Key things about Java classloaders:
1. There is a hierarchy among classloaders:
Bootstrap <---|
Extension <--|
System <---|
Custom
- Child classloaders (typically) delegate class loading to parent classloaders. child class loader’s findClass() method is not called if the parent classloader can load the class.
- Custom classloaders can override the default delegation chain to a certain extent.
- Due to the delegation chain of classloaders, ensure classes to be loaded by custom classloaders are not present on the system class path, boot class path, or extension class path.
2. Bootstrap classloader is a special classloader included with the JVM, written in native code. Bootstrap classloader is tasked with loading all core classes part of the jre.
None of the other classloaders can override the Bootstrap classloader's behvaiour.
3. All other classloaders are written in Java.
4. Extension classloader loads classes from the extension directories: JAVA_HOME/jre/lib/ext/
5. A more popular alternative to using the Extension classloader, is to use the System classloader which loads classes from the CLASSPATH environment variable location.
6. Finally, Custom classloaders can be written to override certain defaults like delegating classloading to parents, etc. Custom classloaders is commonly used by Application servers (such as Tomcat).
7. Separate Namespaces per Classloader:
Same class loaded by two different classloaders, are considered different. Trying to cast an object of one class (loaded by classloader 1) to a reference of the other (loaded by classloader 2, though identical in terms of its fully qualified class name) will result in a ClassCastException.
8. Lazy loading and Caching of Classes:
Classloaders load classes lazily. Once loaded classloaders cache all previously loaded classes for the duration of the JVM.
Key Methods of Classloaders:
To be detailed..
Dynamic Reloading of Classes:
Due to the non-overridable behaviour of caching of classes by classloaders, reloading of classes within a running JVM poses problems. To reload a class dynamically (a common use case for app. servers), a new instance of the classloader itself needs to be created.
Once the earlier classsloader is orphaned/ garbage, classes loaded & cached by it (and reachable only via the now GC'd classloader) also become garbage, which can then be collected by the GC.
