从另一个 jar 实现 class 的 class 的 ClassCastException

Question

I'm getting the below exception when attempting to run the class org.apache.tools.ant.launch.Launcher which comes as part of the ant launcher jar :

testTask: [java] Working directory ignored when same JVM is used.
[java] Unable to locate tools.jar. Expected to find it in C:\Program Files\Java\jre1.8.0_141\lib\tools.jar
[java] java.lang.ClassCastException: org.apache.tools.ant.Main cannot be cast to org.apache.tools.ant.launch.AntMain
[java] at org.apache.tools.ant.launch.Launcher.run(Launcher.java:256)
[java] at org.apache.tools.ant.launch.Launcher.main(Launcher.java:104)
[java] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[java] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
[java] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
[java] at java.lang.reflect.Method.invoke(Unknown Source)
[java] at org.apache.tools.ant.taskdefs.ExecuteJava.run(ExecuteJava.java:217)
[java] at java.lang.Thread.run(Unknown Source)

This is the ant target I'm running:

 <target name="testTask"> <java classname="org.apache.tools.ant.launch.Launcher" failonerror="true" jvm="C:/jdk1.8.0_141-x64/bin/java" dir="."> <classpath> <pathelement location="ant-launcher-1.7.0.jar"/> <pathelement location="ant-1.7.0.jar"/> </classpath> </java> </target>

and if you look inside ant-1.7.0.jar inside the classpath, you will find that org.apache.tools.ant.Main in fact implements org.apache.tools.ant.launch.AntMain :

and inside ant-launcher-1.7.0.jar, org.apache.tools.ant.launch.AntMain exists exactly under the package we're looking for:

Also I made sure that org.apache.tools.ant.Main overrides the method startAnt(String[] paramArrayOfString, Properties paramProperties, ClassLoader paramClassLoader) and takes the same arguments.

The line causing this exception is:

mainClass = Class.forName("org.apache.tools.ant.Main");
AntMain main = (AntMain)mainClass.newInstance();

worth noting that if I create the exact same class as org.apache.tools.ant.Main but rename it and place it on its own (outside of any.jar package), and run the above snippet but using the name of the new class it works perfectly fine.

Why is java insisting that org.apache.tools.ant.Main cannot be cast to org.apache.tools.ant.launch.AntMain ?