dba.html

java 基础的 一点东西,,可以看看

 "jdbc:oracle:thin:username/password@(description=(
 address_list=(address=(protocol=tcp)
 (host=developer)(port=1521)))
 (source_route=yes)(connect_data=(sid=jdcsid)))";
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<P>
The <CODE>actionPerformed</CODE> method calls the 
<CODE>Class.forName(_driver)</CODE> method to load
the driver, and the <CODE>DriverManager.getConnection</CODE> method
to establish the connection. 
The <A HREF="data.html#excep">Exception Handling</A>
section in Lesson 6 describes <CODE>try</CODE> and <CODE>catch</CODE>
blocks. The only thing different here is that this block
uses two <CODE>catch</CODE> statements because 
two different errors are possible.  

<P>
The call to <CODE>Class.forName(_driver);</CODE> throws
<CODE>java.lang.ClassNotFoundException</CODE>, and the call 
to <CODE>c = DriverManager.getConnection(_url);</CODE>
throws <CODE>java.sql.SQLException</CODE>. In the case
of either error, the application tells the user what is
wrong and exits because the program cannot operate in
any meaningful way without a database driver or connection. 

</FONT>

<PRE>
public void actionPerformed(ActionEvent event){
  try{
//Load the driver
     Class.forName(_driver);
//Establish database connection
     c = DriverManager.getConnection(_url);
   }catch (java.lang.ClassNotFoundException e){
     System.out.println(&quot;Cannot find driver class&quot;);
     System.exit(1);
   }catch (java.sql.SQLException e){
     System.out.println(&quot;Cannot get connection&quot;);       
     System.exit(1);
   }
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="final"></A>
<H4>Final and Private Variables</H4>

The member variables used to establish the database connection above are 
declared <CODE>private</CODE>, and two of those variables are also 
declared <CODE>final</CODE>. 

<P>
<STRONG>final</STRONG>:
A <CODE>final</CODE> variable contains
a constant value that can never change once it is initialized. In 
the example, the user name, and password are <CODE>final</CODE>
variables because you would not want to allow an instance
of this or any other class to change this information.

<P>
<STRONG>private</STRONG>:
A <CODE>private</CODE> variable can only be
used (accessed) by the class in which it is declared. No other class 
can read or change <CODE>private</CODE> variables. In the example, the 
database driver, user name, and password variables are
<CODE>private</CODE> to prevent an outside class from accessing
them and jeopardizing the database connection, or compromising
the secret user name and password information. 

You can find more information on this in the
<A HREF="http://java.sun.com/docs/books/tutorial/java/javaOO/index.html">Objects
and Classs</A> lesson in
<A HREF="http://java.sun.com/docs/books/tutorial">The Java Tutorial</A>

<A NAME="rw"></A>
<H4>Writing and Reading Data</H4>

In the write operation, a 
<CODE>Statement</CODE> object is created from the <CODE>Connection</CODE>.
The <CODE>Statement</CODE> object has methods for executing SQL queries 
and updates. Next, a <CODE>String</CODE> object that contains
the SQL update for the write operation is constructed and
passed to the <CODE>executeUpdate</CODE> method of the
<CODE>Statement</CODE> object. 

</FONT>

<PRE>
Object source = event.getSource();
if(source == button){
 JTextArea displayText = new JTextArea();

 try{
//Code to write to database
   String theText = textField.getText();
   Statement stmt = c.createStatement();
   String updateString = &quot;INSERT INTO dba VALUES 
			   ('&quot; + theText + &quot;')&quot;;
   int count = stmt.executeUpdate(updateString);
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

SQL commands are <CODE>String</CODE> objects, and therefore, follow
the rules of <CODE>String</CODE> construction where the string
is enclosed in double quotes (" ") and variable data is appended with a 
plus (+). The variable <CODE>theText</CODE> has single and double quotes 
to tell the database the SQL string has variable rather than literal 
data. 

<P>
In the read operation, a <CODE>ResultSet</CODE> object is created
from the <CODE>executeQuery</CODE> method of the
<CODE>Statement</CODE> object. The <CODE>ResultSet</CODE>
contains the data returned by the query. To retrieve the
data returned, the code iterates through the <CODE>ResultSet</CODE>,
retrieves the data, and appends the data to the text area,
<CODE>displayText</CODE>. 

</FONT>

<PRE>
//Code to read from database
    ResultSet results = stmt.executeQuery(
	&quot;SELECT TEXT FROM dba &quot;);
    while(results.next()){
      String s = results.getString(&quot;TEXT&quot;);
      displayText.append(s + &quot;\n&quot;);
    }
    stmt.close();
  } catch(java.sql.SQLException e){
    System.out.println(e.toString());
  }
//Display text read from database
  panel.removeAll();
  panel.add(&quot;North&quot;, clicked);
  panel.add(&quot;Center&quot;, displayText);
  panel.add(&quot;South&quot;, clickButton);
  panel.validate();
  panel.repaint();
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="applet"></A>
<H3>Database Access by Applets</H3>

The applet version of the example is like the application code described 
above except for the standard differences between applications and applets
described in the <A HREF="applet.html#struct">Structure and Elements</A>
section of Lesson 3.

<P>
However, if you run the applet without a policy file, you get 
a stack trace indicating permission errors.
The <A HREF="data.html#sec">Granting Applets Permission</A> section
in Lesson 6 introduced you to policy files and how to launch an
applet with the permission it needs. The Lesson 6 applet example
provided the policy file and told you how to launch the applet
with it. This lesson shows you how to read the stack trace to
determine the permissions you need in a policy file.

<P>
To keep things interesting, this lesson has two versions
of the database access applet: one uses the  JDBC driver,
and the other uses the the JDBC-ODBC bridge with an 
Open DataBase Connectivity (ODBC) driver.

<P>
Both applets do the same operations to the same database
table using different drivers. Each applet has its own policy file
with different permission lists and has different
requirements for locating the database driver 

<A NAME="jdbc"></A>
<H4>JDBC Driver</H4>

The JDBC driver is used from a program written exclusively
in the Java language (Java program). It
converts JDBC calls directly into the protocol
used by the DBMS. This type of driver is available from
the DBMS vendor and is usually packaged with the DBMS
software.

<P>
<STRONG>Starting the Applet:</STRONG>
To successfully run, the <A HREF="./Code/DbaAppl.java">DbaAppl.java</A> 
applet needs an available database driver and a policy file.
This section walks through the steps to get everything set up. 
Here is the <CODE>DbaAppl.html</CODE> file for running 
the <CODE>DbaAppl</CODE> applet:

</FONT>

<PRE>
&#60;HTML&#62;
&#60;BODY&#62;

&#60;APPLET CODE=DbaAppl.class
  WIDTH=200
  HEIGHT=100&#62;
&#60;/APPLET&#62;

&#60;/BODY&#62;
&#60;/HTML&#62;
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<P>
And here is how to start the applet with appletviewer:

</FONT>

<PRE>
  appletviewer DbaAppl.html
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<P>
<STRONG>Locating the Database Driver:</STRONG>
Assuming the driver is not available to the <CODE>DriverManager</CODE>
for some reason, the following error generates when you click the 
<CODE>Click Me</CODE> button.  

</FONT>

<PRE>
  cannot find driver
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

This error means the DriverManager looked for the JDBC driver in the
directory where the applet HTML and class files are and could not find it. 
To correct this error, copy the driver to the directory
where the applet files are, and if the driver is bundled in a zip file, 
unzip the zip file so the applet can access the driver.

<P>
Once you have the driver in place, launch the applet again. 

</FONT>

<PRE>
  appletviewer DbaAppl.html
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="perm"></A>
<STRONG>Reading a Stack Trace:</STRONG>

Assuming the driver is locally available to the applet, if the 
<A HREF="./Code/DbaAppl.java">DbaAppl.java</A> applet
is launched without a policy file, the following stack trace
is generated when the end user clicks the <CODE>Click Me</CODE>
button. 

</FONT>

<PRE>
java.security.AccessControlException: access denied 
(java.net.SocketPermission developer resolve)
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

The first line in the above stack trace tells you access is denied.
This means this stack trace was generated because the applet tried to 
access a system resource without the proper permission. 
The second line means to correct this condition
you need a <CODE>SocketPermission</CODE> that gives
the applet access to the machine (<CODE>developer</CODE>) where
the database is located. 

<P>
You can use Policy tool to create the policy file you need,
or you can create it with an ASCII editor. Here is the policy
file with the permission indicated by the stack trace:

</FONT>

<PRE>
grant {
  permission java.net.SocketPermission &quot;developer&quot;, 
    &quot;resolve&quot;;
  &quot;accessClassInPackage.sun.jdbc.odbc&quot;;
};
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<P>
Run the applet again, this time with a policy file named
<CODE>DbaApplPol</CODE> that has the above permission in it:

</FONT>

<PRE>
appletviewer -J-Djava.security.policy=DbaApplPol 
                                       DbaAppl.html
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

You get a stack trace again, but this time it is a different
error condition.

</FONT>

<PRE>
  java.security.AccessControlException: access denied 
  (java.net.SocketPermission
  129.144.176.176:1521 connect,resolve)
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

Now you need a <CODE>SocketPermission</CODE> that allows access to the
Internet Protocol (IP) address and port on the <CODE>developer</CODE>
machine where the database is located.

<P>
Here is the <CODE>DbaApplPol</CODE> policy file with the permission 
indicated by the stack trace added to it:

</FONT>

<PRE>
grant {
  permission java.net.SocketPermission &quot;developer&quot;, 
                       &quot;resolve&quot;;
  permission java.net.SocketPermission 
  "129.144.176.176:1521", &quot;connect,resolve&quot;;
};

</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

Run the applet again. If you use the above policy file
with the Socket permissions indicated, it works just fine.

</FONT>

<PRE>
  appletviewer -J-Djava.security.policy=DbaApplPol 
                                         DbaAppl.html
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="odbc"></A>
<H4>JDBC-ODBC Bridge with ODBC Driver</H4>

Open DataBase Connectivity (ODBC) is Microsoft's programming
interface for accessing a large number of relational databases on 
numerous platforms. The JDBC-ODBC bridge is built into the Solaris 
and Windows versions of the Java platform so you can do two 
things:

<OL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">Use ODBC from a Java program</FONT>
<P>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">Load ODBC drivers as 
JDBC drivers.  
This example uses the JDBC-ODBC bridge to load an ODBC driver
to connect to the database. The applet has no ODBC code, however.</FONT>
</OL>

<P>
The <CODE>DriverManager</CODE> uses environment settings to
locate and load the database driver. For this example, 
the driver file does not need to be locally accessible.

<P>
<STRONG>Start the Applet:</STRONG>
Here is the <CODE>DbaOdb.html</CODE> file for running the 
<CODE>DbaOdbAppl</CODE> applet:

</FONT>