Start of Tutorial > Start of Trail > Start of Lesson |
Search
Feedback Form |
Up to this point, you have seen only code fragments. Later in this section you will see sample programs that are complete applications you can run.The first sample code creates the table
COFFEES
; the second one inserts values into the table and prints the results of a query. The third application creates the tableSUPPLIERS,
and the fourth populates it with values. After you have run this code, you can try a query that is a join between the tablesCOFFEES
andSUPPLIERS
, as in the fifth code example. The sixth code sample is an application that demonstrates a transaction and also shows how to set placeholder parameters in aPreparedStatement
object using afor
loop.Because they are complete applications, they include some elements of the Java programming language we have not shown before in the code fragments. We will explain these elements briefly here.
In the Java programming language, any code you want to execute must be inside a class definition. You type the class definition in a file and give the file the name of the class with
.java
appended to it. So if you have a class namedMySQLStatement
, its definition should be in a file namedMySQLStatement.java
.
The first thing to do is to import the packages or classes you will be using in the new class. The classes in our examples all use the
java.sql
package (the JDBC API), which is made available when the following line of code precedes the class definition:import java.sql.*;The star (
*
) indicates that all of the classes in the packagejava.sql
are to be imported. Importing a class makes it visible and means that you do not have to write out the fully qualified name when you use a method or field from that class. If you do not include "import java.sql.*;
" in your code, you will have to write "java.sql.
" plus the class name in front of all the JDBC fields or methods you use every time you use them. Note that you can import individual classes selectively rather than a whole package. Java does not require that you import classes or packages, but doing so makes writing code a lot more convenient.Any lines importing classes appear at the top of all the code samples, as they must if they are going to make the imported classes visible to the class being defined. The actual class definition follows any lines that import classes.
If a class is to be executed, it must contain a
static
public
main
method. This method comes right after the line declaring the class and invokes the other methods in the class. The keywordstatic
indicates that this method operates on a class level rather than on individual instances of a class. The keywordpublic
means that members of any class can access this method. Since we are not just defining classes to be used by other classes but instead want to run them, the example applications in this chapter all include amain
method.
Something else all the sample applications include is
try
andcatch
blocks. These are the Java programming language's mechanism for handling exceptions. Java requires that when a method throws an exception, there be some mechanism to handle it. Generally acatch
block will catch the exception and specify what happens (which you may choose to be nothing). In the sample code, we use twotry
blocks and twocatch
blocks. The firsttry
block contains the methodClass.forName,
from thejava.lang
package. This method throws aClassNotFoundException
, so thecatch
block immediately following it deals with that exception. The secondtry
block contains JDBC methods, which all throwSQLException
s, so onecatch
block at the end of the application can handle all of the rest of the exceptions that might be thrown because they will all beSQLException
objects.
JDBC lets you see the warnings and exceptions generated by your DBMS and by the Java compiler. To see exceptions, you can have a
catch
block print them out. For example, the following twocatch
blocks from the sample code print out a message explaining the exception:try { // Code that could generate an exception goes here. // If an exception is generated, the catch block below // will print out information about it. } catch(SQLException ex) { System.err.println("SQLException: " + ex.getMessage()); } try { Class.forName("myDriverClassName"); } catch(java.lang.ClassNotFoundException e) { System.err.print("ClassNotFoundException: "); System.err.println(e.getMessage()); }If you were to run
CreateCOFFEES.java
twice, you would get an error message similar to this:SQLException: There is already an object named 'COFFEES' in the database. Severity 16, State 1, Line 1This example illustrates printing out the message component of an
SQLException
object, which is sufficient for most situations.There are actually three components, however, and to be complete, you can print them all out. The following code fragment shows a
catch
block that is complete in two ways. First, it prints out all three parts of anSQLException
object: the message (a string that describes the error), the SQL state (a string identifying the error according to the X/Open SQLState conventions), and the vendor error code (a number that is the driver vendor's error code number). TheSQLException
objectex
is caught, and its three components are accessed with the methodsgetMessage
,getSQLState
, andgetErrorCode
.The second way the following
catch
block is complete is that it gets all of the exceptions that might have been thrown. If there is a second exception, it will be chained toex
, soex.getNextException
is called to see if there is another exception. If there is, thewhile
loop continues and prints out the next exception's message, SQLState, and vendor error code. This continues until there are no more exceptions.try { // Code that could generate an exception goes here. // If an exception is generated, the catch block below // will print out information about it. } catch(SQLException ex) { System.out.println("\n--- SQLException caught ---\n"); while (ex != null) { System.out.println("Message: " + ex.getMessage ()); System.out.println("SQLState: " + ex.getSQLState ()); System.out.println("ErrorCode: " + ex.getErrorCode ()); ex = ex.getNextException(); System.out.println(""); } }If you were to substitute the
catch
block above intoCreateCoffees.java
and run it after the tableCOFFEES
had already been created, you would get the following printout:--- SQLException caught --- Message: There is already an object named 'COFFEES' in the database. Severity 16, State 1, Line 1 SQLState: 42501 ErrorCode: 2714SQLState is a code defined in X/Open and ANSI-92 that identifies the exception. Two examples of SQLState code numbers and their meanings follow:
08001 -- No suitable driver HY011 -- Operation invalid at this timeThe vendor error code is specific to each driver, so you need to check your driver documentation for a list of error codes and what they mean.
SQLWarning
objects are a subclass ofSQLException
that deal with database access warnings. Warnings do not stop the execution of an application, as exceptions do; they simply alert the user that something did not happen as planned. For example, a warning might let you know that a privilege you attempted to revoke was not revoked. Or a warning might tell you that an error occurred during a requested disconnection.A warning can be reported on a
Connection
object, aStatement
object (includingPreparedStatement
andCallableStatement
objects), or aResultSet
object. Each of these classes has agetWarnings
method, which you must invoke in order to see the first warning reported on the calling object. IfgetWarnings
returns a warning, you can call theSQLWarning
methodgetNextWarning
on it to get any additional warnings. Executing a statement automatically clears the warnings from a previous statement, so they do not build up. This means, however, that if you want to retrieve warnings reported on a statement, you must do so before you execute another statement.The following code fragment illustrates how to get complete information about any warnings reported on the
Statement
objectstmt
and also on theResultSet
objectrs
:Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery("select COF_NAME from COFFEES"); while (rs.next()) { String coffeeName = rs.getString("COF_NAME"); System.out.println("Coffees available at the Coffee Break: "); System.out.println(" " + coffeeName); SQLWarning warning = stmt.getWarnings(); if (warning != null) { System.out.println("\n---Warning---\n"); while (warning != null) { System.out.println("Message: " + warning.getMessage()); System.out.println("SQLState: " + warning.getSQLState()); System.out.print("Vendor error code: "); System.out.println(warning.getErrorCode()); System.out.println(""); warning = warning.getNextWarning(); } } SQLWarning warn = rs.getWarnings(); if (warn != null) { System.out.println("\n---Warning---\n"); while (warn != null) { System.out.println("Message: " + warn.getMessage()); System.out.println("SQLState: " + warn.getSQLState()); System.out.print("Vendor error code: "); System.out.println(warn.getErrorCode()); System.out.println(""); warn = warn.getNextWarning(); } } }Warnings are actually rather uncommon. Of those that are reported, by far the most common warning is a
DataTruncation
warning, a subclass ofSQLWarning
. AllDataTruncation
objects have an SQLState of01004
, indicating that there was a problem with reading or writing data.DataTruncation
methods let you find out in which column or parameter data was truncated, whether the truncation was on a read or write operation, how many bytes should have been transferred, and how many bytes were actually transferred.
Start of Tutorial > Start of Trail > Start of Lesson |
Search
Feedback Form |
Copyright 1995-2005 Sun Microsystems, Inc. All rights reserved.