Database Access Using JDBC
BCIS 3680 Enterprise Programming
Overview
Connecting to databases
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JDBC driver
Connection steps
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Connection String
Classes used
Methods used
ResultSet
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Reading values from ResultSet
Working with Databases
The java.sql package contains many classes for
working with databases, e.g.,
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Connection
Statement
ResultSet
They are designed to be independent of the vendor of
the DBMS you connect to.
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This abstraction is achieved by adding a driver layer between
these classes and the DBMS.
Database Access from JSP
To connect to a database for dynamic contents, you want
to establish a JDBC connection inside your JSP page.
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Other options are available, e.g., ODBC. But JDBC is
straightforward and you don’t have to configure DSNs on the
local machine.
JDBC is a technology developed by Sun to allow to access
virtually any database system from JSP pages or Java
applications.
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Using JDBC Driver to Work with MySQL
Download the connector zip from the course website.
The file unzips to the mysql-connector-java5.1.7 folder, which contains the JDBC connector file
called mysql-connector-java-5.1.7.bin.jar.
In the deployment folder for your web application, create
a WEB-INF subfolder as usual. Under that subfolder,
create a subfolder named lib.
Copy the connector JAR file - mysql-connectorjava-5.1.7-bin.jar - into \WEB-INF\lib.
Import classes from the java.sql package.
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Do this by using a page directive for importing.
<%@ page import="java.sql.*" %>
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Steps to Use Database in JSP
Load the JDBC driver
Compose the connection string
Establish the connection
Create the statement object
Execute a query or update
Process the results
Close the connection
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Loading the JDBC Driver
The driver acts as the bridge between the JDBC classes
and the database.
The driver is a piece of software that knows how to talk
to the DBMS.
To load a driver:
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Class.forName("com.mysql.jdbc.Driver");
This is just another way to say:
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com.mysql.jdbc.Driver aDriver =
new com.mysql.jdbc.Driver();
Calling the newInstance() method (such as p. 331 in
Metlapalli book) is redundant.
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Defining the Connection String
Use the jdbc: protocol followed by specific subprotocal
and the name of the database.
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For some subprotocals, you may need to specify the database
server host name and port number, user name and password,
etc.
The connection string is different for different database
management systems.
Example:
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jdbc:mysql://localhost:3306/forta?
user=root&password=bcis3680
It’s a good idea to save this as a String variable.
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Parts of a Connection String
jdbc:mysql://localhost:3306/forta?user=root&password=bcis3680
Note what characters (:, ?, &, etc.) connect the different parts.
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Part
Meaning
jdbc
JDBC protocol
mysql
Subprotocol for MySQL
//localhost
Network notation for the local computer
3306
Default TCP port to receive connection for MySQL. If you have
configured a different port, use the number for that port instead.
forta
Name of the MySQL database you want to access.
user=root
Specifying the username of the MySQL user account that accesses the
database on behalf of the WebApp. In production environments you
don’t want to use “root”.
password=b
cis3680
Providing password for the user account used. Note this is visible in
your JSP source code.
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Establishing the Connection
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Create a connection to the database server.
To make the actual network connection, you need to pass
the connection string to the getConnection()
method of the DriverManager class, as follows:
Connection cxn =
DriverManager.getConnection(cxnString);
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Creating the Statement Object
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The connection object only establishes a connection to
the database.
To actually issue SQL statements to the DBMS, you need
a Statement object.
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When we are at a DBMS console (e.g., MySQL console), we
can interact with it directly by running SQL commands
(interactive access).
But JSP code can’t talk to the DBMS console interactively. So
we need to set up a way for the code to send SQL commands
to the DBMS when needed (programmatic access).
We create a Statement object by calling a method of the
connection object.
Statement stm = cxn.createStatement();
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Executing SQL Statements in Code
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A single Command object can be used to run multiple
SQL statements. But we need to call the right method,
depending on the type of the statement.
For SQL statements that modify the database –
executeUpdate()
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For creating, deleting, or updating row(s) in table(s).
This method is a value-returning method.
Returns an integer indicating how many rows were affected.
 Returns 0 if the update fails.
String sql = "CREATE TABLE product " +
"(productID char(5), name varchar(15))";
stm.executeUpdate(sql);
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Executing SQL Statements in Code
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For SQL statements that do not modify the database –
executeQuery()
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For retrieving data from table(s).
This method also is a value-returning method.
But it returns a ResultSet object containing the selected
rows from table(s) in the database.
String sql = "SELECT * FROM customers";
ResultSet rs = stm.executeQuery(sql);
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The next() Method
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The ResultSet class has a next() method.
When a ResultSet is returned, the cursor is “parked”
outside of it. Data in the ResultSet is not read or displayed
automatically.
We must call the next() method to move the cursor into
the rows in the ResultSet to read data.
Each call to next() moves the cursor forward by one row.
The next() method is a value-returning method. It performs
an action (moving into next row) and returns a boolean value:
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true if the cursor moved into a readable row.
false if the cursor didn’t move into a readable row, meaning:
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All rows in the ResultSet have been read, or
The ResultSet is empty.
Non-Empty ResultSet
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If the ResultSet is not empty (the query found matching
data):
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Calling next() for the first time moves the cursor into the first
row in the data – returns true. We know the ResultSet is not
empty.
As long as the last row of data has not been reached, each time
next() is called, the cursor moved into a readable row – returns
true.
Calling next() when the cursor is at the last row of data moves
the cursor out of the last row. Now the cursor is at a place where
there is nothing to read – returns false.
So, in the case of a non-empty ResultSet, when false is
returned from calling next(), we know we’re done with
reading data.
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ResultSet with Three Rows
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Accessing Row 1
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Accessing Row 2
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Accessing Row 3
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Done with Accessing ResultSet
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Empty ResultSet
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If the ResultSet is empty (the query found no
matching data), calling next() for the first time was not
able to move the cursor into any readable data – returns
false.
So, if false is returned after we called next() for the
very first time, we know the ResultSet is empty and
contains no data.
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Empty ResultSet
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How Do You Know It Is Empty
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FYI – Scrollable ResultSet
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ResultSet by default is forward-only.
For some particular reasons, e.g., to check whether a
ResultSet is empty before you ever wants to access it, you
can change its behavior to scrollable or insensitive
to the direction of scrolling.
This is done by creating a different type of Statement object.
For example,
Statement queryStatement =
connection.createStatement(
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);
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Then, you will be able to run the previous() method in
addition to next().
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Reading Data
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While we are at a row in the ResultSet, we can read
data stored in that row, one column at a time.
To read data, use a get???() method, where ???
represents a data type that is compatible with the data
type of that column (as defined in the table’s schema).
For example, if the column is of a text type (e.g.,
varchar), then use getString().
If the column has integer as its data type, then use
getInt().
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Reading Data
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When using get???() methods, an argument needs to
be passed to indicate which column in the row we want
to read.
We can pass the name of the column in the form of a
String.
Alternative, if we know the order of the columns in the
ResultSet, we can pass the column’s position in the
form of an integer.
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Note the counting starts at 1, not 0. So to read the third
column the method call will be get???(3).
Reading Data
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Often a ResultSet contains multiple rows of data.
Since the order and data types of the columns are
uniform throughout the rows, the data reading actions are
the same for every row.
We therefore want to code the actions as the loop body
of a loop and let them be repeated for as many times as
there are rows in the ResultSet.
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Typically a while loop is used.
To get out of the loop, we use the return value of the
next() method as the “sentinel value”.
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After the last row is read, calling next() moves the cursor
into “nothingness”.
The method call returns false – the loop stops.
When You’re Done
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Disconnect from the database connection:
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Close ResultSet
Close Statement
Close Connection