Document that spring-boot-starter-parent sets maven.compiler.release and how to unset it if needed

[brucelwl]

springboot 3.2.0-RC1

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <configuration>
        <compilerArgs>
            <arg>--add-exports=java.base/jdk.internal.misc=ALL-UNNAMED</arg>
            <arg>--add-exports=java.base/jdk.internal.vm=ALL-UNNAMED</arg>
        </compilerArgs>
    </configuration>
</plugin>

the default property <maven.compiler.release>${java.version}</maven.compiler.release>

cause error

exporting a package from system module java.base is not allowed with --release

see

source plexus-compiler-javac-2.13.0.jar

[wilkinsona]

Thanks for trying to RC. Have you only just started seeing this problem? We started setting maven.compiler.release in 3.1.

[brucelwl]

I previously discovered this PR #34365, but it still caused my program to not compile properly, and currently I have not found a corresponding solution

[wilkinsona]

Yes, I suspect that #34365 introduced the problem. However, if that is the case, this would also fail with Spring Boot 3.1. Does it?

[brucelwl]

@wilkinsona I tried using Spring Boot version 3.1 and found the same problem

[wilkinsona]

Thanks.

You can undo spring-boot-starter-parent setting maven.compiler.release by setting it to an empty string:

<maven.compiler.release></maven.compiler.release>

You then have to configure the source and target instead:

<maven.compiler.source>${java.version}</maven.compiler.source>
<maven.compiler.target>${java.version}</maven.compiler.target>

With these three properties in place, a project that uses --add-exports should work.

We'll have to decide if we want to continue to set maven.compiler.release or switch back to source and target.

[brucelwl]

Thank you very much. <maven.compiler.release/> has solved my problem

[mhalbritter]

We'll have to decide if we want to continue to set maven.compiler.release or switch back to source and target.

Upside of release is that the compiler then checks if you're using an API from a newer JDK, which is a definitive pro. Do we know why the JDK developers prohibit opening modules when using --release but allowing it when using --target and --source?

[wilkinsona]

Upside of release is that the compiler then checks if you're using an API from a newer JDK, which is a definitive pro

It's definitely a good thing if you're building with a JDK version other than the one with which you'll run your app. However, if the two match, that benefit disappears and I wonder how common it is for them not to match.

Do we know why the JDK developers prohibit opening modules when using --release but allowing it when using --target and --source?

I don't.

[wilkinsona]

We're going to document that we set maven.compiler.release by default and how and when to unset it. The JEP describes when. It's if you want to use --add-exports, --add-reads, or --patch-module to modify system modules.