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Check out https://www.codejava.net/ides/eclipse/change-java-compiler-version-for-eclipse-project. Apply these changes for the whole of the IDE, not just the selected project.
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Right-click the current project, select your builder (Ant, probably), select Edit, go to the JRE tab, then, in the top box, select Seperate JRE:, then
java-8-openjdk
as you have set up beforehand. -
Save your changes and re-compile!
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You can check your changes by doing the following: (Check for Java 1.8.0)
unzip -p foo.jar META-INF/MANIFEST.MF