Support Java 12 preview features

[fwgreen]

So far, Java 12 has two new language additions:

• Switch Expressions JEP 325
• Raw String Literals JEP 326

Please consider adding support for those of us testing early access builds.

[fbricon]

Unfortunately we can't provide support for new Java versions until very late in the development process, often only after GA, because it depends on the availability of full JDT builds.
We can't consume their earlier patch features in our builds, for technical reasons. Unless someone figures out how to get Tycho to install feature patches.

[fbricon]

Fixed with eclipse-jdtls/eclipse.jdt.ls#970

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]

For anyone trying to run Java 12 code using preview features, with the vscode-java-debug extension, make sure you enable the --enable-preview flag in the launch configuration's vmArgs:

{
    "configurations": [
        {
            "type": "java",
            "name": "CodeLens (Launch) - Demo",
            "request": "launch",
            "mainClass": "foo.bar.Demo",
            "projectName": "Demo",
            "vmArgs": "--enable-preview"
        }
    ]
}

[fbricon]

Similarly, running Unit Tests with vscode-java-test requires setting the --enable-preview flag in the test configuration, in settings.json:

    "java.test.config": [
        {
            "name": "java12",
            "workingDirectory": "${workspaceFolder}",
            "vmargs": [ "--enable-preview" ],
        },
    ],
    "java.test.defaultConfig": "java12"

[stevenzeck]

@fbricon what do we have to do to enable this? Keep receiving the preview features not available error when running in vscode

[fbricon]

Can you be more specific?

• Do you use vscode-java 0.42.1?
• What kind of java code are you looking at? standalone java files? maven, gradle projects?
• is this for actually executing the java code or tests, or compiling?
• what JDK are you using? If executing your code Java 12, did you configure your launch configuration as per Support Java 12 preview features #671 (comment)?

have you tried cleaning up the Java workspace directory?

[stevenzeck]

• 0.42.1 yes
• Standalone Java file
• Executing java code (just a simple hello world)
• JDK 12. Is it necessary to have a launch config? It was working fine until very recently without one

[fbricon]

If you run with a JDK 12, then standalone java files have preview features enabled by default, which means they need to run with the --enable-features flag, which you need to add to a launch configuration for now. Once microsoft/vscode-java-debug#553 is fixed, it should be completely transparent again.

If you don't want to change that flag, you need to downgrade the JDK used to launch the Java language server

[stevenzeck]

Weird, I've been using JDK 12 for quite some time without this issue