Dubbo Filters Failing to Load in Spring Boot Maven Plugin Versions 1.3.8.RELEASE or earlier

[steverao]

Describe the bug

A specific issue occurs when using Spring Boot Maven Plugin 1.3.8.RELEASE or earlier with Dubbo, where SPI-based filters like OpenTelemetryServerFilter fail to load. This is caused by an incompatibility in the LaunchedURLClassLoader implementation of older Spring Boot versions.
Problem Analysis:
The issue arises due to the way resources are loaded via SPI (Service Provider Interface) in Dubbo. Here's the breakdown:

1. SPI Registration:
   The OpenTelemetry filters are registered via SPI in the file:
   META-INF/org.apache.dubbo.rpc.Filter.
   This is managed by the instrumentation module here:
   DubboInstrumentationModule.java#L32-L36.
2. Resource Injection:
   The Javaagent instruments ClassLoader.getResources() to inject these resources:
   ResourceInjectionInstrumentation.java#L80-L113.
3. Resource Lookup Failure:
   In Spring Boot Maven Plugin 1.3.8.RELEASE or earlier, the LaunchedURLClassLoader overrides getResource() in a way that skips the parent classloader (AppClassLoader):
   Faulty Code:
   LaunchedURLClassLoader.java#L71-L77.
   Line 74 explicitly avoids delegating to the parent, which breaks the expected resource lookup path.
   Fix:
   The Spring team resolved this in Spring Boot 1.4.x by ensuring parent classloaders are properly queried.
   Commit Fix.

Steps to reproduce

1. Use spring-boot-maven-plugin 1.3.8.RELEASE or earlier to produce a jar of Dubbo's demo.
2. We can see related filters cannot be loaded.

Expected behavior

Use spring-boot-maven-plugin 1.3.8.RELEASE or earlier to produce a jar of Dubbo's demo, Dubbo’s related filters can be loaded successfully.

Actual behavior

Use spring-boot-maven-plugin 1.3.8.RELEASE or earlier to produce a jar of Dubbo's demo, we can see related filters cannot be loaded.

Javaagent or library instrumentation version

2.14.0

Environment

JDK: JDK 1.8
OS: Linux

Additional context

Workaround: Upgrade to Spring Boot Maven Plugin 1.4.x or later.