Classloader variants and usage in Java

Question

I understand that java.lang.ClassLoader is generally the classloader I extend (when needed). Also, when I see the custom classloaders of tomcat, Jetty etc extend from java.lang.ClassLoader.

I'm curious to understand the purpose and usage of com.sun.org.apache.bcel.internal.util.ClassLoader.

Can someone help me understand it ?

Community · Accepted Answer · 2020-06-20 09:12:55Z

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com.sun.org.apache.bcel.internal is just a repackaging of Apache BCEL. In particular, the class we're interested in (ClassLoader) is documented here...

Drop in replacement for the standard class loader of the JVM. You can use it in conjunction with the JavaWrapper to dynamically modify/create classes as they're requested.

This class loader recognizes special requests in a distinct format, i.e., when the name of the requested class contains with "$$BCEL$$" it calls the createClass() method with that name (everything before the $$BCEL$$ is considered to be the package name. You can subclass the class loader and override that method. "Normal" classes class can be modified by overriding the modifyClass() method which is called just before defineClass().

edited Jun 20, 2020 at 9:12

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answered Sep 9, 2012 at 7:34

obataku

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radai · Accepted Answer · 2012-09-09 07:32:47Z

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judging by the "bcel" part of the package name that classloader doesnt just load classes, it alters the bytecode on the fly (see the bcel homepage).

edit: some more info on what they do with it can be found here : "BCEL is used internally by XSLTC to "compile" XSLT stylesheets into bytecodes for execution"

answered Sep 9, 2012 at 7:32

radai

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