Test for unexpected behaviour in JVMs dynamic class loading

StackOverflow ticket (including solution): http://stackoverflow.com/questions/43792289/unexpected-behaviour-in-jvm-class-loading-classnotfoundexception-before-the-cla/43793110

Tested with Java 1.8.0_131.

This test consists of three artifacts:

A library called libone containing a class called ParentClassFromLibOne.
A library called libtwo containing a class called ParentClassFromLibTwo, which extends from ParentClassFromLibOne. This library has a (compile-scope) dependency on libone.
An application module, with classes DynamicClassLoadingAppOK and DynamicClassLoadingAppKO showing how a change in the type of a method parameter changes the result from the expected behaviour for dynamic class loading to an unexpected one. This module has a (compile-scope) dependency on libone and a (provided-scope) dependency on libtwo.

Code of ParentClassFromLibOne:

public class ParentClassFromLibOne {

    public String message() {
        return "Message from Parent class at LibOne";
    }

}

Code of ChildClassFromLibTwo:

public class ChildClassFromLibTwo extends ParentClassFromLibOne {

    @Override
    public String message() {
        return "Message from Child class at LibTwo";
    }

}

Code of the app class in which the behaviour is as expected and the System.out.println call at the end is executed:

public class DynamicClassLoadingAppOK {

    /*
     * THIS VERSION WORKS OK, BECAUSE showMessage RECEIVES THE CHILD CLASS AS A PARAMETER
     */


    // If this method received ParentClassFromLibOne, we would have a ClassNotFoundException
    private static void showMessage(final ChildClassFromLibTwo obj) {
        System.out.println(obj.message());
    }


    public static void main(final String[] args) throws Throwable {

        try {

            final ChildClassFromLibTwo obj = new ChildClassFromLibTwo();
            showMessage(obj);

        } catch (final Throwable ignored) {
            // ignored, we just wanted to use it if it was present
        }

        System.out.println("This is displayed perfecty fine... as expected.");

    }

}

Code of the app class in which the behaviour obtained is unexpected and the System.out.println call at the end is never executed (a ClassNotFoundException is thrown instead). Note the only difference is the type of the parameter of the showMessage(...) method.

public class DynamicClassLoadingAppKO {

    /*
     * THIS VERSION DOES NOT WORK, A ClassNotFoundException IS THROWN BEFORE EVEN EXECUTING main()
     */


    // If this method received ChildClassFromLibTwo, everything would work OK!
    private static void showMessage(final ParentClassFromLibOne obj) {
        System.out.println(obj.message());
    }


    public static void main(final String[] args) throws Throwable {

        try {

            final ChildClassFromLibTwo obj = new ChildClassFromLibTwo();
            showMessage(obj);

        } catch (final Throwable ignored) {
            // ignored, we just wanted to use it if it was present
        }

        System.out.println("This should be displayed, but no :(");

    }

}

How to test

First install the two libraries:

$ cd libone
$ mvn clean compile install
$ cd ..

$ cd libtwo
$ mvn clean compile install
$ cd ..

Then compile and execute the app:

$ cd app
$ mvn clean compile
$ mvn exec:java -Dexec.mainClass=com.github.danielfernandez.testdynamicclassloading.app.DynamicClassLoadingAppOK
$ mvn exec:java -Dexec.mainClass=com.github.danielfernandez.testdynamicclassloading.app.DynamicClassLoadingAppKO

The first app class executes fine, the second one throws:

java.lang.NoClassDefFoundError: com/github/danielfernandez/testdynamicclassloading/libtwo/ChildClassFromLibTwo
	at java.lang.Class.getDeclaredMethods0(Native Method)
	at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
	at java.lang.Class.privateGetMethodRecursive(Class.java:3048)
	at java.lang.Class.getMethod0(Class.java:3018)
	at java.lang.Class.getMethod(Class.java:1784)
	at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:281)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: com.github.danielfernandez.testdynamicclassloading.libtwo.ChildClassFromLibTwo
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 7 more

Note a ClassNotFoundException is actually thrown in both cases, but in the OK one it happens when the ChildClassFromLibTwo is loaded when a new instance of such class is created, so it is correctly caught by the catch (Throwable t) block and execution can go on.

Nevertheless, in the KO class the ClassNotFoundException is thrown before executing main(), apparently when the DynamicClassLoadingAppKO class itself is loaded, and therefore outside of the try...catch, so no code is executed at all.

danielfernandez/test-dynamic-class-loading

Test for unexpected behaviour in JVMs dynamic class loading

How to test