Archive for the ‘OTDT’ Category
Runtime Specialization – At Last
Between a rock and a hard place
Not long ago, I had to pull Object Teams out of the Eclipse simultaneous release train. Reason: the long standing issue of using BCEL for bytecode weaving, for which no Java 8 compatible version has yet been released. With the Eclipse platform moving to Java 8, this had escalated to a true blocker. During the last weeks I investigated two options in parallel:
- Upgrade BCEL to a release candidate of the upcoming 6.0 release.
- Finalize the alternative weaver for OT/J, which is based on ASM, and thus already capable of handling Java 8 byte codes
I soon found out that even BCEL 6.0 will not be a full solution, because it still has no real support for creating StackMapTable attributes for newly generated bytecode, which however is strictly mandatory for running on a JVM 8.
For that reason I then focussed on the OTDRE, the Object Teams Dynamic Runtime Environment. This has been announced long ago, and after all, I promised to show a sneak preview of this feature in my presentation at EclipseCon Europe:
Runtime Specialization
Java has never been so dynamic before
Success at last
Today I can report success in two regards:
- The Object Teams Development Tooling, which itself is a complex OT/J application, can (mostly) run on the new runtime!
- I created a first demo example that shows the new capability of runtime weaving in action – it works! 🙂
This is a major milestone! Running OTDT on top of OTDRE is a real stress test for that new component – once again I realize that dog-fooding an entire IDE on its own technology is quite an exciting exercise. While a few exceptions need to be ironed out before the Neon release, I’m confident now, that we’re finally and really on the home stretch of this effort.
And after all the hard work on Java 8, also OT/J can finally fully leverage the new version, not only in theory, but also in bytecode.
Less than one week to finalize the presentation. You can be sure this will be a fresh story. Join me on Wednesday, Nov 4, in Ludwigsburg:
PS: The “traditional” Object Teams Runtime Environment isn’t dead, yet. I really want to keep it as an option, because both variants (OTRE / OTDRE) have quite different characteristics, and after all this component has matured over more than 10 years. But with one option already (mostly) working, I can probably wait until a proper release of BCEL 6.0, and still have it back in game before the Neon release.
Object Teams now has Lambdas (and more)
In a previous post I announced that the Luna version of Object Teams will incorporate support for Java 8.
Now that the big rush towards March 18 is over, I took stock how Object Teams fits into the Luna stream, which finally contains all the bits for Java 8 support. The result:
This means, you can now use lambda expressions and more even in OT/J programs, here’s a witness:
While this program may not make a lot of sense, it demonstrates a funny piece of syntax. Here’s what you see:
- The regular class PersonCollection offers a method restrict that takes an argument of a functional type (Predicate)
- Role Club.Members is played by PersonCollection and makes the base method available via a callout binding (lines 19 f)
- The role wants to pass just an int value, where the base method expects a Predicate.
- This incompatibility is bridged by a parameter mapping that maps a lambda expression to the expected parameter (line 20).
This looks funny, because two different uses of the token -> meet on a single line: the first -> is part of the lambda expression, separating the parameter from the body, the second -> is part of the parameter mapping, mapping the expression on the left to the parameter on the right.
Were I to chose the syntax in a green field situation, I would certainly not use the same token for such different purposes. But since we’re basically merging two independent language extensions I had no choice. After some worries that the combined language might not be parseable I’m much relieved to see it actually working!
A very similar situation actually exists concerning type annotations. More on that in a future post.
Remaining steps towards Luna
Merging the entire BETA_JAVA8 code base from JDT into Object Teams is done, but a few regressions (< 30 out of 84,000+ tests) remain from that exercise. But compared to the original JDT efforts for Java 8 this truly is peanuts 🙂
We only have one hurdle still to jump: our bytecode weaver is based on BCEL, for which no update for Java 8 is in sight. This means: we cannot weave into class files that use the Java 8 bytecode version, bummer!
So the real interesting question will be: can we get the long announced alternate weaver using ASM up to speed in time for Luna? I think we can, but it won’t hurt if you wish me luck in this endeavour.
And yes: one day I should start writing meaningful examples using Java 8 features in OT/J code…
Object Teams in the times of Eclipse Luna and Java 8
With Eclipse Luna approaching milestone 5 it’s time to write a bit about what this year will bring for Object Teams.
Overhaul of the OT/Equinox weaver
The Luna cycle started with a surprise (for me): Equinox no longer provides the Adaptor Hooks, which allowed OT/Equinox to hook into the OSGi class loading process and perform its bytecode weaving. Outch!
On the other hand, OSGi meanwhile has a standard WeavingHook for doing this kind of stuff. With some efforts (and lots of help from Tom W. – no help from p2, though) I managed to migrate OT/Equinox to this new standard. At the end of the day, this will bring better encapsulation and hopefully better launch performance, too. Details to be posted some time later.
Lift OT/J to be based on Java 8
Some may have noticed that most my Eclipse-time is currently being spent in helping JDT to get ready for Java 8. And sure this is an exciting endeavour to be part of!
For Object Teams the cool part about this is: I’ve seen the changes for Java 8 happen in JDT, which greatly helps to adopt these changes for OT/J. Once in a while this even reveals a bug in JDT before it ever surfaced 🙂
The integration of OT/J with Java 8 still has some regressions, but my plan is to have this at good “milestone quality” when Java 8 is released in March and to get it to full release quality for Luna GA in June.
Question: does it actually make sense to combine lambdas and type annotations with roles and teams? I strongly believe it does, because these are language improvements in entirely different categories:
- Lambda expressions help to implement algorithms in a more concise and also more modular way – this lets you think differently about functions.
- Type annotations help enhance safety when used together with better program analysis (like, e.g., enhanced null analysis) – these let you think differently about types.
- Roles and teams help improve the program structure at a larger scale – these let you think differently about classes and objects.
So, if each of these additions makes sense, then combining them all in one language will certainly yield a very powerful language. Some examples of combination to follow as we approach the release.
ObjectTeams.org is down, long live ObjectTeams.org
Not all material from the 10+ years of history of Object Teams has moved to Eclipse.org. Notably the language definition (OTJLD) and scientific publications are still hosted on objectteams.org. Until recently, my former University kindly provided the host for publishing these research results. Now that server has gone off service and for a while objectteams.org was dead — but as of today the relevant content is back online – sorry for the disruption. And, btw, this is also the reason why this blog has changed its URL.
Please wish me luck for the work ahead, both on JDT and Object Teams 🙂
Object Teams 2.1 Milestone 7 (finally) brings hot code replacement
As part of the Juno Milestone 7 also Object Teams has delivered its Milestone 7.
As the main new feature in this milestone hot code replacement finally works when debugging OT/J or OT/Equinox applications.
What took us so long to make it work?
- Well, hot code replacement didn’t work out of the box because our load-time weaver only worked the first time each class was loaded. When trying to redefine a class in the VM the weaver was not called and thus class signatures could differ between the first loaded class and the redefined version. The VM would then reject the redefinition due to that signature change.
- Secondly, we didn’t address this issue earlier, because I suspected this would be pretty tricky to implement. When I finally started to work on it, reality proved me wrong: the fix was actually pretty simple 🙂
In fact the part that makes it work even in an Equinox setting is so generic that I proposed to migrate the implementation into either Equinox or PDE/Debug, let’s see if there is interest.
Now when you debug any Object Teams application, your code changes can be updated live in the running debug target – no matter if you are changing teams, roles or base classes. Together with our already great debugging support, this makes debugging Object Teams programs still faster.
More new features can be found in the New&Noteworthy (accumulated since the Indigo release).
Object Teams with Null Annotations
The recent release of Juno M4 brought an interesting combination: The Object Teams Development Tooling now natively supports annotation-based null analysis for Object Teams (OT/J). How about that? 🙂
The path behind us
Annotation-based null analysis has been added to Eclipse in several stages:
- Using OT/J for prototyping
- As discussed in this post, OT/J excelled once more in a complex development challenge: it solved the conflict between extremely tight integration and separate development without double maintenance. That part was real fun.
- Applying the prototype to numerous platforms
- Next I reported that only one binary deployment of the OT/J-based prototype sufficed to upgrade any of 12 different versions of the JDT to support null annotations — looks like a cool product line
- Pushing the prototype into the JDT/Core
- Next all of the JDT team (Core and UI) invested efforts to make the new feature an integral part of the JDT. Thanks to all for this great collaboration!
- Merging the changes into the OTDT
- Now, that the new stuff was mixed back into the plain-Java implementation of the JDT, it was no longer applicable to other variants, but the routine merge between JDT/Core HEAD and Object Teams automatically brought it back for us. With the OTDT 2.1 M4, annotation-based null analysis is integral part of the OTDT.
Where we are now
Regarding the JDT, others like Andrey, Deepak and Aysush have beaten me in blogging about the new coolness. It seems the feature even made it to become a top mention of the Eclipse SDK Juno M4. Thanks for spreading the word!
Ah, and thanks to FOSSLC you can now watch my ECE 2011 presentation on this topic.
Two problems of OOP, and their solutions
Now, OT/J with null annotations is indeed an interesting mix, because it solves two inherent problems of object-oriented programming, which couldn’t differ more:
1.: NullPointerException is the most widespread and most embarrassing bug that we produce day after day, again and again. Pushing support for null annotations into the JDT has one major motivation: if you use the JDT but don’t use null annotations you’ll no longer have an excuse. For no good reasons your code will retain these miserable properties:
- It will throw those embarrassing NPEs.
- It doesn’t tell the reader about fundamental design decisions: which part of the code is responsible for handling which potential problems?
Why is this problem inherent to OOP? The dangerous operator that causes the exception is this:
right, the tiny little dot. And that happens to be the least dispensable operator in OOP.
2.: Objectivity seems to be a central property on any approach that is based just on Objects. While so many other activities in software engineering are based on the insight that complex problems with many stakeholders involved can best be addressed using perspectives and views etc., OOP forces you to abandon all that: an object is an object is an object. Think of a very simple object: a File. Some part of the application will be interested in the content so it can decode the bytes and do s.t. meaningful with it, another part of the application (maybe an underlying framework) will mainly be interested in the path in the filesystem and how it can be protected against concurrent writing, still other parts don’t care about either but only let you send the thing over the net. By representing the “File” as an object, that object must have all properties that are relevant to any part of the application. It must be openable, lockable and sendable and whatnot. This yields
bloated objects and unnecessary, sometimes daunting dependencies. Inside the object all those different use cases it is involved in can not be separated!
With roles objectivity is replaced by a disciplined form of subjectivity: each part of the application will see the object with exactly those properties it needs, mediated by a specific role. New parts can add new properties to existing objects — but not in the unsafe style of dynamic languages, but strictly typed and checked. What does it mean for practical design challenges? E.g, direct support for feature oriented designs – the direct path to painless product lines etc.
Just like the dot, objectivity seems to be hardcoded into OOP. While null annotations make the dot safe(r), the roles and teams of OT/J add a new dimension to OOP where perspectives can be used directly in the implementation. Maybe it does make sense, to have both capabilities in one language 🙂 although one of them cleans up what should have been sorted out many decades ago while the other opens new doors towards the future of sustainable software designs.
The road ahead
The work on null annotations goes on. What we have in M4 is usable and I can only encourage adopters to start using it right now, but we still have an ambitious goal: eventually, the null analysis shall not only find some NPEs in your program, but eventually the absense of null related errors and warnings shall give the developer the guarantee that this piece of code will never throw NPE at runtime.
What’s missing towards that goal:
- Fields: we don’t yet support null annotations for fields. This is next on our plan, but one particular issue will require experimentation and feedback: how do we handle the initialization phase of an object, where fields start as being null? More on that soon.
- Libraries: we want to support null specifications for libraries that have no null annotations in their source code.
- JSR 308: only with JSR 308 will we be able to annotate all occurrences of types, like, e.g., the element type of a collection (think of
List)
Please stay tuned as the feature evolves. Feedback including bug reports is very welcome!
Ah, and one more thing in the future: I finally have the opportunity to work out a cool tutorial with a fellow JDT committer: How To Train the JDT Dragon with Ayushman. Hope to see y’all in Reston!
Builds are like real software – or even more so
Being a part-time release engineer for the Object Teams project I can only agree with every word Kim writes about the job, I wish I could hire her for our project 🙂
She writes:
“Nobody in needs to understand how the build works, they just need to push a button. That’s great. Until the day before a release when your build fails with a cryptic message about unresolved dependencies. And you have no idea how to fix it. And neither does anyone else on the team.”
That puts a sad smile on my face and I’d like to add a little quality metric that seems cruel for today’s build systems, but might actually be useful for any software:
One extreme I experienced was in a PDE/Build-ant-build which I had to set to verbose to get any useful answer but then I had to find the relevant error message deeply buried in literally tens of megabytes of log output. Takes ages to browse that log file. Other tools rank towards the other end of the spectrum saying basically “it didn’t work”.
Why is the worst error message relevant? When you hit that worst message it’s close to saying “game over”. Especially when working on a build I’ve come to the point time and again where all my creativity and productivity came to a grinding halt and for days or weeks I simply made zero progress because I had no idea why that system didn’t work and what it expected me to do to fix the thing. Knock-out.
Obviously I hate that state when I make no progress towards my goal. And typically that state is reached by poor communication from some framework back to me.
Real coolness
I know people usually don’t like to work on improving error messages, but please, don’t think good error messages are any bit less cool than running your software on mars. On the one hand we try to build tools that improve developers’ productivity by a few percent and than the tool will give answers that bring that very productivity down to zero. That’s – inconsistent.
I’m tempted to repeat the p2 story here. Many will remember the merciless dump of data from the sat solver that p2 gave in its early days. Some will consider the problem solved by now. Judge for yourself: what’s the worst-case time a regular Eclipse user will need to understand what p2 is telling him/her by one of its error messages.
The intention of this post is not to blame any particular technology. The list would be quite long anyway. It’s about general awareness (big words, sorry 🙂 ).
Consider the worst case
Again, why worst case? Because the worst case will happen. And it’s enough if it hits you once to easily compensate all the time savings the tool otherwise brought to you.
Communicate!
Framework developers, tool smiths: let your software communicate with the user and let it be especially helpful when the user is in dire need of help.
One small contribution in this field I’d like to share with you: in the OTDT every error/warning raised by the compiler not only tries to precisely describe what’s wrong but it is directly linked to the corresponding paragraph in the language definition that is violated by the current code. At least this should completely explain why the current code is wrong. It’s a small step, but I feel a strong need for linking specific help to every error message.
But first, the software has to anticipate every single error that will occur in order to produce useful messages. That’s the real reason why creating complex software is so challenging. Be it a build system or the “real” software.
Be cool, give superb error messages!
Mix-n-match language support
I’ve been involved in the release of different versions of the JDT lately, supporting different flavors of Java.
Classical release management
At the core we have the plain JDT, of which we published the 3.7.0 release in June and right now first release candidates are being prepared towards the 3.7.1 service release, which will be the first official release to support Java 7. At the same time the first milestones towards 3.8 are being built. OK, this is almost normal business — with the exception of the service release differs more than usual from its base release, due to the unhappy timing of the release of Java 7 vs. Eclipse 3.7.
First variant: Object Teams
The same release plan is mirrored by the Object Teams releases 2.0.0, 2.0.1RC1, 2.1.0M1. Merging the delta from JDT 3.7 to 3.7.1 into the OTDT was a challenge, given that this delta contained the full implementation of all that’s new in Java 7. Still with the experience of regularly merging JDT/Core changes into the OT variant, the pure merging was less than one day plus a couple more days until all 50000+ tests were green again. The nice thing about the architecture of the OTDT: after merging the JDT/Core, I was done. Since all other adaptations of the JDT are implemented using OT/Equinox adopting, e.g., all the new quick assists for Java 7 required a total of zero minutes integration time.
I took the liberty of branching 2.0.x and 2.1 only after integrating the Java 7 support, which also means that 2.1 M1 has only a small number of OT-specific improvements that did not already go into 2.0.1.
Prototyping annotation based null analysis
As I wrote before, I’m preparing a comprehensive new feature for the JDT/Core: static analysis for potential NullPointerException based on annotations in the code. The latest patch attached to the bug had almost 3000 lines. Recent discussions at ECOOP made me change my mind in a few questions, so I changed some implementation strategies. Luckily the code is well modularized due to the use of OT/Equinox.
Now came the big question: against which version of the JDT should I build the null-annotation add-on? I mean, which of the 6 versions I have been involved in during the last 2 months?
As I like a fair challenge every now and then I decided: all six, i.e., I wanted to support adding the new static analysis to all six JDT versions mentioned before.
Integration details
Anybody who has worked on a Java compiler will confirm: if you change one feature of the compiler chances are that any other feature can be broken by the change (I’m just paraphrasing: “it’s complex”). And indeed, applying the nullity plug-in to the OTDT caused some headache at first, because both variants of the compiler make specific assumptions about the order in which specific information is available during the compilation process. It turned out that two of these assumptions where simply incompatible, so I had to make some changes (here I made the null analysis more robust).
At the point where I thought I was done, I tripped over an ugly problem that’s intrinsic to Java.
The nullity plug-in adapts a method in the JDT/Core which contains the following switch statement:
while ((token = this.scanner.getNextToken()) != TerminalTokens.TokenNameEOF) { IExtendedModifier modifier = null; switch(token) { case TerminalTokens.TokenNameabstract: modifier = createModifier(Modifier.ModifierKeyword.ABSTRACT_KEYWORD); break; case TerminalTokens.TokenNamepublic: modifier = createModifier(Modifier.ModifierKeyword.PUBLIC_KEYWORD); break; // more cases } }
I have a copy of this method where I only added a few lines to one of the case blocks.
Compiles fine against any version of the JDT. But Eclipse hangs when I install this plugin on top of a wrong JDT version. What’s wrong?
The problem lies in the (internal) interface TerminalTokens. The required constants TokenNameabstract etc. are of course present in all versions of this interface, however the values of these constants change every time the parser is generated anew. If constants were really abstractions that encapsulate their implementation values, all would be fine, but the Java byte code knows nothing about such an abstraction, all constant values are inlined during compilation. In other words: the meaning of a constant depends solely on the definitions which the compiler sees during compilation. Thus compiling the above switch statement hardcodes a dependency on one particular version of the interface TerminalTokens. BAD.
After recognizing the problem, I had to copy some different versions of the interface into my plug-in, implement some logic to translate between the different encodings and that problem was solved.
What’s next?
Nothing is next. At this point I could apply the nullity plug-in to all six versions of the JDT and all are behaving well.
Mix-n-match
Would you like Java with our without the version 7 enhancements (stable release or milestone)? May I add some role and team classes? How about a dash more static analysis? It turns out we have more than just one product, we have a full little product line with features to pick or opt-out:
| Java 6 | Java 7 | |||
|---|---|---|---|---|
| Indigo | Indigo SR1 | Juno M1 | ||
| no null annotations | Plain JDT | |||
| OTDT | ||||
| with null annotations | Plain JDT | |||
| OTDT | ||||
- OTDT: http://www.eclipse.org/objectteams/download.php
- Null ananlysis: http://wiki.eclipse.org/JDT_Core/Null_Analysis
Just make your choice 🙂
Happy hacking with null annotations and try-with-resources in OT/J.
 |
BTW: if you want to hear a bit more about the work on null annotations, you should really come to EclipseCon Europe — why not drop a comment at this submission 🙂 |
A Short Train Ride
Less than a week ago I happily announced that Object Teams is on the Indigo Train.
Much water has gone under the bridge since then and the above statement is history.
Events where triggered by what was actually a little bug in the b3 aggregator. Only by way of this bug some people noticed that there was a ”’patch feature”’ inside the repository, i.e., a feature (“Object Teams Patch for JDT/Core”) that replaces the jdt.core plugin with a variant.
One part of me is very happy this bug occurred because finally an issue got the attention I had tried to raise at various occasions before. The lesson is:
(see this post, e.g.).
The other part of me got very worried because during that debate some harsh statements occurred that would effectively amount to excluding Object Teams from Eclipse.org. That’s a little more attention than I had intended. As in any heated debate some of the arguments sounded to me more like ideology than anything that could possibly be discussed open-mindedly.
I had mixed feelings regarding the technical scope: the outcry only banned one specific technology: patch features. I don’t see how the goal to protect a project’s bits and bytes against influence from other projects can be achieved without also discussing: access to internal, byte-code weaving and – worst of all, I believe – reflection. To be perfectly open: Object Teams uses all these techniques except for reflection. Personally, I would even argue for banning projects that do use setAccessible(true), but that’s not a realistic option because then quite likely the whole Train would dissolve into a mist.
I am actually guilty of a technical simplification during this debate: I focused too much on the idea that a user would explicitly select features to install, not accounting for the possibility that the jdt.core plugin can well be pulled in invisibly due to dependencies among plugins. So, yes, if Object Teams would still be in the repository, and if a user installed a package without the JDT and if that user did never select to install the JDT and if that user selects another feature that implicitly requires the jdt.core plugin, then that user would unexpectly install the OT variant of the jdt.core. I agree that this is not ideal. I personally would have been happy to take this risk because I know how thoroughly the OT variant is tested for compatibility. And for the remaining minuscule risk I would have been happy to promise same-day fixes. But risk assessment naturally depends on perspective and I understand that others come to different conclusions when weighing the issues.
From two days distance I can already laugh at one implication of the central argument, paraphrased as: the JDT/Core team must be protected against harmful actions from the OT team. When spelling this out in names, one of the sentences reads: “Stephan Herrmann must be protected against harmful actions by Stephan Herrmann”. I should really be careful, because I’ll never be able to escape him!
Where to go?
- As we’re banned from the Train, I willed hurriedly book a plane ticket to Indigo. Make sure we come by a Graduation office on the way.
- I do hope that bug 316702 get’s sufficient attention now. Seriously: if you are so detrimentally determined about patch features, then the UI must report it. And if it reports this, it might as well report other techniques that have similar effects!
- I appreciate any offers for helping OT/J towards a solution that avoids replacing a plugin. As of today and after more than seven years of looking at this, I see no way how this can be done, but that doesn’t mean we shouldn’t try still harder.
 |
Unfortunately, the debate consumed all the time and energy I had planned for preparing a presentation at the EclipseCON Audition. However, Lynn finally made my day by letting me know that my submission is the lucky #42. So I’m making progress towards my all-seasons Eclipse collection 🙂 |
Object Teams is on the Train to Indigo
Object Teams made the next step for even better integration in Eclipse: starting with today’s Indigo Milestone 3 we’re aboard! No further fiddling with URLs of update sites, simply select the Indigo site…
… and directly install the OTDT from there.
For those who haven’t tried Object Teams before, here are steps 1, 2 & 3:
- Walk through the Quick Start
- Read the basic concepts in our OT/J Primer
- Play with more Examples or browse some Object Teams Patterns
Improvements
The main concern of this milestone release was catching up with the Indigo train, so the list of bugs fixed is shorter than usual. But, hey, by upgrading to Eclipse SDK 3.7M3, the OTDT supports also some new JDT coolness, like, e.g., improved static analysis (I happen to have a finger in this pie, too 🙂 ).
Compatiblity
With each new release it is good to test compatibility with older software. So I loaded my previous experiment (“project medal”) into the new milestone. Much as I expected: that little adaptation of the JDT still works as intended also in the new version, which wasn’t possible with the original implementation. This again shows: in-place patching can never give you, what OT/Equinox excels in: evolvable, modular adaptation.
New Honours, New Synergy
I’m flattered to see my name appear on one more Eclipse web-page:
Thanks to the JDT/Core team for accepting me as a new member!
Actually, I’ve been poking about the JDT source code since 2003 when we started implementing the Object Teams Development Tooling (OTDT) on top of the JDT. So it was only natural that I would stumble upon a bug in the JDT every once in a while (incl. one “greatbug” 🙂 ). In this specific situation, reporting bugs and trying to help find solutions was more than just good Eclipse citizenship: the discussions in bugzilla also helped me to better understand the JDT sources and thus it helped me developing the OTDT. And that’s what this post is about: synergy!
Personal Goals
Actually, watching the JDT/Core bug inbox provides a constant stream of cool riddles: spooky Java examples producing unexpected compiler/runtime results. Sometimes those are real fun to crack. Which compiler is right, which one isn’t? What’s the programmer trying to do? I’d say there’s no better way to test your Java knowledge 🙂
Also, watching this stream helps me stay informed, because every patch will eventually need to be merged into the Object Teams branch of the JDT/Core.
But I also have some pet projects that I’d like to actively push forward. Currently I’m thinking about these two:
- Supporting inter-procedural null analysis
- Making the compiler closer match the OSGi semantics
Inter-procedural null analysis
Naturally, the JDT/Core is a facilitator for tons of cool functionality in the IDE, but it hardly communicates directly with the user. But we have one channel, where the compiler can actually talk to the developer and give advice that’s well worth your bucks: errors and warnings. Did you know, that the compiler can immediately tell you what’s wrong about this piece of code:
void foo(String in) { if (in == null) new IllegalArgumentException("Null is not allowed here"); ... // real work done here
(Yep, that was bug 236385, see the New&Noteworthy on how to enable).
Many Java developers share the experience that one of the most frequent problems is also one of the most mundane: NPE. Those, who are aware of this easily produce the opposite problem: cluttering there code with meaningless null-checks (many of which have no better action than the above IllegalArgumentException which isn’t much better than throwing the NPE in the first place). The point is, we’d want to know which values can actually be null (and why!) so we write only the necessary null-checks and for those we should think really hard what would be a suitable action, right? (“This shouldn’t happen” is not an answer!).
Thus I’m happy I could get involved in some improvements of the compiler’s flow analysis. I also experienced that null-analysis combined with aggressive optimization is a delicate issue – remember SDK 3.7M2a? Sorry about that, but to my justification I might add that bug 325755 has always been dormantly present, I only woke’em up. I must admit for those 105 minutes my heart beat was a bit above average 😉 .
Anyway, the current null analysis is still fundamentally limited: it knows nothing about nullness of method arguments nor method call results. So it would be a real cool enhancement if we could feed such information into the compiler. Fortunately, the theory of how to do this (you may either think of improved type systems or of design by contract) is a well-explored subject. Actually, bug 186342 already has some patches in this direction. Since discussing in bugs with long history is a bit tedious I created this wiki page. Sadly, it’s not a technical difficulty that’s keeping us from immediately releasing that patch, but a stalled standardization process 😦 .
However, as outlined in the wiki, here comes another synergy: in case we can’t find a solution that will be sufficiently compatible with future standards, I can easily create an early-adopters release, so sorting out the details of implementation and usage can be done in parallel with the standardization process. How that? I’d simply ship the compiler enhancement as an OT/Equinox-enabled plug-in. This would allow us to ship the compiler enhancement as separate plug-in ready to be tried by early adopters.
OSGi-aware compiler
Some of you may have noticed (here, or here, or here, or here or one of the many duplicates) that the compiler’s concept of a (single, linear) classpath is not a good match for compiling OSGi code. I will soon write another wiki page with my current thinkings about that issue. Stay tuned …
Comments?
For both issues I’d appreciate any comments / feedback. I’ll be checking updates of the bugs I listed, or the wiki page or … talk to me during ESE! Let’s move something together.
See you all in Ludwigsburg!
Stephan
The Object Teams Blog 