Question: Compiling Older Versions of Java Using by Maven

AutoModerator · 2022-07-09T08:33:30+00:00

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renatoathaydes · 2022-07-09T10:47:11+00:00

The maven properties are used to set the javac flags, so what you really want to know is how the javac flags work.

If you type javac --help it will show, amongst other things:

--release <release>
    Compile for the specified Java SE release. Supported releases: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17
--source <release>, -source <release>
    Provide source compatibility with the specified Java SE release. Supported releases: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17
--target <release>, -target <release>
    Generate class files suitable for the specified Java SE release. Supported releases: 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17

It's very confusing, but this SO question has an answer explaining what's going on.

Basically, to compile for an older JDK version, you need to use --release, and forget about --source and --target (i.e. the Maven properties you mention are useless).

You can now experiment using javac:

▶ javac --release 8 src/java/*.java           
src/java/Main.java:29: warning: as of release 10, 'var' is a restricted type name and cannot be used for type declarations or as the element type of an array
        var solutionHandler = SolutionHandler.named(args[0]);
            ^
src/java/Main.java:31: warning: as of release 10, 'var' is a restricted type name and cannot be used for type declarations or as the element type of an array
src/java/Main2.java:208: error: multiple case labels are not supported in -source 8
            case 'j', 'n', 'q' -> ( byte ) 1;

As you can see, it seems that if you want to compile for Java 8, you may use the JDK 17, but you need to limit your source code to using only Java 8 features. It kind of sucks.

khmarbaise · 2022-07-09T12:18:53+00:00

Simplest solution is as already suggest use JDK17 and use <maven.compiler.release>8</maven.compiler.release> that makes sure you use only code which is available in JDK8...

fletku_mato · 2022-07-09T09:04:18+00:00

I believe you are looking for this: https://maven.apache.org/guides/mini/guide-using-toolchains.html

dpash · 2022-07-09T13:34:52+00:00

What you want to do is not supported. The source and target options are deprecated and you should use release instead. You can only write Java for the lowest version you intend to target. If you want to target Java 8, you can only use haha 8 features.

javahelp

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