Java metaprogramming using JavaPoet to generate .java source and the varcode InMemoryJavacCompiler

ex.javapoet.JavaPoetHelloWorldMeta.java

package ex.javapoet;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import javax.lang.model.element.Modifier;

import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.TypeSpec;

import varcode.java.Java;
import varcode.java.javac.InMemoryJavaClassLoader;
import varcode.java.javac.InMemoryJavaCode;
import varcode.java.javac.InMemoryJavac;

/**<PRE>
 * 
 * *1* build a main method
 * *2* build a class and add the main method
 * *3* build a (.java) file
 * *4* write the (.java) file to System.out
 * *5* prepare file to be compiled by javac
 * *6* compile and load the (.class) from the (.java) file
 * *7* invoke the main method on the (.class) 
 */
public class JavaPoetHelloWorldMeta 
{
	public static void main(String[] args) throws IOException
	{
		MethodSpec main = MethodSpec.methodBuilder("main") // *1*
			.addModifiers(Modifier.PUBLIC, Modifier.STATIC)
			.returns(void.class)
			.addParameter(String[].class, "args")
			.addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!")
			.build();

		TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld") // *2*
			.addModifiers(Modifier.PUBLIC, Modifier.FINAL)
			.addMethod(main)
			.build();

		JavaFile javaFile = 
			JavaFile.builder("ex.javapoet", helloWorld)
			    .build();               // *3*
		
		javaFile.writeTo( System.out ); // *4*	
		
		List<InMemoryJavaCode> toCompile =      // *5*   
			new ArrayList<InMemoryJavaCode>();
		
		toCompile.add( 
			InMemoryJavaCode.of( javaFile.toJavaFileObject() ) ); // *5*
		
		Map<String,Class<?>> classNameToClass =  // *5* *6*   
			InMemoryJavac.compileLoadClasses(
				new InMemoryJavaClassLoader(), 
				 toCompile );
		
		Class<?> clazz = 
			classNameToClass.get( "ex.javapoet.HelloWorld" );
		
		Java.invoke( clazz, "main", (Object)new String[ 0 ] ); // *7*
	}
}

Metaprogramming Java (HelloWorld) Examples:

*IMPORTANT NOTE: all examples REQUIRE a Runtime JDK (not a JRE) to compile
Java Code (.java files) to .class files at runtime. (and varcode) *

varcode JavaModel
varcode CodeML
varcode BindML
JavaPoet / varcode

varcode (and in particular varcode JavaModel) makes metaprogramming easy by composing Java source files in a way that is natural and intuitive.

varcode is easy to write, use, test, modify and maintain (it looks just like the Java code you already know.)

Alternatively JavaPoet is clunky, because:

• JavaPoet was designed purely as a code generation engine (not as a way to build AND compile Java code)
  it requires manual code to call the Java Compiler.
• JavaPoet API is fluent and uses the builder pattern, but (unfortunately) doesn't do a good job explaining what the
  generated code "does", therefore it's harder to learn, use, test, and maintain.
  Having the ability to easily invoke the Javac compiler with a simple method makes iteration and testing MUCH easier with varcode)