Download Database Connections

Database Connectivity
For IST410 Students only
1
Objectives
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Overview of JDBC
Executing selection and update
queries
Batched Transactions
Meta-data
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Introduction
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Modern applications often connect to a database
With this need in view, Java provides the JDBC API
through which an application program can establish
database connectivity
The JDBC API supports ANSI SQL and provides a number
of classes and interfaces through which SQL queries of
various types can be executed from within a Java program
Writing a program that uses JDBC requires a working
knowledge of a database management system and SQL
There are many excellent books available for those
interested in a review of database technology
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Introduction
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Two distinct tools are needed to connect to a database
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Java can connect to three different types of databases
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JDBC - Java Database Connectivity drivers
ANSI SQL support by the DBMS
Relational Databases
Object Databases
Object-relational databases
Stored procedures can also be written using JDBC API
Stored procedures are precompiled SQL ‘code snippets’
stored on the database server
For our discussion, we focus only on the client side where
a program issues a SQL query to a database and processes
the result
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JDBC API
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JDBC is a SQL-level API
This API is used by java programs to send SQL statements
to a DBMS; the result of the query is returned back to the
program through the API
A JDBC driver is used as the pipeline through which
queries are sent and results are received
The basic goal of Java developers, in designing JDBC
technology, was to provide a bridge through which Java
programs can interact with any SQL database
JDBC does build on successes of ODBC that defined
industry-wide standards for accessing a database
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JDBC API
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JDBC API is defined in a package named java.sql
java.sql package comprises a number of interfaces, classes,
and exception types
Interfaces
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Array - Used to interact with the SQL array during a transaction
Blob - Used to store/access Binary Large Objects
CallableStatement - Used to execute stored procedures
Connection - Used to establish a connection session with a specific
database
DatabaseMetaData - Provides comprehensive information about
the database as a whole
Driver - Connection pipeline to a database
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JDBC API
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List of interfaces (continued)
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PreparedStatement - A precompiled SQL statement object
Ref - A reference value to a SQL structured type
ResultSet - Table of data
ResultSetMetaData - Types of reference set can be determined
SQLData - Rarely called by programmer directly, used to custom
map user-defined types
SQLInput - Never called by the programmer, used by the driver
behind the scene
SQLOutput - Never called by the programmer, used by the driver
Statement - Object for executing a SQL statement
Struct - Values for SQL structured types
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JDBC API
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The JDBC classes are
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Date - Wrapper object for SQL Date
DriverManager - Manager of database connectivity drivers
DriverPropertyInfo - For advanced programming use only if need
to interact with Driver exists
Time - Wrapper class for SQL Time
Timestamp - Wrapper class to identify SQL TIMESTAMP
Types - Constants to identify generic SQL types
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JDBC API
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JDBC Exception types are
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BatchUpdateException - thrown if error occurs during a batch
update operation
DataTruncation - Data truncation warning
SQLException - Database access error
SQLWarning - Database access warning
All of these interfaces, classes and exceptions have many
methods
A subset of JDBC API features are discussed in this
module
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JDBC Drivers
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Key to establishing a database connection is the JDBC
driver that knows about the database
These drivers can be classified into 4 types
Type 1 - JDBC-ODBC bridge
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Uses ODBC to access a database.
Required in Client systems to be able to connect to a database
These drivers are generic in nature and therefore have some ‘give’
in performance; they have to be able to connect to different type of
databases
Can be used with any database supported by ODBC
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JDBC Drivers
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Type 2 - A native-API, partly Java technology
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Contains calls to native methods (may be in C or C++ ) developed
by commercial DBMS vendors such as Oracle, DB2 etc
Usually delivers better performance and is needed in Client
systems
Type 3 - A net-protocol, fully Java technology-enabled
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Translates JDBC API calls to DBMS-independent net protocol
Java applications use sockets to connect to ‘middleware’ that
translates the database request into API specific request at the
server level
More flexible to use than other two since client machines do not
require additional code support
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JDBC Drivers
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Type 4 - Drivers for databases that have built in
networking capability
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A native-protocol written fully in Java
Built in Java library translates requests to database-specific calls
Databases are called directly using Java sockets and usually
propritary
Network protocol is built into the database engine
There is a very large number of drivers available today
Visit http://java.sun.com/products/jdbc/drivers.html
The next slide shows a partial list of the available drivers
as of February, 2000
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JDBC Drivers
Vendor
Borland
IBM
IBM
NetDirect
Type
4
2,3
4
3
Imaginary
Oracle
Software AG
Sybase
Sun
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2,4
4
3,4
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DBMS Supported
JDataStore
DB2 for OS/390, UDB, AS/400
AS/400, UDB
MS SQL, Oracle, MS Access,
Informix, DB2, Sybase, FoxPro
and others
mSQL
Oracle 8.1.6
ADABAS D
Many
Any with JDBC or ODBC
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Interacting with a Database
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General steps for connecting and interacting with a
database can be described as follows
 Load a JDBC driver
 Establish a connection object
 Create a Statement object
 If return table is expected, establish a Result object
 Execute the query using statement object
 Circle through the result table and use retrieved records
in program processing steps
Specific application is likely to require variations to the
above
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Loading a Database Driver
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A JDBC driver can be loaded by instantiating a driver
object
new sun.jdbc.odbc.JdbcOdbcDriver();
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
The driver being loaded is sun.jdbc.odbc.JdbcOdbc.Driver
A program can choose any of the above two alternatives
The second alternative provides better flexibility since the
driver name can be supplied as a run time argument - it is a
String
The name of the driver depends on the database being used
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Loading a Database Driver
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Java does not limit a program to one JDBC driver supplied
at the run-time
A predefined set of drivers can be specified in a jdbc
property file
jdbc.drivers= sun.jdbc.odbc.JdbcOdbcDriver:secondDriver
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These drivers can then be loaded during run-time
Properties p = new Properties();
FileInputStream fis = new FileInputStream(“jdbc.properties”);
p.load(fis);
String drivers = p.getProperty(“jdbc.drivers”);
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The driver manager iterates through the list of drivers to
select a driver that can interact with the chosen database
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Database Names
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A database name is needed before a connection can be
established
In WINTEL environment, a database name is created by
using the 32bit ODBC connection interface accessible
through the Control Panel
For example, a database name can be stated as
jdbc:odbc:CompanyDB
where a database named CompanyDB will be accessed
using the type 1 driver: JDBC/ODBC bridge
The program, in above example, expects to find the
CompanyDB in the host machine locally
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Database Names
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Programs also need to access remote databases
A complete URL for the database is needed for remote
access
jdbc:odbc://www.myserver.com:4000/CompanyDB
where
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driver used is jdbc/odbc bridge
the server has a URL of www.myserver.com
the database server is at the port number 4000
the database name is CompanyDB
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Database Names
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The general syntax for remote access is
jdbc:sub-protocol:database URL
sub-protocol - specific to the driver used for the database
database URL - the locator, recommended to be a standard URL format
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For our discussion in this module, we will assume that we
have set up a database name called CompanyDB in the
local machine
Our database name is coded as a String
"jdbc:odbc:CompanyDB";
The database name needs to be modified for other
connections
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Establishing a Connection Object
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The DriverManager is called to establish a connection to
the database
String dbName = "jdbc:odbc:CompanyDB";
Connection con =
DriverManager.getConnection(dbName, userid, password);
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The userid and password can be left as null strings if these
attributes are unnecessary to log into the database server
It is the job of the DriverManager to look for the
appropriate driver and establish a connection
A SQLException error is thrown if the DriverManager fails
to connect
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Creating a Statement Object
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Statement is an Interface, i.e objects are created indirectly
Statement objects are used to execute SQL statements and
retrieve results of the execution
Result is returned into an object of type ResultSet
Only one ResultSet object can interact with a Statement
object
Statement object can be created by executing
createStatement method of the Connection object
Statement stmt = con.createStatement();
A number of methods exist to query Statement object
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ResultSet
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ResultSet is an interface in the java.sql package
An object of type ResultSet receives the result, if any, of a
query
Since it is an interfce, it is created indirectly by executing a
method call for statement
ResultSet rs = stmt.executeQuery(
“SELECT fname, lname, dno from employee "+
"where dno = 5");
There is one result set for a statement object
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ResultSet
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ResultSet provides a number of methods through which the
content of each returned record can be examined
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Cursor positioning : absolute, aftterLast, beforeFirst, first, getRow
Extract columns - getString, getDouble, getInt, getDate
Manipulate rows - insertRow, deleteRow, findColumn
Test for things - isBeforeFirst, isFirst
Get metadata information - getMetaData
Move record by record - next(), previous()
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Executing a SQL Query
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SQL queries are executed by embedding the SQL
statement in a method of the statement object
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ResultSet executeQuery(String selects) - Execute a SQL SELECT
query
int executeUpdate(String actionQuery) - Execute INSERT,
DELETE and UPDATEs
executeUpdate can also execute other SQL DDL statements that do
not return records
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Commiting to Changes
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SELECT queries do not leave the database in an
inconsistent state even though the retrieved result may not
be the latest
However, update queries have the potential of not finishing
or leaving the database inconsistent particularly in a multiuser environment
Database management systems provide the facility of
Commit and Rollback to manage these possibilities
A commit request makes all changes made since the
previous commit/rollback permanent and release any
database locks currently held by the Connection
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Commiting to Changes
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Rollback, on the other hand, drops all changes made since
the previous commit/rollback and release any database
locks currently held by the Connection
One can also set the commit mode automatically and in
that case, each individual SQL statement is executed as an
independent transaction
con.commit() - Commit the transaction
con.rollback() - Undo changes made from last commit or
rollback
con.setAutoCommit(boolean) - Sets the automatic commit
mode
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Basic Query Example
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We use a small database to show JDBC examples
The tables are shown next
EMPLOYEE
Social Security
Number
123456789
333445555
453453453
666884444
888665555
987654321
987987987
999887777
First Name
John
Franklin
Joyce
Ramesh
James
Jennifer
Ahmad
Alicia
Last Name
Smith
Wong
English
Narayan
Borg
Wallace
Jabbar
Zelaya
For IST410 Students only
Department
Salary Number
53146.83
40000
25000
38000
55000
43000
25000
25000
5
5
5
5
1
4
4
4
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Basic Query Example
WORKS_ON
ESSN
123456789
123456789
333445555
333445555
333445555
333445555
333445555
453453453
PNO
Hours
10
30
1
2
3
10
20
1
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1
10
10
10
10
20
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SQL Basic Query Example
// Extracts from BasicQueries.java, please see the source for full code
public class BasicQueries {
public static void main(String [] args) {
NumberFormat nf = NumberFormat.getNumberInstance();
nf.setMaximumFractionDigits(2);
nf.setMinimumFractionDigits(2);
Set up JDBC connection
try {
new sun.jdbc.odbc.JdbcOdbcDriver();
String dbName = "jdbc:odbc:CompanyDB";
Connection con = DriverManager.getConnection(dbName, "", "");
con.setAutoCommit(true);
Statement stmt = con.createStatement();
For IST410 Students only
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SQL Basic Query Example
// Query #1 - A simple SELECT query
ResultSet rs = stmt.executeQuery(
"SELECT fname, lname, dno from employee "+
"where dno = 5");
System.out.println("DNO Query completed, printing results");
while(rs.next()) {
String fname = rs.getString(1);
String lname = rs.getString(2);
double dno = rs.getDouble("dno");
System.out.println(lname+", "+fname+" Dept: "+dno);
}
For IST410 Students only
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SQL Basic Query Example
// Query #2 - A bit more involved SELECT query
rs = stmt.executeQuery(
"SELECT fname, lname, dno from employee "+
"where sex = 'M' and salary > 20000");
System.out.println("\n\nSelected names Query completed, printing
results");
while(rs.next()) {
String fname = rs.getString(1);
String lname = rs.getString(2);
double dno = rs.getDouble("dno");
System.out.println(lname+", "+fname+" Dept: "+dno);
}
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SQL Basic Query Example
// Query #3 - an INSERT query, followed by verification
System.out.println("\n\nExecuting Insert query");
stmt.executeUpdate("INSERT into Works_on "+
"VALUES (123456789, 3, 20)");
System.out.println("\n\nVerifying Insertion result");
rs = stmt.executeQuery(
"SELECT * from works_on "+"where essn = 123456789");
while(rs.next()) {
double essn = rs.getDouble(1);
double pno = rs.getDouble(2);
double hours = rs.getDouble(3);
System.out.println("ESSN: "+(long)essn+", PNO: "+(int)pno+
" Hours: "+hours);
}
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SQL Basic Query Example
//Query #4 - a DELETE query
System.out.println("\n\nExecuting a Delete query");
stmt.executeUpdate("DELETE from Works_on "+
"WHERE essn = 123456789 and pno = 3");
//Query #5 - an UPDATE query and verification
System.out.println("\n\nExecuting an UPDATE query");
stmt.executeUpdate("UPDATE employee "+
"set salary = salary*1.10 "+"WHERE fname = 'John'");
rs = stmt.executeQuery(
"SELECT fname, lname, salary "+"from employee "+
"where fname = 'John'");
// rest of the code
For IST410 Students only
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Batch Processing
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Batch processing is a JDBC2.0 feature and needs a
JDBC2.0 driver
We can batch the queries and start a transaction
We start with defining a Buffer to hold the query strings
String sqlLine = ”.......”;
This buffer is then added to the batch using the addBatch
method of the Statement object
stmt.addBatch(sqlLine);
We continue to add SQL statements as String arguments to
the stmt object using addBatch method
Finally, stmt.executeBatch() is executed to run the batch
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Batch Processing
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Since batch transaction can fail, it is generally a good idea
to turn off the AutoCommit
A commit() statement can then be executed when the batch
transaction successfully completes;otherwise rollback()
undoes the effect of the transaction
The executeBatch method returns an array of integers, each
element of the array has a value equal to number of
elements used in the update
A complete example BatchTrans.java is in the directory;
parts of the file extracted in the several slides to highlight
the important issues
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Batch Processing
BatchTrans.java
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Metadata
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Normally, the user of a database would know names of
tables, their attributes and data types
There are cases where this information may be unknown,
or a highly portable program is desired where the program
cannot be bound to fixed table and column names
Java provides a range of JDBC classes that allow the user
to query a database about its type, name etc.
Metadata is ‘data about data’
In the case of JDBC, metadata translates to information
about the system properties of the database
For IST410 Students only
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Metadata
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The details of metadata classes are too voluminous and too
specialized to merit a complete discussion here
An example is presented to introduce the reader to
metadata handling using JDBC
Specifically, the example presented uses 2 classes
ResultSetMetaData - Finds metadata information from a result set
DatabaseMetaData - Finds metadata information about the database
as a whole
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The full example can be found in MetaTest.java
Extracts from this program are presented in the next few
slides
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Metadata
ResultSet rs = stmt.executeQuery ( "SELECT fname, lname, salary ” +
"from employee ” + "where fname = 'John'");
ResultSetMetaData rsm = rs.getMetaData();
Full code in
int colCount = rsm.getColumnCount();
MetaTest.java
String [] colNames = new String[colCount];
System.out.println("Number of columns retrieved = "+colCount);
System.out.println("Column specifications are:");
for (int i=0; i < colCount; i++) {
colNames[i] = rsm.getColumnName(i+1);
String colType = rsm.getColumnTypeName(i+1);
int colSize = rsm.getColumnDisplaySize(i+1);
System.out.println("Name: "+colNames[i]+" Type: "+colType+
" Size: "+colSize);
}
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Metadata
// How many tables in the database
System.out.println("Number of tables in the database");
DatabaseMetaData dmd = con.getMetaData();
rs = dmd.getTables(null,null,null,new String[] {"TABLE"});
while (rs.next()) {
System.out.println(rs.getString(3));
}
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Parameters of getTables
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1. Represents a table catalog - may be null
2. Represents a table schema
3. Represents a table name
4. Represents a table type string with values such as TABLE, VIEW,
SYSTEM TABLE etc.
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