Bug renders Eclipse debugger useless if class has/uses generics:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=344856
Also, Eclipse Glassfish plugins are incapable of launching in debug mode unless the domain.xml is set up to use the default debug port (9009). if your domain.xml has been changed or customized, Eclipse will not be able to attach the debugger when launching the server in debug mode. Unfortunately, the "connection refused" doesn't give you much of a clue what's going on.
http://stackoverflow.com/questions/8344454/oepe-glassfish-debug-connection-refused
Tuesday, March 06, 2012
Wednesday, January 11, 2012
Handling poison JMS messages in Glassfish - infinite loop WTF?
I found this post useful for understanding problems with handling "poison messages" in message-driven beans:
http://weblogs.java.net/blog/felipegaucho/archive/2009/09/24/handling-poison-messages-glassfish
The gist of it is that even if you think you caught an exception, your transaction still might roll back and cause the JMS message to be re-delivered. (The MDB is an EJB, which will default to container-managed transactions, equivalent to having REQUIRED on each method.)
There are two small caveats to add:
1. The specific issue with JPA is that certain persistence exceptions mark the transaction for rollback, even if the exception is actually caught. It matters not whether that JPA activity is happening directly in onMessage() or in a "sub-transaction".
2. Simply annotating another method in the same MDB with @TransactionAttribute(TransactionAttributeType.REQUIRES_NEW) does not actually create a separate transaction context. If you are calling a method locally within the same EJB, "this" refers to the object itself, while the transaction behavior lives in the EJB proxy wrapper. (See http://stackoverflow.com/questions/427452/ejb-transactions-in-local-method-calls) So you actually have to put this transaction in a separate EJB and inject it into the MDB with @Inject or @EJB annotations.
And now for the "WTF" part.
The thing that really surprised me is the retry behavior. If the MDB throws an exception, and the transaction rolls back as a result, Glassfish recognizes that there was an exception and will only re-deliver the message once. There is some setting somewhere that controls how many retries are attempted, I believe. (Haven't found it.)
If you catch an exception that marked the transaction for rollback (or the transaction was marked for rollback programmatically), the transaction still rolls back, and the message is redelivered. However, the rollback without exception does not fire Glassfish's retry counter, so you end up in an infinite loop.
Either way the solution is the same, but still -WTF!
http://weblogs.java.net/blog/felipegaucho/archive/2009/09/24/handling-poison-messages-glassfish
The gist of it is that even if you think you caught an exception, your transaction still might roll back and cause the JMS message to be re-delivered. (The MDB is an EJB, which will default to container-managed transactions, equivalent to having REQUIRED on each method.)
There are two small caveats to add:
1. The specific issue with JPA is that certain persistence exceptions mark the transaction for rollback, even if the exception is actually caught. It matters not whether that JPA activity is happening directly in onMessage() or in a "sub-transaction".
2. Simply annotating another method in the same MDB with @TransactionAttribute(TransactionAttributeType.REQUIRES_NEW) does not actually create a separate transaction context. If you are calling a method locally within the same EJB, "this" refers to the object itself, while the transaction behavior lives in the EJB proxy wrapper. (See http://stackoverflow.com/questions/427452/ejb-transactions-in-local-method-calls) So you actually have to put this transaction in a separate EJB and inject it into the MDB with @Inject or @EJB annotations.
And now for the "WTF" part.
The thing that really surprised me is the retry behavior. If the MDB throws an exception, and the transaction rolls back as a result, Glassfish recognizes that there was an exception and will only re-deliver the message once. There is some setting somewhere that controls how many retries are attempted, I believe. (Haven't found it.)
If you catch an exception that marked the transaction for rollback (or the transaction was marked for rollback programmatically), the transaction still rolls back, and the message is redelivered. However, the rollback without exception does not fire Glassfish's retry counter, so you end up in an infinite loop.
Either way the solution is the same, but still -WTF!
Saturday, January 07, 2012
equals and hashCode on @Entity classes: just say no
I've come to the conclusion that you should avoid defining equals and hashCode on JPA @Entity objects unless you have a really good reason to.
Doing it right is non-trivial, with all the gotchas and caveats. First, the logistics:
So what's the worst thing that happens if you just don't do anything, and take the default based on object identity?
Usually, you won't even miss these methods. The default implementation will work fine unless you're trying to compare objects loaded in two different persistence sessions/transactions,with either equals() or contains(). I've found this is the exception case - much of my collection manipulation in practice is manipulating objects all loaded in the same session to hand off to the view layer, so HashSet recognizes duplicates and contains() works just fine. And I almost never use an entity as a key in a hashmap.
The price you pay is more verbosity when you do need to compare objects between a session-scoped cache and the current request. You have to remember that obj1.equals(obj2) doesn't work, and needs to be replaced with obj1.getId().equals(obj2.getId()). Contains is a bit more verbose, and a utility method like containsById(collection, obj) might be helpful. Some people will say it's confusing that equals doesn't "just work" but I find it less confusing to be explicit about what you're comparing on - and less confusing than a broken or unmaintained equals() method.
Finally, if this were C#, we wouldn't even have this discussion. With closures and LINQ extension methods we would just say "collection.Where(obj => obj.id = foo.id)" and be done with it!
Related links:
http://community.jboss.org/wiki/EqualsAndHashCode
http://onjava.com/pub/a/onjava/2006/09/13/dont-let-hibernate-steal-your-identity.html?page=3
http://stackoverflow.com/questions/1929445/to-equals-and-hashcode-or-not-on-entity-classes-that-is-the-question
http://stackoverflow.com/questions/1638723/equals-and-hashcode-in-hibernate
https://forum.hibernate.org/viewtopic.php?t=928172
http://www.ibm.com/developerworks/java/library/j-jtp05273/index.html: "For mutable objects, the answer is not always so clear. Should equals() and hashCode() be based on the object's identity (like the default implementation) or the object's state (like Integer and String)? There's no easy answer -- it depends on the intended use of the class... It is not common practice to use a mutable object like a List as a key in a HashMap."
http://burtbeckwith.com/blog/?p=53
Doing it right is non-trivial, with all the gotchas and caveats. First, the logistics:
- You can't use the primary key because multiple unsaved objects all have a null PK and would be "equal".
- Using the PK with object identity as fallback means an object won't be equal to itself before and after being persisted, and hash-based collections won't work properly if the hash code changes during the collection's lifespan.
- A unique business key could work, if that's an option - but you have to remember to maintain the equals and hashCode method if those properties or relations change. Also if that key isn't immutable, you have a similar problem as with PK + fallback above.
- Creating a UUID in the constructor just to make equals and hashcode work is... yucky.
- Often these methods are insufficiently covered by unit tests.
So what's the worst thing that happens if you just don't do anything, and take the default based on object identity?
Usually, you won't even miss these methods. The default implementation will work fine unless you're trying to compare objects loaded in two different persistence sessions/transactions,with either equals() or contains(). I've found this is the exception case - much of my collection manipulation in practice is manipulating objects all loaded in the same session to hand off to the view layer, so HashSet recognizes duplicates and contains() works just fine. And I almost never use an entity as a key in a hashmap.
The price you pay is more verbosity when you do need to compare objects between a session-scoped cache and the current request. You have to remember that obj1.equals(obj2) doesn't work, and needs to be replaced with obj1.getId().equals(obj2.getId()). Contains is a bit more verbose, and a utility method like containsById(collection, obj) might be helpful. Some people will say it's confusing that equals doesn't "just work" but I find it less confusing to be explicit about what you're comparing on - and less confusing than a broken or unmaintained equals() method.
Finally, if this were C#, we wouldn't even have this discussion. With closures and LINQ extension methods we would just say "collection.Where(obj => obj.id = foo.id)" and be done with it!
Related links:
http://community.jboss.org/wiki/EqualsAndHashCode
http://onjava.com/pub/a/onjava/2006/09/13/dont-let-hibernate-steal-your-identity.html?page=3
http://stackoverflow.com/questions/1929445/to-equals-and-hashcode-or-not-on-entity-classes-that-is-the-question
http://stackoverflow.com/questions/1638723/equals-and-hashcode-in-hibernate
https://forum.hibernate.org/viewtopic.php?t=928172
http://www.ibm.com/developerworks/java/library/j-jtp05273/index.html: "For mutable objects, the answer is not always so clear. Should equals() and hashCode() be based on the object's identity (like the default implementation) or the object's state (like Integer and String)? There's no easy answer -- it depends on the intended use of the class... It is not common practice to use a mutable object like a List as a key in a HashMap."
http://burtbeckwith.com/blog/?p=53
Tuesday, January 03, 2012
Code coverage - unintended benefit
Unit test coverage is not a panacea. You can reach 100% unit test coverage without a single assertion about outputs or business logic, in which case the tests aren't that useful.
Or are they? Just the act of executing every single line and branch of code does provide one important value: it demonstrates that you can show what inputs or conditions trigger which parts of the code.
More importantly, the act of getting there helps reveal sections of code and conditionals that you might not even need. Having a coverage target also gives you incentives to stay "clean" and avoid gratuitous not-null checks or try/catch blocks, two pet peeves of mine. (I've seen too many not-null checks that looked defensive at first but in reality just kicked the can down the road, further obscuring the real problem.)
The other benefit I've found is that a coverage goal encourages refactoring that you should be doing anyway, because well-factored code is easier to unit test. For instance I had one JSF/JPA application where I was accessing JPA EntityManagers directly in the JSF managed beans. This made unit tests on the managed beans annoying because I had to mock out EntityManager.createQuery and dozens of Query.setParameter calls for each JPA action. By pulling the JPA actions into a separate DAO layer, I could just mock a single call to myDAO.getStuff(arguments). Plus, after isolating the DAO, I could then write an integration test on the DAO hooking up to a real database.
Other related links:
http://www.wakaleo.com/blog/316-code-coverage-as-a-refactoring-tool
http://codebetter.com/patricksmacchia/2009/06/07/high-test-coverage-ratio-is-a-good-thing-anyway/
Or are they? Just the act of executing every single line and branch of code does provide one important value: it demonstrates that you can show what inputs or conditions trigger which parts of the code.
More importantly, the act of getting there helps reveal sections of code and conditionals that you might not even need. Having a coverage target also gives you incentives to stay "clean" and avoid gratuitous not-null checks or try/catch blocks, two pet peeves of mine. (I've seen too many not-null checks that looked defensive at first but in reality just kicked the can down the road, further obscuring the real problem.)
The other benefit I've found is that a coverage goal encourages refactoring that you should be doing anyway, because well-factored code is easier to unit test. For instance I had one JSF/JPA application where I was accessing JPA EntityManagers directly in the JSF managed beans. This made unit tests on the managed beans annoying because I had to mock out EntityManager.createQuery and dozens of Query.setParameter calls for each JPA action. By pulling the JPA actions into a separate DAO layer, I could just mock a single call to myDAO.getStuff(arguments). Plus, after isolating the DAO, I could then write an integration test on the DAO hooking up to a real database.
Other related links:
http://www.wakaleo.com/blog/316-code-coverage-as-a-refactoring-tool
http://codebetter.com/patricksmacchia/2009/06/07/high-test-coverage-ratio-is-a-good-thing-anyway/
Primefaces AJAX callbacks: onstart vs. onclick
I just learned the hard way that onstart and onclick are not the same thing.
In particular, a "return ..." has very different semantics in both cases.
Consider this code:
If "func()" return false, this code will abort the AJAX request and bean.method() won't get called.
If "func()" returns true, the AJAX request processes.
If you replace onstart with onclick, the AJAX request will abort even if func() returns true.
That's because the Primefaces puts the code to generate the AJAX request in the onclick handler, pre-pending your code from the p:commandLink onclick before it. If your code returns, the AJAX request never gets sent.
In particular, a "return ..." has very different semantics in both cases.
Consider this code:
<p:commandLink action="#{bean.method}" onstart="return func()" ...>
If "func()" return false, this code will abort the AJAX request and bean.method() won't get called.
If "func()" returns true, the AJAX request processes.
If you replace onstart with onclick, the AJAX request will abort even if func() returns true.
That's because the Primefaces puts the code to generate the AJAX request in the onclick handler, pre-pending your code from the p:commandLink onclick before it. If your code returns, the AJAX request never gets sent.
Friday, December 30, 2011
Atlas Debugged: The Fountainhead, YAGNI and "clean code"
The last time I re-read The Fountainhead, I felt like many of the ideas in the book could be useful for software developers. It seemed like there are many parallels between the values demonstrated by the book's hero, Howard Roark, and good development practices.
Many of the ideas dovetail nicely with some Agile development principles, especially the various "keep it simple" ones like YAGNI and DRY (along with DRY's cousins,"once and only once", and "three strikes and refactor"). Roark's designs are driven entirely by purpose, function and constraints, with an open disdain for non-functional or ornamental additions. For example, about the house he builds for Austen Heller, Roark says:
Keeping things simple and focused is hard work, just as agile development and YAGNI is not an excuse for sloppiness. If done correctly, it should be quite the opposite. In Roark's designs, “Not a line seemed superfluous, not a needed plane was missing. The structures were austere and simple, until one looked at them and realized what work, what complexity of method, what tension of thought had achieved the simplicity.” [emphasis added]
This made me think of the quote in Uncle Bob's Clean Code: "Learning to write clean code is hard work... You must sweat over it. You must practice it yourself, and watch yourself fail. You must watch others practice it and fail."
There were some other ideas I'm hoping to examine in more depth:
- right tool for the job - using technology idiomatically vs. legacy patterns with new technology
- the architect as a hands-on practitioner (vs. ivory tower)
- leveraging innovations - new methods, technologies etc.
- professional satisfaction / motivation
- interactions with business stakeholders, "people skills" and organizational politics
- making best of bad situations - looking for best possible solution even if you don't agree with the business problem to be solved
Many of the ideas dovetail nicely with some Agile development principles, especially the various "keep it simple" ones like YAGNI and DRY (along with DRY's cousins,"once and only once", and "three strikes and refactor"). Roark's designs are driven entirely by purpose, function and constraints, with an open disdain for non-functional or ornamental additions. For example, about the house he builds for Austen Heller, Roark says:
“Every piece of it is there because the house needs it – and for no other reason… You can see each stress, each support that meets it... But you’ve seen buildings with columns that support nothing, with purposeless cornices, with pilasters, moldings, false arches, false windows… Your house is made by its own needs. The others are made by the need to impress. The determining motive of your house is in the house. The determining motive of others is the audience.”
Keeping things simple and focused is hard work, just as agile development and YAGNI is not an excuse for sloppiness. If done correctly, it should be quite the opposite. In Roark's designs, “Not a line seemed superfluous, not a needed plane was missing. The structures were austere and simple, until one looked at them and realized what work, what complexity of method, what tension of thought had achieved the simplicity.” [emphasis added]
This made me think of the quote in Uncle Bob's Clean Code: "Learning to write clean code is hard work... You must sweat over it. You must practice it yourself, and watch yourself fail. You must watch others practice it and fail."
There were some other ideas I'm hoping to examine in more depth:
- right tool for the job - using technology idiomatically vs. legacy patterns with new technology
- the architect as a hands-on practitioner (vs. ivory tower)
- leveraging innovations - new methods, technologies etc.
- professional satisfaction / motivation
- interactions with business stakeholders, "people skills" and organizational politics
- making best of bad situations - looking for best possible solution even if you don't agree with the business problem to be solved
Saturday, November 12, 2011
Primefaces global AJAX events
You can use jQuery global AJAX events with PrimeFaces to refactor behaviors that appear on multiple components. A good example is if you have a data grid with multiple p:commandLinks and other controls that execute different methods and re-render the grid, and need to run the same onComplete in all of them.
You could pull the updateGrid() statement out into an AJAX listener like this
For example, if you start with something like this:
<h:panelGroup id="grid">...</h:panelGroup>
<p:commandLink update="grid" actionListener="#{bean.update1}" onComplete="updateGrid()"/>
<p:commandLink update="grid" actionListener="#{bean.update2}" onComplete="updateGrid()"/>
<p:commandLink update="grid" actionListener="#{bean.update3}" onComplete="updateGrid()"/>
You could pull the updateGrid() statement out into an AJAX listener like this
jQuery(document).ajaxComplete(function(e, xhr, opts) {
$response = jQuery(xhr.responseXML);
if ($response.find("div[id$='grid']").length > 0) {
updateGrid();
}
});
By using ajaxComplete and parsing the XHR response object, you can see which DIV was going to be impacted by the partial update. That way, if you had some other AJAX controls (say, an autocompleter) that you didn't want to trigger the updateGrid() function, you could filter that out. Another option would be to set global=false on the specific Primefaces components that you don't want to fire the global jQuery ajaxComplete event. The Primefaces autocompleter doesn't support this in Primefaces 2.x but does in 3.0.
Tuesday, November 08, 2011
Primefaces p:ajax and jQuery AJAX events
All of Primefaces' JSF components use the jQuery AJAX engine, which means you can catch global AJAX events with $(document).ajaxStart and $(document).ajaxStop.
Standard JSF components like h:selectOneMenu will also go through jQuery when using a p:ajax facet (vs. standard f:ajax).
Conversely, jsf.ajax.onEvent does not appear to work with Primefaces components as it does for f:ajax.
See also:
IE9 and strict JSON checking
Got an error from IE9 complaining about "invalid character". Turned out this was because the JSON response string had control characters (newlines) in it. These need to be escaped.
Other browsers will let you get away with it, but IE9 is particularly strict and jQuery doesn't check the JSON response string before handing it off to JSON.parse.
Similar issue found here: https://mail.mozilla.org/pipermail/es-discuss/2010-June/011420.html
Other browsers will let you get away with it, but IE9 is particularly strict and jQuery doesn't check the JSON response string before handing it off to JSON.parse.
Similar issue found here: https://mail.mozilla.org/pipermail/es-discuss/2010-June/011420.html
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