Catch `LinkageError` in `ThrowableExtendedStackTraceRenderer#loadClass` (#4028) by chanani · Pull Request #4049 · apache/logging-log4j2

chanani · 2026-02-19T03:58:10Z

Summary

The loadClass method in ThrowableExtendedStackTraceRenderer only catches Exception,
but ClassLoader can throw NoClassDefFoundError, which extends LinkageError (an Error
subclass). This causes the entire log event to fail with AppenderLoggingException instead
of gracefully degrading when a class in the stack trace cannot be resolved.

Changes

ThrowableExtendedStackTraceRenderer.java: Changed catch (Exception) to
catch (Exception | LinkageError) in the loadClass method
ThrowableExtendedStackTraceRendererTest.java: Added two test cases:
- Verifies rendering succeeds when a stack trace references a non-existent class
- Verifies rendering succeeds when a custom ClassLoader throws NoClassDefFoundError
Changelog entry: Added 4028_fix_catch_LinkageError_in_ThrowableExtendedStackTraceRenderer.xml

Root Cause

// Before: Only catches Exception — NoClassDefFoundError (Error) escapes
catch (final Exception ignored)

// After: Also catches LinkageError, which is the parent of NoClassDefFoundError
catch (final Exception | LinkageError ignored)

Java's Throwable hierarchy:

Throwable
├── Exception        ← previously caught
└── Error
       └── LinkageError ← now also caught
                  └── NoClassDefFoundError

Fixes #4028

…s` (apache#4028) Signed-off-by: CHANHAN <130114269+chanani@users.noreply.github.com>

chanani · 2026-02-19T03:59:56Z

Hi @vy, thanks for the encouragement!

Here's what this PR includes:

Fix: Changed catch (Exception) to catch (Exception | LinkageError) in ThrowableExtendedStackTraceRenderer#loadClass
Tests: Two test cases covering NoClassDefFoundError scenarios
Changelog: Added entry for this fix

Let me know if any changes are needed!

vy

@chanani, thanks so much for the (AI-generated?) fix. Please make sure ./mvnw -pl :log4j-core install && ./mvnw -pl :log4j-core-test verify succeeds when wrapping up your changes.

vy · 2026-02-28T19:53:43Z

src/changelog/.2.x.x/4028_fix_catch_LinkageError_in_ThrowableExtendedStackTraceRenderer.xml

+       https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+    <issue id="4028" link="https://github.com/apache/logging-log4j2/issues/4028"/>
+    <description format="asciidoc">Catch `LinkageError` in `ThrowableExtendedStackTraceRenderer#loadClass` to prevent `NoClassDefFoundError` from breaking logging</description>


Suggested change

<description format="asciidoc">Catch `LinkageError` in `ThrowableExtendedStackTraceRenderer#loadClass` to prevent `NoClassDefFoundError` from breaking logging</description>

<description format="asciidoc">Ensure all `Throwable`s are handled while rendering stack traces</description>

vy · 2026-02-28T19:53:56Z

src/changelog/.2.x.x/4028_fix_catch_LinkageError_in_ThrowableExtendedStackTraceRenderer.xml

+       xsi:schemaLocation="https://logging.apache.org/xml/ns
+       https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+    <issue id="4028" link="https://github.com/apache/logging-log4j2/issues/4028"/>


Suggested change

<issue id="4028" link="https://github.com/apache/logging-log4j2/issues/4028"/>

<issue id="4049" link="https://github.com/apache/logging-log4j2/pull/4049"/>

vy · 2026-02-28T19:57:31Z

...test/java/org/apache/logging/log4j/core/pattern/ThrowableExtendedStackTraceRendererTest.java

Instead of adding a new test file, would you mind extending ThrowablePatternConverterTest.StackTraceTest, please?

vy · 2026-02-28T19:57:48Z

...src/main/java/org/apache/logging/log4j/core/pattern/ThrowableExtendedStackTraceRenderer.java

                        return clazz;
                    }
-                } catch (final Exception ignored) {
+                } catch (final Exception | LinkageError ignored) {


Suggested change

} catch (final Exception | LinkageError ignored) {

} catch (final Throwable ignored) {

…che#4028) - Changed catch (Exception) to catch (Throwable) in loadClass method - Moved tests into ThrowablePatternConverterTest.StackTraceTest - Removed standalone ThrowableExtendedStackTraceRendererTest - Updated changelog description and issue link per review feedback Signed-off-by: CHANHAN <130114269+chanani@users.noreply.github.com>

chanani · 2026-03-01T03:07:45Z

Hi @vy, thank you for the review! I've addressed all the feedback.

catch (Throwable) — Applied the suggested change to catch all Throwables instead of Exception | LinkageError
Changelog description — Updated to "Ensure all Throwables are handled while rendering stack traces"
Changelog issue link — Changed from #4028 to #4049
Tests moved — Removed ThrowableExtendedStackTraceRendererTest and added the test cases into ThrowablePatternConverterTest.StackTraceTest as requested

Please let me know if any further changes are needed!

vy · 2026-03-02T09:40:23Z

@chanani, TestFriendlyException is already containing a stack trace element of a non-existent class: ORG_APACHE_REPLACEMENT_STACK_TRACE_ELEMENT. Any ideas on why does this NoClassDefFoundError not surface with the existing test suite?

chanani · 2026-03-03T00:42:28Z

Hi @vy,

The reason NoClassDefFoundError doesn't surface with the existing test suite is the difference between two exception paths:

ClassNotFoundException (extends Exception) — Thrown by ClassLoader.loadClass() when a class simply doesn't exist on the classpath. That's the case for bar.OrgApacheReplacement — it was never on the classpath, so the class loader search fails cleanly with ClassNotFoundException, which was already caught by the previous catch (Exception)
NoClassDefFoundError (extends LinkageError → Error) — Thrown when the class is found but fails during linking/initialization (e.g., a missing dependency at link time). This is the scenario from issue ThrowableExtendedStackTraceRenderer catches Exception but not Error #4028, and it wasn't caught by catch (Exception)

So the existing test with ORG_APACHE_REPLACEMENT_STACK_TRACE_ELEMENT exercises the ClassNotFoundException path, not the NoClassDefFoundError path

Please let me know if any further changes are needed !

vy · 2026-03-02T09:35:33Z

...-test/src/test/java/org/apache/logging/log4j/core/pattern/ThrowablePatternConverterTest.java

+         */
+        @Test
+        @Issue("https://github.com/apache/logging-log4j2/issues/4028")
+        void rendering_should_succeed_with_nonexistent_class_in_stack_trace() {


I've put some more thought into this issue, and this particular test feels like an after-thought. Would you mind removing this particular test method, extending TestFriendlyException with this patch to contain a stack trace element of a non-existent class, and adapt ThrowablePatternConverterTest and its subclasses to make sure they pass, please?

Catch LinkageError in `ThrowableExtendedStackTraceRenderer#loadClas…

c960992

…s` (apache#4028) Signed-off-by: CHANHAN <130114269+chanani@users.noreply.github.com>

github-project-automation bot added this to Log4j pull request tracker Feb 19, 2026

vy requested changes Feb 28, 2026

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github-project-automation bot moved this to Changes requested in Log4j pull request tracker Feb 28, 2026

vy requested changes Mar 16, 2026

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Catch `LinkageError` in `ThrowableExtendedStackTraceRenderer#loadClass` (#4028)#4049

Catch `LinkageError` in `ThrowableExtendedStackTraceRenderer#loadClass` (#4028)#4049
chanani wants to merge 2 commits intoapache:2.xfrom
chanani:fix/4028

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	<description format="asciidoc">Catch `LinkageError` in `ThrowableExtendedStackTraceRenderer#loadClass` to prevent `NoClassDefFoundError` from breaking logging</description>
	<description format="asciidoc">Ensure all `Throwable`s are handled while rendering stack traces</description>

	<issue id="4028" link="https://github.com/apache/logging-log4j2/issues/4028"/>
	<issue id="4049" link="https://github.com/apache/logging-log4j2/pull/4049"/>

	} catch (final Exception \| LinkageError ignored) {
	} catch (final Throwable ignored) {

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chanani commented Feb 19, 2026 •

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