How Can I Dynamically Load and Reload Java Classes and JAR Files in a Running System?

Loading New Code into a Running Java System

Reloading Classes

Reloading existing classes with existing data can lead to instability in your system. Instead, consider using the following approach:

• Instantiate a new URLClassLoader with the path to the updated JAR file.
• Load the new classes into the new class loader and cast them to their appropriate subclasses.
• Create an instance of the loaded class and execute the run method.

Loading New JAR Files

To load a new JAR file into a running Java system:

1. Create a URLClassLoader with the URL of the new JAR file.
2. Use the ClassLoader.newInstance method to create a new instance of the class loader.
3. Load the new classes into the new class loader using Class.forName.
4. Instantiate the loaded classes and execute any necessary actions.

Garbage Collection of Unused Class Loaders

Class loaders that are no longer in use will be garbage collected, provided there are no memory leaks. Common sources of memory leaks include ThreadLocal, JDBC drivers, and java.beans.

Preserving Object Data

If you require object data to persist across class reloads, consider using a persistence mechanism like serialization.

Adding New Classes to Existing Class Loaders

While it is possible to add new classes to existing class loaders using URLClassLoader.addURL, be aware that if a class fails to load during this process, it will not be able to load in that class loader instance.

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