Lesson 5 1. JNI, cont 2. JDBC 3. Intro to Graphics – Image Processing

JDBC Using Java to issue SQL commands

Basic Database Concepts  When to use flat files vs. database? –Data is simple, static, volume is small, accessed by one process at a time on single system. –Cost of database software is prohibitive –Extremely high performance –Database is overkill –Other?

Databases  Built-in methods to source, access, search data.  Application independent of internal data representation – much lower maintenance costs.  Run in server mode, provides security  Built-in support for transactions, concurrency, etc.

Relational Databases  Composed of tables each of which has rows and columns.  Each row or record represents an entity.  Each column or field represents an attribute.  Like an array of structures in C or Java.  Other concepts: primary key, compound key, artificial key, foreign key.

Object-Oriented Databases  Not clear exactly when a db officially becomes OO.  Provide direct support for managing objects and relationships among them – data + methods.  Gaining popularity but still far less common than relational counterpart.  Many SQL vendors support some object extensions.

SQL  Used to stand for “Structured Query Language”.  Standard language for conversing with relational databases.  Composed of three sub-languages: –Data Definition Language (DDL) –Data Control Language (DCL) –Data Manipulation Language (DML)

DDL  Lets you define and revise the structure of relational databases. Examples: Create Database name [options] Create Table name ( columname datatype, … )  Only simple datatypes supported.

DCL  Lets user specify data security and integrity mechanisms that safeguard data  Not very standardized – varies from vendor to vendor.

DML  Functionality for retrieving, manipulating, deleting, sorting, searching data. Examples just to get flavor: –Select * From table; –Select columns From tables [Where condition]; –Select ItemNo, Qty From InvoiceLine; –Insert Into InvoiceLine; (InvoiceNo, LineNo, CustomerNo) Values (101, 100, 10);

How to use SQL  Database vendor typically supplies GUI front-end for issuing SQL queries.  Also usually supplies a scripting front-end for issuing SQL commands. –Called Interactive SQL, good for developing and debugging queries –Of limited use because cannot share data with program variables.  From within a programming language –Embedded SQL

JDBC  Java’s version of Embedded SQL  Interface fully specified in the standard Java language (ie J2SE).  Independent of database vendor’s specific SQL implementation.  Vendor supplies middleware driver to convert JDBC calls to native db hooks.  Similar to Microsoft’s ODBC

Advantages to JDBC model  Application can fairly easily migrate from one DBMS to another. Almost no code needs to be rewritten.  Easy to use since db requests return easy-to- manipulate java objects, with simple methods, java exceptions, etc.

Disadvantages of JDBC  Slower  Cannot take advantage of all SQL extensions of a particular vendor (though it can take advantage of many).

Using JDBC on cluster  To use JDBC on the cs cluster, you’ll need to either install a database or use one of our dbase servers (mysql or sybase).  In this example I’ll show how to use the myql server.  First, you must register for a mysql account  After registering, try logging on and creating a few tables. You should have a database under your login name in which you can create the tables.

Using JDBC  Basic steps for connecting to dbase server 1.Load JDBC driver 2.Define the connection object 3.Establish the connection 4.Create the statement object 5.Execute a query or update with statement object 6.Process the returned ResultSet 7.Close the Connection

Loading the Driver  Each DBMS vendor must supply the driver class which converts JDBC calls to their own native db calls.  This needs to be loaded only once per application.  When loaded, its static initializer is called and the driver is registered with the DriverManager.  Best technique (assuming our sql driver) Class.forName(“org.gjt.mm.mysql.Driver”); –note: you’ll need a copy of mysql-connector-java stable-bin.jar in your classpath.

Define the Connection  Each vendor supplies info on what connection URL to use.  For mysql installed on cluster the following works: String conURL = “jdbc:mysql://dbserver/mydatabase”;

Establish the Connection  Issue the following command to create a single connection to the database java.sql.Connection conn = DriverManager.getConnection(URL);

Create a Statement Object  Once a connection object is obtained, you must use it to create a Statement. import java.sql.Statement; Statement st = conn.createStatement();

Execute Query  To execute standard SQL commands, you need to pass a valid SQL String to the executeQuery method of the statement object. A java object of type ResultSet is returned.  Import java.sql.ResultSet; String query = “SELECT * FROM table”; ResultSet res = st.executeQuery(query);

Process the Results  The ResultSet object is java’s representation of the data returned from the db query. The most typical way of manipulating the ResultSet is something like: While (res.next()) { System.out.println(res.getString(1) + “ “ + res.getString(2) + …);  Study the ResultSet API to see all of the ways in which the data can be accessed, modified, modified locally/globally, etc.

ResultSet in more detail  Like an Iterator or Enumerator.  However, must call next() once to move to first row.  Each call to next then moves to subsequent row.  For the current ResultSet row, there are two ways to access the values of the columns: –by String name Xxx getXxx(int columnNumber); –by column number (starting at 1) Xxx getXxx(String columName);

Execute update  To execute an update, pass appropriate SQL string to executeUpdate method: –e.g. st.executeUpdate(“UPDATE Books SET Price = Price – 5.00”);  Note that execute can be used for both updates and queries, though it is clearer to use one or the other.  executeUpdate returns count of rows modified by update procedure.

Transactions  Transactions are sequences of commands that are only executed if all commands in sequence successfully complete.  If the commands complete successfully, the are commited.  If any command fails, the commands are rolled back.  Fundamental to databases/SQL. How to do with JDBC?

Transactions with JDBC  By default, each command is independently executed and commited.  To change this, execute the following command on a connection object con:con.setAutoCommit(false); st.executeUpdate(command1); st.executeUpdate(command2); con.commit()/con.rollback();

Java/SQL datatype mapping SQL data typeJava data type INTEGERint SMALLINTshort NUMERIC(m,n)java.sql.Numeric FLOAT(n)double REALfloat DOUBLEdouble CHARACTER(n)String VARCHAR(n)String

Java/SQL datatype mapping BOOLEANboolean DATEjava.sql.Date TIMEjava.sql.Time TIMESTAMPjava.sql.Timestamp BLOBjava.sql.Blob CLOBjava.sql.Clob ARRAYjava.sql.Array

Other methods of interest  java.sql.Statement –void cancel(); Aysnchronously cancels an executing SQL request.  java.sql.ResultSet –int findColumn(String columName); gives the column index for column columName -void close(); closes the current result set.

SQLException methods  java.sql.SQLException –String getSQLState(); –int getErrorCode() gets the vendor-specific exception code –SQLException getNextException(); gets the Exception chained to this one for more specific information

Introduction to awt Graphics Reading, displaying images

Awt Image processing  Java has recently added many classes for simplifying image manipulation.  We’ll start by looking at some of these in the context of howto’s for simple things –reading a jpg, gif, etc. from a file –displaying jpg, gif, etc. to a graphics window –constructing an image from raw pixels –manipulating individual pixesl of an image –writing an image to a file (see course examples)

Reading an image  Easiest way to read an image file. Use static read method in javax.image.ImageIO class: BufferedImage img = ImageIO.read(new File(“name”));  Note that “name” can be name of one of many standard Image file formats.

Writing an image  Writing an image is as easy as reading it. Simple use the ImageIO.write method: BufferedImage image; ImageIO.write(new File(name), “gif”,image);  List of supported output file types is can be obtain from: –String[] ImageIO.getWriterFormatNames();

Manipulating image bytes  It is possible to set/access each image pixel independently: image = new BufferedImage(w,h,BufferedImage.TYPE_INT_ARGB); WritableRaster raster = image.getRaster(); raster.setPixel(ival,jval,{rval,gval,bval,alphval}); or int pixel[4]; raster.getPixel(ival,jval,pixel);

Transforming images  It is also possible to transform images without accessing pixels using classes that implement the ImageOp interface.  See ImageProcessor.java example