Why am I getting java.lang.NoClassDefFoundError: java/sql/Driver when using a custom class loader?

Question

This works with Java 8:

public class Test {
  public static void main(String[] args) throws Exception {
    URLClassLoader loader = new URLClassLoader(new URL[0], null);
    System.out.println("" + loader.loadClass("java.sql.Driver"));
  }
}

But throws ClassNotFoundException with Java 17:

Exception in thread "main" java.lang.ClassNotFoundException: java.sql.Driver
    at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:445)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:592)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:525)
    at Test.main(Test.java:16)

Great diagram explaining class loaders (Java 9+):

https://medium.com/@wakefulinsomnia/how-does-java-classloader-system-work-82eaf378f73b

Related/similar issues:

Reto Höhener · Accepted Answer · 2024-07-23 16:37:55Z

-1

My current understanding is that if a null parent is used, really only the java base classes plus the classes in the URL[] parameter will be available.

If 'non-basic' classes from modules like 'java.sql' are also required, then at least the platform classloader needs to be used as parent.

ClassLoader.getPlatformClassLoader() is available since Java 9, as pointed out by @VGR.

public class Test {
  public static void main(String[] args) throws Exception  {
    System.out.println("java.version                                              : " + System.getProperty("java.version"));
    System.out.println("Test.class.getClassLoader()                               : " + Test.class.getClassLoader());
    System.out.println("ClassLoader.getSystemClassLoader()                        : " + ClassLoader.getSystemClassLoader());
    System.out.println("ClassLoader.getSystemClassLoader().getParent()            : " + ClassLoader.getSystemClassLoader().getParent());
    System.out.println("ClassLoader.getPlatformClassLoader()                      : " + ClassLoader.getPlatformClassLoader());
    URLClassLoader loader = new URLClassLoader(new URL[0], ClassLoader.getPlatformClassLoader());
    System.out.println("loader.loadClass('java.sql.Driver')                       : " + loader.loadClass("java.sql.Driver"));
  }
}

Output:

java.version                                              : 17.0.8
Test.class.getClassLoader()                               : jdk.internal.loader.ClassLoaders$AppClassLoader@659e0bfd
ClassLoader.getSystemClassLoader()                        : jdk.internal.loader.ClassLoaders$AppClassLoader@659e0bfd
ClassLoader.getSystemClassLoader().getParent()            : jdk.internal.loader.ClassLoaders$PlatformClassLoader@30f39991
ClassLoader.getPlatformClassLoader()                      : jdk.internal.loader.ClassLoaders$PlatformClassLoader@30f39991
loader.loadClass('java.sql.Driver')                       : interface java.sql.Driver

edited Jul 23, 2024 at 16:37

answered Jul 23, 2024 at 10:07

Reto Höhener

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3 Comments

VGR Over a year ago

ClassLoader.getSystemClassLoader().getParent() is an unreliable hack. Did you happen to try ClassLoader.getPlatformClassLoader() as the parent?

Reto Höhener Over a year ago

We still compile with Java 8, which is probably why I didn't see that method. But for modern Java this is definitely the way to go.

Holger Over a year ago

Or you use java.sql.Driver.class.getClassLoader() as the parent loader…

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