com.thoughtworks.xstream.converters.enums.EnumSetConverter, at its line 31 (rev. 538) uses reflection to address field elementType in class java.util.EnumSet. That field is package private and is not covered by the Java API, and thus may be missing in implementations other than Sun's. Thus, addressing this field limits the XStream's capability to run on Apache Harmony and other implementations lacking that Sun-implementation-specific field. For example, this issue interferes with Apache Geronimo Unit Tests running on Apache Harmony. The stack looks as follows: com.thoughtworks.xstream.XStream$InitializationException: Could not instatiate converter : com.thoughtworks.xstream.converters.enums.EnumSetConverter : null at com.thoughtworks.xstream.XStream.dynamicallyRegisterConverter(XStream.java:395) at com.thoughtworks.xstream.XStream.setupConverters(XStream.java:378) at com.thoughtworks.xstream.XStream.<init>(XStream.java:196) at com.thoughtworks.xstream.XStream.<init>(XStream.java:181) at com.thoughtworks.xstream.XStream.<init>(XStream.java:156) at org.apache.geronimo.timer.jdbc.JDBCWorkerPersistence.serialize(JDBCWorkerPersistence.java:305) at org.apache.geronimo.timer.jdbc.JDBCWorkerPersistence.save(JDBCWorkerPersistence.java:118) at org.apache.geronimo.timer.jdbc.JDBCWorkerPersistenceTestAbstract.testSaveCancel(JDBCWorkerPersistenceTestAbstract.java:66) at java.lang.reflect.VMReflection.invokeMethod(VMReflection.java) at java.lang.reflect.Method.invoke(Method.java:317) at junit.framework.TestCase.runTest(TestCase.java:154) at junit.framework.TestCase.runBare(TestCase.java:127) at junit.framework.TestResult$1.protect(TestResult.java:106) at junit.framework.TestResult.runProtected(TestResult.java:124) at junit.framework.TestResult.run(TestResult.java:109) at junit.framework.TestCase.run(TestCase.java:118) at junit.framework.TestSuite.runTest(TestSuite.java:208) at junit.framework.TestSuite.run(TestSuite.java:203) at java.lang.reflect.VMReflection.invokeMethod(VMReflection.java) at java.lang.reflect.Method.invoke(Method.java:317) at org.apache.maven.surefire.junit.JUnitTestSet.execute(JUnitTestSet.java:210) at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.executeTestSet(AbstractDirectoryTestSuite.java:135) at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.execute(AbstractDirectoryTestSuite.java:122) at org.apache.maven.surefire.Surefire.run(Surefire.java:129) at java.lang.reflect.VMReflection.invokeMethod(VMReflection.java) at java.lang.reflect.Method.invoke(Method.java:317) at org.apache.maven.surefire.booter.SurefireBooter.runSuitesInProcess(SurefireBooter.java:225) at org.apache.maven.surefire.booter.SurefireBooter.main(SurefireBooter.java:747) Caused by: java.lang.reflect.InvocationTargetException at java.lang.reflect.VMReflection.newClassInstance(VMReflection.java) at java.lang.reflect.Constructor.newInstance(Constructor.java:283) at com.thoughtworks.xstream.XStream.dynamicallyRegisterConverter(XStream.java:392) at com.thoughtworks.xstream.XStream.setupConverters(XStream.java:378) ... 26 more Caused by: java.lang.RuntimeException: Could not access java.util.EnumSet.elementType field at com.thoughtworks.xstream.core.util.Fields.find(Fields.java:18) at com.thoughtworks.xstream.converters.enums.EnumSetConverter.<init>(EnumSetConverter.java:31) at java.lang.reflect.VMReflection.newClassInstance(VMReflection.java) ... 29 more