What loads the java system classloader?

Question

We know that we can override the System classloader with:

java -Djava.system.class.loader=com.test.MyClassLoader xxx 

Then, since com.test.MyClassLoader itself is a class, by whom is it loaded?

How do we get the class file of this "meta" classloader?

Answer 1

Bootstrap classloader is the parent of all classloaders and loads the standard JDK classes in lib directory of JRE (rt.jar and i18n.jar). All the java.* classes are loaded by this classloader.

Extensions Classloader is the immediate child of Bootstrap classloader. This classloader loads the classes in lib\ext directory of the JRE.

System-Classpath classloader is the immediate child of Extensions classloader. It loads the classes and jars specified by the CLASSPATH environment variable

You could try to inject your custom class loader by means of the "java.system.class.loader" property (see ClassLoader#getSystemClassLoader).

Default System class loader is parent for MyClassLoader instances,

Answer 2

From the Javadoc for ClassLoader.getSystemClassLoader:

If the system property "java.system.class.loader" is defined when this method is first invoked then the value of that property is taken to be the name of a class that will be returned as the system class loader. The class is loaded using the default system class loader and must define a public constructor that takes a single parameter of type ClassLoader which is used as the delegation parent.

The default system class loader itself is specific to the JVM implementation.