RFR: 8329420: Java 22 (and 23) launcher calls default constructor although main() is static [v3]

[Jan Lahoda]

On Wed, 17 Apr 2024 06:34:25 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> Consider code like:
>> 
>> public class MainClass {
>>     public MainClass() {
>>         System.out.println("Constructor called!");
>>     }
>>     public static void main() {
>>         System.out.println("main called!");
>>     }
>> }
>> 
>> and compile and run it, with preview enabled, like:
>> 
>> $ javac /tmp/MainClass.java 
>> $ java --enable-preview -classpath /tmp MainClass
>> Constructor called!
>> main called!
>> 
>> 
>> That is wrong, as the `main` method is static, and there is no need to create a new instance of the class.
>> 
>> The reason is that as launcher attempts to invoke the main method, it goes in the following order: 1) static-main-with-args; 2) instance-main-with-args; 3) static-main-without-args; 4) instance-main-without-args. But, for the instance variants, the code first creates a new instance of the given class, and only then attempts to lookup the `main` method, and will pass to the next option when the `main` method lookup fails. So, when invoking static-main-without-args, the current class instance may be created for instance-main-with-args, which will then fail due to the missing `main(String[])` method.
>> 
>> The proposed solution to this problem is to simply first do a lookup for the `main` method (skipping to the next variant when the given main method does not exist, without instantiating the current class).
>> 
>> There is also a relatively closely related problem: what happens when the constructor throws an exception?
>> 
>> public class MainClass {
>>     public MainClass() {
>>         if (true) throw new RuntimeException();
>>     }
>>     public void main() {
>>         System.out.println("main called!");
>>     }
>> }
>> 
>> 
>> when compiled an run, this produces no output whatsoever:
>> 
>> $ javac /tmp/MainClass.java 
>> $ java --enable-preview -classpath /tmp MainClass
>> $
>> 
>> 
>> This is because any exceptions thrown from the constructor are effectively ignored, and the launcher will continue with the next variant. This seems wrong - the exception should be printed for the user, like:
>> 
>> $ java --enable-preview -classpath /tmp/ MainClass 
>> Exception in thread "main" java.lang.RuntimeException
>>         at MainClass.<init>(MainClass.java:3)
>> 
>> 
>> This patch proposes to do that by not consuming the exceptions thrown from the constructor, and stop the propagation to the next variant.
>
> Jan Lahoda has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Reflecting code formatting suggestion.
>  - First lookup the main method, and only then the constructor.
>  - Attempting to solve JDK-8329581 by only ignoring j.l.NoSuchMethodError

So, I've pushed an update that:
- attemps to solve [JDK-8329581](https://bugs.openjdk.org/browse/JDK-8329581) by only ignoring `NoSuchMethodError` when looking up methods. Hence any other problem will lead to an error to be printed out, as usually.
https://github.com/openjdk/jdk/pull/18753/commits/dd447e898785a3d7e2def47dd1efcca3db41dd46
- changed the code to first lookup the `main` method, and only then constructors, which is what I think @dholmes-ora was suggesting.
https://github.com/openjdk/jdk/pull/18753/commits/83fabec6e5e8bf6deb9efce22ef6ace408cf3d26
- fixed the comment formatting, as suggested by @AlanBateman
https://github.com/openjdk/jdk/pull/18753/commits/c007f61ebc0660cc6f93e29e71dc635aa50ba538

Please let me know what you think. Thanks!

-------------

PR Comment: https://git.openjdk.org/jdk/pull/18753#issuecomment-2060481245