How Can I Dynamically Load and Reload JARs in My Java Application at Runtime?

How to Dynamically Load and Reload Jars for Runtime Code Extensions

Loading New Jars at Runtime

To dynamically load new JAR files while your application is running, you can leverage the URLClassLoader class:

ClassLoader loader = URLClassLoader.newInstance(
    new URL[] { yourURL },
    getClass().getClassLoader()
);

This creates a new ClassLoader instance that includes the specified URL. You can then use this class loader to load classes from the JAR file:

Class<?> clazz = Class.forName("mypackage.MyClass", true, loader);

Here, clazz represents the class you loaded from the JAR file.

Reloading Existing Classes

Reloading existing classes while your application is running can be trickier, as it may introduce data inconsistencies. However, if necessary, you can follow these steps:

Constructor<?> ctor = runClass.getConstructor();
Runnable doRun = (Runnable) ctor.newInstance();
doRun.run();

Garbage Collection of Class Loaders

Class loaders are no longer used once the associated threads and objects no longer reference them. They can then be garbage collected, unless there are memory leaks caused by ThreadLocal, JDBC drivers, or similar issues.

Handling Persistent Data

If you need to preserve the data associated with existing objects during class reloading, you should consider using a persistence mechanism like serialization to store and restore the data.

Class Loader Extensions

It is also possible to add new classes into an existing class loader using URLClassLoader.addURL. However, failed attempts to load a class will result in the class never being loaded in that class loader instance.

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