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What is JDBC?
JDBC is an API for the Java programming
language that defines how a client may access
a database. It provides methods for querying and
updating data in a database. JDBC is oriented
towards relational databases.
JDBC Guide
JDBC Tutorial
Overview
JDBC has been part of the Java Standard Edition
since the release of JDK 1.1. The JDBC classes
are contained in the Java package java.sql. Starting
with version 3.0, JDBC has been developed under
the Java Community Process. JSR 54 specifies JDBC
3.0 (included in J2SE 1.4), JSR 114 specifies
the JDBC Rowset additions, and JSR 221 is the
specification of JDBC 4.0 (included in Java SE
6).
JDBC allows multiple implementations
to exist and be used by the same application.
The API provides a mechanism for dynamically loading
the correct Java packages and registering them
with the JDBC Driver Manager. The Driver Manager
is used as a connection factory for creating JDBC
connections.
JDBC connections support creating
and executing statements. These statements may
be update statements such as SQL CREATE, INSERT,
UPDATE and DELETE or they may be query statements
using the SELECT statement. Additionally, stored
procedures may be invoked through a statement.
Statements are one of the following types:
Statement :
The statement is sent to the database server each
and every time.
PreparedStatement :
The statement is cached and then the execution
path is pre determined on the database server
allowing it to be executed multiple times in an
efficient manner.
CallableStatement :
This is used for executing stored procedures on
the database.
Update statements :
such as INSERT, UPDATE and DELETE return an update
count that indicates how many rows were affected
in the database. These statements do not return
any other information.
Query statements :
return a JDBC row result set. The row result set
is used to walk over the result set. Individual
columns in a row are retrieved either by name
or by column number. There may be any number of
rows in the result set. The row result set has
metadata that describes the names of the columns
and their types.
There is an extension to the basic JDBC API in
the javax.sql package that allows for scrollable
result sets and cursor support among other things.
Example
The method Class.forName(String) is used to load
the JDBC driver class. The line below causes the
JDBC driver from some jdbc vendor to be loaded
into the application. (Some JVMs also require
the class to be instantiated with .newInstance().)
Class.forName( "com.somejdbcvendor.TheirJdbcDriver"
);
In JDBC 4.0, it's no longer necessary to explicitly
load JDBC drivers using Class.forName(). See JDBC
4.0 Enhancements in Java SE 6
When a Driver class is loaded, it creates an instance
of itself and registers it with the DriverManager.
This can be done by including the needed code
in the driver class's static block. e.g. DriverManager.registerDriver(Driver
driver)
Now when a connection is needed, one
of the DriverManager.getConnection() methods is
used to create a JDBC connection.
Connection conn = DriverManager.getConnection(
"jdbc:somejdbcvendor:other data needed by
some jdbc vendor",
"myLogin",
"myPassword" );
The URL used is dependent upon the particular
JDBC driver. It will always begin with the "jdbc:"
protocol, but the rest is up to the particular
vendor. Once a connection is established, a statement
must be created.
Statement stmt = conn.createStatement();
try {
stmt.executeUpdate( "INSERT INTO MyTable(
name ) VALUES ( 'my name' ) " );
} finally {
//It's important to close the statement when you
are done with it
stmt.close();
}
Note that connections, statements, and resultsets
often tie up operating system resources such as
sockets or file descriptors. In the case of connections
to remote database servers, further resources
are tied up on the server, eg. cursors for currently
open resultsets. It is vital to close() any JDBC
object as soon as it has played its part; garbage
collection should not be relied upon. Forgetting
to close() things properly results in spurious
errors and misbehaviour. The above try-finally
construct is a recommended code pattern to use
with JDBC objects.
Data is retrieved from the database
using a database query mechanism. The example
below shows creating a statement and executing
a query.
Statement stmt = conn.createStatement();
try {
ResultSet rs = stmt.executeQuery( "SELECT
* FROM MyTable" );
try {
while ( rs.next() ) {
int numColumns = rs.getMetaData().getColumnCount();
for ( int i = 1 ; i <= numColumns ; i++ ) {
//Column numbers start at 1.
//Also there are many methods on the result set
to return
// the column as a particular type. Refer to the
Sun documentation
// for the list of valid conversions.
System.out.println( "COLUMN " + i +
" = " + rs.getObject(i) );
}
}
} finally {
rs.close();
}
} finally {
stmt.close();
}
Typically, however, it would be rare for a seasoned
Java programmer to code in such a fashion. The
usual practice would be to abstract the database
logic into an entirely different class and to
pass preprocessed strings (perhaps derived themselves
from a further abstracted class) containing SQL
statements and the connection to the required
methods. Abstracting the data model from the application
code makes it more likely that changes to the
application and data model can be made independently.
An example of a PreparedStatement
query. Using conn and class from first example.
PreparedStatement ps = conn.prepareStatement(
"SELECT i.*, j.* FROM Omega i, Zappa j"
+ "WHERE i = ? AND j = ?" );
try {
// In the prepared statement ps, the question
mark denotes variable input,
// which can be passed through a parameter list,
for example.
// The following replaces the question marks,
// with the string or int, before sending it to
SQL.
// The first parameter corresponds to the first
occurrence of the ?,
// the second parameter tells Java to replace
it with
// the second item.
// The nth parameter corresponds to the nth ?
ps.setString(1, "Poor Yorick");
ps.setInt(2, 8008);
// The ResultSet rs, receives the SQL Query response.
ResultSet rs = ps.executeQuery();
try {
while ( rs.next() ) {
int numColumns = rs.getMetaData().getColumnCount();
for ( int i = 1 ; i <= numColumns ; i++ ) {
//Column numbers start at 1.
//Also there are many methods on the result set
to return
// the column as a particular type. Refer to the
Sun documentation
// for the list of valid conversions.
System.out.println( "COLUMN " + i +
" = " + rs.getObject(i) );
} // for
} // while
} finally {
rs.close();
}
} finally {
ps.close;
} // try
When a database operation fails, an SQLException
is raised. There is typically very little one
can do to recover from such an error, apart from
logging it with as much detail as possible. It
is recommended that the SQLException be translated
into an application domain exception (an unchecked
one) that eventually results in a transaction
rollback and a notification to the user.
Here are examples of host database
types, Java can convert to with a function.
setXXX() Methods Oracle Datatype setXXX()
CHAR setString()
VARCHAR2 setString()
NUMBER setBigDecimal()
setBoolean()
setByte()
setShort()
setInt()
setLong()
setFloat()
setDouble()
INTEGER setInt()
FLOAT setDouble()
CLOB setClob()
BLOB setBlob()
RAW setBytes()
LONGRAW setBytes()
DATE setDate()
setTime()
setTimestamp()
For an example of a CallableStatement
(to call stored procedures in the database), see
the JDBC API Guide.
JDBC
Drivers
JDBC Drivers are client-side adaptors (they are
installed on the client machine, not on the server)
that convert requests from Java programs to a
protocol that the DBMS can understand.
Types
There are commercial and free drivers available
for most relational database servers. These drivers
fall into one of the following types:
Type 1, the JDBC-ODBC bridge
Type 2, the Native-API driver
Type 3, the network-protocol driver
Type 4, the native-protocol drivers
Internal JDBC driver, driver embedded with JRE
in Java-enabled SQL databases. Used for Java stored
procedures.
JDBC URL, all Database Connection String
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