has the odd effect that if you return to

the login page and submit with an empty password while the old session is still valid, the connection succeeds because the old password is remembered by the bean (setPassword() is not called to change it.)

It’s not clear how serious a security risk this is, but if you are concerned, replace the use of the jsp:setProperty action with an explicit scriptlet:<% sql.setPassword(request.getParameter("password")) ;%>

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The sqlerror.jsp JSP page The page directive in sqlconnect.jsp

established an error page sqlerror.jsp. If any bean method or accessor used in the page

throws an exception, control is transferred to the page sqlerror.jsp:<%@ page isErrorPage=“true” %><html><head></head> <body> SQL error: <%= exception.getMessage() %> </body></html>

If a page is declared to be an error page through the page directive, the scripting variable exception is available for use.

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The sqlcommand.jsp JSP page<form action=“sqlquery.jsp”> <h1>SQL <em>query</em></h1> <textarea name=“query” cols=“60”

rows=“3”></textarea><p> <input type=submit value=“Submit Query”/></form><form action=“sqlupdate.jsp”> <h1>SQL <em>update</em></h1> <textarea name=“update” cols=“60”

rows=“3”></textarea><p> <input type=submit value=“Submit Update”></form><form action=“sqlclose.jsp”> Close database connection: <input type=submit value=“Close”/></form>

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Submitting a Command

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Remarks This pages just prints three forms, for SQL queries, SQL

action commands, and disconnecting, respectively. Clearly one could use a static HTML page to do this.

There is a subtle reason why one may prefer a JSP page.– The JSP session tracking mechanism preferentially uses cookies. If

this fails, because the user has configured his/her browser to reject cookies, the JSP engine can still use URL-rewriting for session tracking.

– URLs in dynamically generated pages (in our example the relative URLs sqlquery.jsp, sqlupdate.jsp and sqlclose.jsp) are extended with a session ID, which is thereby returned to the server when the forms are submitted (this happens automatically, if necessary, for a JSP page).

– The server can thus determine session context, even without cookies.

– This is only possible if the HTML is dynamically generated; crossing a static sqlcommand.html page would lose the session identity. Without cookies, the application would fail.

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Handling Queries The most complicated page in our application is

sqlquery.jsp. This will execute an arbitrary SELECT command, and

print out the results as an HTML table. We assume the SQLBean has:

– A write-only trigger property query. Setting this property runs the query, and initializes:

– a read-only property numCols, representing the number of columns in the result, and

– an indexed read-only property colNames, representing the labels for the columns in the result.

Setting query also prepares the bean for a sequence of calls to:– a boolean, no-argument method called nextRow(), which

initializes:– an indexed read-only property, colValues, representing the values

of the columns in the next row of the result.

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The sqlquery.jsp JSP page<%@ page errorPage=“sqlerror.jsp” %><jsp:useBean id=“sql” class=“mybeans.SQLBean” scope=“session”> <jsp:forward page=“sessionerror.html”/></jsp:useBean> <jsp:setProperty name=“sql” property=“query”/><html><head></head> <body> <table border cellspacing=0 cellpadding=5> <% int numCols = sql.getNumCols() ; out.println(“<tr bgcolor=lightblue>”) ; for(int i = 0 ; i < numCols ; i++) out.println(“<td>” +

sql.getColNames(i) + “</td>”) ; out.println(“</tr>”) ; while(sql.nextRow()) { out.println(“<tr>”) ; for(int i = 0 ; i < numCols ; i++) out.println(“<td>” +

sql.getColValues(i) + “</td>”) ; out.println(“</tr>”) ; } %> </table> <jsp:include page=“sqlcommand.jsp”/> </body></html>

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The Response

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Remarks The body of the jsp:useBean element forwards

control to a page called sessionerror.html.– If the bean is newly created here, it will not be correctly

initialized—the password is not set and the connect() call does not get called on the new bean. Most likely this is a symptom of the session having timed out.

– The HTML page should just print a message to this effect. The jsp:setProperty action actually triggers the

database access. The rather complicated scriptlet is only

responsible for generating the body of a suitable HTML table of results.– We used Java out.println() calls in here to generate

HTML. This is to avoid a confusing number of switches between Java inserts and HTML template text.

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Handling Updates The sqlupdate.jsp page is much simpler. We assume the SQLBean has:

– A write-only trigger property update. Setting this property runs the database update, and initializes:

– a read-only property numRows, representing the number of rows modified by the operation.

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The sqlupdate.jsp JSP page

<%@ page errorPage=“sqlerror.jsp” %><jsp:useBean id=“sql” class=“mybeans.SQLBean”

scope=“session”> <jsp:forward page=“sessionerror.html”/></jsp:useBean> <jsp:setProperty name=“sql” property=“update”/><html><head></head> <body> <jsp:getProperty name=“sql”

property=“numRows”/> rows of the database were successfully updated. <p/> <jsp:include page=“sqlcommand.jsp”/> </body></html>

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The sqlclose.jsp JSP page

<% session.invalidate() ; %><html><head></head> <body> Session finished. <p/> Thank you for using the SQL application. </body></html>

The active part of this page is simply a scriptlet that invalidates the current session.

The SQLBean class should be implemented to close the database connection when the session to which it is bound is terminated.

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SQLBean Specification: Summary For connection:

– A write-only property password. – A void, no-argument method called connect(), which attempts to

establish the database connection.– The database connection should be closed when the session terminates.

For queries:– A write-only trigger property query. Setting this property runs the

query, and initializes:– a read-only property numCols, representing the number of columns in

the result, and– an indexed read-only property colNames, representing the labels for

the columns in the result. Setting query also prepares the bean for a sequence of calls to:

– a boolean, no-argument method called nextRow(), which initializes:– an indexed read-only property, colValues, representing the values of

the columns in the next row of the result. For updates:

– A write-only trigger property update. Setting this property runs the database update, and initializes:

– a read-only property numRows, representing the number of rows modified by the operation.

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SQLBean Implementationpackage mybeans ;import java.sql.*;import javax.servlet.http.*;public class SQLBean implements

HttpSessionBindingListener { private String password ; private Connection conn ; private Statement stat ; private ResultSet rs ; private ResultSetMetaData rsmd ; private int numCols, numRows ; static { String driver = "org.gjt.mm.mysql.Driver" ; System.setProperty("jdbc.drivers", driver) ; } ... Method declarations ...}

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Remarks The file imports the JDBC classes and also the

Servlet classes. Latter only needed for

HTTPSessionBindingListener, which the bean class implements. This will be discussed at the end.

A bunch of private instance variables are declared.

The optional static initializer block of a class is called once, when the class is loaded. We define the jdbc.drivers property in here.

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Connecting to the Databasepublic void setPassword(String password) { this.password = password ;}public void connect() throws SQLException { String url =

"jdbc:mysql://localhost/opennet_fall01" ; String user = "tomcat" ; conn = DriverManager.getConnection(url, user,

password); stat = conn.createStatement();}

“Set” method for the password property, and connect method.

Now using the version of getConnection that takes account details.

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Handling Queries 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.

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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.

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Bean Methods for Queriespublic void setQuery(String query) throws SQLException

{ rs = stat.executeQuery(query) ; rsmd = rs.getMetaData() ; numCols = rsmd.getColumnCount() ;}public int getNumCols() { return numCols ;}public String getColNames(int i) throws SQLException { return rsmd.getColumnLabel(i + 1) ;}public boolean nextRow() throws SQLException { return rs.next() ;}public String getColValues(int i) throws SQLException { return rs.getString(i + 1) ;}

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Remarks We assume indexes in the column names and

values properties go from 0, whereas in JDBC they go from 1.

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Bean Methods for Handling Updates

public void setUpdate(String update) throws SQLException {

numRows = stat.executeUpdate(update) ;}public int getNumRows() { return numRows ;}

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Terminating the Session A session may end for various reasons; e.g. it

times out, or the invalidate() method is called on the session (as it is in sqlclose.jsp.)– After this happens, the associated session bean objects

will eventually be garbage collected. Our bean has a fairly costly resource associated

with it—an open database connection. We want to make sure this connection is closed as early as possible after the session ends.– Preferably without waiting for the garbage collector.

One way to do this is to have the object implement the HttpSessionBindingListener interface.

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The HttpSessionBindingListener Interface

package javax.servlet.http;interface HttpSessionBindingListener { public void valueBound(HttpSessionBindingEvent evt) ; public void valueUnbound(HttpSessionBindingEvent

evt) ;}

If an object bound to a Servlet session implements this interface, the event-handling method valueBound() is invoked on the object whenever it is bound to a session (by setAttribute().)

The method valueUnbound() is invoked whenever this object is unbound from a session, either by removeAttribute(), or when the session is invalidated (e.g. when it times out).

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Methods for Closing the connection

...import javax.servlet.http.*;public class SQLBean implements

HttpSessionBindingListener { ... public void valueBound(HttpSessionBindingEvent evt)

{} public void valueUnbound(HttpSessionBindingEvent

evt) { System.out.println("Closing database connection...") ;

// debug try { conn.close() ; } catch (SQLException e) {} // Not much to be done

about it. }}

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Using JDBC From Tomcat Unlike early versions, Tomcat 4 seems to ignore

the CLASSPATH environment variable. It looks for classes in various places within its

own installation directory. You should move the entire org\ folder from

inside the folder C:\mm.mysql-2.0.7\, which you may have installed earlier, into the C:\jakarta-tomcat\classes\ folder (or the classes\ subfolder of your Tomcat installation, wherever it is).– So the class file for the driver is at, e.g.: C:\jakarta-tomcat\classes\org\gjt\mm\mysql\

Driver.class Now the driver class will be found when the Bean

methods try to use it.

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Conclusions We developed a powerful Web application using

JSP and JDBC, glued together with JavaBeans. The implementation is quite clean, with a good

separation between application logic (access to JDBC), and presentation logic.– As noted in earlier examples, session tracking and

error handling are particularly elegant.– The Bean code presumably has some scope for reuse in

other applications. We still had to use scripting inserts fairly freely,

especially for displaying tables.

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Epilogue: The DBTags Tag Library Described at:

http://jakarta.apache.org/taglibs/doc/dbtags-doc/index.html

This is a generic tag library for using SQL commands in JSP pages.

The next slide shows example usage.– Idea similar to Cold Fusion?

If you specifically want to embed SQL commands in your JSP pages, a standardized tag library like this is probably the way to go.

On the other hand, if you want application-specific logic abstracted into tags (and methods), writing your own Bean is much less work than implementing you own tag library.

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