Draft JEP: Launch Single-File Source-Code Programs

Jonathan Gibbons jonathan.gibbons at oracle.com
Mon Feb 12 16:51:47 UTC 2018

On 2/12/18 8:07 AM, Brian Goetz wrote:
> Its simpler than that.  There are NO changes to the JLS, nor even any (IIRC) changes to the compiler.  This is all in the command-line launcher.  This is not a Java language feature. The scope is JDK, not SE.
> There are really two features here:
>   - Launch Java source files from the java launcher.
>   - Additionally, strip off the #! line (if present) before passing the source to the compiler, so that the compiler sees a normal Java compilation unit when compiling, to interact nicely with native #! support.

Yes.  It has always been possible for anyone to write and provide an 
executable that could be specified in a #! line, that would strip the 
first line, and compile and run the class declared in the rest of the 
file.  That could be done by anyone, for any Java compiler and VM, and 
without needing to change JLS. The novelty here is to do it in the Java 
launcher, where, among other benefits, we have access to VM options that 
may be relevant for the compilation, such as the classpath and related 

-- Jon

>> On Feb 12, 2018, at 6:38 AM, Volker Simonis <volker.simonis at gmail.com> wrote:
>> On Mon, Feb 12, 2018 at 3:22 PM, Roger Riggs <Roger.Riggs at oracle.com> wrote:
>>> Hi Jonathan,
>>> It would be  a great convenience to developers to NOT to have two types of
>>> java source files.
>>> Those that can be run directly and those that cannot. According to the JLS,
>>> the former can not
>>> be called java source files, they are something else.
>> Yes, that's exactly what I actually wanted to say. The JavaDoc analogy
>> can't be applied here because JavaDoc is completely transparent to the
>> JLS (because it is hidden within a comment). Not sure how hard and
>> controversial it would be tough, to change the JLS to allow and ignore
>> a shebang in the first line?
>>> $.02, Roger
>>> On 2/10/2018 11:59 AM, Jonathan Gibbons wrote:
>>>> Volker,
>>>> I don't see a compelling need to change the JLS to accommodate "shebang"
>>>> files. Most Java source files don't need the `#!` line, including any source
>>>> file passed to an explicit invocation of the Java launcher. The line is only
>>>> needed when the file is set up to be directly executed by the "shebang"
>>>> mechanism, in which case the line can be thought of as a "wrapper" supported
>>>> by the host system to enable such use. If the feature is sufficiently
>>>> popular, it may even be that editors will recognize and support the use of
>>>> such lines, in the same way that some editors recognize and support the
>>>> manipulation of javadoc comments, which are also not defined in the JLS
>>>> (beyond the basic definition of a comment in JLS 3.7).
>>>> -- Jon
>>>> On 2/9/18 2:51 AM, Volker Simonis wrote:
>>>>> Hi Jonathan,
>>>>> that's an interesting proposal. It is just unfortunate that a Java
>>>>> source file starting with "shebang" won't be a legal Java file
>>>>> according to the Java Language Specification. Or do you plan to change
>>>>> the JLS to allow (and ignore) "shebang" in the first line?
>>>>> Thanks,
>>>>> Volker
>>>>> On Fri, Feb 9, 2018 at 12:18 AM, Jonathan Gibbons
>>>>> <jonathan.gibbons at oracle.com> wrote:
>>>>>> This draft JEP contains a proposal to enhance the java launcher to
>>>>>> support
>>>>>> running a
>>>>>> program supplied as a single file of Java source code. The program will
>>>>>> be
>>>>>> compiled
>>>>>> and run, without the need to explicit invoke javac, or to package up a
>>>>>> jar
>>>>>> file.
>>>>>> For more details, see:
>>>>>> http://openjdk.java.net/jeps/8192920
>>>>>> -- Jon

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