jsr166 runs jtreg via its ant task.
http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/build.xml?view=markup
The key part is this:
    <delete dir="@{workdir}/JTwork" quiet="true"/>
    <delete dir="@{workdir}/JTreport" quiet="true"/>
    <mkdir dir="@{workdir}/JTwork/scratch"/>
    <mkdir dir="@{workdir}/JTreport"/>
    <jtreg dir="${jtreg@{target}.src.dir}"

After upgrading to 4.1-b12 I started seeing:
$ ant jtreg
...
    [jtreg] Test results: passed: 199
    [jtreg] Error while writing report: java.io.FileNotFoundException: /home/martin/jsr166/jtreg-4.2/build/JTreport/text/timeStats.txt (No such file or directory)
    [jtreg] Results written to /home/martin/jsr166/jtreg-4.2/build/JTwork

BUILD SUCCESSFUL

I expect the text subdirectory to be created automatically, and I also expect the build to fail if I get an I/O error.

If I create the directory before running jtreg

     <mkdir dir="@{workdir}/JTreport"/>
+ <mkdir dir="@{workdir}/JTreport/text"/>
     <jtreg dir="${jtreg@{target}.src.dir}"

I get the unexpected

BUILD FAILED
/home/martin/jsr166/jtreg-4.2/build.xml:527: The following error occurred while executing this line:
/home/martin/jsr166/jtreg-4.2/build.xml:318: java.lang.NullPointerException
at com.sun.javatest.regtest.RegressionReporter.fixupReportFiles(RegressionReporter.java:264)
at com.sun.javatest.regtest.RegressionReporter.fixupReports(RegressionReporter.java:248)
at com.sun.javatest.regtest.RegressionReporter.report(RegressionReporter.java:107)
at com.sun.javatest.regtest.Main.batchHarness(Main.java:1899)
at com.sun.javatest.regtest.Main.run(Main.java:1264)
at com.sun.javatest.regtest.Main.access$4900(Main.java:99)
at com.sun.javatest.regtest.Main$Ant.execute(Main.java:886)
at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
at org.apache.tools.ant.Task.perform(Task.java:348)
at org.apache.tools.ant.taskdefs.Sequential.execute(Sequential.java:68)
at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
at org.apache.tools.ant.Task.perform(Task.java:348)
at org.apache.tools.ant.taskdefs.MacroInstance.execute(MacroInstance.java:396)
at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:292)
at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
at org.apache.tools.ant.Task.perform(Task.java:348)
at org.apache.tools.ant.Target.execute(Target.java:435)
at org.apache.tools.ant.Target.performTasks(Target.java:456)
at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1393)
at org.apache.tools.ant.Project.executeTarget(Project.java:1364)
at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
at org.apache.tools.ant.Project.executeTargets(Project.java:1248)
at org.apache.tools.ant.Main.runBuild(Main.java:851)
at org.apache.tools.ant.Main.startAnt(Main.java:235)
at org.apache.tools.ant.launch.Launcher.run(Launcher.java:280)
at org.apache.tools.ant.launch.Launcher.main(Launcher.java:109)


If I go further and create both html and text subdirectories,

    <mkdir dir="@{workdir}/JTreport/html"/>
    <mkdir dir="@{workdir}/JTreport/text"/>
    <jtreg dir="${jtreg@{target}.src.dir}"

I succeed! So that's a pretty good workaround, but please:

- create all needed directories automatically, on demand
- If there is an I/O error, then make sure to fail the build
- For most I/O errors, we should have a meaningful error on stderr - no stack trace unless it's a jtreg bug