how can I generate a lexer and parser on the fly (at runtime)?

Question

I did find an answer to my question: Barts answer is exactly why I need, but it does not function (see below).

Please can some one either give me a working example or show me the where I am going wrong to implement Barts answer?

This is what I get for the other answer, I get the hereunder error from "part 4"

Lexer lexer = (Lexer)Class.forName(grammarName + "Lexer").newInstance();

I've got antlr3.4 complete and JDK libraries. I created it as package createclass; yet I find the answer strange as the class to create has no package??? So I tried adding to the string:

"@lexer::header {\n" +
"   package createclass;\n" +
"}        \n" +
"@parser::header {\n" +
"   package createclass;\n" +
"}\n" +

But still no change.

Here's the output:

debug:
Exception in thread "main" java.lang.ClassNotFoundException: TLexer
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:169)
    at createclass.Createclass.main(Createclass.java:61)
Java Result: 1
BUILD SUCCESSFUL (total time: 31 seconds)

The example code given is:

import java.io.*;
import javax.tools.*;
import java.lang.reflect.*;
import org.antlr.runtime.*;
import org.antlr.Tool;

public class Main {

    public static void main(String[] args) throws Exception {

        // The grammar which echos the parsed characters to theconsole,
        // skipping any white space chars.
        final String grammar =
                "grammar T;                                                  \n" +
                "                                                            \n" +
                "parse                                                       \n" +
                "  :  (ANY {System.out.println(\"ANY=\" + $ANY.text);})* EOF \n" +
                "  ;                                                         \n" +
                "                                                            \n" +
                "SPACE                                                       \n" +
                "  :  (' ' | '\\t' | '\\r' | '\\n') {skip();}                \n" +
                "  ;                                                         \n" +
                "                                                            \n" +
                "ANY                                                         \n" +
                "  :  .                                                      \n" +
                "  ;                                                           ";
        final String grammarName = "T";
        final String entryPoint = "parse";

        // 1 - Write the `.g` grammar file to disk.
        Writer out = new BufferedWriter(new FileWriter(new File(grammarName + ".g")));
        out.write(grammar);
        out.close();

        // 2 - Generate the lexer and parser.
        Tool tool = new Tool(new String[]{grammarName + ".g"});
        tool.process();

        // 3 - Compile the lexer and parser.
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        compiler.run(null, System.out, System.err, "-sourcepath", "", grammarName + "Lexer.java");
        compiler.run(null, System.out, System.err, "-sourcepath", "", grammarName + "Parser.java");

        // 4 - Parse the command line parameter using the dynamically created lexer and 
        //     parser with a bit of reflection Voodoo :)
        Lexer lexer = (Lexer)Class.forName(grammarName + "Lexer").newInstance();
        lexer.setCharStream(new ANTLRStringStream(args[0]));
        CommonTokenStream tokens = new CommonTokenStream(lexer);
        Class<?> parserClass = Class.forName(grammarName + "Parser");
        Constructor parserCTor = parserClass.getConstructor(TokenStream.class);
        Parser parser = (Parser)parserCTor.newInstance(tokens);
        Method entryPointMethod = parserClass.getMethod(entryPoint);
        entryPointMethod.invoke(parser);
    }
}

Answer 1

My guess is that you're using an IDE for this, and don't have your classpath set properly. The demo from my answer works (also using v3.4). Here's what I just did:

I copied the file Main.java in a directory (~/Temp/demo, in my case) and opened a shell in this directory and did the following:

bart@hades:~/Temp/demo$ java -version
> java version "1.6.0_24"
> OpenJDK Runtime Environment (IcedTea6 1.11.4) (6b24-1.11.4-1ubuntu0.12.04.1)
> OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode)

bart@hades:~/Temp/demo$ wget http://www.antlr.org/download/antlr-3.4-complete.jar
> --2012-10-17 19:26:01--  http://www.antlr.org/download/antlr-3.4-complete.jar
> Resolving www.antlr.org (www.antlr.org)... 138.202.170.10
> Connecting to www.antlr.org (www.antlr.org)|138.202.170.10|:80... connected.
> HTTP request sent, awaiting response... 200 OK
> Length: 2388361 (2.3M) [application/java-archive]
> Saving to: `antlr-3.4-complete.jar'
> 
> 100%[===============================================================================================================>] 2,388,361    317K/s   in 8.9s    
> 
> Last-modified header invalid -- time-stamp ignored.
> 2012-10-17 19:26:11 (261 KB/s) - `antlr-3.4-complete.jar' saved [2388361/2388361]

bart@hades:~/Temp/demo$ javac -cp antlr-3.4-complete.jar *.java

bart@hades:~/Temp/demo$ java -cp .:antlr-3.4-complete.jar Main "a b    c"
> ANY=a
> ANY=b
> ANY=c

(the > is output printed to the console)