With Java 8 on Fedora 25 and 26, I get for ant test:
[…]
[java] UmlGraphDoc version 5.7.2, altering javadocs
[java] Building Package view for package gr.spinellis.invoice
[java] UMLDoc test: warning - Error!
[java] UMLDoc test: warning - java.lang.Error: generic is not a supported OS platform.
[java] 2 warnings
[java] java.lang.Error: generic is not a supported OS platform.
[java] at java.lang.UNIXProcess$Platform.get(UNIXProcess.java:164)
[java] at java.lang.UNIXProcess.<clinit>(UNIXProcess.java:168)
[java] at java.lang.ProcessImpl.start(ProcessImpl.java:130)
[java] at java.lang.ProcessBuilder.start(ProcessBuilder.java:1029)
[java] at java.lang.Runtime.exec(Runtime.java:620)
[java] at java.lang.Runtime.exec(Runtime.java:485)
[java] at org.umlgraph.doclet.UmlGraphDoc.runGraphviz(UmlGraphDoc.java:144)
[java] at org.umlgraph.doclet.UmlGraphDoc.generatePackageDiagrams(UmlGraphDoc.java:99)
[java] at org.umlgraph.doclet.UmlGraphDoc.start(UmlGraphDoc.java:67)
[java] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[java] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[java] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[java] at java.lang.reflect.Method.invoke(Method.java:498)
[java] at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:310)
[java] at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:189)
[java] at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:366)
[java] at com.sun.tools.javadoc.Start.begin(Start.java:219)
[java] at com.sun.tools.javadoc.Start.begin(Start.java:205)
[java] at com.sun.tools.javadoc.Main.execute(Main.java:162)
[java] at org.umlgraph.test.UmlDocTest.runDoclet(UmlDocTest.java:155)
[java] at org.umlgraph.test.UmlDocTest.runTest(UmlDocTest.java:77)
[java] at org.umlgraph.test.UmlDocTest.main(UmlDocTest.java:57)
[java] Exception in thread "main" java.io.FileNotFoundException: /home/builder/fedora-scm/umlgraph/UMLGraph-R5_7_2/testdata/umldoc-out/gr/spinellis/invoice/Customer.dot (No such file or directory)
[java] at java.io.FileInputStream.open0(Native Method)
[java] at java.io.FileInputStream.open(FileInputStream.java:195)
[java] at java.io.FileInputStream.<init>(FileInputStream.java:138)
[java] at java.io.FileReader.<init>(FileReader.java:72)
[java] at org.umlgraph.test.DotDiff.readGraphLines(DotDiff.java:130)
[java] at org.umlgraph.test.DotDiff.<init>(DotDiff.java:58)
[java] at org.umlgraph.test.TestUtils.dotFilesEqual(TestUtils.java:99)
[java] at org.umlgraph.test.UmlDocTest.compareDocletOutputs(UmlDocTest.java:120)
[java] at org.umlgraph.test.UmlDocTest.compareDocletOutputs(UmlDocTest.java:118)
[java] at org.umlgraph.test.UmlDocTest.compareDocletOutputs(UmlDocTest.java:118)
[java] at org.umlgraph.test.UmlDocTest.compareDocletOutputs(UmlDocTest.java:118)
[java] at org.umlgraph.test.UmlDocTest.runTest(UmlDocTest.java:79)
[java] at org.umlgraph.test.UmlDocTest.main(UmlDocTest.java:57)
[java] Java Result: 1
With Java 8 on Fedora 25 and 26, I get for ant test: