listener.md

Parse tree listener

In this lab, you will learn the basics of parse tree listeners. It builds on the previous lab where we integrated ANTLR-generated parsers and lexers into a Java program.

This lab is derived from section 7.2 in the ANTLR book. You can look at that when you get stuck doing this lab.

Getting started

Create a new project using the same structure from the previous lab and make a PropertyFile.g4 file in the grammars directory. Here is the core, of course you need to add the grammar header and definitions of those tokens:

file : prop+ ;
prop : ID '=' STRING '\n' ;

Now ask ANTLR to generate the parser/lexer by right clicking in the grammar and saying "Generate recognizer". Then look in the gen directory at the root of your project and you will see PropertyFileListener and PropertyFileBaseListener. Now tell intellij to treat that directory like source code.

Add antlr-4.6-complete.jar as a dependency for the module and try to build.

Copy and paste the main program from the previous lab and then change CDecl to PropertyFile to reflect the new grammar:

You should end up with something like

String prop = "id=\"parrt\"\n";
ANTLRInputStream input = new ANTLRInputStream(prop);
PropertyFileLexer lexer = new PropertyFileLexer(input);
CommonTokenStream tokens = new CommonTokenStream(lexer);
PropertyFileParser parser = new PropertyFileParser(tokens);
ParseTree tree = parser.file();
System.out.println(tree.toStringTree(parser));
Trees.inspect(tree, parser);

Run that and you should get output (file (prop id = "parrt" \n)) as well as a dialog box showing the parse tree.

Listener action

Okay, now that we have a valid parser and we've pulled out the tree, let's walk the tree and perform some actions.

Create a new class called PropertyFileLoader that we will use to fill a Map with properties.

Now modify the main program so that it creates a tree walker and an instance of that property followed her then triggers a walk using walk():

ParseTreeWalker walker = new ParseTreeWalker();
PropertyFileLoader loader = new PropertyFileLoader();
walker.walk(loader, tree);

This will fill the loader.props field, which we can then print out in the main program:

System.out.println(loader.props);

You should get output:

{id="parrt"}