Add Java 12 support

[fbricon]

Signed-off-by: Fred Bricon fbricon@gmail.com

[fbricon]

So far, all kind of projects support Java 12, with some caveats. Experimental features (like advanced switch expressions - JEP-325) may need specific tuning:

• default/invisible projects have features enabled features by default, without warnings

• Maven projects need to have the --enable-preview added to the maven-compiler-plugin configuration, in pom.xml. See https://bugs.eclipse.org/bugs/show_bug.cgi?id=545462#c0, warnings are disabled automatically.

• Eclipse projects need to add the the following preferences to .settings/org.eclipse.jdt.core.prefs:

  org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=enabled
  org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=ignore
  

• Gradle projects can also maintain the same .settings/org.eclipse.jdt.core.prefs. Alternatively adding an eclipse.jdt.file.withProperties hook is possible, but requires the gradle compileJdt task to be invoked manually, as buildship doesn't use those hooks :

plugins {
    // Apply the java-library plugin to add support for Java Library
    id 'java-library'
    id 'eclipse'
}
sourceCompatibility = JavaVersion.VERSION_12
targetCompatibility = JavaVersion.VERSION_12

repositories {
     jcenter()
}
compileJava {
    options.compilerArgs += ["--enable-preview"]
}
compileTestJava {
    options.compilerArgs += ["--enable-preview"]
}
//Buildship doesn't use that hooks (https://discuss.gradle.org/t/when-does-buildship-eclipse-customization-run/20781/2)
//you need to run `gradle eclipseJdt` separately
eclipse.jdt.file.withProperties { props ->
    props['org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures']= 'enabled'
    props['org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures']= 'ignore'
}

test {
    jvmArgs '--enable-preview'
}

[fbricon]

test this please