On Mon, 15 Apr 2024 07:36:05 GMT, Jan Lahoda <jlah...@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 one additional
> commit since the last revision:
>
> Reflecting review feedback.
src/java.base/share/native/libjli/java.c line 419:
> 417: invokeInstanceMainWithArgs(JNIEnv *env, jclass mainClass, jobjectArray
> mainArgs) {
> 418: jmethodID constructor = (*env)->GetMethodID(env, mainClass,
> "<init>", "()V");
> 419: CHECK_EXCEPTION_FAIL();
Shouldn't this also not be checked until you know you have an instance method?
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/18753#discussion_r1567092479
