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  1. JDK
  2. JDK-1236606

GC problem with invalid class name

    Details

    • Type: Bug
    • Status: Closed
    • Priority: P2
    • Resolution: Fixed
    • Affects Version/s: 1.0
    • Fix Version/s: 1.1
    • Component/s: hotspot
    • Labels:
    • Subcomponent:
    • Resolved In Build:
      1.1
    • CPU:
      sparc
    • OS:
      solaris_2.4
    • Verification:
      Not verified

      Description

      From ###@###.### Thu Feb 1 05:59:36 1996
      From: ###@###.### (Jonathan Payne)
      To: ###@###.###, ###@###.###
      Cc: ###@###.###, arthur.vanhoff@Eng, frank.yellin@Eng, lindholm@Eng,
              pavani@Eng, Sami.Shaio@Eng
      Subject: HOLY SHIT I FIGURED IT OUT!

      [For those of you just joining, I have had a horrible bug in Java,
       causing it to segv, because a String literal is arriving in a method
       as an Object handle that's showing major signs of having been garbage
       collected]

      Paul, you will be happy to know this is not your fault!

      It's MY fault, although a major bug in Java.

      I am accidentally looking for a class called:

      ticker..sched

      This should be an error, but it's not. Internally class names are
      stored as

      ticker/sched

      so "/" is the separator. So the Class.forName(foo) method takes a
      name with "." in it, and replaces them with "/".

      ticker//sched

      It then looks for a class by that name, but doesn't find it because
      it's not there. So it looks for a file in the classpath by that name,
      and of course UNIX doesn't mind "//" in a pathname - it silently
      removes the extra slash. So it finds the class, loads it from the
      file system, reads teh actual classname from the .class file, and
      makes an entry in the classes list under the name:

      ticker/sched

      After all, that's the correct name, as stored by the compiler.

      So then later, I ask again for

      ticker..sched

      which is turned into

      ticker//sched

      which is not found when a simple search for it in the classes list is
      made, because it's called

      ticker/sched

      in that list. So it reloads the fucking class, and stores it in the
      classes list, silently REPLACING the previous entry for that name!!
      You see, by the time it gets stuck in the classes list, it has the
      canonical name, and it searches the same class list as it does
      initially to see if it needs to load the class, except this time it
      searches it with the canonical name and finds the match.

      If this doesn't make sense, I will explain again tomorrow (I mean in a
      few hours). The upshot of it is, there's more than one version of
      this class, and the latest version of the class is always the one
      stored in the class list. It's that class list which is used by the
      garbage collector to mark class variables and constant pools. BUT, I
      have my own class list, stored in a hash table, and it's with that
      Class class instance that I create a new instance of the particular
      class, using its constant pool, etc. That constant pool is NOT being
      marked by the GC!

      You might think (as I just did in a panic!) that, hell, if I have a
      reference to a class in a Hashtable, then that must mean that that
      class is garbage collected, and so is its constant pool, so where's
      the bug? But the answer is, Instances of class Class, when
      encountered, are "fake" objects, with a few instance variables, such
      as the class name, I believe, and the garbage collector does not go
      mark static variables and the constant pool when it encounters them,
      probably because it's already marked all of them in the initial "find
      all the roots" pass of the garbage collector.

      HOLY FUCKING SHIT, I AM RIGHT!

      This kills me.

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            • Assignee:
              tlindholsunw Timothy Lindholm (Inactive)
              Reporter:
              ahoffsunw Arthur Hoff (Inactive)
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