loadClass - JDK 1.6.0_18 vs JDK 1.6.0_27

Recently while implementing a custom class loader, I hit upon a issue related to loadClass method of the Classloader.  The custom classloader code of interest is below.

	protected synchronized Class<?> loadClass(String name, boolean resolve)
	throws ClassNotFoundException
	{
	if( name.contains("xxxx") ){
	return null;
	}
	return super.loadClass(name,resolve);
	}

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This classloader worked on JDK 6 Update 27 (dev env) but failed to work on JDK 6 Update 17 (test env).


Its a rare skill to write code in Java that  fails between minor revisions!! :-)

The class loader code was written looking at the Java source code for Classloader. This was the cause of the issue - the custom loader implementation did not adhere to the documented contract for the loadClass method.

The documentation of the loadClass method clearly states

Throws:

ClassNotFoundException - If the class could not be found

While the custom class loader method returns null if the class is not found! So why did it work in latest update of Java?  Following are the Classloader code snippets from the two JDK 6 updates that I have mentioned.

Jdk 1.6 Update 18

	protected synchronized Class<?> loadClass(String name, boolean resolve)
	throws ClassNotFoundException
	{
	// First, check if the class has already been loaded
	Class c = findLoadedClass(name);
	if (c == null) {
	try {
	if (parent != null) {
	c = parent.loadClass(name, false);
	} else {
	c = findBootstrapClass0(name);
	}
	} catch (ClassNotFoundException e) {
	// If still not found, then invoke findClass in order
	// to find the class.
	c = findClass(name);
	}
	}
	if (resolve) {
	resolveClass(c);
	}
	return c;
	}

view raw Jdk1.6.0_18.java hosted with ❤ by GitHub

Jdk 1.6 Update 27

	protected synchronized Class<?> loadClass(String name, boolean resolve)
	throws ClassNotFoundException
	{
	// First, check if the class has already been loaded
	Class c = findLoadedClass(name);
	if (c == null) {
	try {
	if (parent != null) {
	c = parent.loadClass(name, false);
	} else {
	c = findBootstrapClassOrNull(name);
	}
	} catch (ClassNotFoundException e) {
	// ClassNotFoundException thrown if class not found
	// from the non-null parent class loader
	}
	if (c == null) {
	// If still not found, then invoke findClass in order
	// to find the class.
	c = findClass(name);
	}
	}
	if (resolve) {
	resolveClass(c);
	}
	return c;
	}

view raw Jdk1.6.0_27.java hosted with ❤ by GitHub

The if check c==null in the Update 27 made the loadClass method work even though it did not adhere to the documented contract.

The bottom line is to code against documented contract and not look at the implementation to write code!