jamesfredley commented on PR #15541:
URL: https://github.com/apache/grails-core/pull/15541#issuecomment-4200435788
## JSP GrailsLayoutSpec Investigation - Root Cause Analysis
### The Failure
The `"jsp demo"` test in `GrailsLayoutSpec` fails because the response is a
completely empty HTML page (`<html><head></head><body></body></html>`). All 37
other tests in the same spec pass (GSP-based views work fine).
### What Works
1. Jasper initializes correctly - TLD scanning runs at startup
2. The `GroovyPageViewResolver` correctly falls back to JSP when no GSP is
found:
```
Locating GSP view for controller DemoController and path /demo/hello
No GSP view found, falling back to locating JSTL view for name
[/demo/hello]
```
3. The `JstlView` forwards to the correct path:
```
Forwarding to [/WEB-INF/grails-app/views/demo/hello.jsp]
```
4. The JSP compiles successfully via JDT:
```
Compiled [.../hello_jsp.java] 773ms
```
### What Fails
After JSP compilation and execution, the response body is empty. Even after
removing the `<meta name="layout">` tag from the JSP (eliminating SiteMesh
decoration entirely), the response remains empty. The JSP content is never
captured.
### Root Cause
The issue is in `grails-layout`'s response buffering pipeline. The rendering
chain is:
1. `EmbeddedGrailsLayoutView.renderTemplate()` calls `obtainContent()`
2. `obtainContent()` creates a `GrailsContentBufferingResponse` wrapper and
calls `innerView.render(model, request, contentBufferingResponse)`
3. The `JstlView.render()` does a `RequestDispatcher.forward()` to the JSP,
writing to the wrapped response
4. `GrailsContentBufferingResponse.getContent()` tries to retrieve the
captured content
In step 4, `getContent()` returns `null`, causing the entire response to be
silently dropped. This is likely because `GrailsPageResponseWrapper` (from
SiteMesh 2.6.x) doesn't correctly capture output from Tomcat 11's forward
dispatch. Tomcat 11 (Jakarta Servlet 6.1) may have changed how
`RequestDispatcher.forward()` interacts with response wrappers, specifically
around `getOutputStream()` vs `getWriter()` delegation, or response buffer
commit semantics.
### Why GSP Works But JSP Doesn't
GSP views write directly to the `GrailsContentBufferingResponse` via
`GroovyPageView`, which uses `GSPGrailsLayoutPage` (captured at line 90-93 of
`GrailsContentBufferingResponse.getContent()`). JSP views go through a
`RequestDispatcher.forward()` which writes to the underlying
`GrailsPageResponseWrapper` (captured at line 95-98). The forward dispatch path
is what breaks in Tomcat 11.
### Fix Required
This needs a fix in `grails-layout`'s `GrailsPageResponseWrapper` or
`GrailsContentBufferingResponse` to properly capture content written during a
`RequestDispatcher.forward()` on Tomcat 11 / Jakarta Servlet 6.1. The fix
should be tracked as a separate issue since it's a `grails-layout` module bug,
not a configuration problem.
### Impact
Only JSP view rendering through the Grails layout pipeline is affected. Pure
GSP views, REST responses, and direct `render text:` calls all work correctly.
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