ContextClassLoader, SystemClassLoader, CurrentClassLoader

If you run this code as a Java Application (no JEE container):

public class BootstrapCLTest {
    public static void main(String[] args) {
 BootstrapCLTest bootstrapCLTest = new BootstrapCLTest();
 bootstrapCLTest.go();
    }

    private void go() {

 System.out.println("1 " + String.class.getClassLoader());
 System.out.println("2 " + ClassLoader.getSystemClassLoader ());
 System.out.println("3 " + this.getClass().getClassLoader());
 System.out.println("4 " + Thread.currentThread().getContextClassLoader());
 System.out.println("5 " + Thread.currentThread().getContextClassLoader().getParent());
 System.out.println("6 " + Thread.currentThread().getContextClassLoader().getParent().getParent());
    }

}

you get this (parentesis are mine):

1 null (=bootstrap CL)
2 sun.misc.Launcher$AppClassLoader@1bd06bf (=application CL)
3 sun.misc.Launcher$AppClassLoader@1bd06bf (=application CL)
4 sun.misc.Launcher$AppClassLoader@1bd06bf (=application CL)
5 sun.misc.Launcher$ExtClassLoader@1061299 (=extension CL)
6 null (=bootstrap CL)

that is, in this simple case the SystemClassLoader == ApplicationClassLoader == ContextClassLoader

The first null is because the String class is loaded by the Bootstrap classloader, which is NOT represented as a ClassLoader object. The Bootstrap will then load the Extension CL, who in turn loads the Application CL.

Running a similar code in a JSP is more interesting, because here the JEE container (WebLogic in this case) will add its own CL:

 System.out.println("1 " + String.class.getClassLoader());
 System.out.println("2 " + ClassLoader.getSystemClassLoader ());
 System.out.println("3 " + this.getClass().getClassLoader());
 System.out.println("4 " + Thread.currentThread().getContextClassLoader());
 System.out.println("5 " + Thread.currentThread().getContextClassLoader().getParent());
 System.out.println("6 " + Thread.currentThread().getContextClassLoader().getParent().getParent());
 System.out.println("7 " + Thread.currentThread().getContextClassLoader().getParent().getParent().getParent());
 System.out.println("8 " + Thread.currentThread().getContextClassLoader().getParent().getParent().getParent().getParent());
 System.out.println("9 " + Thread.currentThread().getContextClassLoader().getParent().getParent().getParent().getParent().getParent());
 System.out.println("10 " + Thread.currentThread().getContextClassLoader().getParent().getParent().getParent().getParent().getParent().getParent());

the result is:

1 null
2 sun.misc.Launcher$AppClassLoader@fc5b01
3 weblogic.servlet.jsp.JspClassLoader@1bfdb6f finder: weblogic.utils.classloaders.CodeGenClassFinder@137e701 annotation:
4 weblogic.utils.classloaders.ChangeAwareClassLoader@1cc3dd4 finder: weblogic.utils.classloaders.CodeGenClassFinder@11f2691 annotation: _auto_generated_ear_@CLTestWeb
5 weblogic.utils.classloaders.FilteringClassLoader@1d16e3 finder: weblogic.utils.classloaders.CodeGenClassFinder@1c9a7b5 annotation: _auto_generated_ear_@CLTestWeb
6 weblogic.utils.classloaders.GenericClassLoader@ceb98d finder: weblogic.utils.classloaders.CodeGenClassFinder@217181 annotation: _auto_generated_ear_@
7 weblogic.utils.classloaders.FilteringClassLoader@1a308be finder: weblogic.utils.classloaders.CodeGenClassFinder@92f8f0 annotation:
8 weblogic.utils.classloaders.GenericClassLoader@1865594 finder: weblogic.utils.classloaders.CodeGenClassFinder@fcfe6b annotation:
9 sun.misc.Launcher$AppClassLoader@fc5b01
10 sun.misc.Launcher$ExtClassLoader@1353d27

that is, WebLogic inserts 2 layers of GenericClassLoader or ChangeAwareClassLoader plus FilteringClassLoader between the War Application and the System Classloader.

See also http://www.javamonamour.org/2015/09/playing-with-cat-and-classloaders.html