Paul King created GROOVY-11165:
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Summary: JEP 445 compatibility additional functionality
Key: GROOVY-11165
URL: https://issues.apache.org/jira/browse/GROOVY-11165
Project: Groovy
Issue Type: New Feature
Reporter: Paul King
Assignee: Paul King
Fix For: 5.0.0-alpha-1
[JEP 445: Unnamed Classes and Instance Main
Methods|https://openjdk.org/jeps/445] provides features that improve using Java
for scripting purposes. While these features offer a subset of the features
offered by Groovy for scripting, it would be nice to offer some interworking
for cut-n-paste compatibility and leveraging the updated launch protocol.
(i) This description will contain the most update-to-date proposal for the
design but may undergo change during discussions. If needed, a separate GEP
section in the wiki will capture the final design.
h2. Scripting semantics
||Source file characteristics||Semantics determined||
|Contains class declaration|Compiled as a class|
|Contains uncontained non-declaration statements or a no-arg instance {{run}}
method|Compiled as a Groovy script|
|Contains an instance main method, and possibly field and other method
declarations|Compiled as a JEP 445 compatible class|
|Contains a static main method, and possibly field and other method
declarations|Compiled as a JEP 445 compatible class but method signature
"promoted" to public static void main with String[] argument|
Groovy scripts are wrapped into an encompassing class that extends
{{{}groovy.lang.Script{}}}.
Such classes have a {{public static void main}} added which (summarising)
creates a class instance and calls its {{run}} method.
Uncontained statements within the script can be thought of as belonging within
the {{run}} method.
Declaration statements within the script are treated as local variable
definitions (also within the {{run}} method).
Field definitions can be obtained by annotating declaration statements with the
{{@Field}} annotation.
JEP 445 scripts do not extend {{Script}} and do not have any methods added.
Declaration statements within the script are treated as field definitions.
h2. Script runner
Groovy's runner mechanism will be extended to follow a similar protocol to
Java's revised launch protocol. This should be available for JDK11+ and a
JEP-445 compatible class could be invoked using JDK21+ with preview enabled
(with Groovy on the classpath unless a POJO class is created).
h2. Breaking changes
* Previously, if a script had a single instance {{main}} method not taking a
{{String[]}} parameter and no loose statements, that was previously accepted as
a normal Groovy script (and nothing would execute). Now if the method has an
Object parameter, that is treated as a JEP-445 script and its contents executed.
* Previously, if a script had a static main method and no other loose
statements, the main method's contents were moved into the run method and the
script class extended Script. Now, creating a no-arg {{run}} method is
supported for this scenario. If a static {{main}} method is created, it is
treated as a JEP 445 compatible script.
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