I created an issue for this:
https://bugs.openjdk.java.net/browse/JDK-8051713
The proposed patch is still the following:
http://cr.openjdk.java.net/~plevart/jdk9-dev/URL.synchronization/webrev.01/
Regards, Peter
On 07/11/2014 05:11 PM, Peter Levart wrote:
Hi,
java.net.URL is supposed to behave as an immutable object, so URL
instances can be shared among threads and among parts of code without
fear that they will be modified. URL class has an unusual way to
achieve this (or at least it tries to). Partly because of the design
which uses:
- URL constructor(s) that take a 'spec' String to be parsed into URL
object
- parsing is delegated to various URLStreamHandler(s) which are chosen
in the URL constructor (depending on the protocol used in URL string
to be parsed)
An unitialized URL instance (this) is passed from constructor to the
chosen URLStreamHandler which has the responsibility to parse the
string and set back the fields that hold various parts of URL object
being constructed. Consequently, these fields can not be declared as
final, as definite assignment analysis doesn't cross method borders.
It is therefore illegal to unsafely publish URL instances (via data
races) to non-constructing threads because they can appear not fully
initialized. Nevertheless URL, with the help of various
URLStreamHandler implementations, tries hard to make URL appear stable
at least where it is required to be stable. For example:
URL.hashCode() is (almost) stable even if URL instance is unsafely
published. This is achieved by making hashCode() synchronized and
cache the result. At least one way of constructing URLs - constructors
that take 'spec' String to be parsed - is also making sure that
hashCode is computed from fully initialized fields, as parsing is
delegated to URLStreamHandler which uses package-private URL.set()
method to set back the parsed fields and the set() method is also
synchronized. But making URL appear stable even though it is published
unsafely doesn't seem to be the primary concern of URL
synchronization. Other public URL constructors that take individual
URL parts and don't delegate parsing to URLStreamHandler but set
fields directly (not via set() method), are not synchronized.
Primary concern of synchronization in URL appears to be driven from
the fact that some URL operations like hasCode(), equals(), sameFile()
and openConnection() read multiple URL fields and URL.set() which can
be called from custom URLStreamHandler at any time (although this is
not it's purpose - it should only call-back while parsing/constructing
the URL) can set those fields. And those multi-field operations would
like to see a "snapshot" of field values that is consistent. But
synchronization that is performed to achieve that is questionable.
Might be that in Java 1.0 times the JVM implementation assumptions
were different and synchronization was correct, but nowadays Java
memory model makes them invalid.
URL.hasCode() apears to be the only method properly synchronized which
makes it almost stable (doesn't return different results over time)
but even hashCode() has a subtle bug or two. The initializer for
hashCode field sets it to value -1 which represents "not yet computed"
state. If URL is published unsafely, hashCode() method could see the
"default" value of 0, which would be returned. A later call to
hashCode() would see value -1 which would trigger computation and a
different value would be returned. The other subtle bug is a
relatively improbable event that hashCode computation results in value
-1 which means "not yet computed". This can be seen as performance
glitch (as hashCode will never be cached for such URL instance) or an
issue which makes hashCode unstable for one of the reasons why
equals() is unstable too (see below).
If URL.hasCode() method is almost stable (doesn't return different
results over time) and at least one way of URL construction makes sure
that hashCode is also calculated from fully initialized parts, then
URL.equals() and other methods that delegate to URLStreamHandler are a
special story. URL.equals() can't be synchronized on URL instance,
because it would have to be synchronized on both URL instances that
are being compared and this is prone do dead-locks. Imagine:
thread1: url1.equals(url2)
thread2: url2.equals(url1)
So equals() chooses to not be synchronized and therefore risks not
being stable if URL instances are published unsafely. But it
nevertheless uses synchronization. equals() delegates it's work to the
1st URL's URLStreamHandler which synchronizes on itself when
calculating and caching the InetAddress of each individual URL's host
name. InetAddress (if resolvable) is used in preference to host name
for comparison (and also in hashCode()). URL.equals() risks not being
stable for the following reasons:
- URL instances published unsafely can appear not fully initialized to
equals() even though
