Abdera class "this" reference escapes during construction, leading to race
conditions and NPEs
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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
org.apache.abdera.Abdera.newEntry(Abdera.java:114)
Another example was an NPE when calling ClientResponse.getDocument(), Caused
by: java.lang.NullPointerException at
org.apache.abdera.protocol.client.AbstractClientResponse.getDocument(AbstractClientResponse.java:96)
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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