[Java] Generated code gives "this escape" warning on Java 21 in Parser class

[nielsbasjes]

The Java code generated by the antlr4-maven-plugin version 4.13.1 still has this escape warnings when compiling with -Xlint:all.

Note that this issue was partially addressed in #4394 where the @SuppressWarnings on the Lexer class was updated. Apparently this problem is still present on the Parser class aswell.

Minimal reproduction:

In an empty directory create
./pom.xml

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>nl.basjes.bugreports</groupId>
  <artifactId>antlr4-java21</artifactId>
  <version>0.0.1-SNAPSHOT</version>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.build.outputEncoding>UTF-8</project.build.outputEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.antlr</groupId>
      <artifactId>antlr4-runtime</artifactId>
      <version>4.13.1</version>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.antlr</groupId>
        <artifactId>antlr4-maven-plugin</artifactId>
        <version>4.13.1</version>
        <executions>
          <execution>
            <id>antlr</id>
            <goals>
              <goal>antlr4</goal>
            </goals>
            <configuration>
              <visitor>true</visitor>
            </configuration>
          </execution>
        </executions>
      </plugin>

      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.11.0</version>
        <configuration>
          <source>11</source>
          <release>11</release>
          <compilerArgs>
            <arg>-Xlint:all</arg>
          </compilerArgs>
          <showWarnings>true</showWarnings>
          <showDeprecation>true</showDeprecation>
        </configuration>
      </plugin>

    </plugins>
  </build>

</project>

and any grammar in ./src/main/antlr4/

I used ./src/main/antlr4/Expression.g4

grammar Expression;

SPACE: ' ' -> skip;

NUMBER: [0-9]+;
PLUS  : '+';
BOPEN : '(';
BCLOSE: ')';

expression
    : num=NUMBER                                #number
    | left=expression PLUS right=expression     #sumNumbers
    | BOPEN expr=expression BCLOSE              #bracedExpression
    ;

Then using Java 21 run mvn clean compile (I used Linux and maven 3.9.5)

The output contains several lines similar to this:

[WARNING] /home/nbasjes/.../Antlr4Java21/target/generated-sources/antlr4/ExpressionParser.java:[88,50] possible 'this' escape before subclass is fully initialized

[nielsbasjes]

Additional reproduction: Take the reproduction of #4341 located here https://github.com/khmarbaise/issue-antlr-4.13.0

Clone this and change the version to 4.13.1

When you actually build this with Java 21 you'll see that the reported problem on the Lexer class is gone and the (many) problems in the Parser class remain the same.