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Groovy example source code file (MainTransformation.groovy)

This example Groovy source code file (MainTransformation.groovy) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Groovy tags/keywords

annotationnode, asttransformation, classnode, classnode, groovyasttransformation, groovyasttransformation, maintransformation, methodnode, methodnode, public, sourceunit, static, static

The Groovy MainTransformation.groovy source code

package examples.astbuilder

import org.codehaus.groovy.ast.*
import org.codehaus.groovy.control.CompilePhase
import org.codehaus.groovy.transform.GroovyASTTransformation
import org.codehaus.groovy.transform.ASTTransformation
import org.codehaus.groovy.control.SourceUnit
import org.codehaus.groovy.ast.builder.AstBuilder

/**
 * If there is a method in a class with the @Main annotation on it, then this 
 * transformation adds a real main(String[]) method to the class with the same
 * method body as the annotated class. 
 *
 * @author Hamlet D'Arcy
 */
@GroovyASTTransformation(phase = CompilePhase.INSTRUCTION_SELECTION)
public class MainTransformation implements ASTTransformation {

    // normally defined in org.objectweb.asm.Opcodes, but there duplicated
    // here to make the build script simpler. 
    static int PUBLIC = 1
    static int STATIC = 8
    
    void visit(ASTNode[] astNodes, SourceUnit sourceUnit) {

        // use guard clauses as a form of defensive programming. 
        if (!astNodes) return 
        if (!astNodes[0]) return 
        if (!astNodes[1]) return 
        if (!(astNodes[0] instanceof AnnotationNode)) return
        if (astNodes[0].classNode?.name != Main.class.name) return
        if (!(astNodes[1] instanceof MethodNode)) return 

        MethodNode annotatedMethod = astNodes[1]
        ClassNode declaringClass = annotatedMethod.declaringClass
        MethodNode mainMethod = makeMainMethod(annotatedMethod)
        declaringClass.addMethod(mainMethod)
    }

    /**
    * Uses the AstBuilder to synthesize a main method, and then sets the body of
    * the method to that of the source method. Notice how Void.TYPE is used as
    * a return value instead of Void.class. This is required so that resulting method
    * is void and not Void. 
    */ 
    MethodNode makeMainMethod(MethodNode source) {
        def className = source.declaringClass.name
        def methodName = source.name

        def ast = new AstBuilder().buildFromString(CompilePhase.INSTRUCTION_SELECTION, false, """
            package $source.declaringClass.packageName
            
            class $source.declaringClass.nameWithoutPackage {
                public static void main(String[] args) {
                    new $className().$methodName()
                }
            }
        """)
        ast[1].methods.find { it.name == 'main' }
    }
}

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