Github user pwendell commented on a diff in the pull request:

    https://github.com/apache/spark/pull/119#discussion_r10507796
  
    --- Diff: core/src/main/scala/org/apache/spark/SparkContext.scala ---
    @@ -130,6 +130,16 @@ class SparkContext(
     
       val isLocal = (master == "local" || master.startsWith("local["))
     
    +  // Create a classLoader for use by the driver so that jars added via 
addJar are available to the
    +  // driver.  Do this before all other initialization so that any thread 
pools created for this
    +  // SparkContext uses the class loader.
    +  // Note that this is config-enabled as classloaders can introduce subtle 
side effects
    +  private[spark] val classLoader = if 
(conf.getBoolean("spark.driver.loadAddedJars", false)) {
    +    val loader = new SparkURLClassLoader(Array.empty[URL], 
this.getClass.getClassLoader)
    +    Thread.currentThread.setContextClassLoader(loader)
    --- End diff --
    
    Hey Sandy - not sure what you mean exactly by "load them for the 
SparkContext". The SparkContext is just a java object. The scenario we want to 
handle is like this:
    ```
    val sc = new SparkContext(...)
    sc.addJar("jar-containing-lib-foo")
    val x: Seq[Foo] = sc.textFile(...).map(...).collect()
    ```
    There are two ways "Foo" can be visible for the list line. Either it can be 
included in the classpath when launching the JVM or it can be added dynamically 
to the classloader of the calling thread. Is there another way?



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