Question

I have a simple example which tries to connect to a database and I could only do it when I unjarred the mysql*.jar file in the working directory.

1. I tried to export the /path/to/mysql.jar.file to $PATH and $CLASSPATH and that didn’t work.
2. I tried to use include the path like this, but it didn’t work:
   javac -cp “.:/usr/share/java/” HelloWorld.java

What could be the issue. Why is it not finding the jar file?

Any thoughts?

Answer 1

You need to set up the classpath when running the JVM as well as running the compiler. The classpath must include the JAR itself, not the folder containing the JAR.

Your line

javac -cp ".:/usr/share/java" HelloWorld.java

shows that you are passing a classpath to javac, the Java compiler, but not the JVM.

Let’s use the following simple class, which just connects to MySQL and then disconnects again:

import java.sql.*;

public class HelloWorld {
    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/db", "user", "password");
        System.out.println("Got connection");
        con.close();
    }
}

Here’s what happens when I come to run it:

C:\Users\Luke\Java>echo %CLASSPATH%
.;C:\Program Files (x86)\Java\jre6\lib\ext\QTJava.zip;C:\Users\Luke\stuff\ojdbc6.jar

C:\Users\Luke\Java>java HelloWorld
Exception in thread "main" java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Unknown Source)
        at HelloWorld.main(HelloWorld.java:5)

In this case, the MySQL driver JAR isn’t on the classpath, so we get an error when we attempt to load the driver class (com.mysql.jdbc.Driver).

If I add the driver JAR to the classpath using the -cp command-line switch (I have a copy of this JAR in the current directory), it works:

C:\Users\Luke\Java>java -cp .;mysql-connector-java-5.1.18-bin.jar HelloWorld
Got connection

(I’m on Windows, so I use ; as a path separator. You appear to be using some dialect of Unix, so you should use : instead.)

Similarly, if we add the JAR to the CLASSPATH environment variable, it also works:

C:\Users\Luke\Java>set CLASSPATH=%CLASSPATH%;mysql-connector-java-5.1.18-bin.jar

C:\Users\Luke\Java>java HelloWorld
Got connection