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From "Todd Wells (JIRA)" <j...@apache.org>
Subject [jira] Created: (ABDERA-155) Abdera class "this" reference escapes during construction, leading to race conditions and NPEs
Date Fri, 02 May 2008 15:38:55 GMT
Abdera class "this" reference escapes during construction, leading to race conditions and NPEs

                 Key: ABDERA-155
                 URL: https://issues.apache.org/jira/browse/ABDERA-155
             Project: Abdera
          Issue Type: Bug
    Affects Versions: 0.4.0
         Environment: OS X, JVM 1.5, 2 x 2.8 GHz quad-core Xeon
            Reporter: Todd Wells
            Priority: Critical

I have hit a number of NullPointerExceptions when using Abdera in a very generic and straightforward
fashion.  The same code doesn't cause the NPEs in all cases, it seems to depend on the environment
in which it's executing, which leads to a strong suspicion of a race condition.  I've seen
an NPE in code as simple as this groovy code:

def abdera = new Abdera()
Entry entry = abdera.newEntry()  /* NPE here, Caused by: java.lang.NullPointerException at

Another example was an NPE when calling ClientResponse.getDocument(), Caused by: java.lang.NullPointerException

When I run the code in IDEA (calling from a unit test or executing a custom ant task) it works
fine. When I'm calling it from inside of a custom ant task running at the command line, I
get the null pointer exceptions that I've described.  This supports the notion of it being
a race condition -- the NPE's are highly squirrelly and environment-specific.

Digging into the Abdera code, I believe I've found the source of the problems.  The constructor
of the Abdera class calls newFactory(), newParser(), newXPath(), newParserFactory() and newWriter().
 In every one of these methods, a reference to "this" is passed to a different class (or multiple
classes, as it would appear).  The fundamental problem here is since this code is being called
from the constructor the "this" reference is not yet guaranteed to be a completely constructed
object. This can result in unpredictable behavior, such as the NPE mentioned in this bug.
 Since the classes (ServiceUtil and AbderaConfiguration) that get references to "this" appear
to be using "this" in a multi-threaded fashion, this can result in race conditions.

For a detailed discussion, I recommend reviewing this article "Safe construction techniques"
by Brian Goetz: http://www.ibm.com/developerworks/java/library/j-jtp0618.html, or he has a
more detailed discussion of this problem in his book "Java Concurrency in Practice".  I will
quote his article here: "One of the mistakes that can introduce a data race into your class
is to expose the this reference to another thread before the constructor has completed. Sometimes
the reference is explicit, such as directly storing this in a static field or collection,
but other times it can be implicit, such as when you publish a reference to an instance of
a non-static inner class in a constructor. Constructors are not ordinary methods -- they have
special semantics for initialization safety. An object is assumed to be in a predictable,
consistent state after the constructor has completed, and publishing a reference to an incompletely
constructed object is dangerous."

This is a serious set of bugs in the Abdera class and the resultant problems will probably
become more prevalent as machines become faster.

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