Transcript Document

Lesson 3
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
JDBC
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
– High degree of control over performance
– Database is overkill
– Data is not that important
Advantages of 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.
 Much harder to corrupt data, 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:
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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-tomanipulate 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).
 API lacks robustness in certain areas
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
https://www.cs.uchicago.edu/info/services/mysql
 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-3.0.7-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.
General observations
 In executeXXX calls, SQL string need not end
with any specific terminator
st.execute(“Select * from TABLE”);
dbms-specific
terminator automatically added
 Statement objects can be reused for many queries – no
need to create new one each time
st.execute(blah1);
st.execute(blah2);
 Indicate nesting with altnerate single/double quotes
stmt.executeUpdate( "INSERT INTO COFFEES " + "VALUES
('Colombian', 101, 7.99, 0, 0)");
General Observations, cont.
 When using ResultSet object, Java will try to
convert to requested type whenever possible. For
example, rs.getString(...) is valid for any sql type,
rs.getInt can also retrieve floats, etc.
 My have only one open ResultSet object per
Statement! That is, if you want to open two
ResultSets at once (not common), you need to
create a new Statement object.
Closing statements, etc.
 When you are finished with a ResultSet,
Statement, or Connection, you should call
close() immediately.
 The close() statement of a Statement
automatically closes related Connection and
ResultSet objects, and likewise for a
Connection object and its associated
ResultSet.
Sample of ResultSet Conversions
 TINYINT: getByte (recommended)
– Can also be read using getShort, getInt, getLong, getFloat, getDouble,
getBigDecimal, getBoolean, getString, getObject
 SMALLINT: getShort (recommended)
– Can also be read using getByte, getInt, getLong, getFloat, getDouble,
getBigDecimal, getBoolean, getString, getObject
 INTEGER: getInt (recommended)
– Can also be read using getByte, getShort, getLong, getFloat, getDouble,
getBigDecimal, getBoolean, getString, getObject
 BIGINT: getLong (recommended)
– Can also be read using getByte, getShort, getInt, getFloat, getDouble,
getBigDecimal, getBoolean, getString, getObject
Prepared Statements
 A statement that is executed many times can be
optimized by instead using a PreparedStatement
 PreparedStatement objects are sent to the dbms for
pre-compilation.
 Example
PreparedStatement updateSales = con.prepareStatement( "UPDATE
COFFEES SET SALES = ? WHERE COF_NAME LIKE ?");
updateSales.setInt(1, 75);
updateSales.setString(2, "Colombian");
updateSales.executeUpdate():
Prepared statements, cont.
 Can dramatically improve performance
 Can call the set method as many times as you like
for same PreparedStatement object
 Makes it easier also to script multiple updates in a
loop
 Good idea in general!
Transactions
 Transactions are sequences of commands
that are only executed if all commands in
sequence successfully complete.
 If the commands complete successfully,
they 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
execute and commit.
 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();
Transactions – marker points
 Previous technique rolls back all command
since previous commit
 You can explicitly add save points to your
code as an alternative, as:
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Statement stmt = conn.createStatement()
stmt.executeUpdate(command1)
Savepoint svpt = conn.setSavepoint();
Stmt.executeUpdate(command2);
if (…) conn.rollback(svpt);
JDBC2.0 features
 JDBC2.0 features are automatically available in
any version of j2sdk >= 2.0
 List of JDBC2.0 features
– Scroll forward and backward in a result set or move to a
specific row
– Make updates to database tables using methods in the
Java programming language instead of using SQL
commands
– Send multiple SQL statements to the database as a unit,
or batch
– Use the new SQL3 datatypes as column values
Scrollable result sets
Statement stmt = con.createStatement(
ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_READ_ONLY);
Notice two new arguments. Possible values for first are as follows:
arg1: TYPE_SCROLL_SENSITIVE: backward scrollable and
sensitive to DB changes
TYPE_SCROLL_INSENSITIVE: backward scrollable and
NOT sensitive to DB changes
TYPE_FORWARD_ONLY (default): not backward scrollable
arg2: CONCUR_READ_ONLY (default): cannot update directly
CONCUR_UPDATABLE: can be used to update directly
Focus on arg1 first: for first two types, new set of methods defined
to navigate more easily through results.
Methods for scrollable ResultSets
 Many new methods to navigate:
– afterLast(); //position pointer after last row
– previous(); //opposite of next()
– absolute(i);//go to the i’th row
– absolute(-i);//go to the i’th row from end
– relative(i);//go i rows away from this point
– getRow();/returns current row
– isFirst(), isLast(), isBeforeFirst(), is
AfterLast();/ boolean pos query functions
Updatable ResultSets
 Can use Java methods to update table rather
than sql strings.
 Must create statements with
CONCUR_UPDATABLE flag:
Statement stmt = con.createStatement(
ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_UPDATABLE);
Updating a value
 To update a value in a Table, use the updateXXX
procedure. Example:
stmt.executeUpdate("UPDATE COFFEES SET PRICE = 10.99" +
"WHERE COF_NAME = FRENCH_ROAST_DECAF");
uprs.last(); //move to last row
uprs.updateFloat("PRICE", 10.99); //change PRICE column to new
value
 Note that updateXXX works implictly on current row
 Note that updateXXX can operate on col numbers as
well as col names
Committing an updated value
 After previous call to updateXXX, database
is still not updated, only ResultSet is.
 Must call updateRow() to complete
operation with dbms.
 call cancelRowUpdates() to cancel an
update that you no longer wish to commit
 Note that all row references refer to
ResultSet object, which may differ from
database table modulo updates!
Inserting rows
 Also can insert values directly in Java.
 Must move cursor to special row called insert row
and then call insert method.
uprs.moveToInsertRow();
uprs.updateString("COF_NAME", "Kona");
uprs.updateInt("SUP_ID", 150);
uprs.updateFloat("PRICE", 10.99);
uprs.updateInt("SALES", 0);
uprs.updateInt("TOTAL", 0);
uprs.insertRow(); updates both dbase and ResultSet
at same time
Deleting rows
 Simply call deleteRow() on current row
Seeing Changed ResultSets
 Always visible to others once you close and
reopen (or new query executed).
 May or may not be visible to others while
ResultSet still open. Depends on:
– dbms
– driver
– flags used in creating ResultSet
• TypeScrollSensitive  see all updates, might or
might not see inserts/deletes
• Need to use metadata methods to be sure
MetaData methods
 Provide user with rich array of info about
database itself, tables, etc.
 java.sql.DatabaseMetaData
– info about database as a whole. version,
features supported, vendor, etc.
 java.sql.ResultSetMetaData
– data about specific ResultSet – names of
columns, size of types, types of data, etc.
 Study API for more details
Java/SQL datatype mapping
SQL data type
INTEGER
SMALLINT
NUMERIC(m,n)
FLOAT(n)
REAL
DOUBLE
CHARACTER(n)
VARCHAR(n)
Java data type
int
short
java.sql.Numeric
double
float
double
String
String
Java/SQL datatype mapping
BOOLEAN
DATE
TIME
TIMESTAMP
BLOB
CLOB
boolean
java.sql.Date
java.sql.Time
java.sql.Timestamp
java.sql.Blob
java.sql.Clob
ARRAY
java.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
Also see
 RowSet (pp 230 of Horstman)
Stored Procedures
 To create, use executeUpdate with dbms-specific
String to define stored procedure. Example:
String createProcedure = "create procedure SHOW_SUPPLIERS "
+ "as " + "select SUPPLIERS.SUP_NAME,
COFFEES.COF_NAME " + "from SUPPLIERS, COFFEES " +
"where SUPPLIERS.SUP_ID = COFFEES.SUP_ID " + "order by
SUP_NAME";
Statement stmt = con.createStatement();
stmt.executeUpdate(createProcedure);
Calling stored procedures
 Stored procedures can be called using
CallableStatement object. Example:
CallableStatement cs = con.prepareCall("{call
SHOW_SUPPLIERS}");
ResultSet rs = cs.executeQuery();
 Notice the {} around the prepared statement call. This
is required for java to translate into appropriate dbmsspecific sql.
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 pixels 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