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

Jonathan Gibbons jonathan.gibbons at
Wed May 23 21:01:48 UTC 2018

On 5/17/18 1:12 PM, mark.reinhold at wrote:

> The following JEP is proposed to target JDK 11:
>    330: Launch Single-File Source-Code Programs
> 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]
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 

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 
`` might be installed as `helloworld`. And, while the JEP 
describes use cases for executing a small single-file program with `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 ``. 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`, and the execution of 
shebang scripts, such as `helloworld`.

  -- Jon

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