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Making the Bootstrap Loader "just another ClassLoader"

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Making the Bootstrap Loader "just another ClassLoader"

· Java Zone ·
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Microservices. Streaming data. Event Sourcing and CQRS. Concurrency, routing, self-healing, persistence, clustering...learn how Akka enables Java developers to do all this out of the box! Brought to you in partnership with Lightbend.

Recently, I was tweaking MultiSPI to add the following class loading fallback logic:

if (threadContextLoader != null) {
loadFromContextLoader(className);
} else if (systemLoader != null) {
loadFromSystemLoader(className);
} else {
loadFromBootstrapLoader(className);
}

and realized that it's not immediately evident how to do this in a uniform way. But actually, it's quite simple...getting a ClassLoader object for the bootstrap loader is just a couple of lines of code.

So what's the problem?

Well, if you're happy with a literal implementation of the pseudo-code above, there obviously isn't a problem.

ClassLoader loader = Thread.currentThread().getContextClassLoader();
if (loader != null) {
return loader.loadClass(className);
}

loader = ClassLoader.getSystemClassLoader();
if (loader != null) {
return loader.loadClass(className);
}

// null is the bootstrap loader
return Class.forName(className, true, null);

But this is very procedural code, and it would already be much more OO if we could do something like:

getLoader().loadClass(className);

ClassLoader getLoader() {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
if (loader != null) {
return loader;
}

loader = ClassLoader.getSystemClassLoader();
if (loader != null) {
return loader;
}

return ...???????
}

The problem is that there simply isn't a ClassLoader.getBootstrapClassLoader() or equivalent method. You can try to walk up the system loader's parent hierarchy, of course, but what you're likely to find is that the bootstrap loader will be represented by null - not very useful in our circumstances.

Writing BootstrapClassLoader

So what we want is to come up with a ClassLoader implementation that delegates straight to the bootstrap loader. One option would be to override loadClass to simply call Class.forName with the above arguments, but there's actually an even easier way:

class BootstrapClassLoader extends ClassLoader {
BootstrapClassLoader() {
/*
* The default classloader implementation will use the bootstrap loader
* if it finds a null parent.
*/
super(null);
}
}

Now we can finish our getLoader method:

// or simply "new ClassLoader(null) {};"
final ClassLoader BOOTSTRAP_LOADER = new BootstrapClassLoader();

ClassLoader getLoader() {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
if (loader != null) {
return loader;
}

loader = ClassLoader.getSystemClassLoader();
if (loader != null) {
return loader;
}

return BOOTSTRAP_LOADER;
}

Unless we're being really performance-conscious, we can even turn this into the almost functional:

ClassLoader getLoader() {
return Iterables.find(asList(Thread.currentThread().getContextClassLoader(),
ClassLoader.getSystemClassLoader(), BOOTSTRAP_LOADER), Predicates.notNull());
}

There's a demo snippet on PasteBin here.

From http://blog.xebia.com/2011/02/24/making-the-bootstrap-loader-just-another-classloader/

Microservices. Streaming data. Event Sourcing and CQRS. Concurrency, routing, self-healing, persistence, clustering...learn how Akka enables Java developers to do all this out of the box! Brought to you in partnership with Lightbend. 

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