When exactly is a class loaded? [closed]

Question

If I have a static variable that is assigned a value from a class that uses a singleton pattern, does that somehow require that all the classes that could potentially assign a value to that static variable to be loaded?

I am working with code that looks something like this (simplified to protect corporate data):

private static DEVICE_FACTORY;

String factoryType = propertyFile.getDeviceFactoryType();

if(factoryType.equals("REAL"))
   DEVICE_FACTORY = RealDeviceFactory.getInstance();
else
   DEVICE_FACTORY = SimulatedDeviceFactory.getInstance();

The code for each type of factory is in totally different projects. The production version of the application jar does not contain any of the simulated classes. My problem is that when I try to test my production code I am gettting "NoClassDefFoundError" caused by "ClassNotFoundException". I have confirmed that the property is correctly set to "REAL" so I have no idea why the JVM is even attempting to create/load the Simulated classes...

EDIT I will try to create a simplified version that demonstrates the problem tomorrow. The stack trace (edited to protected corporate data) is as follows:

java.lang.NoClassDefFoundError: my/package/SimulatedDeviceFactory
                at java.lang.Class.getDeclaredMethods0(Native Method)
                at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
                at java.lang.Class.getMethod0(Unknown Source)
                at java.lang.Class.getMethod(Unknown Source)
                at sun.launcher.LauncherHelper.getMainMethod(Unknown Source)
                at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)
Caused by: java.lang.ClassNotFoundException: my.package.SimulatedDeviceFactory
                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)

Could it be that I am using the singleton pattern on each type of device factory therefore, even though I am not going to use the SimulatedDeviceFactory, because I am using a static method call from that class the JVM thinks it needs to load it anyway?

If that is the case I could use reflection and a FQCN to resolve that quickly enough...

Answer 1

See http://www.programcreek.com/2013/01/when-and-how-a-java-class-is-loaded-and-initialized/. Classes are loaded when they are referenced in a class file most of the time.