database connectivity

1For IST410 Students only

Database Connectivity


Objectives

Overview of JDBC Executing selection and update

queries Batched Transactions Meta-data

For IST410 Students only 3

Introduction

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

For IST410 Students only 4

Introduction

Two distinct tools are needed to connect to a database JDBC - Java Database Connectivity drivers ANSI SQL support by the DBMS

Java can connect to three different types of databases 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


JDBC API

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


JDBC API

JDBC API is defined in a package named java.sql java.sql package comprises a number of interfaces, classes,

and exception types Interfaces

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


JDBC API

List of interfaces (continued) 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


JDBC API

The JDBC classes are 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


JDBC API

JDBC Exception types are 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


JDBC Drivers

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

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


JDBC Drivers

Type 2 - A native-API, partly Java technology 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

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


JDBC Drivers

Type 4 - Drivers for databases that have built in networking capability

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


JDBC Drivers

Vendor Type DBMS SupportedBorland 4 JDataStoreIBM 2,3 DB2 for OS/390, UDB, AS/400IBM 4 AS/400, UDBNetDirect 3 MS SQL, Oracle, MS Access,

Informix, DB2, Sybase, FoxProand others

Imaginary 4 mSQLOracle 2,4 Oracle 8.1.6Software AG 4 ADABAS DSybase 3,4 ManySun 1 Any with JDBC or ODBC


Interacting with a Database

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


Loading a Database Driver

A JDBC driver can be loaded by instantiating a driver objectnew 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


Loading a Database Driver

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 filejdbc.drivers= sun.jdbc.odbc.JdbcOdbcDriver:secondDriver

These drivers can then be loaded during run-timeProperties p = new Properties();FileInputStream fis = new FileInputStream(“jdbc.properties”);p.load(fis);String drivers = p.getProperty(“jdbc.drivers”);

The driver manager iterates through the list of drivers to select a driver that can interact with the chosen database


Database Names

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


Database Names

Programs also need to access remote databases A complete URL for the database is needed for remote

accessjdbc:odbc://www.myserver.com:4000/

CompanyDBwhere

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


Database Names

The general syntax for remote access isjdbc:sub-protocol:database URL

sub-protocol - specific to the driver used for the databasedatabase URL - the locator, recommended to be a standard URL format

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


Establishing a Connection Object

The DriverManager is called to establish a connection to the databaseString dbName = "jdbc:odbc:CompanyDB";

Connection con = DriverManager.getConnection(dbName, userid,

password); 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


Creating a Statement Object

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 objectStatement stmt = con.createStatement();

A number of methods exist to query Statement object


ResultSet

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


ResultSet

ResultSet provides a number of methods through which the content of each returned record can be examined

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()


Executing a SQL Query

SQL queries are executed by embedding the SQL statement in a method of the statement object

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


Commiting to Changes

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 multi-user 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


Commiting to Changes

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 transactioncon.commit() - Commit the transactioncon.rollback() - Undo changes made from last commit or rollbackcon.setAutoCommit(boolean) - Sets the automatic commit mode


Basic Query Example

We use a small database to show JDBC examples The tables are shown next

Social Security Number First Name Last Name Salary

Department Number

123456789 John Smith 53146.83 5333445555 Franklin Wong 40000 5453453453 Joyce English 25000 5

666884444 Ramesh Narayan 38000 5888665555 James Borg 55000 1987654321 Jennifer Wallace 43000 4987987987 Ahmad Jabbar 25000 4999887777 Alicia Zelaya 25000 4

EMPLOYEE


Basic Query Example

ESSN PNO Hours123456789 10 1123456789 30 7333445555 1 1333445555 2 10333445555 3 10333445555 10 10333445555 20 10453453453 1 20

WORKS_ON


SQL Basic Query Example

// Extracts from BasicQueries.java, please see the source for full codepublic class BasicQueries { public static void main(String [] args) { NumberFormat nf = NumberFormat.getNumberInstance();

nf.setMaximumFractionDigits(2); nf.setMinimumFractionDigits(2);

try { new sun.jdbc.odbc.JdbcOdbcDriver(); String dbName = "jdbc:odbc:CompanyDB"; Connection con = DriverManager.getConnection(dbName, "", "");

con.setAutoCommit(true); Statement stmt = con.createStatement();

Set up JDBC connection



// 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);

}



// 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);

}



// 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); }



//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


Batch Processing

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


Batch Processing

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


Batch Processing

BatchTrans.java


Metadata

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


Metadata

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 classesResultSetMetaData - Finds metadata information from a result setDatabaseMetaData - Finds metadata information about the database as a whole

The full example can be found in MetaTest.java Extracts from this program are presented in the next few

slides


Metadata

ResultSet rs = stmt.executeQuery ( "SELECT fname, lname, salary ” + "from employee ” + "where fname = 'John'");

ResultSetMetaData rsm = rs.getMetaData(); int colCount = rsm.getColumnCount(); 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); }

Full code in MetaTest.java


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)); }

Parameters of getTables 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.

database connectivity

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