Seth, this is not so much "compiler semantics" vs "launcher semantics"
... it's just "ClassLoader semantics", combined with the fact that an
empty class path defaults to the current directory.
In the source launcher, the compiler is not worried about the classes in
the current directory.
But the classes are executed in a class loader, in which the normal
semantics[1] are to check the parent class loader first, before checking
for a local definition. Using those normal semantics, any classes on the
application class path (defaulting to the current directory) are found
before those defined to the class loader. That's the problem that is
being encountered here.
-- Jon
[1]https://docs.oracle.com/javase/10/docs/api/java/lang/ClassLoader.html#loadClass(java.lang.String,boolean)
On 9/14/18 3:00 PM, seth lytle wrote:
ah - compiler semantics vs launcher semantics, and the source code
launcher lives in both worlds
javac refuses to compile two sources for the same class
javac will compile a source file for a class that exists on the
classpath (and even overwrite a class on the classpath)
java accepts two class files for the same class on the classpath
(favors the first)
java -jar accepts a jar containing classes that also exist on the
classpath (favors the jar)
java refuses to launch a source file for a class that exists on the
classpath
the source code launcher "feels" like an outlier to me, but it could
certainly go either way
and i guess the source code launcher is an outlier anyway, as it
decouples the first toplevel class name from the file name
On Fri, Sep 14, 2018 at 1:49 PM, Jonathan Gibbons
<jonathan.gibb...@oracle.com <mailto:jonathan.gibb...@oracle.com>> wrote:
Seth,
The reason for reporting an error is to prevent the "wrong" class
being used!
-- Jon
On 09/14/2018 10:14 AM, seth lytle wrote:
this behavior of throwing an error if the class is found twice
on the
classpath strikes me as unusual - afaik, in all other cases,
java is fine
with finding multiple implementations on the classpath and
uses the first
found (similar to the unix path, which also allows overriding
by providing
an explicit path) and this is really one of the powerful
features of java -
allowing you to swap out one class for another seamlessly. and
for rapid
prototyping, "java -cp target/classes:$cp
src/myPackage/Stuff.java" runs
much faster than "mvn package
-Dexec.mainClass=myPackage.Stuff", and doubly
so for mvnDebug
On Fri, Sep 14, 2018 at 3:33 AM, Peter Levart
<peter.lev...@gmail.com <mailto:peter.lev...@gmail.com>>
wrote:
Hi Jaikiran,
Forwarding to compiler-dev as the core of source file
launcher feature is
produced there...
The check for main class is performed after compilation
(which actually
produces the main class name).
I think it would be possible to check for all classes
compiled from the
source file after compilation without to much
complication. The compilation
produces classes and stores them into a Map<String,
byte[]>. The keySet()
of that map is a Set of compiled class names. Each of them
could be tested
via .getResource() invoked upon the application class
loader. The error
could even point to the URL of the conflicting class file
that way...
Regards, Peter
On 09/14/2018 07:36 AM, Jaikiran Pai wrote:
Please consider this trivial code C.java:
public class C {
public static void main(String[] args) throws Exception {
System.out.println("main() execution started");
}
}
ls
C.java
Similar to a previous discussion[1] while doing random
testing, I ended
up compiling C.java explicitly using javac:
javac C.java
ls
C.java C.class
and then at a later date tried to use the source file
launcher feature
of Java 11 (without realizing C.class was already present
in the dir):
java C.java
This threw the error:
error: class found on application class path: C
Although the error isn't that clear for the reason I note
in [2], having
run into this before, I was aware what this meant and
deleted the
C.class and moved on. The important part here is that the
source
launcher noticed this condition and aborted even before it
auto
compiled(?) and launched and executed the main() of the
program.
Now consider a slight modification to that source file:
public class C {
public static void main(String[] args) throws Exception {
System.out.println("main() execution started");
final B b = new B();
System.out.println("Done");
}
private static class B {
}
}
Again at some point I compiled this explicitly using
javac, so my
directory is (among other things):
ls
C$B.class C.class C.java
Then ran the source file launcher feature:
java C.java
error: class found on application class path: C
As expected, ran into the same previous error. As before,
in order to
move on, deleted C.class:
rm C.class
but forgot to delete the nested static class that belonged
to it. So the
directory now contained:
ls
C$B.class C.java
Now used the source launcher feature again:
java C.java
This time it failed with:
main() execution started
Exception in thread "main" java.lang.IllegalAccessError:
failed to
access class C$B from class C (C$B is in unnamed module of
loader 'app';
C is in unnamed module of loader
com.sun.tools.javac.launcher.Main$MemoryClassLoader @1b1473ab)
at C.main(C.java:4)
The error message isn't clear to pinpoint the issue, but
at least the
reference to MemoryClassLoader was a hint that was enough
for me to
understand where to look. It did take me a few minutes to
realize that
C$B.class was lying around which I needed to remove too.
However, IMO, the important part here is that unlike in
the first case
where the program itself wasn't launched and instead was
aborted early,
in this case the program did get executed (notice the
System.out.println
"main() execution started" message that got printed) and
failed at runtime.
Would it be possible to make these two behaviours
consistent and detect
such cases and abort early here too? Or would that add too
much
complexity to this feature?
Finally, any thoughts on the error messages for this
feature to make it
a bit easier in terms of debugging (classloading) issues
like these?
[1]
http://mail.openjdk.java.net/pipermail/jdk-dev/2018-June/001425.html
<http://mail.openjdk.java.net/pipermail/jdk-dev/2018-June/001425.html>
[2]
http://mail.openjdk.java.net/pipermail/jdk-dev/2018-June/001438.html
<http://mail.openjdk.java.net/pipermail/jdk-dev/2018-June/001438.html>
-Jaikiran
