Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > Java > Thread synchronization

Reply
Thread Tools

Thread synchronization

 
 
Razvan
Guest
Posts: n/a
 
      08-31-2004
Hi




Is the following code correct ?



public class CDummy2 extends Thread
{
Integer threadId;

CDummy2(Integer threadId) { this.threadId = threadId;}

public static void main(String args[])
{
System.out.println("CDummy2.");

CDummy2 cd1 = new CDummy2(new Integer(1));
CDummy2 cd2 = new CDummy2(new Integer(2));

System.out.println("Starting thread 1...");
cd1.start();

try {
Thread.sleep(250);
}
catch(InterruptedException ee) {
System.out.println("The thread was interrupted !");
}


System.out.println("Starting thread 2...");
cd2.start();
}

public void run()
{
int ii = 50;

while(ii-- > 0)
{
printData("threadId: " + threadId + ", ii=" + ii);
try {
Thread.sleep(100);
}
catch(InterruptedException ee) {
System.out.println("The thread was interrupted !");
}
}
}

void printData(String input) {
synchronized (threadId) {
System.out.println(input);
}
}
}



I am synchronizing 2 threads on the Integer 'threadId'. Since the
code is trivial it seems to work. However, each thread class has its
own threadId. Since the object is not common for the 2 threads, that
means no synchronization is taking place in the method printData().

The only solution that I can think of is to make the threadId static.
Being static all the threads will share the same static object thus
synchronization should occur.

If my observation is correct that means that you can only synchronize
on class (static) variables but not on member variables.

My head is spinning. I am thinking to this for over 1 hour. There
should be a very simple explanation.




Regards,
Razvan
 
Reply With Quote
 
 
 
 
Eric Sosman
Guest
Posts: n/a
 
      08-31-2004
Razvan wrote:
>
> [code snipped; see up-thread]
>
> I am synchronizing 2 threads on the Integer 'threadId'. Since the
> code is trivial it seems to work. However, each thread class has its
> own threadId. Since the object is not common for the 2 threads, that
> means no synchronization is taking place in the method printData().
>
> The only solution that I can think of is to make the threadId static.
> Being static all the threads will share the same static object thus
> synchronization should occur.
>
> If my observation is correct that means that you can only synchronize
> on class (static) variables but not on member variables.
>
> My head is spinning. I am thinking to this for over 1 hour. There
> should be a very simple explanation.


You can synchronize on any Object at all. It doesn't
matter whether the references to the Object are static
variables, member variables, or even method-local variables.
The "thing" on which you synchronize is the Object that is
the target of the reference.

... and that's the key to understanding why and when to
synchronize. Synchronization is not about monopolizing the
use of a chunk of code, but about monopolizing the use of
an Object. Consider the Vector class for a moment, as an
example of a familiar class that uses synchronization. It
would be a Bad Thing if two threads both ran the add() method
on the same Vector at the same time, but it's perfectly all
right -- desirable, in fact -- that two threads be able to run
add() on two different Vectors simultaneously. You don't want
to prevent multiple simultaneous executions of add(); that would
just make the program run more slowly than it should. And it
wouldn't make the program correct, either! If thread T1 runs
add() at the same time T2 is running remove() on the same Vector,
there's only one add() running and only one remove() running,
but clearly there's trouble in store.

You don't want to prevent simultaneous execution of a
region of code, you want to prevent simultaneous manipulation
of an object. In the above example, both the add() and remove()
methods synchronize on the Vector they're manipulating: this
means that one of them will wait until the other is finished,
so the Vector is being manipulated by only one thread at a time.
Meanwhile, other threads can call add() and remove() on other
Vectors without interference.

Synchronization is about protecting the object, not about
protecting the code.

With that in mind, reconsider what you're trying to do.
You've got a method that synchronizes on an Integer. As it
happens, the only reference to that Integer is embedded in
the object itself; no other thread can get at the Integer
and hence synchronization accomplishes nothing. That's most
likely not the effect you were looking for, but I don't know
what you're actually trying to achieve.

--
(E-Mail Removed)

 
Reply With Quote
 
 
 
 
xarax
Guest
Posts: n/a
 
      08-31-2004

"Razvan" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Hi
>
> Is the following code correct ?

/snip/
>
> I am synchronizing 2 threads on the Integer 'threadId'. Since the
> code is trivial it seems to work. However, each thread class has its
> own threadId. Since the object is not common for the 2 threads, that
> means no synchronization is taking place in the method printData().
>
> The only solution that I can think of is to make the threadId static.
> Being static all the threads will share the same static object thus
> synchronization should occur.
>
> If my observation is correct that means that you can only synchronize
> on class (static) variables but not on member variables.


You cannot synch on variables or fields, only on objects.
A variable or a field may have a reference to an object.

If thread T1 synchs on variable "fubar", and thread T2
synchs on field "snafu", then they are synch'd with each
if and only if the contents of "fubar" and "snafu" refer
to the same object.

You can use an initialized static field, or you can
pass a reference to a mutex object in the constructor
of the subclass. (I also suggest that you do not extend
Thread, but use the Thread(Runnable) constructor. It's
much more powerful and flexible than extending Thread.)

> My head is spinning. I am thinking to this for over 1 hour. There
> should be a very simple explanation.


You can also use a shared mutex (Mutual Exclusion) object.

http://mindprod.com/products.html

http://mindprod.com/zips/java/mutex10.zip


--
----------------------------
Jeffrey D. Smith
Farsight Systems Corporation
24 BURLINGTON DRIVE
LONGMONT, CO 80501-6906
http://www.farsight-systems.com
z/Debug debugs your Systems/C programs running on IBM z/OS for FREE!



 
Reply With Quote
 
Thomas G. Marshall
Guest
Posts: n/a
 
      09-01-2004
Eric Sosman coughed up:
> Razvan wrote:
>>
>> [code snipped; see up-thread]
>>
>> I am synchronizing 2 threads on the Integer 'threadId'. Since the
>> code is trivial it seems to work. However, each thread class has its
>> own threadId. Since the object is not common for the 2 threads, that
>> means no synchronization is taking place in the method printData().
>>
>> The only solution that I can think of is to make the threadId static.
>> Being static all the threads will share the same static object thus
>> synchronization should occur.
>>
>> If my observation is correct that means that you can only synchronize
>> on class (static) variables but not on member variables.
>>
>> My head is spinning. I am thinking to this for over 1 hour. There
>> should be a very simple explanation.

>
> You can synchronize on any Object at all. It doesn't
> matter whether the references to the Object are static
> variables, member variables, or even method-local variables.
> The "thing" on which you synchronize is the Object that is
> the target of the reference.
>
> ... and that's the key to understanding why and when to
> synchronize. Synchronization is not about monopolizing the
> use of a chunk of code, but about monopolizing the use of
> an Object. Consider the Vector class for a moment, as an
> example of a familiar class that uses synchronization. It
> would be a Bad Thing if two threads both ran the add() method
> on the same Vector at the same time, but it's perfectly all
> right -- desirable, in fact -- that two threads be able to run
> add() on two different Vectors simultaneously. You don't want
> to prevent multiple simultaneous executions of add(); that would
> just make the program run more slowly than it should. And it
> wouldn't make the program correct, either! If thread T1 runs
> add() at the same time T2 is running remove() on the same Vector,
> there's only one add() running and only one remove() running,
> but clearly there's trouble in store.
>
> You don't want to prevent simultaneous execution of a
> region of code, you want to prevent simultaneous manipulation
> of an object. In the above example, both the add() and remove()
> methods synchronize on the Vector they're manipulating: this
> means that one of them will wait until the other is finished,
> so the Vector is being manipulated by only one thread at a time.
> Meanwhile, other threads can call add() and remove() on other
> Vectors without interference.
>
> Synchronization is about protecting the object, not about
> protecting the code.


uh.................... You seem to understand this issue very well, but
you've used a sentence that I'd never suggest anyone use.

Synchronization is about protecting sections of code. Whether or not there
is an object involved to be protected as such is secondary. The object used
can merely be to "hold the lock".

For example,

public class YoiksAndAwayWham
{
Object lock = new Object();

...

void dangerousThingy()
{
....lots of safe stuff...
synchronized(lock)
{
...stuff not protecting current object, nor protecting
"lock" but meant to protect this section of code from
collision with other similar methods. Protecting
YoiksAndAwayWham instances may never even
enter the picture...
}
}

void anotherDangerousThingy()
{
....lots of safe stuff...
synchronized(lock)
{
...stuff not protecting current object, nor protecting
"lock" but meant to protect this section of code from
collision with other similar methods. Protecting
YoiksAndAwayWham instances may never even
enter the picture...
}
}
}






>
> With that in mind, reconsider what you're trying to do.
> You've got a method that synchronizes on an Integer. As it
> happens, the only reference to that Integer is embedded in
> the object itself; no other thread can get at the Integer
> and hence synchronization accomplishes nothing. That's most
> likely not the effect you were looking for, but I don't know
> what you're actually trying to achieve.


--
Onedoctortoanother:"Ifthisismyrectalthermometer,wh erethehell'smypen???"



 
Reply With Quote
 
Thomas G. Marshall
Guest
Posts: n/a
 
      09-01-2004
Thomas G. Marshall coughed up:
> Eric Sosman coughed up:
>> Razvan wrote:
>>>
>>> [code snipped; see up-thread]
>>>
>>> I am synchronizing 2 threads on the Integer 'threadId'. Since the
>>> code is trivial it seems to work. However, each thread class has its
>>> own threadId. Since the object is not common for the 2 threads, that
>>> means no synchronization is taking place in the method printData().
>>>
>>> The only solution that I can think of is to make the threadId
>>> static. Being static all the threads will share the same static
>>> object thus synchronization should occur.
>>>
>>> If my observation is correct that means that you can only
>>> synchronize on class (static) variables but not on member variables.
>>>
>>> My head is spinning. I am thinking to this for over 1 hour. There
>>> should be a very simple explanation.

>>
>> You can synchronize on any Object at all. It doesn't
>> matter whether the references to the Object are static
>> variables, member variables, or even method-local variables.
>> The "thing" on which you synchronize is the Object that is
>> the target of the reference.
>>
>> ... and that's the key to understanding why and when to
>> synchronize. Synchronization is not about monopolizing the
>> use of a chunk of code, but about monopolizing the use of
>> an Object. Consider the Vector class for a moment, as an
>> example of a familiar class that uses synchronization. It
>> would be a Bad Thing if two threads both ran the add() method
>> on the same Vector at the same time, but it's perfectly all
>> right -- desirable, in fact -- that two threads be able to run
>> add() on two different Vectors simultaneously. You don't want
>> to prevent multiple simultaneous executions of add(); that would
>> just make the program run more slowly than it should. And it
>> wouldn't make the program correct, either! If thread T1 runs
>> add() at the same time T2 is running remove() on the same Vector,
>> there's only one add() running and only one remove() running,
>> but clearly there's trouble in store.
>>
>> You don't want to prevent simultaneous execution of a
>> region of code, you want to prevent simultaneous manipulation
>> of an object. In the above example, both the add() and remove()
>> methods synchronize on the Vector they're manipulating: this
>> means that one of them will wait until the other is finished,
>> so the Vector is being manipulated by only one thread at a time.
>> Meanwhile, other threads can call add() and remove() on other
>> Vectors without interference.
>>
>> Synchronization is about protecting the object, not about
>> protecting the code.

>
> uh.................... You seem to understand this issue very well,
> but you've used a sentence that I'd never suggest anyone use.
>
> Synchronization is about protecting sections of code. Whether or not
> there is an object involved to be protected as such is secondary.
> The object used can merely be to "hold the lock".
>
> For example,
>
> public class YoiksAndAwayWham
> {
> Object lock = new Object();


static Object lock = new Object();

That is a better example for a newbie.

The other way allows for multiple yoiks /each/ with their own handful of
threads accessing them, which was the example I was going for.

But that would be a disaster for a newbie, since it adds a layer of
complexity not needed here.

I'm sorry.





>
> ...
>
> void dangerousThingy()
> {
> ....lots of safe stuff...
> synchronized(lock)
> {
> ...stuff not protecting current object, nor
> protecting "lock" but meant to protect this
> section of code from collision with other similar
> methods. Protecting YoiksAndAwayWham instances
> may never even enter the picture...
> }
> }
>
> void anotherDangerousThingy()
> {
> ....lots of safe stuff...
> synchronized(lock)
> {
> ...stuff not protecting current object, nor
> protecting "lock" but meant to protect this
> section of code from collision with other similar
> methods. Protecting YoiksAndAwayWham instances
> may never even enter the picture...
> }
> }
> }
>
>
>
>
>
>
>>
>> With that in mind, reconsider what you're trying to do.
>> You've got a method that synchronizes on an Integer. As it
>> happens, the only reference to that Integer is embedded in
>> the object itself; no other thread can get at the Integer
>> and hence synchronization accomplishes nothing. That's most
>> likely not the effect you were looking for, but I don't know
>> what you're actually trying to achieve.


--
It'salwaysbeenmygoalinlifetocreateasignaturethaten dedwiththeword"blarphoogy"
..


 
Reply With Quote
 
Eric Sosman
Guest
Posts: n/a
 
      09-01-2004
Thomas G. Marshall wrote:
> Eric Sosman coughed up:
>
>> Synchronization is about protecting the object, not about
>>protecting the code.

>
> uh.................... You seem to understand this issue very well, but
> you've used a sentence that I'd never suggest anyone use.
>
> Synchronization is about protecting sections of code. Whether or not there
> is an object involved to be protected as such is secondary. The object used
> can merely be to "hold the lock".
>
> For example,
>
> public class YoiksAndAwayWham
> {
> Object lock = new Object();
>
> ...
>
> void dangerousThingy()
> {
> ....lots of safe stuff...
> synchronized(lock)
> {
> ...stuff not protecting current object, nor protecting
> "lock" but meant to protect this section of code from
> collision with other similar methods. Protecting
> YoiksAndAwayWham instances may never even
> enter the picture...
> }
> }
>
> void anotherDangerousThingy()
> {
> ....lots of safe stuff...
> synchronized(lock)
> {
> ...stuff not protecting current object, nor protecting
> "lock" but meant to protect this section of code from
> collision with other similar methods. Protecting
> YoiksAndAwayWham instances may never even
> enter the picture...
> }
> }
> }


In what way can the synchronized blocks (in this example
or in your follow-up with a static lock) "collide?" The code
is immutable whenever it's executable (that is, from the time
it's been loaded to the time when it's unloaded, if ever), so
it doesn't seem to be in need of much protection ...

The only kind of "collision" I can envision is if the two
pieces of code both manipulate some kind of shared resource.
Usually, that resource is a Java Object, and the Object is
the thing that needs the protection. Sometimes the resource
is something outside Java, so Java cannot protect it: Java
cannot synchronize access to an on-disk file, for example.
But almost invariably any such extra-Java resource will have
a Java Object acting as its "proxy," and you protect the
actual resource by "protecting" its proxy: you synchronize
on the File object that represents the file on disk.

Sometimes the shared resource has little physical reality.
You might be generating output with System.out.print() calls,
for example, and trying to ensure that the output from
different threads doesn't get intermixed on a single line.
The synchronized blocks in your example would contain several
System.out.print() calls followed by System.out.println(),
and synchronization would prevent intermixture. You might want
to think of this as protecting the code, but to me it seems more
useful to think of the lock object as a proxy for "the current
line," a resource outside Java that Java manipulates through the
System.out object. (You could, I suppose, use the System.out
object itself for this purpose -- I wouldn't, myself, but it
"should" work, I think.)

One of the characteristics of object-oriented programming
is that it has (duh...) an object-centric orientation rather
than a code-centric orientation. That being the case, it seems
far more natural to think about protecting the objects than
about which disparate pieces of code are or are not running
simultaneously. It happens that I find this viewpoint helpful
when thinking about synchronization in non-O-O languages, too --
but for an O-O language it seems almost a foregone conclusion
that one would think in this way. YMMV.

--
http://www.velocityreviews.com/forums/(E-Mail Removed)

 
Reply With Quote
 
Steve Horsley
Guest
Posts: n/a
 
      09-01-2004
Razvan wrote:
> Hi
>

<snip>
>
>
> I am synchronizing 2 threads on the Integer 'threadId'. Since the
> code is trivial it seems to work. However, each thread class has its
> own threadId. Since the object is not common for the 2 threads, that
> means no synchronization is taking place in the method printData().
>


Right.

> The only solution that I can think of is to make the threadId static.
> Being static all the threads will share the same static object thus
> synchronization should occur.
>
> If my observation is correct that means that you can only synchronize
> on class (static) variables but not on member variables.
>


That's not strictly true. To make code blocks exclusive, they must
synchronize on the same Object, but this need not be by a static reference.
It could be an Object handed as a method argument, or returned from
another Object.method, e.g. myHashMap.get("BackDoorLock");

Actually, a dirty trick using interned String constants goes like this:

synchronized("BackDoorLock") {
...
}

Actually, I recommend not extending Thread, but always using Runnable
objects instead. This is because it gets very confusing when you start
to synchronize by having one Thread subclass trying to lock on a different
Thread subclass's member Objects or call another Thread subclass's methods.
It's simpler if there are just Runnables that may be visited by more than
one thread than if a Thread is being visited by a different Thread's thread.
If that makes any sense.

Steve
 
Reply With Quote
 
Thomas G. Marshall
Guest
Posts: n/a
 
      09-02-2004
Eric Sosman coughed up:
> Thomas G. Marshall wrote:
>> Eric Sosman coughed up:
>>
>>> Synchronization is about protecting the object, not about
>>> protecting the code.

>>
>> uh.................... You seem to understand this issue very well,
>> but you've used a sentence that I'd never suggest anyone use.
>>
>> Synchronization is about protecting sections of code. Whether or
>> not there is an object involved to be protected as such is
>> secondary. The object used can merely be to "hold the lock".
>>
>> For example,
>>
>> public class YoiksAndAwayWham
>> {
>> Object lock = new Object();
>>
>> ...
>>
>> void dangerousThingy()
>> {
>> ....lots of safe stuff...
>> synchronized(lock)
>> {
>> ...stuff not protecting current object, nor
>> protecting "lock" but meant to protect this
>> section of code from collision with other
>> similar methods. Protecting YoiksAndAwayWham
>> instances may never even enter the picture...
>> }
>> }
>>
>> void anotherDangerousThingy()
>> {
>> ....lots of safe stuff...
>> synchronized(lock)
>> {
>> ...stuff not protecting current object, nor
>> protecting "lock" but meant to protect this
>> section of code from collision with other
>> similar methods. Protecting YoiksAndAwayWham
>> instances may never even enter the picture...
>> }
>> }
>> }

>
> In what way can the synchronized blocks (in this example
> or in your follow-up with a static lock) "collide?" The code
> is immutable whenever it's executable (that is, from the time
> it's been loaded to the time when it's unloaded, if ever), so
> it doesn't seem to be in need of much protection ...


And how on earth do you know what threads are involved here?

What if: The first method is called within one thread. The 2nd in another.
Both of these access something that just cannot be accessed by more than
one. This can involve something as innocuous as simple arithmetic on an
integer primitive. Multiple threads doing very simple non-atomic things all
at once can result in unpredictable results.


> The only kind of "collision" I can envision is if the two
> pieces of code both manipulate some kind of shared resource.
> Usually, that resource is a Java Object, and the Object is
> the thing that needs the protection.


It can be several objects, or simply an algorithm that cannot be
interrupted. But it's not about saving the objects involved. It's about
keeping multiple executions of segments of code blocked until one execution
is done.


> Sometimes the resource
> is something outside Java, so Java cannot protect it: Java
> cannot synchronize access to an on-disk file, for example.
> But almost invariably any such extra-Java resource will have
> a Java Object acting as its "proxy," and you protect the
> actual resource by "protecting" its proxy: you synchronize
> on the File object that represents the file on disk.
>
> Sometimes the shared resource has little physical reality.
> You might be generating output with System.out.print() calls,
> for example, and trying to ensure that the output from
> different threads doesn't get intermixed on a single line.
> The synchronized blocks in your example would contain several
> System.out.print() calls followed by System.out.println(),
> and synchronization would prevent intermixture.


That is the precise example I use when I teach java.


> You might want
> to think of this as protecting the code,


And you should. "protection" has a great many meanings. It seems that you
are comfortable saying that anything that allows an algorithm to stay
functional is protecting the objects. But your statements go beyond that.
The object is not what is protected, it is a section of code. That may
result in objects behaving properly.


> but to me it seems more
> useful to think of the lock object as a proxy for "the current
> line," a resource outside Java that Java manipulates through the
> System.out object.


This makes no sense whatsoever.


> (You could, I suppose, use the System.out
> object itself for this purpose -- I wouldn't, myself, but it
> "should" work, I think.)
>
> One of the characteristics of object-oriented programming
> is that it has (duh...) an object-centric orientation rather
> than a code-centric orientation.


No, it has object orientation rather than /procedural/ orientation. "code"
lives in both paradigms. Wow, you're all over the map on this post.


> That being the case, it seems
> far more natural to think about protecting the objects than
> about which disparate pieces of code are or are not running
> simultaneously. It happens that I find this viewpoint helpful
> when thinking about synchronization in non-O-O languages, too --
> but for an O-O language it seems almost a foregone conclusion
> that one would think in this way. YMMV.


It is not the object in total that is protected. It is a section of code
that you specifically force blockage into. If you instruct people to think
that synchronization is protecting objects, then you are misleading them
horribly. The bottom line is not that they are objects. The bottom line is
that a section of executable code is protected from re-entrance by another
thread.

--
"It's easier to be terrified by an enemy you admire."
-Thufir Hawat, Mentat and Master of Assassins to House Atreides


 
Reply With Quote
 
Thomas G. Marshall
Guest
Posts: n/a
 
      09-02-2004
Steve Horsley coughed up:
> Razvan wrote:
>> Hi
>>

> <snip>
>>
>>
>> I am synchronizing 2 threads on the Integer 'threadId'. Since the
>> code is trivial it seems to work. However, each thread class has its
>> own threadId. Since the object is not common for the 2 threads, that
>> means no synchronization is taking place in the method printData().
>>

>
> Right.
>
>> The only solution that I can think of is to make the threadId static.
>> Being static all the threads will share the same static object thus
>> synchronization should occur.
>>
>> If my observation is correct that means that you can only synchronize
>> on class (static) variables but not on member variables.
>>

>
> That's not strictly true. To make code blocks exclusive, they must
> synchronize on the same Object, but this need not be by a static
> reference. It could be an Object handed as a method argument, or
> returned from
> another Object.method, e.g. myHashMap.get("BackDoorLock");
>
> Actually, a dirty trick using interned String constants goes like
> this:
>
> synchronized("BackDoorLock") {
> ...
> }
>
> Actually, I recommend not extending Thread, but always using Runnable
> objects instead. This is because it gets very confusing when you start
> to synchronize by having one Thread subclass trying to lock on a
> different Thread subclass's member Objects or call another Thread
> subclass's methods. It's simpler if there are just Runnables that may
> be visited by more than one thread than if a Thread is being visited
> by a different Thread's thread. If that makes any sense.
>
> Steve


For similar reasons I *strongly* recommend that a newbie not use
synchronized methods such as:

public synchronized void hoohaa()
{
//...goblidy gook...
}

but instead use

public void hoohaa()
{
synchronized (myLock)
{
//...goblidy gook...
}
}

and have myLock be an Object instance devoted to nothing but holding the
lock.

The reason for this is important:

a. it keeps the confusion down in general about whether or not a lock is
held by the same instance as used elsewhere.

b. it keeps the following bug out. This bug I've seen time and time again:
someone tries to add a synchronized method, but later makes it static for
some other reason. But he forgets that synchronized then is locking on the
class object when used with static. Two locks in that case.

<bug>
public synchronized void hoohaa()
{
}

//and then someday someone adds this:
public static synchronized void anotherMethod()
{
// not protected against hoohaa() collisions...
}
</bug>


--
"It's easier to be terrified by an enemy you admire."
-Thufir Hawat, Mentat and Master of Assassins to House Atreides


 
Reply With Quote
 
Chris Uppal
Guest
Posts: n/a
 
      09-02-2004
Thomas G. Marshall wrote:

> It is not the object in total that is protected. It is a section of code
> that you specifically force blockage into. If you instruct people to
> think that synchronization is protecting objects, then you are misleading
> them horribly. The bottom line is not that they are objects. The bottom
> line is that a section of executable code is protected from re-entrance
> by another thread.


I disagree with this completely.

First off, it is not -- no matter what your stance on OO, or even whether you
are using an OO language -- code that is being protected, it is /state/. There
are one or more variables whose values must be "managed" to allow correct
concurrent access. So at minimum it is misleading to talk of protecting code,
you should be talking about protecting data.

Now in the OO world (and I'll accept, that Java is OO in this matter),
data/state is normally bunched into semantically coherent units called objects.
So the most natural thing to do is to manage concurrency at the level of the
overall state of one object. In that case we are clearly using synchronisation
to protect the (state of) the object. Just as Eric says. And the normal way
to express that is with code synchronised on 'this' -- synchronised methods for
instance.

Now in some cases, the state that needs to be protected will be either
distributed across more than one object, or be less than the entire state of an
object. Such cases are rare -- as you'd expect because the object is (or
should be) expressing a semantically coherent unit of state, and the protection
will normally be expected to follow the same boundaries, precisely /because/ it
is protecting semantic coherence. Still, such cases do occur, and in those
cases, and /only/ in those cases, your more "advanced" technique of using a
lock object is appropriate.

BTW, I don't think "lock" is a good name for a lock object. For the reasons
given above, it is misleading to use it to protect exactly the state of 'this'.

So either it is being used as a shared lock -- to act as a "channel" whereby
two or more objects can maintain some semantic interdependency. In such cases,
I would submit that "sharedLock" is the minimum meaningful name. OTOH, you may
have an object that only needs to protect some of its state. Offhand, I can't
think of a convincing example, but say that an object needs to protect its
output logging stream, and independently needs to protect the state of its
interaction with some network server; in such cases it seems that the minimum
meaningful names would be loggingLock and networkLock. The occurrence of the
undecorated name "lock" is an indication that the author may not have
understood what s/he was doing.

(As, BTW, is casually moving a method between static and non-static -- that is
a change at least as great, in OO terms, as moving a method between classes
since it is changing the behaviour of two objects (treating the 'class' as if
it were an object for these purposes).)

-- chris



 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Thread Synchronization vijufri@yahoo.com Java 0 07-18-2006 12:41 PM
Thread Synchronization vijufri@yahoo.com Java 0 07-18-2006 12:41 PM
Thread Synchronization vijufri@yahoo.com Java 0 07-18-2006 12:40 PM
Thread safe access to elements of a growing array without synchronization Adam Warner Java 13 03-28-2006 01:14 AM
Thread synchronization problem Vincent Lascaux Java 9 09-02-2004 07:53 PM



Advertisments