Martin Leitner created FOP-3323:
-----------------------------------
Summary: NPE in layout manager when `fo:title` carries an `id`
attribute — `Page.getPageViewport()` on null `this.curPage`
Key: FOP-3323
URL: https://issues.apache.org/jira/browse/FOP-3323
Project: FOP
Issue Type: Bug
Components: layout/unqualified, renderer/pdf
Affects Versions: 2.11
Reporter: Martin Leitner
Attachments: no-title.fo, title-no-id.fo, title-with-id.fo
*Description*
FOP throws a {{NullPointerException}} in the layout manager when a {{fo:title}}
element carries an {{id}} attribute. The failure happens while wiring the
per-page ID->element map: {{Page.getPageViewport()}} is invoked on
{{{}this.curPage{}}}, which is still {{null}} because {{fo:title}} is processed
at {{fo:page-sequence}} scope, before any page has been constructed.
{code:java}
java.lang.NullPointerException: Cannot invoke
"org.apache.fop.layoutmgr.Page.getPageViewport()" because "this.curPage" is
null{code}
Note: {{fo:title}} is _not permitted_ to carry {{id}} per XSL-FO 1.1 §6.4.20
(its allowed properties do not include the {{id}} property). So this is invalid
FO input. The request here is not to _accept_ the {{{}id{}}}, but for FOP to
fail gracefully (validation error / skipped registration) instead of NPE-ing.
This is consistent with the existing documented limitation on the Known Issues
page ("the 'id' attributes are not properly handled for all block-level /
inline-level FO elements").
*Steps to reproduce*
Three sibling FO files isolate the trigger. They are identical except for the
presence of {{fo:title}} and its {{{}id{}}}.
# title-with-id.fo: triggers the NPE
# title-no-id.fo: renders OK (same file, {{id}} removed from {{{}fo:title{}}})
# no-title.fo: renders OK (same file, {{fo:title}} removed entirely)
Run each:
{code:java}
fop -fo title-with-id.fo -pdf out.pdf
{code}
||Input ||Result ||
|no-title.fo|OK - renders to PDF |
|title-no-id.fo|OK - renders to PDF |
|title-with-id.fo|NPE - {{Page.getPageViewport()}} on null {{this.curPage}}|
File 1 differs from file 2 only by {{id="t1"}} on {{{}fo:title{}}}.
*Expected*
Either a clean validation error ("property 'id' is not applicable to fo:title")
or silently skip ID registration for {{{}fo:title{}}}. No
{{{}NullPointerException{}}}.
*Actual*
Uncaught {{NullPointerException}} from the layout manager; processing aborts.
*Suggested fix*
* AreaTreeHandler / PageSequenceLayoutManager - defer wiring fo:title/@id into
the ID->element map until the first page is constructed, _or_ skip ID
registration for fo:title (it lives outside any page viewport).
* Defensive guard on {{this.curPage == null}} before the
Page.getPageViewport() call.
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