JEP 118 works with Lambda ?

[Brian Goetz]

The short answer is: the code being referred to (that synthesizes "map 
literals") has no claim to ever work.  (And in fact, it does not work on 
the reference implementation, but I've heard tell it does "work" when 
compiled with ECJ.)  And that it does not work is not a bug.

Lambda expressions are expressions, not methods.  There is no reflection 
on expressions.  A lambda expression evaluates to an instance of a class 
implementing its target type.  There is (by design) no guarantee what 
the name of that class is, whether that class is shared with that of 
other lambda expressions, or what the parameter names of its methods are.

A compiler implementation may or may not choose to desugar lambda 
expressions to methods.  If it does, it may or may not preserve the 
parameter names as they appear in source code.  And if it does, this may 
freely change from day to day.

To claim there is no "exception" for lambdas has it backwards; lambdas 
are not methods, and there is no way to reflect over them.



On 4/16/2014 2:59 PM, Alex Buckley wrote:
> // Adding compiler-dev
>
> Since lambda expressions are expressions, they cannot be reflected over
> directly. However, if you know the method that a lambda expression is
> compiled to, then you can obviously call getParameters() on the
> java.lang.reflect.Method object.
>
> That said, even if you compiled with -parameters, there are bugs in the
> name generation for parameters of the method, so Parameter.getName()
> doesn't work. This has been discussed on compiler-dev and is being fixed
> for JDK 8u20.
>
> Alex
>
> On 4/16/2014 11:43 AM, Luan Cestari wrote:
>> Hi, Good morning/afternoon/evening! =)
>>
>> I got this email from a reply of help at openjdk.java.net ( from
>> https://bugs.openjdk.java.net/secure/Dashboard.jspa , where I was
>> thinking
>> to create an issue (maybe for enhance or bug, not really sure).
>>
>> I got a problem during a test using the getName from Parameter (from JEP
>> 118, new in Java 8) with Lambda expression (also new in Java 8). I made a
>> simple class to reproduce the issue which I posted here
>> https://gist.github.com/luan-cestari/10902418 , where the getName works
>> fine using other classes like in this other code I made
>> https://gist.github.com/luan-cestari/10902332. I started to investigate
>> that after I saw a gist ( https://gist.github.com/galdosd/10823529, which
>> also created a discussion on
>> http://www.reddit.com/r/java/comments/2360is/pretty_map_literals_for_java_8/)
>>
>> saying that feature worked fine for him, so I talked with some guys in
>> IRC ( OFTC#openjdk ), but we wasn't sure if it is a bug or not.
>>
>> I searched a bit and I found the following:
>>
>>     - In http://openjdk.java.net/jeps/118 , it doesnt say about any
>>     exception around the Lambda functions
>>     - In Oracle documentation it mentions the JVM parameter that have
>> to be
>>     used during the compilation to that work fine
>>
>> http://docs.oracle.com/javase/tutorial/reflect/member/methodparameterreflection.html
>>
>>     - In javadoc there is no more details about that (I think it could
>> make
>>     more clear that there is a JVM parameter or maybe it is possible
>> to have
>>     this kind of parameter. Also it could say about execptions such as
>> Lambda
>>     expressions)
>>
>> http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Parameter.html#getName--
>>
>>     - I also saw a mail list (
>>
>> http://mail.openjdk.java.net/pipermail/lambda-dev/2013-January/007548.html)
>>
>> where Remi and Brian exchanged some mails where he said it is had the
>> JEP
>>     118 due there are people that want thing in a way and people who
>> doesn't
>>     want some changes
>>     - In that mail, they cited this document
>>     http://cr.openjdk.java.net/~abuckley/8misc.pdf which I think that
>> some
>>     parts of it are in JVM spec (
>>     - http://docs.oracle.com/javase/specs/ like
>>     http://docs.oracle.com/javase/specs/jvms/se8/jvms8.pdf and
>>     http://docs.oracle.com/javase/specs/jls/se8/jls8.pdf
>>     - ) but it just say the class structure in bytecode, nothing about
>>     Lambda and JEP 118
>>
>> As far as I know, Lambda will create some symbolic invocation using
>> invokedynamic which is different than creating anonymous classes, so this
>> might be the root cause of the issue. But maybe there is some
>> difference in
>> the native implementations (for example, in my case I'm using Fedora 20
>> with OpenJDK Runtime Environment (build 1.8.0-b132) / OpenJDK 64-Bit
>> Server
>> VM (build 25.0-b70, mixed mode)). In the IRC, people said that maybe the
>> IDE was generating the inner classes to make it work, but that would
>> make a
>> problem during the runtime if a different tool (like maven) compiles the
>> project.
>>
>> In summary, is the right behavior for JEP 118 return argx due lambda
>> expressions? Maybe we could enhance it to make some new features (for
>> Java
>> 9 ).
>>
>> Thank you in advance,
>>
>> Kindest regards,
>>
>> Luan Cestari
>>
>> "All the gold which is under or upon the earth is not enough to give in
>> exchange for virtue."
>> Plato
>> "At his best, man is the noblest of all animals; separated from law and
>> justice he is the worst."
>> "A true friend is one soul in two bodies."
>> Aristotle
>> "The only limit to your impact is your imagination and commitment."
>> Tony Robbins
>>