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<h1>How Groovy works behind the scenes</h1>

<p>With your Groovy scripts, you have multiple options on how to use them:</p>
<ul>
	<li>run groovyc on them to create Java *.class files</li>
	<li>evaluate the script at runtime (either from a file or from a Java String)</li>
	<li>make your script available as Java Class-objects through the GroovyClassLoader (no *.class files generated!)</li>
</ul>

<p><em>No *.java source files are ever generated.</em></p>

<p>All the above may seem like magic and it somehow is: the magic of imaginative, mindful software engineering.</p>

<h2>Everything starts with the Groovy grammar.</h2>

<p><em>There is the notion of the 'new' and the 'old' parser. Only the new one (as of Feb.05) is described here.</em></p>

<p>The Groovy grammar is written in terms of an ANTLR (= <em>ANother Tool for Language Recognition</em>) grammar. The tool can handle grammars of type LL(k), where the Java grammar is of type LL(2) and Groovy is of type LL(3).</p>

<p>The difference is in the number of tokens that the parser needs to look ahead for recognizing e.g. &quot;==&quot; (2 tokens for Java) or &quot;===&quot; (3 tokens for Groovy).  To be more correct, the problem is in recognizing  the first &quot;=&quot; character. The parser needs to &quot;look ahead&quot; to derive its meaning.</p>

<p>ANTLR formulates the grammar in terms of &quot;rules&quot; that fully implement EBNF (Extended Backus-Naur Form) enriched with Java code  blocks, special functions and some other things.</p>

<p>With the recognition of a rule, actions can be triggered that come as usual Java code. ANTLR dumps out Javacode representing a parser capable to  recognize the given grammar. And this parser executes the embedded code  blocks from the grammar - the &quot;action&quot;.</p>

<h2>Parser Generation and AST</h2>

<p>ANTLR takes the Groovy grammar file &quot;Groovy.g&quot; to create the Groovy parser.  When the parser is fed with  the source code of a Groovy script, it produces the AST (= <em>Abstract Syntax Tree</em>) that represents that code as a run-time structure.</p>

<h2>Byte Code Generation</h2>

<p>From the AST, it is possible to create Java Byte Code:  either for making it persistent as *.class files or for making it directly  available as Class objects through the GroovyClassLoader.</p>

<p>This ClassGeneration is done with the help of objectweb's ASM tool.  (The ASM name does not mean anything: it is just a reference to the &quot;asm&quot; keyword in C, which allows some functions to be implemented in assembly language.)</p>

<p>ASM provides a Java API to construct or modify Byte Code on a given AST.</p>

<p>The API for bytecode generation heavily relies on the Visitor Pattern.  The main entry point for the class generation is   org.org.codehaus.groovy.classgen.AsmClassGenerator.java.</p>

<p>It is a large class. There are visitXYExpression  methods called when converting the AST to bytecode. For example  visitArrayExpression is called when creating arrays in bytecode.</p>

<h2>More Links on the topic</h2>

<p><a class="confluence-link" href="/display/GROOVY/Groovy+Method+Invokation" data-linked-resource-id="26770" data-linked-resource-type="page" data-linked-resource-default-alias="Groovy Method Invokation" data-base-url="http://docs.codehaus.org">Groovy Method Invokation</a></p>

<p>Tools:</p>
<ul>
	<li><a href="http://www.antlr.org/">http://www.antlr.org/</a></li>
	<li><a href="http://asm.objectweb.org/">http://asm.objectweb.org/</a></li>
</ul>

<p>General:</p>
<ul>
	<li><a href="http://compilers.iecc.com/comparch/article/99-02-109">http://compilers.iecc.com/comparch/article/99-02-109</a></li>
	<li><a href="http://en.wikipedia.org/wiki/Category:Parsing_algorithms">http://en.wikipedia.org/wiki/Category:Parsing_algorithms</a></li>
	<li><a href="http://lambda.uta.edu/cse5317/spring02/notes/notes.html">http://lambda.uta.edu/cse5317/spring02/notes/notes.html</a></li>
</ul>