JavaDowngrader

Standalone program and library which can downgrade Java classes/programs down to Java 8.

To use JavaDowngrader as a library in your application, check out the Usage (As a library) section.
If you just want to downgrade .jar files you can check out the Usage (Standalone) section.

Features

Supports up to Java 21 and down to Java 8
Downgrades language features
Downgrades important Java API calls
Can be applied during runtime
Can downgrade .jar files

Releases

Executable Jar File

If you want the executable jar file you can download a stable release from GitHub or the latest dev version from this Jenkins.

Gradle/Maven

To use JavaDowngrader with Gradle/Maven you can use this Maven server or Jitpack.
You can also find instructions how to implement it into your build script there.

Usage (Standalone/Bootstrap)

Download the latest version from the Releases section
Run the jar file with java -jar JavaDowngrader-whateverversion.jar to see the usage

Downgrade ahead of time

Here is an example command to convert the jar input.jar to Java 8 and output it to output.jar: java -jar JavaDowngrader-Standalone-whateverversion.jar -i "input.jar" -o "output.jar" -v 8

Downgrade during runtime

Here is an example command to run a jar file and downgrade it during runtime by using the bootstrap java agent: java -javaagent:JavaDowngrader-Bootstrap-whateverversion.jar -jar the_real_jar_you_want_to_run.jar

Additionally, you can set the -DspoofJavaVersion=<java version> property to bypass application specific checks for the Java version.

Usage (As a library)

To transform a ClassNode you can use the JavaDowngrader class.
As a low level class modification framework in your application ClassTransform is recommended. JavaDowngrader provides the impl-classtransform submodule which contains various utility classes for ClassTransform.

Usage (In Gradle)

To use JavaDowngrader in gradle (To downgrade a whole jar or one of your source sets) you have to add the following to the top of your build.gradle:

buildscript {
    repositories {
        maven {
            name = "Lenni0451 Releases"
            url "https://maven.lenni0451.net/releases"
        }
    }

    dependencies {
        classpath "net.raphimc.javadowngrader:gradle-plugin:1.0.0"
    }
}

Downgrade the main source set

tasks.register("java8Main", DowngradeSourceSetTask) {
    sourceSet = sourceSets.main
}.get().dependsOn("classes")
classes.finalizedBy("java8Main")

Downgrade the built jar (If you use Java 8+ libraries)

tasks.register("java8Jar", DowngradeJarTask) {
    input = tasks.jar.archiveFile.get().asFile
    outputSuffix = "+java8"
    compileClassPath = sourceSets.main.compileClasspath
}.get().dependsOn("build")
build.finalizedBy("java8Jar")

Some of the optional properties include:

targetVersion: The target classfile version (Default: 8)
outputSuffix: The suffix to append to the output jar file (Default: "-downgraded")
copyRuntimeClasses: Whether to copy the JavaDowngrader runtime classes to the output jar (Default: true). Should be set to false if your jar already contains JavaDowngrader itself

Contact

If you encounter any issues, please report them on the issue tracker.
If you just want to talk or need help using JavaDowngrader feel free to join my Discord.

ayunami2000 / JavaDowngrader