Vscode-java: The java builder should use the correct compiler compliance to build the project

[testforstephen]

Environment

• Operating System: MacOS and Windows 10
• JDK version: any
• Visual Studio Code version: 1.41.x or 1.42.0-insider
• Java extension version: 0.57.0

Steps To Reproduce

1. Have java.home to point to JDK 13, and launch the language server.
2. Have Java 8 installed in the machine.
3. Configure multiple JDKs in the user setting.

    "java.configuration.runtimes": [
        {
            "name": "JavaSE-13",
            "path": "C:\\Program Files\\Java\\jdk-13.0.1"
        },
        {
            "name": "JavaSE-1.8",
            "path": "C:\\Program Files\\AdoptOpenJDK\\jdk-8.0.242.08-hotspot"
        }
    ]

1. Open an eclipse Java 8 project 26.environmentVariables.zip
   .
   

Current Result:

See the screenshot above, the PROBLEMS viewlet reports a warning on the project: "The compiler compliance specified is 13 but a JRE 1.8 is used". This means the classfile built by the ls is targeting to java 13, that would make it not able to be launched in JDK 8.

Besides, the debugger has fixed a bug https://github.com/microsoft/vscode-java-debug/issues/753 about using the project's Java runtime to launch the application. The latest debugger would fail at this case.

Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.UnsupportedClassVersionError: EnvrionmentVariable has been compiled by a more recent version of the Java Runtime (class file version 57.0), this version of the Java Runtime only recognizes class file versions up to 52.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:757)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
        at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:419)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:352)
        at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)

Expected

The language server should generate the compatible .class files with the project's Java Runtime.

// @fbricon @snjeza

[snjeza]

@testforstephen

Have java.home to point to JDK 13, and launch the language server.

That causes Java LS to use the workspace Compiler Compliance level 13.
Your project inherits it and reports the warning.
You should configure your project by adding 26.environmentVariables/.settings/org.eclipse.jdt.core.prefs with following contents:

eclipse.preferences.version=1
org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.8
org.eclipse.jdt.core.compiler.compliance=1.8
org.eclipse.jdt.core.compiler.release=disabled
org.eclipse.jdt.core.compiler.source=1.8

or use the following settings:

 {
      "name": "JavaSE-1.8",
      "path": "C:\\Program Files\\AdoptOpenJDK\\jdk-8.0.242.08-hotspot",
      "default": true
}

[testforstephen]

@snjeza Most of users don't have knowledge about the .settings. If project compiler compliance setting is undefined, how about auto assign a compiler compliance compatible with the project JRE_CONTAINER during eclipse project importer?

[snjeza]

@testforstephen I agree. @fbricon What do you think?

[marlonpatrick]

I had the same problem when not using the properties:

   <properties>
     <maven.compiler.source> XX </maven.compiler.source>
     <maven.compiler.target> XX </maven.compiler.target>
   </properties>

As I'm experimenting with Java 14 the way to configure the java version in maven has changed to:

<plugin>
  <artifactId> maven-compiler-plugin </artifactId>
  <configuration>
    <release>14</release>
  </configuration>
</plugin>

Thus, from the point of view of maven, it is no longer necessary to use maven.compiler.target/source, however, for VS Code it is still necessary to specify these properties so that it correctly interprets the version of Java used in the project.

Then, I solve my scenario using this two settings above.

[lincolnthree]

Well hello there :)

It does seem that VSCode-Java stuggles to properly detect and apply maven compiler settings from maven's pom.xml.

I've tried a number of configurations, and no matter what I do, the Java version defaults to 1.6 in org.eclipse.jdt.core.prefs. It can, however be manually configured, as pointed out earlier in this thread.

This matter is made worse because the vscode-java plugin will overwrite manually configured settings back to 1.6 when updating project configurations, making this supremely disruptive.

I also had to to set configurations manually in the .classpath file manually:

    <classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.8">
        <attributes>
            <attribute name="maven.pomderived" value="true"/>
        </attributes>
    </classpathentry>

[snjeza]

@lincolnthree could you attach your project or pom.xml?

[lincolnthree]

Update. Looks like adding a newer version of the maven-compiler-plugin and setting the following configuration has gotten things mostly working:

            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.8.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>

[lincolnthree]

The broken version was maven-compiler-plugin:3.2.0

[lincolnthree]

Perhaps a warning could be issued if using an obsolete/incompatible version of the maven-compiler-plugin.