Jabel - Use modern Java 9-25 syntax when targeting Java 8

Because life is too short to wait for your users to upgrade their Java!

Motivation

While Java is evolving and introduces new language features, the majority of OSS libraries are still using Java 8 as their target because it still dominates.

But, since most of features after Java 8 did not require a change in the bytecode, javac could emit Java 8 bytecode even when compiling Java 12 sources.

How Jabel works

Although Jabel is a javac compiler plugin, it does not run any processing, but instruments the java compiler classes and makes it treat some new Java 9+ languages features as they were supported in Java 8.

The result is a valid Java 8 bytecode for your switch expressions, var declarations, and other features unavailable in Java 8.

Why it works

The JVM has evolved a lot for the past years. However, most language features that were added are simply a syntatic sugar.
They do not require new bytecode, hence can be compiled to the Java 8.

But, since the Java language was always bound to the JVM development, new language features require the same target as the JVM because they get released altogether.

As was previously described, Jabel makes the compiler think that certain features were developed for Java 8, and removes the checks that otherwise will report them as invalid for the target.

It is important to understand that it will use the same desugaring code as for Java 9+ but won't change the result's classfile version, because the compilation phase will be done with Java 8 target.

Limitations

Please remember that Jabel is not magic! It does its best to adapt the syntax transformation, according to the currently used JDK.
So yes, in specific cases, the syntax transformation is more efficient than the compiler's, but most of the time, it's not as efficient as JDK+JVM implementation.

Jabel has been tested with Oracle JDKs, on versions 8, 9, 12, 16, 17, 19, 21 and 25.
In the vast majority of cases, everything works correctly. However, if you encounter a problem with another version or JDK, feel free to open an issue.

How to use

Gradle

Use the following snippet to add Jabel to your Gradle build:

repositories {
    maven { url "https://maven.xpdustry.com/releases" }
}

dependencies {
    annotationProcessor "com.xpdustry:jabel:1.1.0"
}

compileJava {
    sourceCompatibility = 16 // for the IDE support
    options.release = 8

    javaCompiler = javaToolchains.compilerFor {
        languageVersion = JavaLanguageVersion.of(16)
    }
    
    // Can be omitted on Java 14 and higher
    options.compilerArgs += "-Xplugin:jabel"
}

Short version:

repositories { maven { url "https://maven.xpdustry.com/releases" } }
dependencies.annotationProcessor "com.xpdustry:jabel:1.1.0"
compileJava.options.release = 8

You can also force your tests to run with Java 8:

compileTestJava {
    sourceCompatibility = targetCompatibility = 8
}

test {
    javaLauncher = javaToolchains.launcherFor {
        languageVersion = JavaLanguageVersion.of(8)
    }
}

Compile your project and verify that the result is still a valid Java 8 bytecode (52.0):

$ ./gradlew --no-daemon clean :examples:run

> Task :examples:run
=== Jabel Feature Examples ===

--- Java 9 Features ---
Diamond with anonymous: [TEST]
Private interface method: Hello World!

--- Java 10 Features ---
item
item0
item1
item2
var keyword works!

--- Java 11 Features ---
var in lambda: helloworld

--- Java 14 Features ---
Switch expression: weekend
Switch with yield: 2

--- Java 15 Features ---
Text block: Hello\nWorld\n

--- Java 16 Features ---
5
HELLO
Record: Point[x=3,y=4]

--- Java 17 Features ---
sealed classes works!
strictfp method: 9.668071586715866

--- Java 21 Features ---
case null: null
pattern switch: String: test
record pattern: 12
switch yield: 10

--- Java 22 Features ---
Unnamed variables work!

--- Java 25 Features ---
ComplexPrologue: value=20
ManyLocals: value=14 label=10/4 ratio=2.5
SpoofChild: value=99 spoofed=false
Implicit classes work!

=== All features work! ===

BUILD SUCCESSFUL in 32s
7 actionable tasks: 7 executed