Question

I am trying to add jar file to classpath at runtime. I use this code

 public static void addURL(URL u) throws IOException {

            URLClassLoader sysloader = (URLClassLoader) ClassLoader
                    .getSystemClassLoader();
            Class<URLClassLoader> sysclass = URLClassLoader.class;

            try {
                Method method = sysclass.getDeclaredMethod("addURL", parameters);
                method.setAccessible(true);
                method.invoke(sysloader, new Object[] { u });
                System.out.println(u);
            } catch (Throwable t) {
                t.printStackTrace();
                throw new IOException("Error");
            }

        }

System out prints this url:

file:/B:/Java/Tools/mysql-connector-java-5.1.18/mysql-connector-java-5.1.18/mysql-connector-java-5.1.18-bin.jar

I was check this path carefully, this jar exist. Even this test show that com.mysql.jdbc.
Driver class exists.

javap -classpath "B:\Java\Tools\mysql-connector-java-5.1.18\
mysql-connector-java-5.1.18\mysql-connector-java-5.1.18-bin.jar" com.mysql.jdbc.
Driver
Compiled from "Driver.java"
public class com.mysql.jdbc.Driver extends com.mysql.jdbc.NonRegisteringDriver i
mplements java.sql.Driver{
    public com.mysql.jdbc.Driver()       throws java.sql.SQLException;
    static {};
}

But I still get java.lang.ClassNotFoundException when I use this Class.forName(driver).
What is wrong with this code?

Answer 1

The URL is ok, nevertheless you try to load a jar from classpath, so it means that yo need to have the file in cp first.
In your case you want to load a jar that is not in classpath so you have to use
URLClassLoader and for JAR you can use also the JARClassLoader
If you want some sample lesson on it:
http://docs.oracle.com/javase/tutorial/deployment/jar/jarclassloader.html

Here a sample I ran by myself see if helps you. It search the Logger class of Log4j that is not in my classpath, of course i got exception on invocation of the constructor since i did not pass the right params to the constructor

package org.stackoverflow;
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;

public class URLClassLoaderSample
{
  public static void main(String[] args) throws Exception
  {
    File f = new File("C:\\_programs\\apache\\log4j\\v1.1.16\\log4j-1.2.16.jar");
    URLClassLoader urlCl = new URLClassLoader(new URL[] { f.toURL()},System.class.getClassLoader());
    Class log4jClass = urlCl.loadClass("org.apache.log4j.Logger");
    log4jClass.newInstance();
  }
}



Exception in thread "main" java.lang.InstantiationException: org.apache.log4j.Logger
    at java.lang.Class.newInstance0(Class.java:357)
    at java.lang.Class.newInstance(Class.java:325)
    at org.stackoverflow.URLClassLoaderSample.main(URLClassLoaderSample.java:19)

Exception due to the wrong invocation, nevertheless at this stage we already found the class