JongminChung opened a new pull request, #4033:
URL: https://github.com/apache/logging-log4j2/pull/4033
**Context**
Issue #4019 reports that Pattern Layout no longer preserves custom
`Throwable.toString()` output after upgrading from 2.24.3 to 2.25.x.
The official docs for the `exception` converter state that the default
behavior (no options) should match `Throwable#printStackTrace()`, which relies
on `Throwable.toString()` for the header line.
**Suspected change: PR #2691**
PR #2691 (commit `b55c4d3fa2` - "Consolidate stack trace rendering in
Pattern Layout") introduced a new renderer pipeline:
- Added `ThrowableStackTraceRenderer` (and extended/inverted variants) to
manually render stack traces.
- Updated `ThrowablePatternConverter` to delegate to the new renderer
instead of calling `Throwable#printStackTrace()` directly.
**What changed in rendering behavior**
Before PR #2691 (e.g., tag `rel/2.24.3`):
- `ThrowablePatternConverter` used `Throwable#printStackTrace()` directly
(either into a `StringWriter` or a `StringBuilderWriter`).
- The output header line was always produced by `Throwable.toString()`.
- Any custom override of `toString()` or `printStackTrace()` would be
reflected in Pattern Layout output.
After PR #2691:
- Rendering moved into `ThrowableStackTraceRenderer` which builds the header
line manually.
- In `ThrowableStackTraceRenderer.renderThrowableMessage(...)`, the header
was constructed as:
- `throwable.getClass().getName()` + `": " +
throwable.getLocalizedMessage()`
- This bypasses `Throwable.toString()` entirely.
- The new renderer also manually iterates stack trace elements, suppressed
exceptions, and causes using cached metadata.
**Why that causes #4019**
`Throwable#printStackTrace()` uses `Throwable.toString()` for its first
line.
By replacing `printStackTrace()` with a manual renderer that uses
`getClass().getName()` and `getLocalizedMessage()`
**, any customized `toString()` logic is lost.**
This matches the reporter's symptoms: stack traces remain, but the custom
header details disappear.
In short:
- **Regression introduced by PR #2691:** default `%ex` output no longer
matches `printStackTrace()` for custom `toString()` implementations.
- **Root cause:** `renderThrowableMessage` changed from `toString()`
semantics to explicit class/message concatenation.
**Related issue: #3623 (Pattern Layout vs. JSON Template Layout)**
Issue #3623 reports a mismatch between Pattern Layout (`%ex`) and JSON
Template Layout (`exception` resolver):
- JSON Template Layout uses `Throwable.printStackTrace()` for
`stackTrace.stringified`.
- Pattern Layout uses
`ThrowableStackTraceRenderer.renderThrowableMessage(...)`.
The JSON Template Layout implementation lives in:
-
`log4j-layout-template-json/src/main/java/org/apache/logging/log4j/layout/template/json/resolver/StackTraceStringResolver.java`
Specifically, `StackTraceStringResolver.resolve(...)` calls
`throwable.printStackTrace(...)` (line ~84), which means its header line always
comes from `Throwable.toString()`.
This makes the mismatch visible when exceptions override `toString()`.
Aligning `ThrowableStackTraceRenderer` to `throwable.toString()` closes both
#4019 (regression) and #3623 (layout inconsistency).
## Checklist
* Base your changes on `2.x` branch if you are targeting Log4j 2; use `main`
otherwise
* `./mvnw verify` succeeds ([the build
instructions](https://logging.apache.org/log4j/2.x/development.html#building))
* Non-trivial changes contain an entry file in the `src/changelog/.2.x.x`
directory
* Tests are provided
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