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This exception occurs when using an older version of java. Mainly when trying to run a jar compiled with a newer Java version in an older Java version. We can avoid this issue by executing the jar in the same version which it was compiled.
This might be caused by and outdated version of Java, SonarQube now uses Java 8 as default. You might upgrade java or use an older version of SonarQube.
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java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)