https://bz.apache.org/bugzilla/show_bug.cgi?id=62952
--- Comment #1 from Jaikiran Pai <jaiki...@apache.org> --- Hello Javier, It's true that AntClassLoader currently out of the box doesn't support multi-release jar files. The java.util.jar.JarFile documentation[1] has an interesting and useful information which states: <quote> Implementation Note: If the API can not be used to configure a JarFile (e.g. to override the configuration of a compiled application or library), two System properties are available. jdk.util.jar.version can be assigned a value that is the String representation of a non-negative integer <= Runtime.version().major(). The value is used to set the effective runtime version to something other than the default value obtained by evaluating Runtime.version().major(). The effective runtime version is the version that the JarFile(File, boolean, int, Runtime.Version) constructor uses when the value of the last argument is JarFile.runtimeVersion(). jdk.util.jar.enableMultiRelease can be assigned one of the three String values true, false, or force. The value true, the default value, enables multi-release jar file processing. The value false disables multi-release jar processing, ignoring the "Multi-Release" manifest attribute, and the versioned directories in a multi-release jar file if they exist. Furthermore, the method isMultiRelease() returns false. The value force causes the JarFile to be initialized to runtime versioning after construction. It effectively does the same as this code: (new JarFile(File, boolean, int, JarFile.runtimeVersion()). </quote> So I set up the 'jdk.util.jar.enableMultiRelease' system property to 'force' through ANT_OPTS as follows: export ANT_OPTS="-Djdk.util.jar.enableMultiRelease=force" and ran your example: ant -f ant-test.xml on a Java 9 setup and it worked fine and used the Java 9 version of the task. Here's the snippet from the output: run.task: [testTask] Executing Java9+ Task. MRJAR is supported! [echo] ======================================== [echo] Now using the same jar in a jvm process [echo] ======================================== [java] Executing Java9+ Task. MRJAR is supported! Having said that, we can certainly make AntClassLoader support multi-release jar file out of the box and I'll go ahead and commit a change for that so it's available in the next release. [1] https://docs.oracle.com/javase/9/docs/api/java/util/jar/JarFile.html -- You are receiving this mail because: You are the assignee for the bug.