JEP proposed to target JDK 11: 330: Launch Single-File Source-Code Programs

[Robert Zenz]

I'm sorry, I still don't get why the shebang line is required at all for this
proposal. I might argue that shebang lines are system dependent features and are
typically only used with languages which treat `#` as a comment which is not the
case in Java. Additionally, we can already "emulate" shebangs in Java (or any C
like language) source files using a polyglot. Which is fore sure not that easy
to understand, but it still gets the job done.

I know what you're trying to do, but I fail to see how the achieved goal (of
easily executing a Java source file by doing `./HelloWorld.java` versus `java
HelloWorld.java`) validates the added special case just to support the shebang
line (including possible confusion of newcomers why the first line is allowed to
start with `#` but everywhere else is a syntax error).

On 23.05.2018 23:01, Jonathan Gibbons wrote:
> On 5/17/18 1:12 PM, mark.reinhold at oracle.com wrote:
> 
>> The following JEP is proposed to target JDK 11:
>>
>>    330: Launch Single-File Source-Code Programs
>>         http://openjdk.java.net/jeps/330
>>
>> Feedback on this proposal is more than welcome, as are reasoned
>> objections.  If no such objections are raised by 23:00 UTC on Thursday,
>> 24 May, or if they're raised and then satisfactorily answered, then
>> per the JEP 2.0 process proposal [1] I'll target this JEP to JDK 11.
>>
>> - Mark
>>
>>
>> [1] http://cr.openjdk.java.net/~mr/jep/jep-2.0-02.html
> A number of points have been raised, regarding the interaction with javac and
> shebang scripts.
> 
> It was never a goal to modify javac to support shebang-java scripts. This can be
> seen indirectly in the JEP in the description of how the first line may be
> removed before passing the rest of the file as a normal CompilationUnit to the
> compiler. With hindsight, this deserves to be stated as an explicit Non-Goal.
> 
> There are various reasons to not want to change JLS or javac:
> 
> 1. Changing JLS is a Big Deal, and comes with its own costs and constraints.
> Further, shebang files are a platform-specific feature, and are not even defined
> in the POSIX standard. The feature does not warrant changing a JCP-controlled
> specification.
> 
> 2. Changing javac to accept shebang-java files is also a Big Deal. Modifying the
> command-line options to accept files that do not follow the standard naming
> conventions would introduce complexity and potential ambiguity.
> 
> 3. There is no compelling need to change JLS or javac.  As demonstrated by the
> proposed implementation, no change to JLS or javac is actually necessary in
> order to implement the feature. It is therefore at most a convenience if javac
> were to be adapted to ignore shebang lines.
> 
> Shebang scripts are an executable format defined on some, but not all,
> platforms. Creating a shebang script is typically more than just adding an
> initial first line to a file; it typically involves a number of steps:
> 
> a. Add an initial shebang line to the file
> b. Rename the file to a "command-friendly" name
> c. Make the file executable
> d. Install the file in some standard location
> 
> While renaming the file to a command-friendly name is optional, it is also
> expected to be common practice. For example, a source file named
> `HelloWorld.java` might be installed as `helloworld`. And, while the JEP
> describes use cases for executing a small single-file program with `java
> HelloWorld.java` or executing it as a platform-specific shebang script with just
> `helloworld`, it does not seem like there is a common use case to execute
> `HelloWorld.java`. So, if the shebang script is typically renamed to a
> command-friendly name, it will not be possible to compile it directly, with
> "javac helloworld", because that is not a valid command line for javac. This
> reduces any potential convenience of having javac ignore shebang lines.
> 
> Since Java source files are different artifacts to platform-specific executable
> scripts, it makes sense to treat them differently, and since we do not want to
> change the Java language to support shebang lines, the suggestion is to amend
> the JEP and implementation so that shebang lines are never stripped from Java
> source files, i.e. files ending in `.java`. This avoids the problem of having
> the ecosystem of tools handling Java source files having to deal with arbitrary
> artifacts like shebang lines.  The change would still permit the direct
> execution of Java source files, such as `java HelloWorld.java`, and the
> execution of shebang scripts, such as `helloworld`.
> 
>  -- Jon