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Connectionobject represents a connection with a database. A connection session includes the SQL statements that are executed and the results that are returned over that connection. A single application can have one or more connections with a single database, or it can have connections with many different databases.
DriverManager.getConnection. This method takes a string containing a URL. The
DriverManagerclass, referred to as the JDBC management layer, attempts to locate a driver than can connect to the database represented by that URL. The
DriverManagerclass maintains a list of registered
Driverclasses, and when the method
getConnectionis called, it checks with each driver in the list until it finds one that can connect to the database specified in the URL. The
connectuses this URL to actually establish the connection.
A user can bypass the JDBC management layer and call
directly. This could be useful in the rare case that two drivers can connect to a
database and the user wants to explicitly select a particular driver. Normally, however, it is much easier to just let the
DriverManager class handle opening a connection.
The following code exemplifies opening a connection to a database located at
"jdbc:odbc:wombat" with a user ID of
"12Java" as the password :
String url = "jdbc:odbc:wombat"; Connection con = DriverManager.getConnection(url, "oboy", "12Java");
A URL (Uniform Resource Locator) gives information for locating a resource on the Internet. It can be thought of as an address.
The first part of a URL specifies the protocol used to access information, and it is always followed by a colon. Some common protocols are "ftp", which specifies "file transfer protocol," and "http," which specifies "hypertext transfer protocol." If the protocol is "file," it indicates that the resource is in a local file system rather than on the Internet. (Underlining in the examples below is used to indicate the part being described; it is not part of the URL.)
ftp://javasoft.com/docs/JDK-1_apidocs.zip http://java.sun.com/products/jdk/CurrentRelease file:/home/haroldw/docs/books/tutorial/summary.htmlThe rest of a URL, everything after the first colon, gives information about where the data source is located. If the protocol is
file, the rest of the URL is the path to a file. For the protocols
http, the rest of the URL identifies the host and may optionally give a path to a more specific site. For example, below is the URL for the JavaSoft home page. This URL identifies only the host:
http://java.sun.comBy navigating from this home page, one can go to many other pages, one of which is the JDBC home page. The URL for the JDBC home page is more specific and looks like this:
Since JDBC URLs are used with various kinds of drivers, the conventions are
of necessity very flexible. First, they allow different drivers to use different
schemes for naming databases. The
odbc subprotocol, for example, lets the URL
contain attribute values (but does not require them).
Second, JDBC URLs allow driver writers to encode all necessary connection information within them. This makes it possible, for example, for an applet that wants to talk to a given database to open the database connection without requiring the user to do any system administration chores.
Third, JDBC URLs allow a level of indirection. This means that the JDBC URL may refer to a logical host or database name that is dynamically translated to the actual name by a network naming system. This allows system administrators to avoid specifying particular hosts as part of the JDBC name. There are a number of different network name services (such as DNS, NIS, and DCE), and there is no restriction about which ones can be used.
The standard syntax for JDBC URLs is shown below. It has three parts, which are separated by colons:
jdbc:<subprotocol>:<subname>The three parts of a JDBC URL are broken down as follows:
jdbc-the protocol. The protocol in a JDBC URL is always
<subprotocol>-the name of the driver or the name of a database connectivity mechanism, which may be supported by one or more drivers. A prominent example of a subprotocol name is "odbc", which has been reserved for URLs that specify ODBC-style data source names. For example, to access a database through a JDBC-ODBC bridge, one might use a URL such as the following:
jdbc:odbc:fred In this example, the subprotocol is "odbc", and the subname "fred" is a local ODBC data source.
If one wants to use a network name service (so that the database name in the JDBC URL does not have to be its actual name), the naming service can be the subprotocol. So, for example, one might have a URL like:
jdbc:dcenaming:accounts-payable In this example, the URL specifies that the local DCE naming service should resolve the database name "accounts-payable" into a more specific name that can be used to connect to the real database.
<subname>-a way to identify the database. The subname can vary, depending on the subprotocol, and it can have a subsubname with any internal syntax the driver writer chooses. The point of a subname is to give enough information to locate the database. In the previous example, "fred" is enough because ODBC provides the remainder of the information. A database on a remote server requires more information, however. If the database is to be accessed over the Internet, for example, the network address should be included in the JDBC URL as part of the subname and should follow the standard URL naming convention of
//hostname:port/subsubnameSupposing that "dbnet" is a protocol for connecting to a host on the Internet, a JDBC URL might look like this:
odbcis a special case. It has been reserved for URLs that specify ODBC-style data source names and has the special feature of allowing any number of attribute values to be specified after the subname (the data source name). The full syntax for the odbc subprotocol is:
Thus all of the following are valid
DriverManagerclass presents this name to its list of registered drivers, the driver for which this name is reserved should recognize it and establish a connection to the database it identifies. For example,
odbcis reserved for the JDBC- ODBC Bridge. If there were, for another example, a Miracle Corporation, it might want to register "miracle" as the subprotocol for the JDBC driver that connects to its Miracle DBMS so that no one else would use that name.
JavaSoft is acting as an informal registry for JDBC subprotocol names. To register a subprotocol name, send email to:
JDBC provides three classes for sending SQL statements to the database, and
three methods in the
Connection interface create instances of these classes.
These classes and the methods which create them are listed below:
Statement- -created by the method
Statementobject is used for sending simple SQL statements.
PreparedStatement- -created by the method
PreparedStatementobject is used for SQL statements that take one or more parameters as input arguments (IN parameters).
PreparedStatementhas a group of methods which set the value of IN parameters, which are sent to the database when the statement is executed. Instances of
Statementand therefore include
PreparedStatementobject has the potential to be more efficient than a
Statementobject because it has been pre-compiled and stored for future use.
CallableStatement- -created by the method
CallableStatementobjects are used to execute SQL stored procedures- -a group of SQL statements that is called by name, much like invoking a function. A
CallableStatementobject inherits methods for handling IN parameters from
PreparedStatement; it adds methods for handling OUT and INOUT parameters.
Connectionmethod is appropriate for creating different types of SQL statements:
createStatementmethod is used for
prepareStatementmethod is used for
prepareCallmethod is used for
rollbackis called, the current transaction ends and another one begins.
A new connection is in auto-commit mode by default, meaning that when a
statement is completed, the method
commit will be called on that statement automatically. In this case, since each statement is committed individually, a transaction consists of only one statement. If auto-commit mode has been disabled, a
transaction will not terminate until the method
rollback is called
explicitly, so it will include all the statements that have been executed since the
last invocation of the
rollback method. In this second case, all the
statements in the transaction are committed or rolled back as a group.
commit makes permanent any changes an SQL statement makes
to a database, and it also releases any locks held by the transaction. The method
rollback will discard those changes.
Sometimes a user doesn't want one change to take effect unless another one
does also. This can be accomplished by disabling auto-commit and grouping both
updates into one transaction. If both updates are successful, then the
method is called, making the effects of both updates permanent; if one fails or
both fail, then the
rollback method is called, restoring the values that existed
before the updates were executed.
Most JDBC drivers will support transactions. In fact, a JDBC-compliant
driver must support transactions.
DatabaseMetaData supplies information
describing the level of transaction support a DBMS provides.
conis the current connection:
con.setTransactionIsolation(TRANSACTION_READ_UNCOMMITTED);The higher the transaction isolation level, the more care is taken to avoid conflicts. The
Connectioninterface defines five levels, with the lowest specifying that transactions are not supported at all and the highest specifying that while one transaction is operating on a database, no other transactions may make any changes to the data read by that transaction. Typically, the higher the level of isolation, the slower the application executes (due to increased locking overhead and decreased concurrency between users). The developer must balance the need for performance with the need for data consistency when making a decision about what isolation level to use. Of course, the level that can actually be supported depends on the capabilities of the underlying DBMS.
When a new
Connection object is created, its transaction isolation level
depends on the driver, but normally it is the default for the underlying database. A
user may call the method
setIsolationLevel to change the transaction isolation
level, and the new level will be in effect for the rest of the connection session. To
change the transaction isolation level for just one transaction, one needs to set it
before the transaction begins and reset it after the transaction terminates. Changing the transaction isolation level during a transaction is not recommended, for it
will trigger an immediate call to the method
commit, causing any changes up to
that point to be made permanent.