Doesn't compile on Java 8 about secon-tool HOT 4 CLOSED

That's intentional. For building the JARs, a JDK >= 9 is required. At runtime, only a JRE >= 1.8 is required. You can download ready-made JARs and distributions from here: https://github.com/DieTechniker/kks-encryption/actions

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The workaround doesn't figure the use of Java API > 1.8, that's why the options.release.set(8) is preferable.

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If source level needs to be Java >= 9 it's fair enough if it works for Java 8 at runtime

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No worries, source and target level are indeed set to 1.8. The build just needs a JDK >= 9 to set the "--release 1.8" switch, which ensures that the boot classpath matches 1.8, too. Otherwise, you could run the build with the latest JDK and use it's JRE API, which is to be avoided.

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