If you run a java command in process using <java ... failonerror="false"> and do
not provide permissions, then the tasks never defines a perm object. 
Subsequently, Permissions$SM is not installed and if the java program invokes
System.exit, it kills the VM instead of being cought and resulting in an
ExitException being tossed out of the task execute.

However, when failonerror is set to true, if perm==null, a new default
Permissions object is defined.  The side effect of doing this is that the
Permissions object installs a security manager that traps RuntimePermission
exitVM checks ant tosses ExitExceptions.  So if the java program calls
System.exit it doesn't kill the VM. 

In short, you don't get protection against a VM exit when you don't declare
failonerror="true".   Since these two concepts are completely distinct, the task
should be updated to ensure that the the security manager is either always
installed or never installed regardless of the failonerror value.

If the choice is to always define a default Permissions when perm is null, then
Java.executejava could explicitly trap the ExitException and rethrow it if
failonerror=true and the status code is non-zero or smother it otherwise.

A build file snippet compares the behavior with and without failonerror:
<project ...>
   <!-- make a TryToExit class that just calls System.exit(1); in it's main:
 public class TryToExit { public static void main(String[] args) {
System.exit(1); } }
   -->

   <target name="invoke1">
      <echo message="About to invoke with failonerror=false"/>
      <java classname="TryToExit"/> <!-- VM exit will cause process to abort
without ant's consent -->
      <echo message="TryToExit returned"/>
   </target>

   <target name="invoke2">
      <echo message="About to invoke with failonerror=true"/>
      <java classname="TryToExit"/> <!-- VM exit will cause process to abort
without ant's consent -->
      <echo message="TryToExit returned"/>
   </target>
</project>

ant invoke1 results in output of:

---- ant invoke1 output ---
Using default logger
Buildfile: build.xml

invoke1:
     [echo] About to invoke with failonerror=false
---------------------------


---- ant invoke2 output ---
Using default logger
Buildfile: build.xml

invoke2:
     [echo] About to invoke with failonerror=true

BUILD FAILED
d:\temp\build.xml:14: Java returned: 1
---------------------------