Find a way out of the ClassLoader maze (2)

Find a way out of the ClassLoader maze

System, current, context? Which ClassLoader should you use?

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What is a Java programmer to do?
If your implementation is confined to a certain framework with articulated resource loading rules, stick to them. Hopefully, the burden of making them work will be on whoever has to implement the framework (such as an application server vendor, although they don't always get it right either). For example, always use Class.getResource() in a Web application or an Enterprise JavaBean.

In other situations, you might consider using a solution I have found useful in personal work. The following class serves as a global decision point for acquiring the best classloader to use at any given time in the application (all classes shown in this article are available with the download):

public abstract class ClassLoaderResolver
{
/**
* This method selects the best classloader instance to be used for
* class/resource loading by whoever calls this method. The decision
* typically involves choosing between the caller's current, thread context,
* system, and other classloaders in the JVM and is made by the {@link IClassLoadStrategy}
* instance established by the last call to {@link #setStrategy}.
*
* @return classloader to be used by the caller ['null' indicates the
* primordial loader]
*/
public static synchronized ClassLoader getClassLoader ()
{
final Class caller = getCallerClass (0);
final ClassLoadContext ctx = new ClassLoadContext (caller);

return s_strategy.getClassLoader (ctx);
}

public static synchronized IClassLoadStrategy getStrategy ()
{
return s_strategy;
}

public static synchronized IClassLoadStrategy setStrategy (final IClassLoadStrategy strategy)
{
final IClassLoadStrategy old = s_strategy;
s_strategy = strategy;

return old;
}


/**
* A helper class to get the call context. It subclasses SecurityManager
* to make getClassContext() accessible. An instance of CallerResolver
* only needs to be created, not installed as an actual security
* manager.
*/
private static final class CallerResolver extends SecurityManager
{
protected Class [] getClassContext ()
{
return super.getClassContext ();
}

} // End of nested class


/*
* Indexes into the current method call context with a given
* offset.
*/
private static Class getCallerClass (final int callerOffset)
{
return CALLER_RESOLVER.getClassContext () [CALL_CONTEXT_OFFSET +
callerOffset];
}


private static IClassLoadStrategy s_strategy; // initialized in <clinit>

private static final int CALL_CONTEXT_OFFSET = 3; // may need to change if this class is redesigned
private static final CallerResolver CALLER_RESOLVER; // set in <clinit>

static
{
try
{
// This can fail if the current SecurityManager does not allow
// RuntimePermission ("createSecurityManager"):

CALLER_RESOLVER = new CallerResolver ();
}
catch (SecurityException se)
{
throw new RuntimeException ("ClassLoaderResolver: could not create CallerResolver: " + se);
}

s_strategy = new DefaultClassLoadStrategy ();
}

} // End of class.

You acquire a classloader reference by calling the ClassLoaderResolver.getClassLoader() static method and use the result to load classes and resources via the normal java.lang.ClassLoader API. Alternatively, you can use this ResourceLoader API as a drop-in replacement for java.lang.ClassLoader:

public abstract class ResourceLoader
{
/**
* @see java.lang.ClassLoader#loadClass(java.lang.String)
*/
public static Class loadClass (final String name)
throws ClassNotFoundException
{
final ClassLoader loader = ClassLoaderResolver.getClassLoader (1);

return Class.forName (name, false, loader);
}

/**
* @see java.lang.ClassLoader#getResource(java.lang.String)
*/
public static URL getResource (final String name)
{
final ClassLoader loader = ClassLoaderResolver.getClassLoader (1);

if (loader != null)
return loader.getResource (name);
else
return ClassLoader.getSystemResource (name);
}

... more methods ...

} // End of class

The decision of what constitutes the best classloader to use is factored out into a pluggable component implementing the IClassLoadStrategy interface:

public interface IClassLoadStrategy
{
ClassLoader getClassLoader (ClassLoadContext ctx);

} // End of interface

To help IClassLoadStrategy make its decision, it is given a ClassLoadContext object:

public class ClassLoadContext
{
public final Class getCallerClass ()
{
return m_caller;
}

ClassLoadContext (final Class caller)
{
m_caller = caller;
}

private final Class m_caller;

} // End of class

ClassLoadContext.getCallerClass() returns the class whose code calls into ClassLoaderResolver or ResourceLoader. This is so that the strategy implementation can figure out the caller's classloader (the context loader is always available as Thread.currentThread().getContextClassLoader()). Note that the caller is determined statically; thus, my API does not require existing business methods to be augmented with extra Class parameters and is suitable for static methods and initializers as well. You can augment this context object with other attributes that make sense in your deployment situation.

All of this should look like a familiar Strategy design pattern to you. The idea is that decisions like "always context loader" or "always current loader" get separated from the rest of your implementation logic. It is hard to know ahead of time which strategy will be the right one, and with this design, you can always change the decision later.

I have a default strategy implementation that should work correctly in 95 percent of real-life situations:

public class DefaultClassLoadStrategy implements IClassLoadStrategy
{
public ClassLoader getClassLoader (final ClassLoadContext ctx)
{
final ClassLoader callerLoader = ctx.getCallerClass ().getClassLoader ();
final ClassLoader contextLoader = Thread.currentThread ().getContextClassLoader ();

ClassLoader result;

// If 'callerLoader' and 'contextLoader' are in a parent-child
// relationship, always choose the child:

if (isChild (contextLoader, callerLoader))
result = callerLoader;
else if (isChild (callerLoader, contextLoader))
result = contextLoader;
else
{
// This else branch could be merged into the previous one,
// but I show it here to emphasize the ambiguous case:
result = contextLoader;
}

final ClassLoader systemLoader = ClassLoader.getSystemClassLoader ();

// Precaution for when deployed as a bootstrap or extension class:
if (isChild (result, systemLoader))
result = systemLoader;

return result;
}

... more methods ...

} // End of class

The logic above should be easy to follow. If the caller's current and context classloaders are in a parent-child relationship, I always choose the child. The set of resources visible to a child loader is normally a superset of classes visible to its parent, so this feels like the right decision as long as everybody plays by J2SE delegation rules.

It is when the current and the context classloaders are siblings that the right decision is impossible. Ideally, no Java runtime should ever create this ambiguity. When it happens, my code chooses the context loader: a decision based on personal experience of when things work correctly most of the time. Feel free to change that code branch to suit your taste. It is possible that the context loader is a better choice for framework components, and the current loader is better for business logic.

Finally, a simple check ensures that the selected classloader is not a parent of the system classloader. This is a good thing to do if you are developing code that might be deployed as an extension library.

Note that I intentionally do not look at the name of resources or classes that will be loaded. If nothing else, the experience with Java XML APIs becoming part of the J2SE core should have taught you that filtering by class names is a bad idea. Nor do I trial load classes to see which classloader succeeds first. Examining classloader parent-child relationships is a fundamentally better and more predictable approach.

Although Java resource loading remains an esoteric topic, J2SE relies on various load strategies more and more with every major platform upgrade. Java will be in serious trouble if this area is not given some significantly better design considerations. Whether you agree or not, I would appreciate your feedback and any interesting pointers from your personal design experience.

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About the author
Vladimir Roubtsov has programmed in a variety of languages for more than 13 years, including Java since 1995. Currently, he develops enterprise software as a senior engineer for Trilogy in Austin, Texas.

Resources

• Download the source code for all classes discussed in this article:
  http://www.javaworld.com/javaworld/javaqa/2003-06/load/01-qa-0606-load.zip
  
• "Get a Load of That Name!" Vladimir Roubtsov (JavaWorld, March 2003):
  http://www.javaworld.com/javaworld/javaqa/2003-03/01-qa-0314-forname.html
  
• Ted Neward's "Understanding Class.forName()" examines similar topics in great detail:
  http://www.javageeks.com/Papers/ClassForName/index.html
  
• Sun's bug database contains multiple issues related to getContextClassLoader. See these bug IDs, for example (requires login): 4648098, 4630895, 4452042, 4340158, 4155645:
  http://developer.java.sun.com/developer/bugParade/
  
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  http://www.javaworld.com/columns/jw-qna-index.shtml
  
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