Question

I’m trying to construct a jar file using the Amazon Mechanical Turk API. The SDK comes with a helloworld file that I’m trying to jar up as a sanity check – it is located here:

http://aws.amazon.com/code/SDKs/695

After setting everything up, I’m able to use ant to build and execute correctly, using the supplied build.xml file.

bash-3.2$ ant helloworld
ant helloworld
Buildfile: /Users/astorer/Work/dtingley/java-aws-mturk-1.2.2/build.xml

compile-sample:
     [echo] Compiling the sample java source files...
    [javac] /Users/astorer/Work/dtingley/java-aws-mturk-1.2.2/build.xml:252: warning: 'includeantruntime' was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds

helloworld:
     [echo] Running the Hello World sample application...
     [java] Got account balance: 10000.00
     [java] Created HIT: 2RB2D5NQYN5F41KJ2IKYPNCW2H3A60
     [java] You may see your HIT with HITTypeId '22R58B727M0IHQ4HZEXYISVF4XCWBC' here: 
     [java] http://workersandbox.mturk.com/mturk/preview?groupId=22R58B727M0IHQ4HZEXYISVF4XCWBC
     [java] Success.

BUILD SUCCESSFUL
Total time: 11 seconds

I want the helloworld to be executable by someone else without needing to install the libraries. It seems like the “correct” way to do this is to construct a jar from within ant.

My understanding is that I need to include:

• the necessary libraries, build from the sdk itself (and provided as a .jar)
• the built helloworld class file
• the manifest attribute specifying the main class to run

I don’t know if I need to include anything else. I know that there is a big, complicated classpath at runtime, and that I can specify the classpath on the command line, but I suspect hard coding the classpath will prevent me from distributing the .jar file, which is the entire point.

Here is the build.xml snippet for the jar:

  <target name="hellojar" depends="helloworld" description="Creates Jar of helloworld" >
    <jar destfile="helloworld.jar">
      <fileset file="${sdk.jar}" />
      <fileset dir="${sample.classes.dir}/helloworld/" />
      <fileset dir="."/>
      <manifest>
        <attribute name="Main-Class" value="MTurkHelloWorld" />
      </manifest>
    </jar>
  </target>

This builds. When I run the jar, it crashes, however:

bash-3.2$ java -jar helloworld.jar 
java -jar helloworld.jar 
Exception in thread "main" java.lang.NoClassDefFoundError: MTurkHelloWorld (wrong name: helloworld/MTurkHelloWorld)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

This makes sense, because my MTurkHelloWorld is actually in the helloworld package. Thus, I should change to:

  <manifest>
    <attribute name="Main-Class" value="helloworld.MTurkHelloWorld" />
  </manifest>

This builds successfully. When I run it:

bash-3.2$ java -jar helloworld.jar 
java -jar helloworld.jar 
Exception in thread "main" java.lang.NoClassDefFoundError: helloworld/MTurkHelloWorld
Caused by: java.lang.ClassNotFoundException: helloworld.MTurkHelloWorld
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

We can investigate the files within the jar:

jar tf helloworld.jar | grep hello
build/private/classes/samples/helloworld/
samples/helloworld/
build/private/classes/samples/helloworld/MTurkHelloWorld.class
samples/helloworld/MTurkHelloWorld.java

Which suggests that maybe if the classpath were set to build/private/classes/samples/ it would work properly:

<attribute name="Class-Path" value="build/private/classes/samples/helloworld" />

This leads to an identical error. I think there is something pretty fundamental that I’m missing here, and I’d appreciate any help!

Answer 1

Your package-folder have to start directly, you can’t put them in any sub-directory in the jar file:

It would have to look like this:

helloworld/
helloworld/MTurkHelloWorld.class

Your jar-task would have to look like this:

<jar destfile="helloworld.jar" basedir="${sample.classes.dir}">>
  <manifest>
    <attribute name="Main-Class" value="MTurkHelloWorld" />
  </manifest>
</jar>

The basedir is the source of your compiled packages.
With <fileset /> you just add normal files.

Furthermore, you can’t put external libs into jar files. They have to be in the same folder as the jar, in the java classpath or in a folder specified by the classpath-attribute of the manifest.
Or you could use the one jar task, to include the classes into your jar.