"release" parameter should only be set for javac version >= 9

[kwin]

Sometimes it is difficult to set up the plugin configuration in a way to support different JDKs (maybe even using toolchains). For example look at MPOM-264. To ease that the JavacCompiler should only use the -release parameter with version 9 or above as using it with earlier versions leads to errors.

An additional condition should be added to

plexus-compiler/plexus-compilers/plexus-compiler-javac/src/main/java/org/codehaus/plexus/compiler/javac/JavacCompiler.java

Line 344 in 0a0aab0

if ( !StringUtils.isEmpty( config.getReleaseVersion() ) )

[rfscholte]

This should be done at maven-compiler-plugin level, to ensure that every compiler implementation gets the same set of arguments. The more I think about it, the more it looks like https://issues.apache.org/jira/browse/MCOMPILER-270 from almost 5 years ago.
Reading the conclusion I still agree with myself: using the release flag should be a choice of the project, it should not be calculated as there are situation where you don't want to use the release flag. As long as the compiler supports source/target it must be possible to pass it as is.

[kwin]

IMHO plexus-compiler and m-compiler-p is a higher level abstraction and therefore should try to translate the intent of the parameter into the compiler specific flags. Release is clearly a parameter for javac >= 9 but does mot make sense for earlier versions nor for some other compilers.

[slachiewicz]

a lot of code in JavacCompiler related to Java < 8 can be deleted

[rfscholte]

a lot of code in JavacCompiler related to Java < 8 can be deleted

I don't think so. Keep in mind that you can still use toolchain, and best practice is to use the targeted JDK to compile.

[kwin]

There is already

plexus-compiler/plexus-compilers/plexus-compiler-javac/src/main/java/org/codehaus/plexus/compiler/javac/JavacCompiler.java

Line 370 in 0a0aab0

else if ( !suppressSource( config ) )

which implements the same logic for parameter -source (only supported in javac 1.4 or newer) or

plexus-compiler/plexus-compilers/plexus-compiler-javac/src/main/java/org/codehaus/plexus/compiler/javac/JavacCompiler.java

Line 321 in 0a0aab0

if ( !isPreJava18(config) && config.isParameters() )

for parameter -parameters.