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Posted to dev@activemq.apache.org by Cunningham <he...@gmail.com> on 2021/04/01 11:52:57 UTC
What does "passed by reference" mean in ActiveMQ's context?
Java's built-in evaluation strategy [1] is 100% pass-by-value [2].
A question regarding that fact arose for me when I read this note in the
ActiveMQ documentation: "One thing to note with the VM transport is that
messages are passed by reference" [3]. That contradicts the well-known fact
about Java.
Another question arose about this: "disable the automatic serialization of
ObjectMessage payloads so that the objects are passed by value in 4.x by
setting the objectMessageSerializationDefered flag to true" [4].
The documentation leads you to expect that setting
objectMessageSerializationDefered to false somehow transforms Java's
built-in pass-by-value evaluation strategy to instead do pass-by-reference.
ActiveMQ's documentation is incomplete and not 100% crystal clear. Since
Java being pass-by-value is a given, the ActiveMQ documentation would be
more complete and less confusing if its contradictory use of
pass-by-reference and pass-by-value were explained in more detail.
To try to understand what context ActiveMQ uses
pass-by-reference/pass-by-value, I've looked at some unit tests here [5]
and here [6]. Neither of those unit tests exercise what's claimed in the
documentation I quoted. I didn't see anything that looked like
pass-by-reference in the VMTransport code either [7].
So I did some experimenting. I instantiated a new ActiveMQObjectMessage in
a method that implements one of several consumers in the receiving end of a
VM transport. In that one mutating consumer's method, I then assigned the
locally instantiated new message to the original reference received from
the destination that several consumers subscribed to.
If ActiveMQ truly does pass-by-reference in the standard sense of the term,
then I would expect the assignment of the newly instantiated message
created in the one mutating consumer's method would be seen by all the
other consumers. I would expect the producer that sent the original message
to also be able to observe — after sending the original message — that its
original reference would eventually refer to the new message instantiated
and assigned in the mutating consumer.
Neither of those expectations were borne out in my experiment. Which is not
a surprise, since Java doesn't do pass-by-refrence.
Therefore my questions are:
1. In what sense does the ActiveMQ documentation mean "passed by reference"?
2. Where can I find existing unit tests that exercise and verify ActiveMQ's
expected pass-by-reference/pass-by-value behavior?
3. Is that ActiveMQ documentation even relevant for ActiveMQ 5+? Or only
for 3x and 4x?
----
[1] https://bit.ly/WikiPbV
[2] https://bit.ly/JavaDudePbV
[3] https://bit.ly/VMTransPbR
[4] https://bit.ly/DisObjSerPbV
[5] https://bit.ly/ObjSerTest
[6] https://bit.ly/ObjMsgNotSer
[7] https://bit.ly/VMTrans
Re: What does "passed by reference" mean in ActiveMQ's context?
Posted by Arthur Naseef <ar...@amlinv.com>.
May I ask where this is going? The vm-transport's handling of
copying/not-copying messages isn't intended to be a feature. Changing a
message that's in-flow within the broker is a bad idea for a lot of reasons.
Art
On Thu, Apr 1, 2021 at 1:01 PM Christopher Shannon <
christopher.l.shannon@gmail.com> wrote:
> I haven't spent a ton of time using the VM transport personally (I
> exclusively use SSL/TCP) but at this point it is pretty clear to me that
> this is just a case of the documentation is old and doesn't apply to
> ActiveMQ 5.x and it's safe to assume the messages are copied.
>
> ActiveMQ has been around for something like 15 years so it is likely that
> the behavior used to be one way and then changed and that specific piece of
> documentation was never updated or removed. ActiveMQ version 5.x has been
> around since 2008 so I don't know that too many people remember older
> versions like version 4.x and how things used to be (I know I wasn't around
> that long ago). Ideally all the documentation should be up to date but
> sometimes things get missed.
>
> Besides what I already sent to show the message is being copied on send, I
> also just did a quick search and it looks like it also gets copied on
> dispatch to a consumer therefore you are never going to get "pass by
> reference" behavior which is what you have verified in your own testing.
> See:
>
> https://github.com/apache/activemq/blob/activemq-5.16.1/activemq-client/src/main/java/org/apache/activemq/ActiveMQConnection.java#L1840
> .
>
> There are certainly lots of unit tests that use the VMTransport but I don't
> know that there are any specific tests to demonstrate pass by value or
> reference, I'm guessing there is nothing else than what you already found
> and testing it yourself is probably the best thing which you have done.
>
> The main action point here is that documentation is quite confusing and
> needs to be fixed up (probably just deleted honestly) as it doesn't appear
> to be relevant anymore.
>
> On Thu, Apr 1, 2021 at 2:48 PM Cunningham <he...@gmail.com> wrote:
>
> > > "...that was just referring to the fact that when the message is sent
> by
> > VM transport the same reference is used..."
> >
> > That answers my question #1. Thanks.
> >
> >
> > > "...changes to the original message after sending would also affect the
> > message that was already sent since they are the same object still..."
> >
> > Thanks. My experiments show that is true only if
> > ActiveMQObjectMessage.setObject(new Object()) is done in the producer
> after
> > the original send. The same change when done in consumers has no effect.
> In
> > both cases I set ActiveMQObjectMessage.setReadOnlyBody(false) beforehand,
> > by the way.
> >
> > If you have running code that implements what you're describing, I'd be
> > grateful if you could share it. Please? TIA.
> >
> >
> > > "...See: .../org/apache/activemq/ActiveMQSession.java#L1955..."
> >
> > Forgive me if I sound ungrateful. But I had already seen that. Thanks
> just
> > the same though.
> >
> > I've also seen the unit test for it [1]. And I've seen where it's used
> with
> > objectMessageSerializationDefered [2]. ActiveMQObjectMessage.setObject()
> > also uses objectMessageSerializationDefered [3].
> >
> > Another test I've seen is one that verifies that an ObjectMessage's body
> is
> > serialized over TCP transport but not serialized over VM transport when
> > objectMessageSerializationDefered is true [4].
> >
> >
> > > "...a copy of the message is still sent even with the VM transport by
> > default..."
> >
> > That might explain why I haven't had any luck producing pass-by-reference
> > behavior (as I understand it [5]) in my experiments.
> >
> > Starting with this ActiveMQ-supplied example code [6] as a base, I tried
> > every combination of values for objectMessageSerializationDefered and
> > copyMessageOnSend I could think of.
> >
> > That documentation I linked to in my first post claims "Messages are
> passed
> > by reference when objectMessageSerializationDefered is false" and
> "Messages
> > are passed by value when objectMessageSerializationDefered is true". I
> > would love to find a test that either verifies either of those claims or
> > disproves either of them.
> >
> > Until I do though, I will still have these unanswered questions...
> >
> > 1. ...
> > 2. Where can I find existing unit tests that exercise and verify
> ActiveMQ's
> > expected pass-by-reference/pass-by-value behavior?
> > 3. Is that ActiveMQ documentation even relevant for ActiveMQ 5+? Or only
> > for 3x and 4x?
> >
> > ----
> >
> > [1] https://bit.ly/testAMQcopy
> > [2] https://bit.ly/AMQcopy
> > [3] https://bit.ly/setObject
> > [4] https://bit.ly/MsgNotSerTest
> > [5] https://bit.ly/JavaDudePbV
> > [6] https://bit.ly/HelloAMQApp
> >
> >
> > ----
> >
> > On Thu, Apr 1, 2021 at 2:39 PM Christopher Shannon <
> > christopher.l.shannon@gmail.com> wrote:
> >
> > > I forgot to add that your experiment probably didn't show changes
> > because I
> > > believe by default the ActiveMQConnection has the flag
> > "copyMessageOnSend"
> > > set to true. So the message would have been copied anyways into a new
> > > object which means the whole copy by reference wording is even less
> > > relevant and more wrong since a copy of the message is still sent even
> > with
> > > the VM transport by default.
> > >
> > > See:
> > > https://activemq.apache.org/connection-configuration-uri
> > >
> > >
> >
> https://github.com/apache/activemq/blob/activemq-5.16.1/activemq-client/src/main/java/org/apache/activemq/ActiveMQSession.java#L1955
> > >
> > > On Thu, Apr 1, 2021 at 8:28 AM Christopher Shannon <
> > > christopher.l.shannon@gmail.com> wrote:
> > >
> > > > Yes, Java is pass by value but the value is a copy of the reference
> and
> > > > not an independent copy of the message (unlike C or C++). The
> > > documentation
> > > > wording should be fixed as it is wrong as you pointed out but I
> assume
> > > > whoever wrote that was just referring to the fact that when the
> message
> > > is
> > > > sent by VM transport the same reference is used so you need to be
> > careful
> > > > as changes to the original message after sending would also affect
> the
> > > > message that was already sent since they are the same object still.
> The
> > > > messages are just stored in a data structure in the JVM (I think a
> > > > LinkedBlockingQueue). So this is just (poorly) trying to convey the
> > fact
> > > > that the messages are not independent copies of the message like they
> > > > would be if sent over TCP or another protocol.
> > > >
> > >
> >
>
Re: What does "passed by reference" mean in ActiveMQ's context?
Posted by Christopher Shannon <ch...@gmail.com>.
I haven't spent a ton of time using the VM transport personally (I
exclusively use SSL/TCP) but at this point it is pretty clear to me that
this is just a case of the documentation is old and doesn't apply to
ActiveMQ 5.x and it's safe to assume the messages are copied.
ActiveMQ has been around for something like 15 years so it is likely that
the behavior used to be one way and then changed and that specific piece of
documentation was never updated or removed. ActiveMQ version 5.x has been
around since 2008 so I don't know that too many people remember older
versions like version 4.x and how things used to be (I know I wasn't around
that long ago). Ideally all the documentation should be up to date but
sometimes things get missed.
Besides what I already sent to show the message is being copied on send, I
also just did a quick search and it looks like it also gets copied on
dispatch to a consumer therefore you are never going to get "pass by
reference" behavior which is what you have verified in your own testing.
See:
https://github.com/apache/activemq/blob/activemq-5.16.1/activemq-client/src/main/java/org/apache/activemq/ActiveMQConnection.java#L1840
.
There are certainly lots of unit tests that use the VMTransport but I don't
know that there are any specific tests to demonstrate pass by value or
reference, I'm guessing there is nothing else than what you already found
and testing it yourself is probably the best thing which you have done.
The main action point here is that documentation is quite confusing and
needs to be fixed up (probably just deleted honestly) as it doesn't appear
to be relevant anymore.
On Thu, Apr 1, 2021 at 2:48 PM Cunningham <he...@gmail.com> wrote:
> > "...that was just referring to the fact that when the message is sent by
> VM transport the same reference is used..."
>
> That answers my question #1. Thanks.
>
>
> > "...changes to the original message after sending would also affect the
> message that was already sent since they are the same object still..."
>
> Thanks. My experiments show that is true only if
> ActiveMQObjectMessage.setObject(new Object()) is done in the producer after
> the original send. The same change when done in consumers has no effect. In
> both cases I set ActiveMQObjectMessage.setReadOnlyBody(false) beforehand,
> by the way.
>
> If you have running code that implements what you're describing, I'd be
> grateful if you could share it. Please? TIA.
>
>
> > "...See: .../org/apache/activemq/ActiveMQSession.java#L1955..."
>
> Forgive me if I sound ungrateful. But I had already seen that. Thanks just
> the same though.
>
> I've also seen the unit test for it [1]. And I've seen where it's used with
> objectMessageSerializationDefered [2]. ActiveMQObjectMessage.setObject()
> also uses objectMessageSerializationDefered [3].
>
> Another test I've seen is one that verifies that an ObjectMessage's body is
> serialized over TCP transport but not serialized over VM transport when
> objectMessageSerializationDefered is true [4].
>
>
> > "...a copy of the message is still sent even with the VM transport by
> default..."
>
> That might explain why I haven't had any luck producing pass-by-reference
> behavior (as I understand it [5]) in my experiments.
>
> Starting with this ActiveMQ-supplied example code [6] as a base, I tried
> every combination of values for objectMessageSerializationDefered and
> copyMessageOnSend I could think of.
>
> That documentation I linked to in my first post claims "Messages are passed
> by reference when objectMessageSerializationDefered is false" and "Messages
> are passed by value when objectMessageSerializationDefered is true". I
> would love to find a test that either verifies either of those claims or
> disproves either of them.
>
> Until I do though, I will still have these unanswered questions...
>
> 1. ...
> 2. Where can I find existing unit tests that exercise and verify ActiveMQ's
> expected pass-by-reference/pass-by-value behavior?
> 3. Is that ActiveMQ documentation even relevant for ActiveMQ 5+? Or only
> for 3x and 4x?
>
> ----
>
> [1] https://bit.ly/testAMQcopy
> [2] https://bit.ly/AMQcopy
> [3] https://bit.ly/setObject
> [4] https://bit.ly/MsgNotSerTest
> [5] https://bit.ly/JavaDudePbV
> [6] https://bit.ly/HelloAMQApp
>
>
> ----
>
> On Thu, Apr 1, 2021 at 2:39 PM Christopher Shannon <
> christopher.l.shannon@gmail.com> wrote:
>
> > I forgot to add that your experiment probably didn't show changes
> because I
> > believe by default the ActiveMQConnection has the flag
> "copyMessageOnSend"
> > set to true. So the message would have been copied anyways into a new
> > object which means the whole copy by reference wording is even less
> > relevant and more wrong since a copy of the message is still sent even
> with
> > the VM transport by default.
> >
> > See:
> > https://activemq.apache.org/connection-configuration-uri
> >
> >
> https://github.com/apache/activemq/blob/activemq-5.16.1/activemq-client/src/main/java/org/apache/activemq/ActiveMQSession.java#L1955
> >
> > On Thu, Apr 1, 2021 at 8:28 AM Christopher Shannon <
> > christopher.l.shannon@gmail.com> wrote:
> >
> > > Yes, Java is pass by value but the value is a copy of the reference and
> > > not an independent copy of the message (unlike C or C++). The
> > documentation
> > > wording should be fixed as it is wrong as you pointed out but I assume
> > > whoever wrote that was just referring to the fact that when the message
> > is
> > > sent by VM transport the same reference is used so you need to be
> careful
> > > as changes to the original message after sending would also affect the
> > > message that was already sent since they are the same object still. The
> > > messages are just stored in a data structure in the JVM (I think a
> > > LinkedBlockingQueue). So this is just (poorly) trying to convey the
> fact
> > > that the messages are not independent copies of the message like they
> > > would be if sent over TCP or another protocol.
> > >
> >
>
Re: What does "passed by reference" mean in ActiveMQ's context?
Posted by Cunningham <he...@gmail.com>.
> "...that was just referring to the fact that when the message is sent by
VM transport the same reference is used..."
That answers my question #1. Thanks.
> "...changes to the original message after sending would also affect the
message that was already sent since they are the same object still..."
Thanks. My experiments show that is true only if
ActiveMQObjectMessage.setObject(new Object()) is done in the producer after
the original send. The same change when done in consumers has no effect. In
both cases I set ActiveMQObjectMessage.setReadOnlyBody(false) beforehand,
by the way.
If you have running code that implements what you're describing, I'd be
grateful if you could share it. Please? TIA.
> "...See: .../org/apache/activemq/ActiveMQSession.java#L1955..."
Forgive me if I sound ungrateful. But I had already seen that. Thanks just
the same though.
I've also seen the unit test for it [1]. And I've seen where it's used with
objectMessageSerializationDefered [2]. ActiveMQObjectMessage.setObject()
also uses objectMessageSerializationDefered [3].
Another test I've seen is one that verifies that an ObjectMessage's body is
serialized over TCP transport but not serialized over VM transport when
objectMessageSerializationDefered is true [4].
> "...a copy of the message is still sent even with the VM transport by
default..."
That might explain why I haven't had any luck producing pass-by-reference
behavior (as I understand it [5]) in my experiments.
Starting with this ActiveMQ-supplied example code [6] as a base, I tried
every combination of values for objectMessageSerializationDefered and
copyMessageOnSend I could think of.
That documentation I linked to in my first post claims "Messages are passed
by reference when objectMessageSerializationDefered is false" and "Messages
are passed by value when objectMessageSerializationDefered is true". I
would love to find a test that either verifies either of those claims or
disproves either of them.
Until I do though, I will still have these unanswered questions...
1. ...
2. Where can I find existing unit tests that exercise and verify ActiveMQ's
expected pass-by-reference/pass-by-value behavior?
3. Is that ActiveMQ documentation even relevant for ActiveMQ 5+? Or only
for 3x and 4x?
----
[1] https://bit.ly/testAMQcopy
[2] https://bit.ly/AMQcopy
[3] https://bit.ly/setObject
[4] https://bit.ly/MsgNotSerTest
[5] https://bit.ly/JavaDudePbV
[6] https://bit.ly/HelloAMQApp
----
On Thu, Apr 1, 2021 at 2:39 PM Christopher Shannon <
christopher.l.shannon@gmail.com> wrote:
> I forgot to add that your experiment probably didn't show changes because I
> believe by default the ActiveMQConnection has the flag "copyMessageOnSend"
> set to true. So the message would have been copied anyways into a new
> object which means the whole copy by reference wording is even less
> relevant and more wrong since a copy of the message is still sent even with
> the VM transport by default.
>
> See:
> https://activemq.apache.org/connection-configuration-uri
>
> https://github.com/apache/activemq/blob/activemq-5.16.1/activemq-client/src/main/java/org/apache/activemq/ActiveMQSession.java#L1955
>
> On Thu, Apr 1, 2021 at 8:28 AM Christopher Shannon <
> christopher.l.shannon@gmail.com> wrote:
>
> > Yes, Java is pass by value but the value is a copy of the reference and
> > not an independent copy of the message (unlike C or C++). The
> documentation
> > wording should be fixed as it is wrong as you pointed out but I assume
> > whoever wrote that was just referring to the fact that when the message
> is
> > sent by VM transport the same reference is used so you need to be careful
> > as changes to the original message after sending would also affect the
> > message that was already sent since they are the same object still. The
> > messages are just stored in a data structure in the JVM (I think a
> > LinkedBlockingQueue). So this is just (poorly) trying to convey the fact
> > that the messages are not independent copies of the message like they
> > would be if sent over TCP or another protocol.
> >
>
Re: What does "passed by reference" mean in ActiveMQ's context?
Posted by Christopher Shannon <ch...@gmail.com>.
I forgot to add that your experiment probably didn't show changes because I
believe by default the ActiveMQConnection has the flag "copyMessageOnSend"
set to true. So the message would have been copied anyways into a new
object which means the whole copy by reference wording is even less
relevant and more wrong since a copy of the message is still sent even with
the VM transport by default.
See:
https://activemq.apache.org/connection-configuration-uri
https://github.com/apache/activemq/blob/activemq-5.16.1/activemq-client/src/main/java/org/apache/activemq/ActiveMQSession.java#L1955
On Thu, Apr 1, 2021 at 8:28 AM Christopher Shannon <
christopher.l.shannon@gmail.com> wrote:
> Yes, Java is pass by value but the value is a copy of the reference and
> not an independent copy of the message (unlike C or C++). The documentation
> wording should be fixed as it is wrong as you pointed out but I assume
> whoever wrote that was just referring to the fact that when the message is
> sent by VM transport the same reference is used so you need to be careful
> as changes to the original message after sending would also affect the
> message that was already sent since they are the same object still. The
> messages are just stored in a data structure in the JVM (I think a
> LinkedBlockingQueue). So this is just (poorly) trying to convey the fact
> that the messages are not independent copies of the message like they
> would be if sent over TCP or another protocol.
>
> On Thu, Apr 1, 2021 at 7:53 AM Cunningham <he...@gmail.com> wrote:
>
>> Java's built-in evaluation strategy [1] is 100% pass-by-value [2].
>>
>> A question regarding that fact arose for me when I read this note in the
>> ActiveMQ documentation: "One thing to note with the VM transport is that
>> messages are passed by reference" [3]. That contradicts the well-known
>> fact
>> about Java.
>>
>> Another question arose about this: "disable the automatic serialization of
>> ObjectMessage payloads so that the objects are passed by value in 4.x by
>> setting the objectMessageSerializationDefered flag to true" [4].
>>
>> The documentation leads you to expect that setting
>> objectMessageSerializationDefered to false somehow transforms Java's
>> built-in pass-by-value evaluation strategy to instead do
>> pass-by-reference.
>>
>> ActiveMQ's documentation is incomplete and not 100% crystal clear. Since
>> Java being pass-by-value is a given, the ActiveMQ documentation would be
>> more complete and less confusing if its contradictory use of
>> pass-by-reference and pass-by-value were explained in more detail.
>>
>> To try to understand what context ActiveMQ uses
>> pass-by-reference/pass-by-value, I've looked at some unit tests here [5]
>> and here [6]. Neither of those unit tests exercise what's claimed in the
>> documentation I quoted. I didn't see anything that looked like
>> pass-by-reference in the VMTransport code either [7].
>>
>> So I did some experimenting. I instantiated a new ActiveMQObjectMessage in
>> a method that implements one of several consumers in the receiving end of
>> a
>> VM transport. In that one mutating consumer's method, I then assigned the
>> locally instantiated new message to the original reference received from
>> the destination that several consumers subscribed to.
>>
>> If ActiveMQ truly does pass-by-reference in the standard sense of the
>> term,
>> then I would expect the assignment of the newly instantiated message
>> created in the one mutating consumer's method would be seen by all the
>> other consumers. I would expect the producer that sent the original
>> message
>> to also be able to observe — after sending the original message — that its
>> original reference would eventually refer to the new message instantiated
>> and assigned in the mutating consumer.
>>
>> Neither of those expectations were borne out in my experiment. Which is
>> not
>> a surprise, since Java doesn't do pass-by-refrence.
>>
>> Therefore my questions are:
>>
>> 1. In what sense does the ActiveMQ documentation mean "passed by
>> reference"?
>> 2. Where can I find existing unit tests that exercise and verify
>> ActiveMQ's
>> expected pass-by-reference/pass-by-value behavior?
>> 3. Is that ActiveMQ documentation even relevant for ActiveMQ 5+? Or only
>> for 3x and 4x?
>>
>>
>> ----
>>
>>
>> [1] https://bit.ly/WikiPbV
>> [2] https://bit.ly/JavaDudePbV
>> [3] https://bit.ly/VMTransPbR
>> [4] https://bit.ly/DisObjSerPbV
>> [5] https://bit.ly/ObjSerTest
>> [6] https://bit.ly/ObjMsgNotSer
>> [7] https://bit.ly/VMTrans
>>
>
Re: What does "passed by reference" mean in ActiveMQ's context?
Posted by Christopher Shannon <ch...@gmail.com>.
Yes, Java is pass by value but the value is a copy of the reference and not
an independent copy of the message (unlike C or C++). The documentation
wording should be fixed as it is wrong as you pointed out but I assume
whoever wrote that was just referring to the fact that when the message is
sent by VM transport the same reference is used so you need to be careful
as changes to the original message after sending would also affect the
message that was already sent since they are the same object still. The
messages are just stored in a data structure in the JVM (I think a
LinkedBlockingQueue). So this is just (poorly) trying to convey the fact
that the messages are not independent copies of the message like they
would be if sent over TCP or another protocol.
On Thu, Apr 1, 2021 at 7:53 AM Cunningham <he...@gmail.com> wrote:
> Java's built-in evaluation strategy [1] is 100% pass-by-value [2].
>
> A question regarding that fact arose for me when I read this note in the
> ActiveMQ documentation: "One thing to note with the VM transport is that
> messages are passed by reference" [3]. That contradicts the well-known fact
> about Java.
>
> Another question arose about this: "disable the automatic serialization of
> ObjectMessage payloads so that the objects are passed by value in 4.x by
> setting the objectMessageSerializationDefered flag to true" [4].
>
> The documentation leads you to expect that setting
> objectMessageSerializationDefered to false somehow transforms Java's
> built-in pass-by-value evaluation strategy to instead do pass-by-reference.
>
> ActiveMQ's documentation is incomplete and not 100% crystal clear. Since
> Java being pass-by-value is a given, the ActiveMQ documentation would be
> more complete and less confusing if its contradictory use of
> pass-by-reference and pass-by-value were explained in more detail.
>
> To try to understand what context ActiveMQ uses
> pass-by-reference/pass-by-value, I've looked at some unit tests here [5]
> and here [6]. Neither of those unit tests exercise what's claimed in the
> documentation I quoted. I didn't see anything that looked like
> pass-by-reference in the VMTransport code either [7].
>
> So I did some experimenting. I instantiated a new ActiveMQObjectMessage in
> a method that implements one of several consumers in the receiving end of a
> VM transport. In that one mutating consumer's method, I then assigned the
> locally instantiated new message to the original reference received from
> the destination that several consumers subscribed to.
>
> If ActiveMQ truly does pass-by-reference in the standard sense of the term,
> then I would expect the assignment of the newly instantiated message
> created in the one mutating consumer's method would be seen by all the
> other consumers. I would expect the producer that sent the original message
> to also be able to observe — after sending the original message — that its
> original reference would eventually refer to the new message instantiated
> and assigned in the mutating consumer.
>
> Neither of those expectations were borne out in my experiment. Which is not
> a surprise, since Java doesn't do pass-by-refrence.
>
> Therefore my questions are:
>
> 1. In what sense does the ActiveMQ documentation mean "passed by
> reference"?
> 2. Where can I find existing unit tests that exercise and verify ActiveMQ's
> expected pass-by-reference/pass-by-value behavior?
> 3. Is that ActiveMQ documentation even relevant for ActiveMQ 5+? Or only
> for 3x and 4x?
>
>
> ----
>
>
> [1] https://bit.ly/WikiPbV
> [2] https://bit.ly/JavaDudePbV
> [3] https://bit.ly/VMTransPbR
> [4] https://bit.ly/DisObjSerPbV
> [5] https://bit.ly/ObjSerTest
> [6] https://bit.ly/ObjMsgNotSer
> [7] https://bit.ly/VMTrans
>