Maven compiler plugin configuration should not include source and target as maven.compiler.release is already set

[arend-von-reinersdorff]

Follow-up on #34365.

When the compiler's release argument is set, arguments source and target are not used. No need to keep them around.

See https://docs.oracle.com/en/java/javase/17/docs/specs/man/javac.html#option-release

Note: When using --release, you cannot also use the --source/-source or --target/-target options.

Maven actually allows setting both. But using both directly with javac would result in an error.

[paulvi]

Indeed a073ef8 was not enough,
removing 2 older option in 1f71fb5 is correct

Note that with javac --release option
it is possible to to target even older releases 7,8 (while --release was introduced in JDK 9)

e.g. for JDK 19

  --release <release>
        Compile for the specified Java SE release. Supported releases: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19

I wonder it is possible to target Spring and Spring Boot to JDK 8 via this cross compilation?

But for sure simpler project can use this to target jdk 8 or jdk 11

[paulvi]

And I am sure this switch to --release option should be refrected somewhere in docs
as in real world Spring Boot projects ther eis still mix of using

    <maven.compiler.source>17</maven.compiler.source>
    <maven.compiler.target>17</maven.compiler.target>

or configuration section for maven-compiler-plugin
https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html

I wonder what exact error maven or javac give when there is mix of --target and --release options?

[khmarbaise]

There is also an explicit reference for JDK9+ https://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-release.html

[paulvi]

Maven docs however do not make it clear that there is big difference between --source --target vs --release
as now in Java it became common that APIs can appear and go away, e.g. Nashorn,
so it is imperative to compare against what APIs are actually avaiable for targeted release

https://stackoverflow.com/questions/43102787/what-is-the-release-flag-in-the-java-9-compiler

[paulvi]

I have tried Spring Boot 3.0 with added section like

			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.11.0</version>
				<configuration>
					<source>1.8</source>
					<target>1.8</target>
					<release>17</release>
				</configuration>
			</plugin>

In this case maven-compiler-plugin 3.11.0 just ignores source and target

[INFO] --- maven-compiler-plugin:3.11.0:compile (default-compile) @ ecbrates ---
[INFO] Changes detected - recompiling the module! :source
[INFO] Compiling 10 source files with javac [debug release 17] to target/classes

But when <release>8</release> the error message is misleading:
records are not supported in -source 8

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project ecbrates: Compilation failure
[ERROR] /Users/paulverest/Workspaces/Try/ecbrates/src/main/java/com/currencies/ecbrates/model/Quote.java:[7,8] records are not supported in -source 8
[ERROR]   (use -source 16 or higher to enable records)

That is telling about --source option not about --release;
so of course configuration like

					<source>17</source>
					<target>17</target>
					<release>8</release>

will not make difference.

https://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html page is also misleading
it says there is Default value is: 1.8. for
<source>
But says nothing about default value for
<release>
or that it will override <source> configuration.

---

In short this PR will make --release work, though some developer will come through not so nice hints when running into errors. But that is actually problem for maven-compiler-plugin to solve.

But whatever wording of error messages, the Stackoverflow.com solution will be:

for Spring Boot 3.1+ use

<properties>
		<java.version>17</java.version>

[snicoll]

@arend-von-reinersdorff thank you for making your first contribution to Spring Boot.