Writing JDBC Applications with MySQL

文章目錄

• Issuing Queries That Return No Result Set
• Issuing Queries That Return a Result Set
• Error Handling

 

Paul DuBois
paul@kitebird.com

Document revision: 1.01
Last update: 2003-01-24

Table of Contents

• Preliminary Requirements
• Connecting to the MySQL Server
• Issuing Queries
  • Issuing Queries That Return No Result Set
  • Issuing Queries That Return a Result Set
    • Using Placeholders
  • Error Handling
• Resources
• Acknowledgment

You can write MySQL applications in a variety of languages. The languages that most people use with MySQL are PHP and Perl, but a sometimes overlooked option is the MySQL Connector/J driver, which allows you to develop Java applications that interact with your MySQL server.

MySQL Connector/J works within the framework of the Java JDBC interface, an API that allows Java programs to use database servers in a portable way. JDBC is based on an approach similar to that used in the design of Perl and Ruby DBI modules, Python's DB-API module, and PHP's PEAR::DB class. This approach uses a two-tier architecture:

• The top level is visible to application programs and presents an abstract interface for connecting to and using database engines. The application interface does not depend on details specific to particular engines.
• The lower level consists of drivers for individual database engines. Each driver handles the details necessary to map the abstract application interface onto operations that a specific engine will understand.

The JDBC interface allows developers to write applications that can be used with different databases with a minimum of porting effort. Once a driver for a given server engine is installed, JDBC applications can communicate with any server of that type. By using MySQL Connector/J, your Java programs can access MySQL databases.

Note: MySQL Connector/J is the successor to the MM.MySQL driver. If you have JDBC programs written for MM.MySQL, they should work with MySQL Connector/J as well, although you may want to update the driver class name used in your programs. Just replace instances of org.gjt.mm.mysql in your Java source files with com.mysql.jdbc and recompile.

Preliminary Requirements

To use Java applications with MySQL, you may need to install some additional software:

• If you want to compile and run Java programs, you'll need a Java compiler (such as javac or jikes) and a runtime environment. If these are not already installed on your system, you can get them by obtaining a Java Software Development Kit (SDK) from java.sun.com.
• If you want only to run precompiled applications, no compiler is necessary, but you'll still need a Java Runtime Environment (JRE). This too may be obtained from java.sun.com.

This article assumes that you'll write and compile your own programs, and thus that you have a Java SDK installed. Once you compile a Java program, however, you can deploy it to other machines, even ones that have only a runtime environment. This works even in heterogenous installations, because Java is platform-independent. Applications compiled on one platform can be expected to work on other platforms. For example, you can develop on a Linux box and deploy on Windows.

Connecting to the MySQL Server

To connect to the MySQL server, register the JDBC driver you plan to use, then invoke its getConnection() method. The following short program, Connect.java, shows how to connect to and disconnect from a server running on the local host. It accesses a database named test, using a MySQL account with a user name and password of testuser and testpass:

   import java.sql.*;public class Connect{public static void main (String[] args){Connection conn = null;try{String userName = "testuser";String password = "testpass";String url = "jdbc:mysql://localhost/test";Class.forName ("com.mysql.jdbc.Driver").newInstance ();conn = DriverManager.getConnection (url, userName, password);System.out.println ("Database connection established");}catch (Exception e){System.err.println ("Cannot connect to database server");}finally{if (conn != null){try{conn.close ();System.out.println ("Database connection terminated");}catch (Exception e) { /* ignore close errors */ }}}}}

Compile Connect.java to produce a class file Connect.class that contains executable Java code:

   % javac Connect.java

Then invoke the class file as follows and it should connect to and disconnect from your MySQL server:

   % java ConnectDatabase connection establishedDatabase connection terminated

If you have trouble compiling Connect.java, double check that you have a Java Software Development Kit installed and make sure that the MySQL Connector/J driver is listed in your CLASSPATH environment variable.

The arguments to getConnection() are the connection URL and the user name and password of a MySQL account. As illustrated by Connect.java, JDBC URLs for MySQL consist of jdbc:mysql:// followed by the name of the MySQL server host and the database name. An alternate syntax for specifying the user and password is to add them as parameters to the end of the connection URL:

   jdbc:mysql://localhost/test?user=testuser&password=testpass

When you specify a URL using this second format, getConnection() requires only one argument. For example, the code for connecting to the MySQL server in Connect.java could have been written like this:

   String userName = "testuser";String password = "testpass";String url = "jdbc:mysql://localhost/test?user="+ userName+ "&password="+ password;Class.forName ("com.mysql.jdbc.Driver").newInstance ();conn = DriverManager.getConnection (url);

getConnect() returns a Connection object that may be used to interact with MySQL by issuing queries and retrieving their results. (The next section describes how to do this.) When you're done with the connection, invoke its close() method to disconnect from the MySQL server.

To increase the portability of your applications, you can store the connection parameters (host, database, user name, and password) in a Java properties file and read the properties at runtime. Then they need not be listed in the program itself. This allows you to change the server to which the program connects by editing the properties file, rather than by having to recompile the program.

Issuing Queries

To process SQL statements in a JDBC-based application, create a Statement object from your Connection object. Statement objects support an executeUpdate() method for issuing queries that modify the database and return no result set, and an executeQuery() method for queries that do return a result set. The query-processing examples in this article use the following table, animal, which contains an integer id column and two string columns, name and category:

   CREATE TABLE animal(id          INT UNSIGNED NOT NULL AUTO_INCREMENT,PRIMARY KEY (id),name        CHAR(40),category    CHAR(40))

id is an AUTO_INCREMENT column, so MySQL automatically assigns successive values 1, 2, 3, ... as records are added to the table.

Issuing Queries That Return No Result Set

The following example obtains a Statement object from the Connection object, then uses it to create and populate the animal table. DROP TABLE, CREATE TABLE, and INSERT all are statements that modify the database, so executeUpdate() is the appropriate method for issuing them:

   Statement s = conn.createStatement ();int count;s.executeUpdate ("DROP TABLE IF EXISTS animal");s.executeUpdate ("CREATE TABLE animal ("+ "id INT UNSIGNED NOT NULL AUTO_INCREMENT,"+ "PRIMARY KEY (id),"+ "name CHAR(40), category CHAR(40))");count = s.executeUpdate ("INSERT INTO animal (name, category)"+ " VALUES"+ "('snake', 'reptile'),"+ "('frog', 'amphibian'),"+ "('tuna', 'fish'),"+ "('racoon', 'mammal')");s.close ();System.out.println (count + " rows were inserted");

The executeUpdate() method returns the number of rows affected by a query. As shown above, the count is used to report how many rows the INSERT statement added to the animal table.

A Statement object may be used to issue several queries. When you're done with it, invoke its close() method to dispose of the object and free any resources associated with it.

Issuing Queries That Return a Result Set

For statements such as SELECT queries that retrieve information from the database, use executeQuery(). After calling this method, create a ResultSet object and use it to iterate through the rows returned by your query. The following example shows one way to retrieve the contents of the animal table:

   Statement s = conn.createStatement ();s.executeQuery ("SELECT id, name, category FROM animal");ResultSet rs = s.getResultSet ();int count = 0;while (rs.next ()){int idVal = rs.getInt ("id");String nameVal = rs.getString ("name");String catVal = rs.getString ("category");System.out.println ("id = " + idVal+ ", name = " + nameVal+ ", category = " + catVal);++count;}rs.close ();s.close ();System.out.println (count + " rows were retrieved");

executeQuery() does not return a row count, so if you want to know how many rows a result set contains, you should count them yourself as you fetch them.

To obtain the column values from each row, invoke getXXX() methods that match the column data types. The getInt() and getString() methods used in the preceding example return integer and string values. As the example shows, these methods may be called using the name of a result set column. You can also fetch values by position. For the result set retrieved by the SELECT query in the example, id, name, and category are at column positions 1, 2 and 3 and thus could have been obtained like this:

   int idVal = rs.getInt (1);String nameVal = rs.getString (2);String catVal = rs.getString (3);

ResultSet objects, like Statement objects, should be closed when you're done with them.

To check whether or not a column value is NULL, invoke the result set object's wasNull() method after fetching the value. For example, you could check for a NULL value in the name column like this:

   String nameVal = rs.getString ("name");if (rs.wasNull ())nameVal = "(no name available)";

Using Placeholders

Sometimes it's necessary to construct queries from values containing characters that require special treatment. For example, in queries, string values are written enclosed within quotes, but any quote characters in the string itself should be doubled or escaped with a backslash to avoid creating malformed SQL. In this case, it's much easier to let JDBC handle the escaping for you, rather than fooling around trying to do so yourself. To use this approach, create a different kind of statement (a PreparedStatement), and refer to the data values in the query string by means of placeholder characters. Then tell JDBC to bind the data values to the placeholders and it will handle any special characters automatically.

Suppose you have two variables nameVal and catVal from which you want to create a new record in the animal table. To do so without regard to whether or not the values contain special characters, issue the query like this:

   PreparedStatement s;s = conn.prepareStatement ("INSERT INTO animal (name, category) VALUES(?,?)");s.setString (1, nameVal);s.setString (2, catVal);int count = s.executeUpdate ();s.close ();System.out.println (count + " rows were inserted");

The '?' characters in the query string act as placeholders--special markers indicating where data values should be placed. The setString() method takes a placeholder position and a string value and binds the value to the appropriate placeholder, performing any special-character escaping that may be necessary. The method you use to bind a value depends on the data type. For example, setString() binds string values and setInt() binds integer values.

Error Handling

If you want to trap errors, execute your JDBC operations within a try block and use an exception handler to display information about the cause of any problems that occur. JDBC provides getMessage() and getErrorCode() methods that may be invoked when an exception occurs to obtain the error message and the numeric error code. The following example deliberately issues a malformed query. When it runs, the executeQuery() method fails and raises an exception that is handled in the catch block:

   try{Statement s = conn.createStatement ();s.executeQuery ("XYZ"); // issue invalid querys.close ();}catch (SQLException e){System.err.println ("Error message: " + e.getMessage ());System.err.println ("Error number: " + e.getErrorCode ());}

Resources

The following sites provide information about the tools discussed in this article:

• Sun's Java site is a clearinghouse for all kinds of Java-related information:

     http://java.sun.com/        

  You can obtain the Java Software Development Kit or Java Runtime Environment here. The specification for the JDBC API is also available on this site, should you wish to read more about it.

• The MySQL Connector/J driver is supported by MySQL AB. Connector/J author Mark Matthews maintains the driver, which is available at:

     http://www.mysql.com/        

• Introductory articles describing other MySQL APIs based on an architecture similar to that used for JDBC may be found at:

     http://www.kitebird.com/articles/        

  APIs discussed in the articles include Perl DBI, PHP PEAR::DB, Python DB-API, and Ruby DBI.

Acknowledgment

The original version of this article was written for NuSphere Corporation. The current version is an updated revision of the original.