Скачать или смотреть Solving the Pf4j Extension Loading Issue in a Spring Boot Application

Discover how to address the `Pf4j` extension loading issue in your Spring Boot application by configuring a custom plugin loader with the proper classloader.
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Tackling the Pf4j Extension Loading Issue in Spring Boot

When building a Spring Boot application that utilizes plugins via Pf4j (Plugin Framework for Java), you might encounter an issue where the application fails to find the extensions you've created. This can be frustrating, especially if you're following a guide and expect everything to function smoothly. In this guide, we'll explore the symptoms of this problem and walk through a clear solution to ensure extensions are properly loaded in your application.

Understanding the Problem

You might find yourself in a situation where your attempt to retrieve extensions using the plugin manager doesn't yield any results. Here’s a common approach that illustrates this issue:

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Despite following the code execution, you may notice that the application does not enter the expected conditions in the extension loading mechanisms, particularly due to the use of different class loaders—PluginClassLoader for your extension and RestartClassLoader from Spring for the interface of the extension point.

The primary concern here is that the application is not recognizing the interface of your extension point, which can stem from how these class loaders interact with one another.

Why ClassLoaders Cause Issues

Java's class loader mechanism is designed to maintain separation between different classes loaded by different class loaders. In your case, a mismatch between the plugin class loader and the restart class loader leads to situations where the application fails to identify the appropriate extensions, as the interface cannot be found.

The following points summarize the issue:

Multiple ClassLoaders: You have a PluginClassLoader for extensions and a RestartClassLoader from Spring.

Class Visibility: JAR files loaded in one class loader may not be visible to another.

Debugging Difficulty: It can be tricky to identify the exact point of failure without proper debugging.

The Solution

To resolve the issue of class loader separation, you can customize the development plugin loader to ensure that it utilizes the parent class loader correctly. This involves extending the DevelopmentPluginLoader class. Here’s how you can accomplish this:

Step 1: Extend the DevelopmentPluginLoader

Create a custom plugin loader that extends the functionality of DevelopmentPluginLoader to pass the Spring class loader:

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Step 2: Create the Custom Plugin Manager

Next, create a custom PluginManager that uses your new plugin loader:

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Step 3: Compile and Test

Once you've made these changes, compile your application and run it. The adjustments made in the plugin loader should ensure that the correct class loader recognizes the extensions you've defined, allowing them to be loaded properly.

Conclusion

Navigating class loader issues when working with plugins can be challenging. By extending the DevelopmentPluginLoader and properly configuring your PluginManager, you can resolve the issues related to class visibility within your Spring Boot application. With this guidance, you can confidently tackle any similar issues that may arise in your future development endeavors.

For more insights and tips on Spring Boot and Pf4j, stay tuned to our blog!