This actually gets worse if the second thread is an internal non static
class, since it gets an implicit 'this' reference to the partially
constructed object.
Peter Firmstone wrote:
Hmm, :|
To quote
http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4:
A call to |start()| on a thread /happens-before/ any actions in the
started thread.
<comment>
But does that guarantee that construction of objects whose references
will be written to final fields (guaranteed after construction
completes) in the constructor of an object that starts that thread,
will happen before or after the new thread is started? Remembering
the jvm is free to not initialize and reorder something it doesn't
think it needs now, but must after construction is complete.
So in other words the second thread which started during object
construction might not see the objects the first thread has created in
a fully constructed state as they haven't yet been published.
</comment>
In some cases, such as deserialization, the system will need to
change the |final| fields of an object after construction. |final|
fields can be changed via reflection and other
implementation-dependent means. The only pattern in which this has
reasonable semantics is one in which an object is constructed and
then the |final| fields of the object are updated. The object should
not be made visible to other threads, nor should the |final| fields
be read, until all updates to the |final| fields of the object are
complete. Freezes of a |final| field occur both at the end of the
constructor in which the |final| field is set, and immediately after
each modification of a |final| field via reflection or other special
mechanism.
Even then, there are a number of complications. If a |final| field
is initialized to a compile-time constant expression (ยง15.28
<http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.28>)
in the field declaration, changes to the |final| field may not be
observed, since uses of that |final| field are replaced at compile
time with the value of the constant expression.
Another problem is that the specification allows aggressive
optimization of |final| fields. Within a thread, it is permissible
to reorder reads of a |final| field with those modifications of a
|final| field that do not take place in the constructor.
An implementation may provide a way to execute a block of code in a
/|final|-field-safe context/. If an object is constructed within a
|final|-field-safe context, the reads of a |final| field of that
object will not be reordered with modifications of that |final|
field that occur within that |final|-field-safe context.
A |final|-field-safe context has additional protections. If a thread
has seen an incorrectly published reference to an object that allows
the thread to see the default value of a |final||final|-field-safe
context, reads a properly published reference to the object, it will
be guaranteed to see the correct value of the |final| field. In the
formalism, code executed within a |final|-field-safe context is
treated as a separate thread (for the purposes of |final| field
semantics only).
In an implementation, a compiler should not move an access to a
|final| field into or out of a |final|-field-safe context (although
it can be moved around the execution of such a context, so long as
the object is not constructed within that context).
Dan Creswell wrote:
On 1 April 2013 09:24, Peter Firmstone <[email protected]> wrote:
Dan Creswell wrote:
On 1 April 2013 08:11, Peter Firmstone <[email protected]> wrote:
Food for thought: After our pending release, it might be an idea
to make
a combined effort to identify and address as many concurrency
issues as
possible, we need to modernize our implementation code so we stay
relevant.
An important task will be updating all our service implementations so
they
DON'T start threads during construction.
The ActiveObject pattern often does start threads at construction. I'd
like
to understand why that is such a problem for you? It surely isn't a
big
deal for me but....
It allows fields to be declared final, if a thread is started during
construction the JMM makes no guarantee that thread will see the final
state of that objects fields after construction completes.
Not sure that's true, at least in JDK 7:
http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4
"An action that starts a thread *synchronizes-with* the first action
in the
thread it starts. "
"Two actions can be ordered by a *happens-before* relationship. If one
action *happens-before* another, then the first is visible to and
ordered
before the second. "
"If an action *x* *synchronizes-with* a following action *y*, then we
also
have *hb(x, y)*. "
i.e. If thread A is doing construction and then starts another thread,
variable assignments prior will be visible to the newly created thread.
That in turn means so long as all critical assignments are done prior to
starting that second thread, there's no problem?
And if that's true, starting a thread in a constructor needn't be
avoided,
merely done "carefully". Thus it would be sufficient to ensure all final
variables are assigned prior to thread starting, which isn't so hard
to do
or assure. I guess my point is, yes there's some care required but
outright
banning thread start() in constructors is overkill.
?
This is important when that thread accesses fields in the constructed
object.
See:
https://www.securecoding.cert.**org/confluence/display/java/**
TSM03-J.+Do+not+publish+**partially+initialized+objects<https://www.securecoding.cert.org/confluence/display/java/TSM03-J.+Do+not+publish+partially+initialized+objects>
https://www.securecoding.cert.**org/confluence/display/java/**
TSM01-J.+Do+not+let+the+this+**reference+escape+during+**
object+construction<https://www.securecoding.cert.org/confluence/display/java/TSM01-J.+Do+not+let+the+this+reference+escape+during+object+construction>
This doesn't mean you can't start a thread during construction, but it
does mean you must be very careful if you do; our old code isn't that
careful. ;)
Cheers,
Peter.
