www.developer.com/java/data/article.php/3417381
October 5, 2004 Java Programming Notes # 662
PrefacePurpose The purpose of this lesson is to get you beyond the initial hurdles involved in:
What is JDBC? JDBC technology is an API (included in both J2SE and J2EE) that provides cross-DBMS connectivity to a wide range of SQL databases and access to other tabular data sources, such as spreadsheets or flat files. What is MySQL? The MySQL database server is probably the world's most popular open source database software, with more than five million active installations as of September 2004. The database server software from MySQL is available under a "dual licensing" model. Under this model, users may choose to use MySQL products under the free software/open source GNU General Public License (commonly known as the "GPL") or under a commercial license. A powerful combination Simply stated, JDBC makes it possible to write platform independent Java programs that can be used to manipulate the data in a wide range of SQL databases without the requirement to modify and/or recompile the Java programs when moving from platform to platform or from DBMS to DBMS. MySQL is available for a wide variety of platforms. Since both JDBC and MySQL are freely available for many purposes, the combination of JDBC and MySQL is a powerful combination that should be of interest for a wide variety of applications. Viewing tip You may find it useful to open another copy of this lesson in a separate browser window. That will make it easier for you to scroll back and forth among the different listings and figures while you are reading about them. Supplementary material I recommend that you also study the other lessons in my extensive collection of online Java tutorials. You will find those lessons published at Gamelan.com. However, as of the date of this writing, Gamelan doesn't maintain a consolidated index of my Java tutorial lessons, and sometimes they are difficult to locate there. You will find a consolidated index at www.DickBaldwin.com. PreviewDownload, install, and prepare the database server It is often possible to use the same Java program to manipulate the data in a wide variety of SQL databases without a requirement to modify and/or recompile the Java program. However, the installation and preparation procedures for different SQL databases vary widely. A large part of the battle in using JDBC with a particular database is getting the database installed and properly prepared for use with JDBC. In this lesson, I will show you how to download, install, and prepare a MySQL database as a localhost server on a Windows platform for use with JDBC. Then I will show you how to write three simple JDBC programs to administer the database server and to manipulate data stored on the database server after it is installed.
URLs and version numbers for downloading I will provide specific URLs and version numbers for downloading MySQL software and documentation as of September 2004. The version numbers will certainly change over time as new versions of the software are released. Hopefully, the folks at MySQL will preserve the integrity of the URLs. At least one URL, http://www.mysql.com/ should remain constant over time. If the other links to MySQL in this lesson become broken with time, you should revert to the main MySQL URL given above and begin your search for the software and documentation from that point. Saving time If you are a newcomer to the installation of database server software and the preparation of that software for use with JDBC, the information that I will provide in this lesson should save you several days of effort in pouring through documentation trying to figure out how to download, install, and tie everything together. Even if you are experienced in these matters, this information should save you several hours of effort. Installation and Preparation of MySQLGetting things up and running As a minimum, getting up and running with MySQL and JDBC involves at least the following steps:
Additional MySQL software Beyond the minimum, there are a variety of additional software packages, (such as GUI administrator packages) that can be downloaded from MySQL and installed on your computer. Since the main thrust of this lesson has to do with JDBC rather than database administration, I won't get into that. Rather, I will show you how to use a command-line monitor program that is included with the MySQL database software to perform the minimal database administrative tasks required to satisfy the objectives of this lesson. DocumentationI will begin with a discussion of the available documentation for both the MySQL database server and the MySQL Connector/J. MySQL database server documentation The MySQL Reference Manual can be downloaded from http://dev.mysql.com/doc/. In addition, there is an online searchable version of the Reference Manual available at http://dev.mysql.com/doc/mysql/en/Reference.html. The downloadable version is available in several different formats, including: You would probably do well to have both of these formats locally available on your computer if you have sufficient disk space. The one-page-per-chapter formatted manual The first format in the above list consists of a large number of HTML files. There is one HTML file for the table of contents plus about thirty-three additional files containing the text of the reference manual. This format has a major advantage over the second format in terms of speed. It is relatively fast to click on a hyperlink in the table of contents and to see that material appear in the browser window. There are a couple of downsides to this format, however. One downside is that this format is not very useful for searching the entire manual for keywords, (using your browser) because it is broken down into a large number of separate HTML files. A second downside is that even though MySQL 4.0 is the recommended release in September of 2004, this manual contains information up through version 5.0.1-alpha. Sometimes information about the newer versions tends to obscure information about version 4.0. Installing the one-page-per-chapter formatted manual All that you need to do to install the database server documentation in this format is to download the zip file and extract the various HTML files into a folder on your disk. Then open the file named manual_toc.html in your browser to view the manual. For convenience, I created a desktop icon that links to the table of contents file. The all-on-one-page formatted manual This format is very useful for searching (using your browser) because all of the text is in a single HTML file.
The primary downside to this format is speed, or lack thereof. The HTML file containing the text of the manual is about four megabytes in size. On my machine, navigating this manual in a browser is a very slow process. The downloadable version in this format also contains information up through version 5.0.1-alpha, resulting in the same disadvantage mentioned earlier. Included in the software distribution When you download and install the currently recommended version of MySQL, (which is version 4.0.21), the Docs folder in the installation tree structure will contain a copy of the manual in the all-on-one-page format that purports to be for version 4.0.21. Thus, you don't need to download this format separately. You will get it when you download the software.
Installation of the all-on-one-page formatted manual As mentioned above, you don't need to do anything special to install this format of the manual. It will be installed automatically when you install the MySQL 4.0.21 version of the database.
Once you have installed the database, the Docs folder of the installation tree will contain the files named manual_toc.html and manual.html. The first file contains a hyperlinked table of contents, and the second file contains the entire text of the manual. Once again, for convenience, I created a desktop icon linked to the table of contents file to make it convenient to open in my browser. Using both formats Because I have plenty of space on my disk, I have both formats installed on my computer with an icon on the desktop for each. I occasionally open the version that contains the entire manual in a single HTML file when I need to search the entire document for something. Most of the time, however, I open and use the multi-file version due to its increased speed. MySQL Connector/J documentation I did not find a separate downloadable version of the connector documentation at the MySQL site. However, I did find an online version at http://dev.mysql.com/doc/connector/j/en/index.html. I was able to save the connector documentation locally by selecting the Save Page As... item on the File menu of my Netscape 7.2 browser.
Saving the page in Netscape 7.2 resulted in a local file named index.html and an associated folder named index_files. The file contains the text of the connector documentation. The folder contains style sheets and other related material. Installation of the connector documentation consisted simply of saving this material locally and creating a desktop icon linked to the file named index.html. Downloading the MySQL Database ServerThe download page The main download page for both the database server and the connector as of September 2004 is http://dev.mysql.com/downloads/. Hopefully, this URL will also remain intact as MySQL releases later versions of the software. Several different versions of the database server are available for downloading as of September 2004, including:
This list can be expected to change over time as new versions of the database server are released. Thus, the links in the above list will become obsolete. When that happens, you should revert back to the download page at http://dev.mysql.com/downloads/ and download the version that best suits your needs at that time. The different versions of the database server As of September 2004, the database server documentation has this to say about these different versions:
I elected MySQL 4.0 I elected to download MySQL 4.0 since it is the stable production quality version as of September 2004. This resulted in the downloading of a distribution file named mysql-4.0.21-win.zip.
Installing MySQL Database ServerInstallation instructions Installation instructions for the database server are provided in the database server documentation, Section 2, entitled Installing MySQL. Since I was installing on Windows XP and had no desire to deal with source code, I quickly skipped down to Section 2.2.1.2 entitled Installing a Windows Binary Distribution. In my case, installation was easy Because I did not have an earlier version of MySQL installed and I was logged onto Windows as an administrator, all that I needed to do was to execute the following instructions from the database server documentation to install the MySQL database server on my computer.
Because I didn't want to deal with option files, I elected to allow the software to be installed in the default directory, C:\mysql. Testing the installation After completing the installation, I performed some of the procedures shown in the database server documentation, Section 2.4.1 entitled Windows Post-Installation Procedures. Although I didn't get exactly the same results as those shown in the documentation, my results were close enough to convince me that the MySQL database server was correctly installed on my computer.
Not installed as a Windows service Section 2.2.1.7 of the database server documentation entitled Starting MySQL as a Windows Service contains the following:
I have no desire for the MySQL database server to start running every time I start Windows running. I already waste enough time waiting for Windows XP to become ready for use on my laptop each time I start it. Therefore, I did not install the database server as a service. I will explain how I manually start and stop the database server whenever I need to use it later in this lesson. Downloading MySQL Connector/JWhat is MySQL Connector/J? For those who don't know, let me begin by explaining the purpose of MySQL Connector/J. The JDBC API is designed to make it possible for you to write a single Java program and to use it to manipulate the data in a variety of different SQL database servers without a requirement to modify and/or recompile the program. In order to do this, it is necessary for you to:
The programming interface The programming interface deals with the interface peculiarities of the different database servers. Sun refers to the process of providing this information to the program as registering the database server with the Java program. You will see how this is done in the sample programs later in this lesson. The connector download page The download page for MySQL Connector/J is http://dev.mysql.com/downloads/index.html. As of September 2004, the following versions are available for downloading from this page:
As with the MySQL database server software, these individual links are likely to become obsolete as new versions of the software are released. Hopefully the link to http://dev.mysql.com/downloads/index.html will remain intact. The MySQL Connector/J 3.0 distribution file Because I was very interested in stability, I elected to download and install MySQL Connector/J 3.0, identified above as the production release. This resulted in the download of a file named mysql-connector-java-3.0.15-ga.zip. This zip file encapsulates 194 individual files in different folders including source code files, class files, pdf files, xml files, jar files, license files, files with no extensions, a manifest file, HTML files, and other file types not listed here. The zip file also contains several java programs in a folder named testsuite that can be used to test your installation. You may find them useful for that purpose. In addition, these programs illustrate a variety of database operations using JDBC, so you may find them useful as example programs as well. Fortunately, as I will explain below, all but one of these files can be ignored insofar as installation of the connector software is concerned. Installing MySQL Connector/JGeneral installation instructions The following statement appears in the connector documentation Section 2.2.1. entitled Setting the CLASSPATH (For Standalone Use).
My installation Actually, the above quotation describes three options instead of just two. To make a long story short, I elected the third option. I extracted the jar file named mysql-connector-java-3.0.15-ga-bin.jar from the zip file and copied it into the folder named c:\j2sdk1.4.2\jre\lib\ext, which is the installation directory tree for the currently installed version of Java on my computer. The advantage of doing it this way was that I didn't have to modify the classpath environment variable. The disadvantage is that the next time I upgrade to a new version of Java, I must remember to save the MySQL connector jar file and copy it into the directory tree for my new Java installation. Your installation If you prefer the first option, the connector documentation contains a wealth of information to help you perform the necessary steps to modify the classpath, etc. Testing the installation I didn't use any of the test programs mentioned above in the folder named testsuite. Rather, I tested my installation using JDBC programs that I had developed earlier using a different SQL database server. You can use the test programs mentioned earlier in the testsuite folder to test your installation. Also, I will provide and explain three sample JDBC programs later in this lesson that you can use to test your installation. Before you can test the installation, however, you must start the MySQL database server running. Starting the database server At this point, all of the software necessary to use the database server in a JDBC program should be installed on your computer ready for use. The next step is to confirm that you can start the database server running. Selecting a Windows server I found it necessary to pull together several pieces of information from the database server documentation to determine the best way to start the server from a command line. For example, the following table is found in the database server documentation, Section 2.2.1.4 entitled Selecting a Windows Server.
Explaining the different types of servers The following explanation follows the table:
Changes in MySQL 4.1.2 This is followed by another explanation indicating that beginning with MySQL 4.1.2, the server names were changed eliminating the server name mysqld-opt and replacing the debug version (mysqld) with mysqld-debug. Therefore, if you are installing MySQL 4.1.2 or a later version, you should use the syntax mysqld instead of mysqld-opt to start the server running from an optimized binary file. Starting MySQL 4.0.21 Since I am running MySQL 4.0.21 and need to make certain that what I am doing is compatible with a large number of students having different operating systems, I concluded that I should start the server running by using the syntax mysqld-opt. Section 2.2.1.5 of the database server documentation entitled Starting the Server for the First Time indicates that the following command should be used at the command prompt to start the server running: C:\mysql\bin\mysqld --console As I understand it, the purpose of --console is to cause error messages to be displayed on the standard error device (typically the screen) rather than to be entered into an error log file. This is what I want to happen. Combining the two pieces of information given above, I concluded that I should start the MySQL database server by entering the following command at a command prompt: C:\mysql\bin\mysqld-opt --console Encapsulated in a batch file Therefore, I created a batch file named MySqlStart.bat and linked that file to an icon on the desktop for convenience. The batch file contains the two commands shown in Listing 1 and repeated later in Listing 30 near the end of the lesson.
The startup screen output Figure 1 shows the screen output following the execution of the batch file named MySqlStart.bat.
The process window shown in Figure 1 remains open and active until the server is stopped. It should be possible to connect to the server using JDBC during this period. Stopping the database server It is probably a good idea to shut down the server before shutting down the computer. Section 2.2.1.6 of the database server documentation entitled Starting MySQL from the Windows Command Line states that you can stop the MySQL server by executing the following command: C:\mysql\bin\mysqladmin -u root shutdown Therefore, I created a batch file named MySqlStop.bat and linked that file to an icon on my desktop to make it convenient to stop the server. The batch file contains the two commands shown in Listing 2 and repeated in Listing 31 near the end of the lesson.
The screen output at server shutdown Figure 2 shows the screen output in the server process window when the file
named MySqlStop.bat is executed.
Once the server is shut down, attempts to connect to the server from JDBC will fail.
Creating a New Database using the Monitor ProgramNow that you know how to start the MySQL database server, it's time to learn how to:
Three different approaches I'm going to show you three different ways to accomplish this:
I will illustrate the first approach in this and the later section entitled Creating a New User using the Monitor Program . I will illustrate the second approach in the section entitled Administering the Database Server using Text Files. I will illustrate the third approach in the section entitled Discussion and Sample Code, which shows how to use JDBC to manage and manipulate the database server. The monitor program and manual data entry The monitor program is named mysql.exe. It is located in c:\mysql\bin. This program makes it possible to log onto the database server and to enter commands at the command line to:
To really learn MySQL ... In order to really learn how to use MySQL, you will need to study the MySQL database server documentation in detail and probably some good SQL books as well. The purpose of this lesson is to teach you just enough to get you started. When you finish this lesson, you should be able to successfully write and execute simple JDBC programs that will manipulate database tables on the MySQL database server. The default administrative user As I understand it, when the MySQL database server is first installed, there is a default user named root with full administrative privileges and no password. At this point, the server is totally wide open and insecure.
The user named root has the ability to create new databases as well as to create new users and to register those users on the databases. Existing databases at MySQL installation Also, as I understand it, there are two existing databases on the server when it is first installed. There is a database named test, which is wide open with no password requirements. Any user can access this database. There is also a database named mysql, which is apparently used to keep track of things such as databases, users, etc. I believe that this database is accessible only by users having administrative privileges. Adding a new database using the monitor program The first step in adding a new database using the monitor program is to log onto the database server as the administrative user named root with access to the database named mysql. Until a password is assigned to the root user, login can be accomplished by entering the following command at the command prompt: c:\mysql\bin\mysql --user=root mysql
The screen output Assuming that the MySQL database server is running, the screen output produced by entering this command is shown in Figure 3.
The monitor program Note that this program refers to itself as the MySQL monitor. That explains why I refer to it as the monitor or the monitor program. The purpose of this program is to make it possible for you to enter SQL database commands from the keyboard. Note in particular the prompt shown in boldface at the end of Figure 3, which reads: mysql> This is not a command-line prompt, which typically looks something like: C:\jnk> Rather, this is a program-generated prompt where the monitor program is requesting input from the user. SQL command terminators As indicated in Figure 3, SQL commands end with either a semicolon character or \g (note the difference in typeface for the character g in this text relative to that shown in Figure 3). SQL commands are often quite long. You can enter successive portions of SQL commands at successive program prompts.
It is not until you enter a semicolon character or a \g that the program responds to and attempts to execute the entire SQL command. Terminating the monitor program You can terminate the monitor program by entering a \q at the program prompt.
Creating a new database named JunkDB Figure 4 shows the screen output for the use of the monitor program by the
user named root to create a new database named JunkDB and then to
terminate the monitor program.
The new material in Figure 4 is shown in boldface in the bottom half of the figure. The material in the top half of Figure 4 is a repeat of the material shown in Figure 3. Using a batch file and a text file Shortly, I will show you how to create a new database using a Windows batch file and an associated text file. Later on, I will show you how to create a new database using a Java JDBC program. Creating a New User with the Monitor ProgramFigure 5 shows the screen output for the use of the monitor program by the user named root to add a new user named auser. The new material is shown in boldface in the lower half of the figure.
A longer SQL command This SQL command is much longer than the command used to create the new database. This command requires several continuation lines to complete to prevent it from exceeding the screen width.
I won't try to explain the SQL command is detail. I will simply refer you to the MySQL database documentation and a good SQL book for that purpose. However, the SQL command is relatively self explanatory. The meaning of the SQL command This SQL command grants a list of six different privileges on the database named JunkDB to a user named auser who will be accessing the database from localhost.
The user named auser will be allowed to access the database named JunkDB using the password drowssap, (which is password spelled backwards to make it easy for me to remember). Administering the Database Server using Text FilesAs mentioned earlier, unless you really enjoy typing, using the monitor program in manual data entry mode is not a fun way to work with the database. For example, if you make a typing error, you must go back and retype the entire command from the beginning. Fortunately, there is a better approach. That approach is to provide the commands to the monitor program using a text file as input. Then if you make an error, you can simply edit the text file and rerun the process. How does it work? To make a long story short, you start the monitor program by redirecting the input so that the input is derived from a text file instead of from the keyboard. This process is described in the database server documentation, Section 3.5 entitled Using mysql in Batch Mode. Creating a new database There are probably several ways to set this process up. I elected to use a combination of a batch file and a text file. The batch file starts the monitor program, logging in as root, and redirects input to the associated text file. For example, the files used to create a new database named JunkDB are shown in Listing 32 and Listing 33 near the end of the lesson. Making a new user The files used to make a new user named auser are shown in Listing 34 and Listing 35. Compare the contents of these two files with the manual data entry shown earlier in Figure 5. The new user is granted six different privileges on the database named JunkDB from localhost with a password of drowssap. The files used to remove the user named auser are shown in Listing 36 and Listing 37. The procedure for revoking a user's privileges and removing the user is explained near the end of Section 14.5.1.1 entitled DROP USER Syntax in the database server documentation. Deleting the database named JunkDB The files used to delete the database named JunkDB are shown in Listing 38 and Listing 39. The procedure for deleting a database is explained in the database server documentation, Section 14.2.8 entitled DROP DATABASE Syntax. Discussion and Sample Code for JDBC ProgramsThree separate programs I will explain three programs. The first program, named Jdbc11 shows how to:
The second program named Jdbc12 shows how to:
The third program named Jdbc10 shows how to log onto the server as the user named auser and to manipulate the database named JunkDB in a variety of ways. I will break each of these programs down into fragments and discuss the fragments. Complete listings of all the programs are shown in Listings 40, 41, and 42 near the end of the lesson. Jdbc11 - Create a database and make a new user The purpose of the program named Jdbc11 is to log onto the master database named mysql as the default administrator named root whose password is blank in order to perform the following updates on the MySQL database server:
The output, or lack thereof These two operations produce no visible output when successful. However, they produce error messages in the output when unsuccessful.
Server must be running The MySQL server must be running on localhost before this program is started. Instructions for starting and stopping the database server were provided earlier (see Listing 1, Figure 1, Listing 2, and Figure 2). The program was tested using Java SDK 1.4.2 under WinXP, MySQL version 4.0.21-win, and JDBC connector version mysql-connector-java-3.0.15-ga. There are five critical steps in using JDBC to manipulate a database:
I will highlight these five steps in the discussion of the sample program that follows. Beginning of class definition for Jdbc11 The first program fragment for the program named Jdbc11 is shown in Listing 3. The entire program is shown in Listing 40 near the end of the lesson.
The code in Listing 3 is straightforward, showing the beginning of the class, the beginning of the main method, and a print statement. Listing 3 also declares a local variable of type Statement. I will have more to say about the Statement interface later. Register the JDBC driver for MySQL Listing 4 shows the statement that implements the first critical step listed earlier (load and register the JDBC driver classes). This statement registers the MySQL driver classes with the Java program, making it possible for this program to manipulate data on the MySQL server.
Reference to the driver class The following statement appears in the MySQL Connector documentation, Section 2.2.1. entitled Setting the CLASSPATH (For Standalone Use).
This information is also provided in the connector documentation in Section 2.2.2. entitled Driver Class Name and JDBC URL Format. The Driver interface Note the reference to the Driver interface in the above quotation. Here is some of what Sun has to say about the Driver interface:
The name of the driver class In order to use a JDBC program with a specific database server, you must obtain the name of this critical driver class from the database vendor (or from some third party that supports the database server). You must then cause your program to load the class. This is the class that connects the other classes in the connector package to the Java program. Without it, the Java program would be unable to communicate successfully with the database server. Loading the driver class The statement in Listing 4 causes this class to be loaded as described in the Sun documentation quoted above.
Specification of Driver class as a String There are several alternative ways to register the Driver class, only one of which is shown in Listing 4. The statement in Listing 4 makes it possible to specify the Driver class as a String. The primary advantage of this approach is that this string can be obtained by the program in a variety of ways at runtime. Although the string was hard coded into this simple program, that is not a requirement. For a more general program intended to be used with two or more database servers, this string would most likely be provided as some form of user input. The URL of the database server MySQL and other similar database engines behave as servers on a network. They are identified by a URL much as other types of servers (such as HTTP servers and FTP servers) are identified. The next fragment defines the URL for the MySQL database server that I used in this sample program. The code in Listing 5 defines the URL of the master database named mysql on the MySQL database server residing on localhost and servicing the default port number 3306.
The URL format is provided in the connector documentation in Section 2.2.2. entitled Driver Class Name and JDBC URL Format. The URL format contains several optional elements. This URL will be referenced in the statement in Listing 6 for the purpose of getting a connection to the database. Get a connection to the database The code in Listing 6 implements the second critical step listed earlier (get a connection object). Listing 6 gets a connection to the database at the specified URL (mysql on localhost port 3306) for a user named root with a blank password. As you are already aware, this user is the default administrator having full privileges to do anything, including creating new databases and registering new users on those databases.
In effect, Listing 6 logs the JDBC program onto the MySQL database server in a manner that is analogous to the first line in Figures 3, 4, and 5 as well as the statements in the batch files shown in Listings 32, 34, 36, and 38. The getConnection method The getConnection method is a static method of the DriverManager class. When getConnection is invoked, the DriverManager will attempt to locate a suitable driver from among those loaded at initialization and those loaded explicitly using the same classloader as the current applet or application. There are several overloaded versions of the getConnection method. The version used in Listing 6 attempts to establish a connection to the given database URL for a specific user with a specific password. If the attempt to get a connection to the database server is successful, the method returns an object of type Connection. In this program, a reference to the Connection object is stored in the reference variable named con. If the attempt is not successful, an exception of type SQLException will be thrown. Information pertinent to the nature of the problem will be encapsulated in the SQLException object. As you will see later, SQL statements are executed and results are returned within the context of a connection. Display some information The code in Listing 7 is not critical to the program. This code simply displays information about the URL and the connection.
Get a Statement object The code in Listing 8 implements the third critical step listed earlier (get one or more Statement objects). This code invokes the createStatement method of the Connection interface to get an object of type Statement.
Recall that con is a reference to an object of type Connection. A Connection object defines a connection (session) with a specific database. SQL statements are executed and results are returned within the context of a connection. According to Sun, a Statement object is:
The results that are returned The results, if any, are returned in the form of a ResultSet object. I will have more to say about the ResultSet interface later in conjunction with the discussion of the program named Jdbc10.
Methods of the Statement interface The Statement interface declares many methods that can be used to access the database server and to manipulate the data in the database. One of those methods is executeUpdate, which will be used in this program. The executeUpdate method has a single String parameter. This parameter must be a valid SQL command. The method is used to execute SQL INSERT, UPDATE or DELETE statements. In addition, other SQL statements that return nothing can be executed using this method. Create the new database The code in Listing 9 implements the fourth critical step in the list provided earlier, (use of a Statement object to manipulate the database). Listing 9 invokes the executeUpdate method on the Statement object to create the new database named JunkDB.
The executeUpdate method Sun has this to say about the executeUpdate method:
The method parameter is an SQL command Note the similarity of the method parameter in Listing 9 to the interactive input shown in Figure 4 and the contents of the text file shown in Listing 33. In all three cases, an SQL command is invoked on the database server to cause a new database named JunkDB to be created. This SQL command returns nothing, so it is suitable for use with the executeUpdate method described above. In all three cases, the SQL command is invoked by the default administrator named root who has the ability to create new databases. Make a new user The code in Listing 10 also implements the fourth critical step listed earlier, (use of a Statement object to manipulate the database). The code in Listing 10 invokes the executeUpdate method once again to make a new user named auser who is capable of accessing the database named JunkDB from localhost using the password drowssap with six different privileges.
Another SQL command Once again, note the similarity of the executeUpdate method parameter in Listing 10 to the interactive input shown in Figure 5 and to the contents of the text file shown in Listing 35. Figure 5, Listing 10, and Listing 35 illustrate three different ways for the default administrator named root to invoke the same SQL command on the database server. Close the connection and terminate the program Listing 11 implements the fifth critical step listed earlier, closing the connection and terminating the program.
The results of running the program named Jdbc11 Once this program has been run successfully, the MySQL database server contains a database named JunkDB, as well as a user named auser, having various privileges relative to that database with a password of drowssap. At this point, it is possible to execute JDBC programs by which the user named auser manipulates the contents of the database named JunkDB. That will be the purpose of the program named Jdbc10, which I will explain later. First, however, I am going to show you how to write a JDBC program to remove the user named auser and to delete the database named JunkDB from the MySQL database server. JDBC12 - Remove a user and delete a database The purpose of the program named Jdbc12 is to log onto the master database named mysql as the default administrator named root whose password is blank in order to perform the following updates on the MySQL database server:
The output These two operations produce no visible output when successful. However, they produce error messages in the output when unsuccessful.
A reversal This program is the reverse of the program named Jdbc11, discussed earlier, which creates the database named JunkDB and registers the user named auser on that database. Server must be running The MySQL server must be running on localhost before this program is started. Instructions for starting and stopping the database server were provided earlier (see Listing 1, Figure 1, Listing 2, and Figure 2). Testing This program was tested using Java SDK 1.4.2 under WinXP, MySQL version 4.0.21-win, and JDBC connector version mysql-connector-java-3.0.15-ga. Beginning of the Jdbc12 class Listing 12 shows the beginning of the class definition and the beginning of the main method for the program named Jdbc12. A complete listing of the program is provided in Listing 41 The code in Listing 12 is the same as the code discussed earlier for the program named Jdbc11. Therefore, I won't repeat that discussion.
Revoke privileges and remove user named auser Listing 13 invokes the executeUpdate method four times in succession to cause the database server to revoke privileges for and to remove the user named auser. The SQL commands required to accomplish this were discussed earlier and illustrated in Listings 36 and 37.
Delete the database named JunkDB Listing 14 invokes the executeUpdate method to delete the database named JunkDB, using an SQL command discussed earlier and illustrated in Listings 38 and 39.
Closing the connection and terminating the program Listing 15 closes the connection and terminates the program.
Jdbc10 - Manipulating the data in the database The behavior of each of the two previous programs has been in the nature of administering or managing the database server. In particular, those programs added and removed databases and users from the database server. The user perspective The purpose of the program named Jdbc10 is to illustrate the ability to use JDBC to access a MySQL database server on localhost and to manipulate the data stored in that database. Server must be running The MySQL server must be running on localhost before the program named Jdbc10 is started. Instructions for starting and stopping the MySQL database server were provided earlier (see Listing 1, Figure 1, Listing 2, and Figure 2). Server must be prepared In addition, a database named JunkDB must have been created on the server and a user named auser must have been registered on that database with a password of drowssap before this program is started. The user named auser must have privileges that allow for the creation of tables in the database and the insertion of data into the tables. In addition the user must be allowed to perform SELECT queries on the tables in the database. Creating the database and adding the user The database may have been created and the user may have been added in at least three different ways:
Behavior of Jdbc10 program This program:
Two different approaches are used to display the contents of the table. The first approach displays all of the data in the table. The second approach displays only the data in a specific row in the table. A precaution As a precaution, before attempting to create the new table, the program attempts to delete a table having the same name. If a table having the same name already exists as residue from a previous run, it is deleted. If it doesn't already exist when the attempt is made to delete it, an exception is thrown. This exception is caught, displayed, and ignored. The program output This program produces the output shown in Figure 6 under normal conditions
where the table named myTable does not exist when the program is started
(the specifics regarding the Connection object may vary from one run to the
next).
Testing This program was tested using Java SDK 1.4.2 under WinXP, MySQL version 4.0.21-win, and JDBC connector version mysql-connector-java-3.0.15-ga. Beginning of the class definition The class definition begins in Listing 16. A complete listing of the program is shown in Listing 42 near the end of the lesson.
Except for the declaration of a variable of type ResultSet, the code in Listing 16 is the same as the code in the two previous programs. I will have more to say about the ResultSet type later. Define the database URL Listing 17 defines the URL of the database server and the database that will be accessed later by this program.
Note that the previous two programs accessed the master database named mysql, whereas this program accesses the user database named JunkDB. Otherwise, the code in Listing 17 is the same as in the two previous programs. Get a connection to the database Listing 18 gets a connection to the database.
Recall that the code in Listing 18 is analogous to a specific user logging onto a specific database using a specific password. The previous two programs got a connection to the master database named mysql for the default administrator named root whose password was blank. This program gets a connection to the database named JunkDB for the user named auser whose password is drowssap. Display some information and get a statement object As in the previous two programs, Listing 19 displays some information about the URL and the connection, and then gets a Statement object.
Delete the table named myTable if it exists A table named myTable may already exist in the database named JunkDB for some reason such as the premature ending of a previous run of this program. If the table already exists, it will not be possible to create a new empty table having that name. A requirement of this program is to create an empty table named myTable in a particular format. Therefore, as a precaution, before attempting to create the new table, the code in Listing 20 attempts to delete a table named myTable. If a table having that name already exists, it is deleted. If it doesn't already exist when the attempt is made to delete it, an exception is thrown. This exception is simply caught, displayed, and ignored.
The code in Listing 20 invokes the executeUpdate method, with which you are already familiar. Only the syntax of the SQL command provided as a parameter to the method is new to this program. Create the new table named myTable The code in Listing 22 invokes the executeUpdate method to create a new table named myTable. I will refer you to an SQL book for a full understanding of the SQL command that is passed as a parameter to the method.
Briefly, the new table will have two columns. The first column will be named test_id and will be designed to contain integer data. The second column will be named test_val and will be designed to contain character data up to 15 characters in length (in Java, we might refer to that as String data, except that there is no limit to the length of a String in Java). Insert some values into the table Listing 23 invokes the executeUpdate method to insert one value into each column of the first row in the table.
The integer value 1 is inserted into the column named test_id. The three characters, One, are inserted into the column named test_val. Insert values into four more rows Listing 24 invokes the executeUpdate method four more times in succession to insert values into the columns in rows 2 through 5. You should be able to examine the SQL commands and determine the values inserted into the table.
The table is populated At this point, the first five rows in the table named myTable have been populated with data. The remaining code in the program will:
Get another Statement object The code in Listing 25 gets a different Statement object, initialized as shown by the parameters that are passed to the createStatement method.
I won't try to explain the meaning of the initialization parameters. Rather, I will let you look them up in the Sun documentation for the ResultSet interface, and then perhaps do further research in an SQL book. The ResultSet interface provides about ten symbolic constants that can be used as parameters to this method. The values of the parameters exercise control over the behavior of the ResultSet object returned by a subsequent query based on the Statement object. Query the database All of the database operations to this point have been based on the use of the executeUpdate method of the Statement interface. Recall that I told you earlier that the executeUpdate method can be used to execute SQL commands that don't return anything. We have now reached the point where we want to execute an SQL command that does return something. For this, we will invoke the executeQuery method on the Statement object. What does Sun have to say? Here is part of what Sun has to say about this method:
In other words, this method will execute the SQL command on the database and encapsulate the returned values in an object of type ResultSet. A ResultSet object A ResultSet object provides access to an encapsulated table of data. The object maintains a cursor pointing to its current row of data. Initially the cursor is positioned before the first row. The next method moves the cursor to the next row (similar to an iterator or an enumerator in Java). Once the results are encapsulated in a ResultSet object, the ResultSet interface provides several methods that can be used to extract the information from the object. Methods of a ResultSet object The get methods (such as getString) retrieve column values for the current row. You can retrieve values using either the index number of the column or the name of the column. I have read that using the column index is more efficient but I can't give you a reference on that.
For the get methods, the JDBC driver attempts to convert the underlying data to the specified Java type and returns a suitable Java value. The life of a ResultSet object A ResultSet object is automatically closed by the Statement object that generated it when that Statement object is closed, re-executed, or used to retrieve the next result from a sequence of multiple results. Perform the query Listing 26 invokes the executeQuery method to perform the query on the database selecting all columns in all rows, and sorting the results in order by the values in the column named test_id. The results are encapsulated in the ResultSet object referred to by the reference variable named rs.
Display all of the results in the ResultSet object Listing 27 uses a while loop to:
The output The output produced by the code in Listing 27 is shown in Figure 7.
Display the data in row number 2 The code in Listing 28 invokes the absolute method on the same ResultSet object to get, save, and display the data in the two columns of row number 2.
The output Figure 8 shows the output produced by the code in Listing 28.
Delete the table, close the connection, and terminate Listing 29 invokes the executeUpdate method to delete the table named myTable from the database named JunkDB.
Then Listing 29 closes the connection (logs off the database) and
terminates the program. Run the ProgramsI encourage you to download and install the MySQL database server and the MySQL connector as described in this lesson. Then copy the code from the listings near the end of this lesson. Execute the batch files. Compile and execute the programs. Experiment with the files and the programs, making changes, and observing the results of your changes. SummaryI showed you how to download, install, and prepare a MySQL database as a localhost server on a Windows platform for use with JDBC. Then I showed you how to write three simple JDBC programs to administer the database server and to manipulate data stored in a MySQL database. Complete Program Listings
Listing 30 Contents of MySqlStart.bat
Listing 31 Contents of MySqlStop.bat
Listing 32 MySqlCreateDatabase01.bat
Listing 33 MySqlCreateDatabase01.txt
Listing 34 MySqlMakeUser01.bat
Listing 35 MySqlMakeUser01.txt
Listing 36 MySqlRemoveUser01.bat
Listing 37 MySqlRemoveUser01.txt
Listing 38 MySqlDropDatabase01.bat
Listing 39 MySqlDropDatabase01.txt
Listing 40 Jdbc11.java
Listing 41 Jdbc12.java
Listing 42 Jdbc10.java Copyright 2004, Richard G. Baldwin. Reproduction in whole or in part in any form or medium without express written permission from Richard Baldwin is prohibited. About the authorRichard Baldwin is a college professor (at Austin Community College in Austin, TX) and private consultant whose primary focus is a combination of Java, C#, and XML. In addition to the many platform and/or language independent benefits of Java and C# applications, he believes that a combination of Java, C#, and XML will become the primary driving force in the delivery of structured information on the Web.Richard has participated in numerous consulting projects, and he frequently provides onsite training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Programming Tutorials, which have gained a worldwide following among experienced and aspiring programmers. He has also published articles in JavaPro magazine. Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems. -end- |
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