Please review draft JEP: JMX Specific Annotations for Registration of Managed Resources

[Eamonn McManus]

A couple more comments on the draft as it stands.

I like the approach of @NotificationSupport on fields. However, the
name is not great I think. Maybe nest an @Inject annotation inside
NotificationSender, so you could write
  @NotificationSender.Inject private NotificationSender sender;
?

I think you are right about MBeanRegistration not being a great
alternative to annotating an explicit lifecycle method, even if
MBeanRegistration acquires default implementations of its methods.
Having classes implement callback interfaces is generally not very
clean, unless that is all they do. @RegistrationHandler on a method
with a RegisterEvent parameter is a good alternative. The
RegisterEvent parameter would be required, I think; otherwise the
MBean object would have to keep track of whether it is already
registered in order to know that a second call means it has been
unregistered.

Éamonn


2015-03-25 9:53 GMT-07:00 Jaroslav Bachorik <jaroslav.bachorik at oracle.com>:
> On 23.3.2015 13:12, Jaroslav Bachorik wrote:
>>
>> On 18.3.2015 23:28, Eamonn McManus wrote:
>>>>
>>>> Mainly because the long term goal (beyond the scope of this JEP,
>>>> anyway) would be to get users to slowly migrate to the annotation
>>>> based M(X)Beans. Not giving them the chance to specify the service
>>>> interface via annotations will mean keeping this dichotomy forever.
>>>
>>>
>>> I'm not sure that is a good goal. M(X)Bean interfaces allow clients to
>>> make proxies, and there's no obvious equivalent with annotations.
>>
>>
>> You still can create proxies for MXBeans defined through annotations -
>> the 'service' attribute of '@ManagedBean' annotation serves exactly this
>> purpose. The value of this attribute will be used in the associated
>> Descriptor under the 'interfaceClassName' key.
>>
>> In fact, the resulting instance registered in the MBeanServer for an
>> annotation based MXBean is undistinguishable from the one based on
>> MXBean interface.
>>
>>> Though I suppose you could provide a standard annotation processor
>>> that would generate the implied interface from the annotations.
>>
>>
>> Yes, this might be an option. But probably beyond the scope of this JEP.
>> I need to keep the change as simple as possible - otherwise it might not
>> make it for JDK 9.
>>
>>>
>>> Re @Notification: Please don't add types to the JMX API with the same
>>> name as types that are already there. That will make the API very
>>> unpleasant to use.
>>
>>
>> Agreed. A nice, simple name for this annotation will have to be found.
>>
>>>
>>> I don't understand what this text means: "It can also be used as a
>>> parameter annotation for a field of type NotificationSender." Is it
>>
>>
>> Should read '... for an argument of type NotificationSender'
>>
>>> applied to parameters or fields? The code example shows the former,
>>> but that seems a bit limiting. What if the MBean wants to send a
>>> notification at some point unrelated to method invocation?
>>
>>
>> For the sakes of simplicity I opted for something that seemed to be the
>> common case - sending notification from within the managed operations or
>> attribute getters/setters. Could you come up with a use case when it is
>> inevitable to send notification from a code not reachable either through
>> a managed operation or attribute getter/setter? If it is something
>> generally needed I might make the @Notification applicable to fields as
>> well.
>
>
> I was able to cleanup the notifications part a bit - moving the declaration
> from the top level annotation and the per-parameter annotations to just one
> place - an annotated field of type NotificationSender. This will also solve
> the problem with emitting notifications from the methods associated with
> neither the managed operations nor attributes. Basically a custom dependency
> injection - but very simple one without all the bells-and-whistles.
> Unfortunately, the @Resource annotation has been moved to jaxws in JDK 9 :(
>
> I also simplified the @RegistrationHandler - the solution you proposed,
> extending the MBeanRegistration interface, is not something I would really
> like to do now - mostly because a logical part of this interface is hidden
> in DynamicMBean2 (preRegister2 method) and consolidating this will take a
> major effort on its own.
>
> Hopefully I was able to come up with concise and simple naming for the
> annotations - conveying their purpose and not being too chatty.
>
> Eamonn, thank you once again for taking your time to review this draft. I am
> planning to submit this JEP in the next two days. Submitting the JEP does
> not mean freezing the specification - just acknowledging that the JEP is
> worth of pursuing. There will be at least one more additional JEP review in
> the process and then the final code review before push.
>
>
> -JB-
>
>
>>
>> Thanks,
>>
>> -JB-
>>
>>>
>>> Éamonn
>>>
>>>
>>> 2015-03-04 10:38 GMT-08:00 Jaroslav Bachorik
>>> <jaroslav.bachorik at oracle.com>:
>>>>
>>>> Thanks for taking the time to review this.
>>>> I apologize for the formatting mess - clearly the JIRA's markdown
>>>> processor
>>>> is rather confused with more extensive usage of the code blocks.
>>>>
>>>> On 4.3.2015 18:42, Eamonn McManus wrote:
>>>>>
>>>>>
>>>>> Thank you for updating the JEP text referencing JSR 255.
>>>>>
>>>>> Perhaps unsurprisingly I disagree with many of the differences between
>>>>> this proposal and the one we carefully thought out in JSR 255. Even
>>>>> though a lot has changed in the meanwhile, I don't see anything that
>>>>> invalidates our assumptions of the time.
>>>>>
>>>>> For reference, a snapshot of the JSR 255 javadoc is at
>>>>> http://hg.openjdk.java.net/jmx2/jmx2/file/f417598a7b72/javadoc.
>>>>> Unfortunately, it appears that the Mercurial server now serves these
>>>>> files as application/binary, probably because of the change here:
>>>>>
>>>>>
>>>>> http://mercurial.selenic.com/wiki/UpgradeNotes#A1.9.1:_guessmime.2C_revert_behavior_restored.
>>>>>
>>>>>
>>>>> To address some specific points:
>>>>>>
>>>>>>
>>>>>> would you care to elaborate what you find to be not "even correct
>>>>>> Java"?
>>>>>
>>>>>
>>>>>
>>>>> As of Java 8, annotation elements cannot have null values so the
>>>>> "default null" clauses are nonsense. I have not seen any proposal to
>>>>> change this in Java 9. The @ManagedBean definition also has an obvious
>>>>> syntax error.
>>>>
>>>>
>>>>
>>>> Agreed. They should not be there. During the updates JIRA failed to
>>>> update
>>>> the field and I failed to notice that my edits didn't apply.
>>>>
>>>>>
>>>>>> - ability to annotate fields turning them into attributes
>>>>>
>>>>>
>>>>>
>>>>> This might be useful for read-only attributes. I'd question whether it
>>>>> is useful for read/write attributes, because I think it will be very
>>>>> unusual for you to want neither validation of the new value nor
>>>>> behaviour to be triggered by the set.
>>>>
>>>>
>>>>
>>>> On the other hand it gives the possibility to expose those read-only
>>>> fields
>>>> (eg. metrics or settings being immutable via JMX) without the
>>>> necessity to
>>>> conjure the getter.
>>>>
>>>>>
>>>>>> - ability to provide metadata directly in the annotations, not relying
>>>>>> solely on inferring them from the annotated element
>>>>>
>>>>>
>>>>>
>>>>> I'm not sure what specifically this refers to. Do you mean for example
>>>>> that it is possible to add @ManagedAttribute to a method that does not
>>>>> look like getFoo() and nevertheless have the annotation say that the
>>>>> attribute is called foo? I don't see any particular advantage to that
>>>>> flexibility. The getFoo() pattern is already familiar, and having a
>>>>> second, completely different way of specifying the name just
>>>>> complicates the spec for not much benefit.
>>>>
>>>>
>>>>
>>>> And yet it can be done in DynamicMBeans. My starting point was the
>>>> attempt
>>>> to give the user the same flexibility she would have if she were
>>>> hand-crafting the various MBean*Info classes.
>>>>
>>>>>
>>>>>> - missing @ManagedConstructor to expose a constructor
>>>>>
>>>>>
>>>>>
>>>>> We deliberately omitted this. The fact that MBeanConstructorInfo
>>>>> exists at all is in my opinion a mistake in the original JMX
>>>>> specification. What does it mean for an MBean to tell you how to
>>>>> construct another instance of itself? And if the purpose is to specify
>>>>> which constructors from this class are available to the MBean Server,
>>>>> there's no use for names and descriptions, and there's no particular
>>>>> advantage over just saying that all public constructors are available.
>>>>
>>>>
>>>>
>>>> I don't know the meaning. I was not involved in the inception of this
>>>> API.
>>>> My reasoning is that if you can do it by hand than it should probably be
>>>> achievable by annotation as well. The other route would be
>>>> deprecating the
>>>> MBeanConstructorInfo now and removing it in a subsequent release.
>>>>
>>>>>
>>>>>> - optional 'service' argument to @ManagedBean annotation which will be
>>>>>> reflected in the descriptor's 'interfaceClassName' field to provide a
>>>>>> guidance about the recommended service interface when using
>>>>>> JMX.newMXBeanProxy()
>>>>>
>>>>>
>>>>>
>>>>> If you have such an interface, why wouldn't you just use it to define
>>>>> the MBean and dispense with annotations?
>>>>
>>>>
>>>>
>>>> Mainly because the long term goal (beyond the scope of this JEP, anyway)
>>>> would be to get users to slowly migrate to the annotation based
>>>> M(X)Beans.
>>>> Not giving them the chance to specify the service interface via
>>>> annotations
>>>> will mean keeping this dichotomy forever.
>>>>
>>>>>
>>>>> Some other comments:
>>>>>
>>>>> * @ManagedBean.
>>>>>
>>>>> We called this @MBean because we also had an @MXBean annotation. That
>>>>> annotation exists today, but JSR 255 allowed it to be applied to a
>>>>> class as well as to an interface. It appears that @ManagedBean only
>>>>> defines MXBeans, which is a legitimate choice but, first, it should be
>>>>> called out more explicitly, and, second, wouldn't it then make sense
>>>>> to extend the existing @MXBean annotation?
>>>>
>>>>
>>>>
>>>> I thought about this and extending an existing annotation is pretty
>>>> sensitive from the compatibility PoV. Also, giving the annotation
>>>> different
>>>> meanings depending whether it is used on interface or class is rather
>>>> wobbly. I am still open to suggestions for better naming, though.
>>>>
>>>>>
>>>>> The specification is inconsistent as to whether the annotation is
>>>>> @ManagedBean or @MBean.
>>>>>
>>>>> I think it is a fair idea to have an objectName field, but the idea of
>>>>> randomly appending numbers to the name for disambiguation is broken.
>>>>
>>>>
>>>>
>>>> Ok. Valid point.
>>>>
>>>>> Something like @ObjectNameTemplate from JSR 255 is more appropriate.
>>>>
>>>>
>>>>
>>>> Yes, but it brings even more complexity.
>>>>
>>>>>
>>>>> The text for the notifications() member references @TypeMapping but
>>>>> does not say what that is. The declared type is Notification[] and the
>>>>> text defines an annotation @Notification, but there is already a class
>>>>> called Notification in javax.management.
>>>>
>>>>
>>>>
>>>> The annotations should be placed in a sub-package of
>>>> "javax.management". The
>>>> "javax.management" is pretty crowded already.
>>>>
>>>>>
>>>>> I think that the simple "name=value" syntax used by JSR 255's
>>>>> @DescriptorFields is preferable to the unspecified and verbose type
>>>>> @Tag. I don't see an advantage to making people write @Tag(name =
>>>>> "foo", value = "bar") rather than just "foo=bar". This syntax is
>>>>> already present in the JMX spec, for example in the
>>>>> ImmutableDescriptor constructor.
>>>>
>>>>
>>>>
>>>> IMO, having just plain text there will open door for spurious errors
>>>> due to
>>>> typos in delimiters. But that's just my experience.
>>>>
>>>>>
>>>>> * @Notification.
>>>>>
>>>>> As I mentioned, you can't use that name.
>>>>>
>>>>> The first paragraph of the description is indecipherable.
>>>>>
>>>>> The NotificationSender interface is unspecified. Based on the example,
>>>>> I think it could potentially be a major usability improvement but it's
>>>>> hard to be sure.
>>>>
>>>>
>>>>
>>>> I can add this interface to the proposal. The reason for it not being
>>>> explicitly specified is that it is still very prototypical.
>>>>
>>>>>
>>>>> I think it's extremely ugly to propagate the misspelling clazz into an
>>>>> API where people will have to write it. Also, if it must extend
>>>>> Notification then it should be specified as Class<? extends
>>>>> Notification>.
>>>>
>>>>
>>>>
>>>> The problem is that using the rather obvious "type" creates confusion
>>>> with
>>>> the "types". I can't use "class", of course. I am not too happy about
>>>> this
>>>> name either but anything else will just be more verbose.
>>>>
>>>>>
>>>>> * @ManagedAttribute
>>>>>
>>>>> It's extremely strange for this to have getter and setter fields. Why
>>>>> wouldn't it just be applied to those methods?
>>>>
>>>>
>>>>
>>>> Less boilerplate. One wouldn't need to retype the whole
>>>> @ManagedAttribute
>>>> definition twice.
>>>>
>>>>>
>>>>> Promoting units from a descriptor field to a separate annotation
>>>>> member seems like a good idea. The specified value would be copied
>>>>> into the Descriptor.
>>>>
>>>>
>>>>
>>>> Exactly.
>>>>
>>>>>
>>>>> * @ManagedOperation
>>>>>
>>>>> I don't see any reason to allow the name to be different from the
>>>>> method name. It just complicates the spec.
>>>>
>>>>
>>>>
>>>> Well, you can do it manually. But I am open here - it would be nice
>>>> to hear
>>>> from potential users whether this would make sense.
>>>>
>>>>>
>>>>> Instead of repeating a description member inside every annotation, JSR
>>>>> 255 defined a top-level @Description, which included elements for
>>>>> internationalization. Hardcoded strings are a step backwards.
>>>>
>>>>
>>>>
>>>> Until we have support for providing the client locale to the JMX
>>>> server any
>>>> internationalization is rather illusionary.
>>>>
>>>>>
>>>>> Defining Impact inside this annotation is questionable. I'd expect
>>>>> user code and possible future API changes to want to reference it
>>>>> independently of the annotation. Also, the JSR 255 enum Impact had
>>>>> methods to convert to and from the integer codes used by
>>>>> MBeanOperationInfo.
>>>>
>>>>
>>>>
>>>> Please, consider class packaging being transitional. The classes may
>>>> change
>>>> their locations during the draft review.
>>>>
>>>>>
>>>>> * @ManagedParameter
>>>>>
>>>>> The text repeatedly says operation name and method name when it means
>>>>> parameter name. I assume that if the name member is empty then the
>>>>> parameter name from reflection is used, which since Java 8 could be
>>>>> the actual name of the parameter if the class was compiled with
>>>>> -parameters.
>>>>
>>>>
>>>>
>>>> Precisely.
>>>>
>>>>>
>>>>> * @RegistrationHandler
>>>>>
>>>>> It seems like an API smell for an annotation to say that it must be
>>>>> applied to methods with a certain signature. I think a much better
>>>>> approach would be to change the existing MBeanRegistration interface
>>>>> so that its methods have default implementations that do nothing. That
>>>>> removes the main reason that this interface is a pain: having to
>>>>> implement four methods when you're usually only interested in one. You
>>>>> could also add a preDeregister overload with MBeanServer and
>>>>> ObjectName parameters, again with a default implementation.
>>>>
>>>>
>>>>
>>>> Well, @ManagedAttribute must be applied to methods of certain signatures
>>>> only, too.
>>>>
>>>> I wanted to avoid the necessity for the annotated M(X)Bean to
>>>> implement any
>>>> other JMX specific interfaces explicitly. Therefore I proposed this
>>>> annotation.
>>>>
>>>> -JB-
>>>>
>>>>
>>>>
>>>>>
>>>>> Éamonn
>>>>>
>>>>>
>>>>> 2015-03-04 0:47 GMT-08:00 Jaroslav Bachorik
>>>>> <jaroslav.bachorik at oracle.com>:
>>>>>>
>>>>>>
>>>>>> On 4.3.2015 02:09, Eamonn McManus wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Could you explain what you mean by this, regarding the annotations
>>>>>>> that were already agreed on by the JSR 255 Expert Group:
>>>>>>>
>>>>>>> * Smaller scope compared to the proposed solution
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> This is a leftover from the previous proposal which had wider scope
>>>>>> than
>>>>>> what is presented now. Still a few points remain.
>>>>>>
>>>>>> - ability to annotate fields turning them into attributes
>>>>>> - ability to provide metadata directly in the annotations, not relying
>>>>>> solely on inferring them from the annotated element
>>>>>> - missing @ManagedConstructor to expose a constructor
>>>>>> - optional 'service' argument to @ManagedBean annotation which will be
>>>>>> reflected in the descriptor's 'interfaceClassName' field to provide a
>>>>>> guidance about the recommended service interface when using
>>>>>> JMX.newMXBeanProxy()
>>>>>>
>>>>>>> * Conceptually in pre JDK7 era
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> I am afraid this relates more to the implementation - or at least the
>>>>>> code I
>>>>>> was able to reconstruct from the repo history. Shouldn't be mentioned
>>>>>> here.
>>>>>>
>>>>>>>
>>>>>>> I have a number of other comments, but procedurally I'm not sure what
>>>>>>> the precedent is for summarily discarding work previously done in the
>>>>>>> JCP on the same subject. I'd certainly have expected this JEP to
>>>>>>> start
>>>>>>> from that work, rather than proposing a starting point that isn't
>>>>>>> even
>>>>>>> correct Java.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Well, this is a draft review. The JSR 255 annotations work is not
>>>>>> discarded.
>>>>>> It is mentioned in the alternatives. The purpose of the open review
>>>>>> is to
>>>>>> find out whether it is ok to continue with proposed feature, modify
>>>>>> it to
>>>>>> use eg. JSR 255 work or abandon it completely.
>>>>>>
>>>>>> -JB-
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Éamonn McManus, former JSR 255 Spec Lead
>>>>>>>
>>>>>>> 2015-03-03 8:27 GMT-08:00 Jaroslav Bachorik
>>>>>>> <jaroslav.bachorik at oracle.com>:
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Hi all,
>>>>>>>>
>>>>>>>> Please review this draft JEP for JMX Specific Annotations for
>>>>>>>> Registration of Managed Resources:
>>>>>>>>
>>>>>>>>         https://bugs.openjdk.java.net/browse/JDK-8044507
>>>>>>>>
>>>>>>>> Background:
>>>>>>>> Current mechanism of defining an MBean requires to provide an MBean
>>>>>>>> interface and its implementation. The interface and the
>>>>>>>> implementation
>>>>>>>> must
>>>>>>>> conform to the strict naming and visibility rules in order for the
>>>>>>>> introspection to be able to bind them.
>>>>>>>>
>>>>>>>> At least the same level of verbosity is required when adding an
>>>>>>>> MBeanInfo
>>>>>>>> to generate MBean metadata.
>>>>>>>>
>>>>>>>> All this leads to a rather verbose code containing a lot of
>>>>>>>> repeating
>>>>>>>> boilerplate parts even for the most simple MBean registrations.
>>>>>>>>
>>>>>>>> This JEP proposes to add a set of annotations for registration and
>>>>>>>> configuration of manageable resources (in other word 'MBeans').
>>>>>>>> These
>>>>>>>> annotations will be used to generate all the metadata necessary
>>>>>>>> for a
>>>>>>>> resources to be accepted by the current JMX system.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>> -JB-
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>
>>
>