----- Original Message -----
Sent: Sunday, September 03, 2000 12:54
PM
Subject: Re: [java-list] SOS
javaneses
--- luizbarbosa <[EMAIL PROTECTED]> escribió:
>
Estou com erro na conexao atraves de um Applet.
>
>
import java.net.URL;
> import java.sql.*;
> import
java.awt.*;
> import java.applet.*;
>
> public class Teste
extends Applet
> { public void init()
>
{ try
> { String url =
"c:jdbc:odbc:cadastros";
>
>
Class.forName
> ("sun.jdbc.odbc.JdbcOdbcDriver");
>
> Connection con
=
> DriverManager.getConnection (url, "my_user",
>
"my_passwd");
>
>
proba url
"String url =
"jdbc:odbc:cadastros"; ?
nao "String url = "C:
...."
Daniel
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2 - Connection
This overview is excerpted from
JDBCTM Database Access from JavaTM: A Tutorial and Annotated Reference,
currently in progress at JavaSoft. This book, both a tutorial and the
definitive reference manual for JDBC, will be published in the spring of 1997
by Addison-Wesley Publishing Company as part of the Java series.
2.1 Overview
A
Connection object 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.
2.1.1 Opening a Connection
The
standard way to establish a connection with a database is to call the method
DriverManager.getConnection. This method takes a string
containing a URL. The DriverManager class, referred to as the
JDBC management layer, attempts to locate a driver than can connect to the
database represented by that URL. The DriverManager class
maintains a list of registered Driver classes, and when the
method getConnection is called, it checks with each driver in the
list until it finds one that can connect to the database specified in the URL.
The Driver method connect uses this URL to actually
establish the connection.
A user can bypass the JDBC management layer and call
Driver methods 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 the URL "jdbc:odbc:wombat" with a user ID
of "oboy" and "12Java" as the password :
String url = "jdbc:odbc:wombat";
Connection con = DriverManager.getConnection(url, "oboy", "12Java");
2.1.2 URLs in General Use
Since URLs
often cause some confusion, we will first give a brief explanation of URLs in
general and then go on to a discussion of JDBC URLs.
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.html
The 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 ftp and 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.com
By 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:
http://java.sun.com/products/jdbc
2.1.3 JDBC URLs
A JDBC URL provides
a way of identifying a database so that the appropriate driver will recognize
it and establish a connection with it. Driver writers are the ones who
actually determine what the JDBC URL that identifies their particular driver
will be. Users do not need to worry about how to form a JDBC URL; they simply
use the URL supplied with the drivers they are using. JDBC's role is to
recommend some conventions for driver writers to follow in structuring their
JDBC URLs.
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
jdbc.
<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/subsubname
Supposing that "dbnet" is a protocol for connecting to a host on the Internet, a
JDBC URL might look like this:
jdbc:dbnet://wombat:356/fred
2.1.4 The "odbc" Subprotocol
The
subprotocol odbc is 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:
jdbc:odbc:<data-source-name>[;<attribute-name>=<attribute-value>]*
Thus all of the following are valid
jdbc:odbc names:
jdbc:odbc:qeor7
jdbc:odbc:wombat
jdbc:odbc:wombat;CacheSize=20;ExtensionCase=LOWER
jdbc:odbc:qeora;UID=kgh;PWD=fooey
2.1.5 Registering Subprotocols
A
driver developer can reserve a name to be used as the subprotocol in a JDBC
URL. When the DriverManager class 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, odbc is 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:
[EMAIL PROTECTED]
2.1.6 Sending SQL Statements
Once a
connection is established, it is used to pass SQL statements to its underlying
database. JDBC does not put any restrictions on the kinds of SQL statements
that can be sent; this provides a great deal of flexibility, allowing the use
of database-specific statements or even non-SQL statements. It requires,
however, that the user be responsible for making sure that the underlying
database can process the SQL statements being sent and suffer the consequences
if it cannot. For example, an application that tries to send a stored
procedure call to a DBMS that does not support stored procedures will be
unsuccessful and generate an exception. JDBC requires that a driver provide at
least ANSI SQL-2 Entry Level capabilities in order to be designated JDBC
COMPLIANTTM. This means that users can count
on at least this standard level of functionality.
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
createStatement. A Statement object is used for
sending simple SQL statements.
PreparedStatement- -created by the method
prepareStatement. A PreparedStatement object is
used for SQL statements that take one or more parameters as input arguments
(IN parameters). PreparedStatement has a group of methods which
set the value of IN parameters, which are sent to the database when the
statement is executed. Instances of PreparedStatement extend
Statement and therefore include Statement methods.
A PreparedStatement object has the potential to be more
efficient than a Statement object because it has been
pre-compiled and stored for future use.
CallableStatement- -created by the method
prepareCall. CallableStatement objects are used to
execute SQL stored procedures- -a group of SQL statements that is called by
name, much like invoking a function. A CallableStatement object
inherits methods for handling IN parameters from
PreparedStatement; it adds methods for handling OUT and INOUT
parameters.
The following list gives a quick way
to determine which Connection method is appropriate for creating
different types of SQL statements:
createStatement method is used
for
- simple SQL statements (no parameters)
prepareStatement method is used
for
- SQL statements with one or more IN parameters
- simple SQL statements that are executed frequently
prepareCall method is used for
- call to stored procedures
2.1.7 Transactions
A transaction
consists of one or more statements that have been executed, completed, and
then either committed or rolled back. When the method commit or
rollback is 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 commit or rollback is
called explicitly, so it will include all the statements that have been
executed since the last invocation of the commit or
rollback method. In this second case, all the statements in the
transaction are committed or rolled back as a group.
The method 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 commit 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.
2.1.8 Transaction Isolation Levels
If a DBMS supports transaction processing, it will have some
way of managing potential conflicts that can arise when two transactions are
operating on a database at the same time. A user can specify a transaction
isolation level to indicate what level of care the DBMS should exercise in
resolving potential conflicts. For example, what happens when one transaction
changes a value and a second transaction reads that value before the change
has been committed or rolled back? Should that be allowed, given that the
changed value read by the second transaction will be invalid if the first
transaction is rolled back? A JDBC user can instruct the DBMS to allow a value
to be read before it has been committed ("dirty reads") with the following
code, where con is the current connection:
con.setTransactionIsolation(TRANSACTION_READ_UNCOMMITTED);
The higher the transaction isolation level, the more
care is taken to avoid conflicts. The Connection interface
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.
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Copyright © 1996, 1997 Sun Microsystems, Inc. All rights
reserved.
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