http-url-connection instrumentation fails with read-only filesystem on Java 25

[kchrusciel]

Describe the bug

When running on Java 25 in an environment with a read-only filesystem (e.g. Kubernetes with readOnlyRootFilesystem: true), the http-url-connection instrumentation fails at startup with:

ERROR io.opentelemetry.javaagent.tooling.HelperInjector - Error preparing helpers while processing class sun.net.www.protocol.http.HttpURLConnection for http-url-connection. Failed to inject helper classes into instance <bootstrap>
java.lang.IllegalStateException: Failed to create temporary directory.
	at io.opentelemetry.javaagent.tooling.HelperInjector.createTempDir(HelperInjector.java:432)
	at io.opentelemetry.javaagent.tooling.HelperInjector.injectBootstrapClassLoader(HelperInjector.java:416)
...
Caused by: java.nio.file.FileSystemException: /tmp/opentelemetry-temp-jars5955764542960470820: Read-only file system

Steps to reproduce

1. Run any Java 25 application with the OTel Java agent (tested with v2.25.0)
2. Use a Kubernetes pod with readOnlyRootFilesystem: true and no writable /tmp volume
3. Application makes an HTTPS connection via HttpURLConnection

Expected behavior

No error at startup. The http-url-connection instrumentation should work on Java 25 without requiring a writable /tmp, the same way java-util-logging was fixed in #15682.

Actual behavior

Errors at startup

Javaagent or library instrumentation version

2.25.0

Environment

JDK: 25
OS: Linux

Additional context

On Java 25, Unsafe is no longer available for defining classes in the bootstrap classloader. HelperInjector falls back to Instrumentation.appendToBootstrapClassLoaderSearch, which requires creating a temporary JAR in /tmp.
PR #15682 solved this for java-util-logging by introducing JulLookupSupplier and registering it via addPackageLookup() in HelperInjector. The same approach is needed for http-url-connection.

Currently registered LookupSuppliers in HelperInjector:

• ExecutorLookupSupplier
• JulLookupSupplier ← added by Allow defining jul helper classes without using temp directory #15682
• LambdaLookupSupplier
• ReflectionLookupSupplier
• VirtualFieldLookupSupplier
• AdviceForwardLookupSupplier

http-url-connection has no equivalent supplier, so it always falls back to the /tmp path on Java 25.

Environment

• OTel Java agent version: v2.25.0
• Java version: 25
• OS: Linux (Kubernetes, readOnlyRootFilesystem: true)

Related

• Issue Errors at startup in read-only filesystems environment #15666 — original report for read-only filesystem errors
• PR Allow defining jul helper classes without using temp directory #15682 — fix for java-util-logging, same approach needed here

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