Illegal field name when trying to compile with a javax.swing.Action

[Remi Forax]

Hi David, 
thanks for the bug report. 

I was able to reproduce the issue with the latest early acces of 22, 
There is no issue if "-g" is not specified, so the bug seems to be in the debug information that are added to the classfile. 

javac -g --enable-preview --source 22 GUI.java 

Here is a simpler reproducer 
import java.util.function.Consumer ; 

public class GUI { 
public static void main ( String [] args) { 
Consumer < String > op = _ -> {}; 
} 
} 

javap shows the issue, the LocalVariableTable is generated with the name "" 

private static void lambda$main$0(java.lang.String); 
descriptor: (Ljava/lang/String;)V 
flags: (0x100a) ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC 
Code: 
stack=0, locals=1, args_size=1 
0: return 
LineNumberTable: 
line 5: 0 
LocalVariableTable: 
Start Length Slot Name Signature 
0 1 0 Ljava/lang/String; 

The JVM spec 4.7.13 said 
" 
name_index 

The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must contain a CONSTANT_Utf8_info structure representing a valid unqualified name denoting a local variable ( [ https://docs.oracle.com/javase/specs/jvms/se20/html/jvms-4.html#jvms-4.2.2 | §4.2.2 ] ). 

" 

And 4.2.2 said that the string must have at least one code point, so "" is an invalid name, rightly rejected by the VM (the error message is just not great, it's not about a field but a local variable name, here a parameter inside the LocalVariableTable). 

So it's a bug in the compiler, I see two ways to fix it, either do not generate a line in the LocalVariableTable if the parameter is unamed or use "_" instead. 

regards, 
Rémi 

> From: "David Alayachew" <davidalayachew at gmail.com>
> To: "amber-dev" <amber-dev at openjdk.org>
> Sent: Sunday, August 6, 2023 4:31:54 PM
> Subject: Re: Illegal field name when trying to compile with a javax.swing.Action

> Also, here is my version info.

> $ java --version
> openjdk 21-ea 2023-09-19
> OpenJDK Runtime Environment (build 21-ea+31-2444)
> OpenJDK 64-Bit Server VM (build 21-ea+31-2444, mixed mode, sharing)

> $ javac --version
> javac 21-ea

> On Sun, Aug 6, 2023 at 7:31 PM David Alayachew < [
> mailto:davidalayachew at gmail.com | davidalayachew at gmail.com ] > wrote:

>> Hello Amber Dev Team,

>> I ran into a problem that I am not sure what the cause is. It only seems to
>> occur when I am using unnamed variables.

>> Here are the commands that I am using, as well as their outputs.

>> $ javac -g -Xlint:all,-preview,-serial -Xdiags:verbose --enable-preview --source
>> 21 GUI.java
>> Note: GUI.java uses preview features of Java SE 21.
>> Note: Recompile with -Xlint:preview for details.

>> $ java --enable-preview GUI
>> Error: LinkageError occurred while loading main class GUI
>> java.lang.ClassFormatError: Illegal field name "" in class GUI

>> And here is the file that I am using.

>> import javax.swing.JPanel;
>> import javax.swing.JLabel;
>> import javax.swing.JButton;
>> import java.util.function.BiFunction;

>> public class GUI
>> {

>> public static void main(String[] args)
>> {

>> new GUI(null, null);

>> }

>> public GUI(final Object characterType, final Object icon)
>> {

>> }

>> private Object createConstraintPanel()
>> {

>> final JPanel constraintPanel = new JPanel();
>> final JButton button = new JButton();

>> CREATE_INPUT_SANITIZATION:
>> {

>> BUTTON_ACTION_LISTENER:
>> {

>> button
>> .addActionListener
>> (
>> _ ->
>> {

>> final BiFunction<String, Integer, JLabel> createLabel =
>> (text, width) ->
>> {

>> return null;

>> }
>> ;

>> final JButton remove = new JButton();
>> remove
>> .addActionListener
>> (
>> _ ->
>> {

>> }
>> );

>> }
>> )
>> ;

>> }

>> }

>> return constraintPanel;

>> }

>> }

>> Technically, this issue also occurs if even 1 of the 2 unnamed variables is
>> turned into a real variable. But regardless, the point is, this fails at run
>> time.

>> Please let me know if there is anything I am missing.

>> Thank you for your time and help!
>> David Alayachew

>> On Tue, Jul 18, 2023 at 1:54 AM David Alayachew < [
>> mailto:davidalayachew at gmail.com | davidalayachew at gmail.com ] > wrote:

>>> Ok, ty for the heads up.

>>> On Tue, Jul 18, 2023 at 1:18 AM - < [ mailto:liangchenblue at gmail.com |
>>> liangchenblue at gmail.com ] > wrote:

>>>> Hi David,
>>>> Please do not ask for technical support in the development mailing list. There's
>>>> no proof that this issue is with the core libraries instead of the Java
>>>> compiler or another component.
>>>> I have already downloaded your zip, compiled the 4 files with JDK 22 (my local
>>>> dev build for another unrelated patch) javac --enable-preview -release 22
>>>> *.java and java --enable-preview ToDoList, which just shows up a UI without any
>>>> crash. Without enable-preview, yoru code does not compile at all for the usage
>>>> of _ as an identifier.

>>>> This is most likely a problem with the compiler you are using; you have a gpj
>>>> file, which probably means you are using jGrasp. Please report this issue to
>>>> the relevant parties (i.e. the 3rd-party IDE or whatever supports using _ as
>>>> identifer without --enable-preview) instead.

>>>> Chen Liang

>>>> On Tue, Jul 18, 2023 at 11:43 AM David Alayachew < [
>>>> mailto:davidalayachew at gmail.com | davidalayachew at gmail.com ] > wrote:

>>>>> Wait, nevermind. Don't ignore. I was able to recreate this issue on the command
>>>>> line using the abovementioned JDK. Could someone come back and take a look at
>>>>> this please?

>>>>> Sorry for jerking everyone around here. I have multiple JDK's and got mixed up
>>>>> on which one I was using.

>>>>> On Sun, Jul 16, 2023 at 2:26 AM David Alayachew < [
>>>>> mailto:davidalayachew at gmail.com | davidalayachew at gmail.com ] > wrote:

>>>>>> Hmmmm, this one might actually be IDE specific. Please ignore.
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