perf: cache compiled XSLT Templates per XSL file in MessageCodeGenerator

[Copilot]

Cache compiled XSLT Templates per XSL file in MessageCodeGenerator to avoid recompiling the same stylesheet on every generated file.

Problem

MessageCodeGenerator.createTransformer() was creating a new TransformerFactory instance and calling newTransformer(styleSource) on every invocation — once per output file. newTransformer(source) compiles the XSL stylesheet from scratch each time. For large FIX versions (e.g. FIX Latest) with hundreds of message/component/field files, the same handful of XSL stylesheets were being compiled hundreds of times unnecessarily.

Changes

quickfixj-codegenerator/src/main/java/org/quickfixj/codegenerator/MessageCodeGenerator.java

• Shared TransformerFactory — moved from a local variable inside createTransformer() to a final instance field so Saxon is initialised once per MessageCodeGenerator instance.
• Templates cache — added a ConcurrentHashMap<String, Templates> instance field. A javax.xml.transform.Templates object is a thread-safe, pre-compiled representation of an XSL stylesheet.
• Cache key — computed per stylesheet:
  • Filesystem XSL: the absolute path of the file.
  • Classpath XSL: "classpath:" + xsltFile.
• The first call for a given key compiles the stylesheet via transformerFactory.newTemplates(styleSource) and stores the result. Subsequent calls reuse the cached Templates and call the cheap templates.newTransformer() to get a fresh (non-thread-safe) Transformer instance — as required by the JAXP spec.
• TransformerConfigurationException is still propagated correctly via the existing CodeGenerationException wrapper.

What is styleSource?

styleSource is a javax.xml.transform.stream.StreamSource — it wraps the raw byte stream of the XSL stylesheet file, loaded either from the filesystem (task.getTransformDirectory()/<file>.xsl) or from the classpath. It is passed once to transformerFactory.newTemplates(styleSource) to compile the stylesheet into a reusable Templates object. After the first compilation for a given stylesheet, styleSource is no longer needed; all subsequent calls reuse the cached Templates.

Before / After

// Before: new factory + full stylesheet compilation on every file generated
TransformerFactory transformerFactory = new net.sf.saxon.TransformerFactoryImpl();
return transformerFactory.newTransformer(styleSource);  // compiles XSL every time

// After: compile once, reuse pre-compiled Templates
Templates templates = templatesCache.computeIfAbsent(cacheKey,
    k -> transformerFactory.newTemplates(styleSource));  // compiled only on first call
return templates.newTransformer();                       // cheap — no recompilation

[chrjohn]

@copilot the pr description is not correct. Please add the changes that were done in this PR.

[Copilot]

@copilot the pr description is not correct. Please add the changes that were done in this PR.

Updated the PR description. It now explains the actual change: caching compiled XSLT Templates per XSL file in MessageCodeGenerator so that each stylesheet is compiled only once instead of once per generated file. It also explains what styleSource is — a StreamSource wrapping the raw XSL file bytes, passed to newTemplates() for the one-time compilation.