A playground for dark magic with Java class loaders. This is a work in progress. There are two implmentations to call methods on dynamically loaded classes currently:

• Using reflection to call a method by name on a class file passed in an multi-part HTTP POST request
• Using the interface Callable and invoking the call method on the object created from the class file passed

This library is an attempt to understand how dynamic class loaders work and how in languages like Clojure, code is evaluated.

Start the server with:

mvn clean install exec:java

Send a class file to be executed via reflection:

curl -XPOST "localhost:8080/loadclass/?classname=SystemInfo&methodname=info" -F "[email protected]"
curl -XPOST "localhost:8080/loadclass/?classname=RandomNumber&methodname=sayHello" -F "[email protected]"

To run the call method on a Callable class:

curl -XPOST "localhost:8080/callable/?classname=CallableAPI" -F "[email protected]"
  _              _
 | |            | |
 | |     ___ ___| |_ _ __ __ _ _ __   __ _  ___
 | |    / _ \ __| __| '__/ _` | '_ \ / _` |/ _ \
 | |____  __\__ \ |_| | | (_| | | | | (_| |  __/
 |______\___|___/\__|_|  \__,_|_| |_|\__, |\___|
                                      __/ |
                                     |___/

Ensure that clases are public and the method being accessed is public.

cd utils
javac RandomNumber.java
javac HelloClass.java
javac SystemInfo.java
javac CallableAPI.java

We are creating a new classloader for any class passed to the /loadclass route. We do this because if we don't, then it is impossible to reload a class that is already loaded.

This will not cause a memory leak because we are not holding on to any object of the classes, which therefore can be garbage collected, and once that happens, no object will reference the classloader, which can also be garbage collected. Recall that:

A classloader will be removed by the garbage collector only if nothing else refers to it. All classes hold a reference to their classloader and all objects hold references to their classes.

This can be demonstrated using:

MAVEN_OPTS="-XX:+PrintGC -Xmx20m -XX:+HeapDumpOnOutOfMemoryError-XX:+CrashOnOutOfMemoryError" mvn exec:java

while true;
  do
      curl -XPOST "localhost:8080/loadclass/?classname=SystemInfo&methodname=info" -F "[email protected]";
      curl -XPOST "localhost:8080/loadclass/?classname=HelloClass&methodname=sayHello" -F "[email protected]";
      curl -XPOST "localhost:8080/loadclass/?classname=RandomNumber&methodname=sayHello" -F "[email protected]";
      curl -XPOST "localhost:8080/callable/?classname=CallableAPI" -F "[email protected]";
  done

The GC logs will show metaspace cleanup of the form:

[GC (Metadata GC Threshold)  14670K->12241K(18944K), 0.0041918 secs]
[Full GC (Metadata GC Threshold)  12241K->8766K(18944K), 0.0548479 secs]
[Full GC (Ergonomics)  13400K->7388K(16896K), 0.0567170 secs]

Investigating further, we can check the memory performance in Visual VM. Also, let us stop firing requests and then force a GC via VisualVM and take a heap dump to confirm none of the classloaders we created on the fly are remaining.

Copyright © 2020 Mourjo Sen

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.