Sometimes NMT tests failed when executed in parallel with following error:
java.lang.NoClassDefFoundError: jdk/test/lib/JDKToolFinder
at jdk.test.lib.ProcessTools.createJavaProcessBuilder(ProcessTools.java:150)
at jdk.test.lib.ProcessTools.createJavaProcessBuilder(ProcessTools.java:146)
at CommandLineEmptyArgument.main(CommandLineEmptyArgument.java:38)
at sun.reflect.NativeMethodAccessorImpl.invoke0(java.base@9-internal/Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(java.base@9-internal/NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(java.base@9-internal/DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(java.base@9-internal/Method.java:531)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:110)
at java.lang.Thread.run(java.base@9-internal/Thread.java:804)
Caused by: java.lang.ClassNotFoundException: jdk.test.lib.JDKToolFinder
at jdk.internal.loader.BuiltinClassLoader.loadClass(java.base@9-internal/BuiltinClassLoader.java:366)
at jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(java.base@9-internal/ClassLoaders.java:184)
at java.lang.ClassLoader.loadClass(java.base@9-internal/ClassLoader.java:419)
... 9 more

Adding explicit @build statements for test library classes can help to avoid such errors.