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commit 978dff9999815fcdeb0ef26908d609723e120fb6
Author: jenkins <[email protected]>
AuthorDate: Wed May 6 02:54:10 2026 +0000
2026/05/06 02:54:10: Generated dev website from groovy-website@61dcbd6
---
search/search-index.json | 9 +-
wiki/GEP-22.html | 848 +++++++++++++++++++++++++++++++++++++++++++++++
wiki/geps.html | 2 +-
3 files changed, 857 insertions(+), 2 deletions(-)
diff --git a/search/search-index.json b/search/search-index.json
index 69f3eef..c57e1fc 100644
--- a/search/search-index.json
+++ b/search/search-index.json
@@ -790,6 +790,13 @@
"url": "wiki/initial-release-process-proposal.html",
"site": "dev"
},
+ {
+ "id": "wiki/GEP-22.html",
+ "title": "The Apache Groovy programming language - Developer docs -
GEP-22",
+ "content": "The Apache Groovy programming language - Developer docs -
GEP-22 Socialize Discuss on the mailing list Groovy on X Groovy on Bluesky
Groovy on Mastodon Groovy on LinkedIn Events and conferences Source code on
GitHub Report issues in Jira Stack Overflow questions Slack Community You are
using an outdated browser. Please upgrade your browser to improve your
experience. Apache Groovy™ Learn Documentation Download Support
Contribute Ecosystem Blog posts Socialize GE [...]
+ "url": "wiki/GEP-22.html",
+ "site": "dev"
+ },
{
"id": "wiki/GEP-8.html",
"title": "The Apache Groovy programming language - Developer docs -
GEP-8",
@@ -877,7 +884,7 @@
{
"id": "wiki/geps.html",
"title": "The Apache Groovy programming language - GEPs",
- "content": "The Apache Groovy programming language - GEPs Socialize
Discuss on the mailing list Groovy on X Groovy on Bluesky Groovy on Mastodon
Groovy on LinkedIn Events and conferences Source code on GitHub Report issues
in Jira Stack Overflow questions Slack Community You are using an outdated
browser. Please upgrade your browser to improve your experience. Apache
Groovy™ Learn Documentation Download Support Contribute Ecosystem Blog
posts Socialize GEPs GEP-1 GEP-2 GEP- [...]
+ "content": "The Apache Groovy programming language - GEPs Socialize
Discuss on the mailing list Groovy on X Groovy on Bluesky Groovy on Mastodon
Groovy on LinkedIn Events and conferences Source code on GitHub Report issues
in Jira Stack Overflow questions Slack Community You are using an outdated
browser. Please upgrade your browser to improve your experience. Apache
Groovy™ Learn Documentation Download Support Contribute Ecosystem Blog
posts Socialize GEPs GEP-1 GEP-2 GEP- [...]
"url": "wiki/geps.html",
"site": "dev"
},
diff --git a/wiki/GEP-22.html b/wiki/GEP-22.html
new file mode 100644
index 0000000..a873990
--- /dev/null
+++ b/wiki/GEP-22.html
@@ -0,0 +1,848 @@
+<!DOCTYPE html>
+<!--[if lt IE 7]> <html class="no-js lt-ie9 lt-ie8 lt-ie7"> <![endif]-->
+<!--[if IE 7]> <html class="no-js lt-ie9 lt-ie8"> <![endif]-->
+<!--[if IE 8]> <html class="no-js lt-ie9"> <![endif]-->
+<!--[if gt IE 8]><!--> <html class="no-js"> <!--<![endif]--><head>
+ <meta charset='utf-8'/><meta http-equiv='X-UA-Compatible'
content='IE=edge'/><meta name='viewport' content='width=device-width,
initial-scale=1'/><title>The Apache Groovy programming language - Developer
docs - GEP-22</title><link href='../img/favicon.ico' type='image/x-ico'
rel='icon'/><script src='../js/matomo.js'></script><link rel='stylesheet'
type='text/css' href='../css/bootstrap.css'/><link rel='stylesheet'
type='text/css' href='../css/fontawesome.min.css'/><link rel='styleshe [...]
+</head><body>
+ <div id='fork-me'>
+ <a href='https://github.com/apache/groovy'>
+ <img style='position: fixed; top: 20px; right: -58px; border: 0;
z-index: 100; transform: rotate(45deg);'
src='../img/horizontal-github-ribbon.png'/>
+ </a>
+ </div><div id='st-container' class='st-container st-effect-9'>
+ <nav class='st-menu st-effect-9' id='menu-12'>
+ <h2 class='icon icon-lab'>Socialize</h2><ul>
+ <li>
+ <a href='https://groovy-lang.org/mailing-lists.html'
class='icon'><span class='fa fa-classic fa-regular fa-envelope'></span> Discuss
on the mailing list</a>
+ </li><li>
+ <a href='https://x.com/ApacheGroovy' class='icon'><span
class='fa fa-brands fa-x-twitter'></span> Groovy on X</a>
+ </li><li>
+ <a href='https://bsky.app/profile/groovy.apache.org'
class='icon'><span class='fa fa-brands fa-bluesky'></span> Groovy on Bluesky</a>
+ </li><li>
+ <a href='https://fosstodon.org/@ApacheGroovy'
class='icon'><span class='fa fa-brands fa-mastodon'></span> Groovy on
Mastodon</a>
+ </li><li>
+ <a
href='https://www.linkedin.com/company/106402668/admin/dashboard/'
class='icon'><span class='fa fa-brands fa-linkedin'></span> Groovy on
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+ </li><li>
+ <a href='https://groovy-lang.org/events.html'
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+ </li><li>
+ <a href='https://github.com/apache/groovy'
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+ </li><li>
+ <a href='https://groovy-lang.org/reporting-issues.html'
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+ </li><li>
+ <a href='http://stackoverflow.com/questions/tagged/groovy'
class='icon'><span class='fa fa-brands fa-stack-overflow'></span> Stack
Overflow questions</a>
+ </li><li>
+ <a href='http://www.groovycommunity.com/'
class='icon'><span class='fa fa-brands fa-slack'></span> Slack Community</a>
+ </li>
+ </ul>
+ </nav><div class='st-pusher'>
+ <div class='st-content'>
+ <div class='st-content-inner'>
+ <!--[if lt IE 7]>
+ <p class="browsehappy">You are using an
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href="http://browsehappy.com/";>upgrade your browser</a> to improve your
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+ <![endif]--><div><div class='navbar navbar-default
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+ <div class='container'>
+ <div class='navbar-header'>
+ <button type='button'
class='navbar-toggle' data-toggle='collapse' data-target='.navbar-collapse'>
+ <span class='sr-only'></span><span
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+ </button><a class='navbar-brand'
href='../index.html'>
+ <i class='fa-classic fa-solid
fa-star'></i> Apache Groovy™
+ </a>
+ </div><div class='navbar-collapse collapse'>
+ <ul class='nav navbar-nav navbar-right'>
+ <li class=''><a
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class=''><a href='/download.html'>Download</a></li><li class=''><a
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+ <a data-effect='st-effect-9'
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+ </li><li class=''>
+ <a href='../search.html'>
+ <i class='fa-classic fa-solid
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+ </a>
+ </li><li>
+ <button id='theme-switcher'
class='theme-switcher' type='button' title='Toggle theme' aria-label='Toggle
theme'>
+ <span
class='theme-icon'></span>
+ </button>
+ </li>
+ </ul>
+ </div>
+ </div>
+ </div><div id='content' class='page-1'><div
class='row'><div class='row-fluid'><div class='col-lg-3'><ul
class='nav-sidebar'><li><a href='geps.html'>GEP index</a></li><li
class='active'><a href='#doc'>GEP-22</a></li><li><a href='#_abstract_traits'
class='anchor-link'>Abstract: Traits</a></li><li><a href='#_specification'
class='anchor-link'>Specification</a></li><li><a
href='#_cross_version_evolution' class='anchor-link'>Cross-version
evolution</a></li><li><a href [...]
+<div class="sectionbody">
+<div class="sidebarblock">
+<div class="content">
+<div class="title">Metadata</div>
+<div class="hdlist">
+<table>
+<tr>
+<td class="hdlist1">
+<strong>Number</strong>
+</td>
+<td class="hdlist2">
+<p>GEP-22</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Title</strong>
+</td>
+<td class="hdlist2">
+<p>Traits</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Version</strong>
+</td>
+<td class="hdlist2">
+<p>1</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Type</strong>
+</td>
+<td class="hdlist2">
+<p>Feature</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Status</strong>
+</td>
+<td class="hdlist2">
+<p>Final</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Comment</strong>
+</td>
+<td class="hdlist2">
+<p>Delivered in Groovy 2.3 and refined in later versions; static-member
support remains incubating</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Leader</strong>
+</td>
+<td class="hdlist2">
+<p>Cédric Champeau</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Created</strong>
+</td>
+<td class="hdlist2">
+<p>2026-05-06</p>
+</td>
+</tr>
+<tr>
+<td class="hdlist1">
+<strong>Last modification</strong>
+</td>
+<td class="hdlist2">
+<p>2026-05-06</p>
+</td>
+</tr>
+</table>
+</div>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_abstract_traits">Abstract: Traits</h2>
+<div class="sectionbody">
+<div class="paragraph">
+<p>Traits are a structural construct of the Groovy language that allow:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>composition of behaviour</p>
+</li>
+<li>
+<p>runtime implementation of interfaces</p>
+</li>
+<li>
+<p>behaviour overriding</p>
+</li>
+<li>
+<p>compatibility with static type checking and static compilation</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>Conceptually a trait is an <strong>interface</strong> that may carry both
<strong>default implementations</strong>
+and <strong>state</strong>. A class declares its participation in a trait via
the <code>implements</code>
+clause exactly as it would for any other interface. Traits compose into the
+implementing class at compile time (or, optionally, at runtime via coercion)
+without requiring multiple inheritance of classes.</p>
+</div>
+<div class="paragraph">
+<p>This GEP specifies the language semantics of traits, captures the bytecode
+compilation model, and records the evolution of the feature across Groovy
+releases. Worked examples and tutorial-style code samples live in the
+language specification (<code>_traits.adoc</code>); this document is
intentionally
+terse and prescriptive.</p>
+</div>
+<div class="sect2">
+<h3 id="_motivation">Motivation</h3>
+<div class="paragraph">
+<p>Object-oriented codebases routinely need to reuse behaviour across class
+hierarchies that cannot share a common ancestor. The available pre-existing
+mechanisms each carry trade-offs:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p><strong>Single inheritance</strong> forces all reuse through a single line,
leading to
+deep hierarchies, fragile base-class problems, and inability to mix
+unrelated capabilities.</p>
+</li>
+<li>
+<p><strong>Java 8 default methods</strong> attach behaviour to interfaces but
are stateless
+by design, do not support stackable composition through <code>super</code>, and
+their resolution rules are designed around binary compatibility rather
+than composition.</p>
+</li>
+<li>
+<p><strong>Runtime mixins</strong> (Groovy’s pre-2.3 <code>@Mixin</code>
annotation, since deprecated)
+weave behaviour at runtime but the resulting object is not an
+<code>instanceof</code> the mixed-in type, defeating type-based dispatch and
+compile-time checks.</p>
+</li>
+<li>
+<p><strong>Delegation via <code>@Delegate</code></strong> expresses
<em>has-a</em> relationships well but is
+verbose for genuine <em>is-a</em> composition and does not support overriding
+delegated behaviour transparently.</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>Traits address these gaps. Drawing on the work of Schärli, Ducasse, Black
+and Nierstrasz (ECOOP 2003), and on Scala’s analogous construct, a trait
+combines the contract role of an interface with the reusable-implementation
+role of a class while imposing deterministic composition rules. As Cédric
+Champeau put it when introducing them: traits <strong>extend the benefit of
+interfaces to concrete classes</strong> without producing inheritance pyramids,
+allowing new APIs to be layered onto existing classes without modification.</p>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_specification">Specification</h2>
+<div class="sectionbody">
+<div class="sect2">
+<h3 id="_declaration">Declaration</h3>
+<div class="paragraph">
+<p>A trait is declared with the <code>trait</code> keyword:</p>
+</div>
+<div class="listingblock">
+<div class="content">
+<pre class="prettyprint highlight"><code data-lang="groovy">trait
FlyingAbility {
+ String fly() { "I'm flying!" }
+}</code></pre>
+</div>
+</div>
+<div class="paragraph">
+<p>The annotation <code>@groovy.transform.Trait</code> is an exact synonym and
may be
+applied to any class declaration that would otherwise satisfy the trait
+shape; the AST transform produces equivalent output. The <code>trait</code>
keyword
+is preferred in source.</p>
+</div>
+<div class="paragraph">
+<p>A trait may declare any of: methods (public or private, abstract or
+concrete, static or instance), properties, fields (public or private,
+instance or static), and an <code>implements</code> clause naming interfaces
and/or
+super-traits. A trait may extend at most one super-trait via
<code>extends</code>,
+or any number of super-traits by listing them in <code>implements</code>.</p>
+</div>
+<div class="paragraph">
+<p>Constructors are not permitted on traits.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_methods">Methods</h3>
+<div class="ulist">
+<ul>
+<li>
+<p>Public and private methods are supported. <code>protected</code> and
package-private
+are <strong>not</strong> supported.</p>
+</li>
+<li>
+<p>Abstract methods are permitted; concrete classes that implement the
+trait must provide an implementation unless they too are abstract.</p>
+</li>
+<li>
+<p>Private methods do not appear on the generated trait interface and are
+not visible to other classes.</p>
+</li>
+<li>
+<p>The <code>final</code> modifier records the intended modifier of the woven
method
+in the implementing class. Mixing <code>final</code> and
non-<code>final</code> declarations of
+the same signature across multiple inherited traits is permitted; normal
+trait method-selection rules apply and the resulting method takes the
+modifier of the selected source.</p>
+</li>
+</ul>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_fields">Fields</h3>
+<div class="paragraph">
+<p>Fields declared in a trait are not stored on the generated trait
+interface (interfaces cannot hold instance state). Instead they are stored
+on a synthetic <em>field helper</em> class and woven into the implementing
class
+under name-mangled identifiers:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>For a field <code>bar</code> of type <code>T</code> declared in trait
<code>my.pkg.Foo</code>, the
+implementing class gains a field named <code>my_pkg_Foo__bar</code> of type
<code>T</code>.</p>
+</li>
+<li>
+<p>The mangling rule is: replace each <code>.</code> in the package name with
<code><em></code>,
+append <code></em><TraitSimpleName>__<fieldName></code>. The
double underscore separates
+the trait identity from the field identity.</p>
+</li>
+<li>
+<p>Private fields follow the same scheme but are also marked private; the
+mangling guarantees no collisions when a class implements multiple
+traits that declare fields of the same name (the diamond problem for
+state).</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>Use of public fields in traits is discouraged in favour of properties.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_properties">Properties</h3>
+<div class="paragraph">
+<p>A property declared in a trait yields the usual auto-generated accessor
+pair (<code>getX</code>/<code>setX</code>, or <code>isX</code> for
<code>boolean</code>) on the generated trait
+interface, and a backing field on the implementing class woven via the
+field-helper mechanism. Property semantics are otherwise identical to
+those of a regular Groovy class.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_this_super_and_stackable_traits"><code>this</code>,
<code>super</code>, and stackable traits</h3>
+<div class="paragraph">
+<p>Inside a trait method:</p>
+</div>
+<div class="olist arabic">
+<ol class="arabic">
+<li>
+<p><code>this</code> refers to the <strong>implementing instance</strong>,
never to a trait-level
+artefact. A trait should be reasoned about as if it were a superclass of
+the implementing class, despite the fact that no such class exists in
+the runtime hierarchy.</p>
+</li>
+<li>
+<p>An unqualified <code>super.m(…​)</code> call delegates to the
<strong>next trait in the
+implementation chain</strong> (in the order produced by the
<code>implements</code> clause
+of the implementing class, walked right-to-left). When the chain is
+exhausted, <code>super</code> resolves to the actual superclass of the
implementing
+class.</p>
+</li>
+<li>
+<p>A qualified <code>T.super.m(…​)</code> call resolves to trait
<code>T’s implementation
+of `m</code>, regardless of position in the chain. This is the explicit
+override form.</p>
+</li>
+</ol>
+</div>
+<div class="paragraph">
+<p>This rule set yields <strong>stackable traits</strong>: behaviours that
delegate to the
+next trait via unqualified <code>super</code> can be composed in different
orders
+to produce different overall behaviours, without those traits needing to
+know about each other.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_multiple_inheritance_and_conflict_resolution">Multiple inheritance
and conflict resolution</h3>
+<div class="paragraph">
+<p>When a class implements two or more traits that supply conflicting
+implementations of a method with the same signature, the <strong>last trait
+listed in the <code>implements</code> clause wins</strong>. This is the
deterministic
+default. The implementing class may override the resolution:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>by providing its own implementation of the method, or</p>
+</li>
+<li>
+<p>by calling <code>T.super.m(…​)</code> for the desired trait
<code>T</code>.</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>The same rule applies to property accessors and field-helper-backed
+state. Diamond conflicts on field state cannot arise: each trait’s
+fields live under their own mangled names.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_sam_coercion">SAM coercion</h3>
+<div class="paragraph">
+<p>A trait that declares exactly one abstract method is a <strong>Single
Abstract
+Method</strong> type for the purposes of closure coercion. A
<code>Closure</code> may be
+assigned or coerced to such a trait, in which case the closure body
+becomes the implementation of the abstract method. SAM coercion of
+traits composes with the rules in GEP-12.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_runtime_application">Runtime application</h3>
+<div class="paragraph">
+<p>Traits may be applied to an existing object at runtime:</p>
+</div>
+<div class="listingblock">
+<div class="content">
+<pre class="prettyprint highlight"><code data-lang="groovy">def proxy =
subject as SomeTrait
+def proxy = subject.withTraits(TraitA, TraitB)</code></pre>
+</div>
+</div>
+<div class="paragraph">
+<p>Both forms produce a <strong>new proxy instance</strong> — never the
original object —
+that:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>implements <code>SomeTrait</code> (or <code>TraitA</code> and
<code>TraitB</code>),</p>
+</li>
+<li>
+<p>implements every interface that the original object implemented, and</p>
+</li>
+<li>
+<p>delegates to the original object for behaviour not supplied by the
+applied trait(s).</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>A trait method always takes precedence over the corresponding method on
+the proxied object when both exist. The proxy is <strong>not</strong> an
instance of
+the original class; consumers that rely on <code>instanceof</code> against the
+original concrete class must continue to use the underlying object.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_static_members">Static members</h3>
+<div class="paragraph">
+<p>Static methods, properties and fields in a trait are supported with
+the following constraints (collectively flagged <strong>incubating</strong> /
+experimental):</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>Each implementing class receives its <strong>own copy</strong> of static
members.
+Static state is not shared across implementing classes — a static field
+declared in a trait is conceptually a template, instantiated per
+implementer.</p>
+</li>
+<li>
+<p>Static members are <strong>not</strong> exposed on the generated trait
interface.
+Calls of the form <code>Trait.staticMethod(…​)</code> are not
statically valid.</p>
+</li>
+<li>
+<p>Static members are accessed dynamically and are not subject to static
+type checking; traits with static methods cannot be
<code>@CompileStatic</code>.</p>
+</li>
+<li>
+<p>Mixing static and instance methods of the same signature across
+multiple traits is undefined behaviour and should be avoided. If
+selection chooses a static variant where an instance variant is
+required, a compilation error is raised; conversely, an instance
+selection silently shadows the static variant.</p>
+</li>
+</ul>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_selftype"><code>@SelfType</code></h3>
+<div class="paragraph">
+<p><code>@groovy.transform.SelfType</code> declares a list of types that any
class
+implementing the annotated trait must extend or implement. The compiler
+verifies the constraint at compile time and reports a clear error if it
+is violated. Self types make traits that depend on the surrounding
+class’s API safe to type-check and statically compile, without resorting
+to explicit <code>(this as SomeBaseClass)</code> casts inside trait method
bodies.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_interaction_with_sealed">Interaction with <code>@Sealed</code></h3>
+<div class="paragraph">
+<p>Traits may be sealed (see GEP-13). <code>@Sealed</code> and
<code>@SelfType</code> are
+orthogonal:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p><code>@SelfType</code> constrains <strong>what an implementing class must
already be</strong>
+(its supertype shape).</p>
+</li>
+<li>
+<p><code>@Sealed</code> constrains <strong>which specific classes are allowed
to implement</strong>
+the trait at all.</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>A trait may carry both annotations. For the degenerate single-permitted-
+implementer case, <code>@SelfType(Foo)</code> is preferred over
+<code>@Sealed(permittedSubclasses = ['Foo'])</code> for clarity.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_bytecode_and_stub_model">Bytecode and stub model</h3>
+<div class="paragraph">
+<p>A trait <code>pkg.T</code> produces, after compilation, the following
artefacts:</p>
+</div>
+<div class="olist arabic">
+<ol class="arabic">
+<li>
+<p><strong>An interface</strong> <code>pkg.T</code> containing the abstract
and public-method
+signatures, plus accessor signatures for any properties. This is the
+type that participates in <code>instanceof</code>, generics, and Java interop.
+The interface contains <strong>no</strong> default methods — trait method
bodies are
+not bytecode-default-method bodies, even on JDK 8+.</p>
+</li>
+<li>
+<p><strong>A trait helper class</strong> named <code>pkg.T$Trait$Helper</code>
(constant
+<code>Traits.TRAIT_HELPER</code>). Each non-abstract trait method
<code>m(args)</code> is
+emitted as a <code>static</code> method on the helper, taking the receiver as
an
+explicit first parameter (and threading any captured trait state
+through field-helper accessors). Bridge methods are woven into each
+implementing class to forward instance calls to the helper.</p>
+</li>
+<li>
+<p><strong>A field helper class</strong> named
<code>pkg.T$Trait$FieldHelper</code> (constant
+<code>Traits.FIELD_HELPER</code>). It exposes get/set accessors for trait
+instance fields, parameterised by the receiver. The implementing class
+is given mangled fields and small bridge accessors that delegate to
+this helper.</p>
+</li>
+<li>
+<p><strong>A static field helper class</strong> named
<code>pkg.T$Trait$StaticFieldHelper</code>
+(constant <code>Traits.STATIC_FIELD_HELPER</code>) when the trait declares
static
+fields, providing per-implementer storage for static state.</p>
+</li>
+</ol>
+</div>
+<div class="paragraph">
+<p>This four-class model is what makes traits visible to Java callers as
+plain interfaces (with no default methods to surprise Java consumers),
+while still supplying state, stackable <code>super</code>, and conflict
resolution
+that a Java-default-method approach cannot.</p>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_limitations">Limitations</h3>
+<div class="ulist">
+<ul>
+<li>
+<p><strong>AST transform compatibility</strong> is best-effort. Some transforms
+(<code>@CompileStatic</code>, <code>@TypeChecked</code>, logging transforms)
apply cleanly to
+traits; others apply only to the implementing class; others are
+unsupported. New transforms should be evaluated case by case.</p>
+</li>
+<li>
+<p><strong>Prefix and postfix operators</strong> (<code>+`, `--`) on a trait
field are
+rejected at compile time. The workaround is `=</code> / <code>-=</code>. The
reason is
+that the field-helper indirection cannot represent the read-modify-write
+sequence atomically.</p>
+</li>
+<li>
+<p><strong>Constructors</strong> on traits are not supported. Initialisation
logic
+should be expressed via abstract methods supplied by the implementing
+class, or via property defaults.</p>
+</li>
+<li>
+<p><strong>Inheritance-of-state gotcha</strong>: trait method bodies that read
a trait
+field directly read the <strong>trait’s own</strong> field, not any
same-named property
+declared by the implementing class. To pick up an overriding value,
+trait code should access state through accessors (<code>getX()</code>), not by
+direct field reference.</p>
+</li>
+</ul>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_cross_version_evolution">Cross-version evolution</h2>
+<div class="sectionbody">
+<table class="tableblock frame-all grid-all stretch">
+<colgroup>
+<col style="width: 14.2857%;">
+<col style="width: 14.2857%;">
+<col style="width: 71.4286%;">
+</colgroup>
+<thead>
+<tr>
+<th class="tableblock halign-left valign-top">Version</th>
+<th class="tableblock halign-left valign-top">Year</th>
+<th class="tableblock halign-left valign-top">Change</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2.3.0</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2014</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">Initial
release of traits. <code>trait</code> keyword and <code>@Trait</code> annotation
+introduced; public/private methods; abstract methods; instance fields
+with name mangling; properties; multiple inheritance with last-wins
+conflict resolution; explicit <code>T.super.m()</code> resolution; stackable
+unqualified <code>super</code>; runtime application via <code>as</code> and
+<code>Object.withTraits(…​)</code>; SAM coercion of
single-abstract-method
+traits.</p></td>
+</tr>
+<tr>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2.4.0</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2015</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock"><code>@SelfType</code> annotation introduced (GROOVY-7134),
enabling statically
+checked traits whose method bodies depend on members supplied by the
+implementing class’s superclass hierarchy.</p></td>
+</tr>
+<tr>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">3.0</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2020</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">Default
methods declared in <code>interface</code> types using the Java 8 syntax
+were accepted by the parser and implemented under the hood by
+delegating to the trait machinery (incubating). No spec-level change to
+trait semantics.</p></td>
+</tr>
+<tr>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">4.0</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2022</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">Sealed
traits supported via <code>@Sealed</code> / the <code>sealed</code> keyword and
the
+<code>permits</code> clause (see GEP-13). Distinction between
<code>@Sealed</code> and
+<code>@SelfType</code> formalised in the spec. Java stub generation for static
+trait properties hardened.</p></td>
+</tr>
+<tr>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">5.0</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">2024</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">Default,
private and static methods in <code>interface</code> types are now
+implemented as <strong>native</strong> JVM bytecode rather than via the trait
+machinery (GROOVY-8299). This decouples the interface-default-method
+feature from traits and improves Java interop. Multiple bug fixes
+landed for trait + <code>@TupleConstructor</code> default-value handling
+(GROOVY-8219, GROOVY-8788) and for trait static-field generation under
+static compilation (GROOVY-11817, GROOVY-11907).</p></td>
+</tr>
+<tr>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">6.0+</p></td>
+<td class="tableblock halign-left valign-top"><p
class="tableblock">TBD</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">No
spec-level changes to traits planned at the time of writing. This
+GEP is the canonical location to record any future change.</p></td>
+</tr>
+</tbody>
+</table>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_non_goals_and_potential_future_extensions">Non-goals and potential
future extensions</h2>
+<div class="sectionbody">
+<div class="paragraph">
+<p>The following items are deliberately out of scope for this GEP and for
+the current implementation. Any of them would warrant a follow-up
+revision of this document or a successor GEP.</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p>Promoting static-member support out of incubating status. The current
+per-implementer template semantics is a deliberate JVM-shaped
+compromise; a "true" shared-static-member model would require either
+breaking that semantics or moving static state into a separate runtime
+holder.</p>
+</li>
+<li>
+<p>Emitting trait methods as native interface default methods on JDK 8+.
+The four-class helper model deliberately avoids this so that Java
+callers see traits as plain interfaces. Switching would be a
+Java-interop change.</p>
+</li>
+<li>
+<p>First-class compatibility guarantees for arbitrary AST transforms on
+traits.</p>
+</li>
+<li>
+<p>Constructor support on traits.</p>
+</li>
+<li>
+<p><code>protected</code> or package-private trait methods.</p>
+</li>
+</ul>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_references_and_useful_links">References and useful links</h2>
+<div class="sectionbody">
+<div class="ulist">
+<ul>
+<li>
+<p>Schärli, Ducasse, Nierstrasz, Black. <em>Traits: Composable Units of
+Behaviour.</em> ECOOP 2003. — the academic foundation.</p>
+</li>
+<li>
+<p><a href="https://docs.scala-lang.org/tour/traits.html";>Scala traits</a> —
the
+closest sibling construct in another JVM language.</p>
+</li>
+<li>
+<p><a href="https://openjdk.org/jeps/126";>JEP 126: Default Methods</a> —
Java’s
+stateless analogue, contrasted in the Motivation section.</p>
+</li>
+<li>
+<p><a href="https://groovy.apache.org/wiki/GEP-12.html";>GEP-12: SAM
coercion</a> —
+underlies trait SAM coercion.</p>
+</li>
+<li>
+<p><a href="https://groovy.apache.org/wiki/GEP-13.html";>GEP-13: Sealed
classes</a> —
+applies to traits (sealed traits).</p>
+</li>
+<li>
+<p><a
href="https://speakerdeck.com/melix/rethinking-api-design-with-traits";>Cédric
+Champeau, <em>Rethinking API design with traits</em></a> — original design
+motivation and worked examples.</p>
+</li>
+<li>
+<p>The language specification chapter on traits in the Groovy
+documentation contains worked tutorial examples that complement this
+spec-only document.</p>
+</li>
+</ul>
+</div>
+<div class="sect2">
+<h3 id="_reference_implementation">Reference implementation</h3>
+<div class="paragraph">
+<p>Package: <code>org.codehaus.groovy.transform.trait</code></p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p><code>TraitASTTransformation</code> — entry point invoked for every trait
+declaration; produces the interface, helper, field-helper and (where
+applicable) static-field-helper artefacts.</p>
+</li>
+<li>
+<p><code>TraitComposer</code> — weaves trait methods, accessors and bridge
methods
+into each implementing class.</p>
+</li>
+<li>
+<p><code>TraitReceiverTransformer</code> — rewrites references inside trait
method
+bodies so that <code>this</code>, <code>super</code> and field accesses
resolve to the
+implementing-instance receiver and to helper-routed state.</p>
+</li>
+<li>
+<p><code>Traits</code> — utility class holding helper-class naming constants
+(<code>$Trait$Helper</code>, <code>$Trait$FieldHelper</code>,
<code>$Trait$StaticFieldHelper</code>),
+trait detection predicates, and self-type collection logic.</p>
+</li>
+<li>
+<p><code>TraitTypeCheckingExtension</code> — integrates traits with the static
type
+checker (GEP-8).</p>
+</li>
+</ul>
+</div>
+<div class="paragraph">
+<p>Public API:</p>
+</div>
+<div class="ulist">
+<ul>
+<li>
+<p><code>groovy.transform.Trait</code> (since 2.3.0)</p>
+</li>
+<li>
+<p><code>groovy.transform.SelfType</code> (since 2.4.0)</p>
+</li>
+</ul>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_representative_jira_issues">Representative JIRA issues</h3>
+<div class="ulist">
+<ul>
+<li>
+<p><a href="https://issues.apache.org/jira/browse/GROOVY-7134";>GROOVY-7134</a>:
+introduce <code>@SelfType</code> for statically checked traits.</p>
+</li>
+<li>
+<p><a
href="https://issues.apache.org/jira/browse/GROOVY-8233";>GROOVY-8233</a>: Java
+stub generation for static properties of traits.</p>
+</li>
+<li>
+<p><a
href="https://issues.apache.org/jira/browse/GROOVY-8299";>GROOVY-8299</a>: native
+default/private/static methods in interfaces (decoupling from trait
+machinery in 5.0).</p>
+</li>
+<li>
+<p><a href="https://issues.apache.org/jira/browse/GROOVY-8219";>GROOVY-8219</a>,
+<a href="https://issues.apache.org/jira/browse/GROOVY-8788";>GROOVY-8788</a>:
+<code>@TupleConstructor</code> interaction with traits and default values.</p>
+</li>
+<li>
+<p><a
href="https://issues.apache.org/jira/browse/GROOVY-8951";>GROOVY-8951</a>: trait
+getter conflicts with generated getter (pre-compiled case).</p>
+</li>
+<li>
+<p><a
href="https://issues.apache.org/jira/browse/GROOVY-11674";>GROOVY-11674</a>:
+deterministic ordering of trait methods for reproducible builds.</p>
+</li>
+<li>
+<p><a
href="https://issues.apache.org/jira/browse/GROOVY-11907";>GROOVY-11907</a>:
+trait field reference transform restructure.</p>
+</li>
+</ul>
+</div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_update_history">Update history</h2>
+<div class="sectionbody">
+<div class="paragraph">
+<p>1 (2026-05-06) Initial draft. Retrospective specification capturing
+trait semantics as shipped in 2.3.0 and refined through 5.0, the
+four-class bytecode model, and the cross-version evolution table.</p>
+</div>
+</div>
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