Roaster is a library that allows easy parsing and formatting of java source files. Roaster introduces a fluent interface to manipulate Java source files, like adding fields, methods, annotations and so on.

Below is one example:

import java.io.File;
import java.io.IOException;
import java.io.Serializable;
import java.util.Date;

import org.apache.commons.io.FileUtils;
import org.jboss.forge.roaster.Roaster;
import org.jboss.forge.roaster.model.source.JavaClassSource;

public class POJOGenerator {
	
	public static void main(String[] args) throws IOException {
		//create an empty instance
		final JavaClassSource javaClass = Roaster.create(JavaClassSource.class);
		
		//set the package name, and Java class name
		javaClass.setPackage("demo.JavaCodeGenerator.roaster").setName("PersonPojo");

		//add interface, can be one or more		
		javaClass.addInterface(Serializable.class);
		
		//extend an abstract class
		javaClass.extendSuperType(Date.class);
		
		//add a Long/private/static/final Long field, with value '-1L' 
		javaClass.addField()
		  .setName("serialVersionUID")
		  .setType("long")
		  .setLiteralInitializer("-1L")
		  .setPrivate().setStatic(true).setFinal(true);

		//add a String property, setter/getter functions are generated
		javaClass.addProperty(String.class, "firstName");
		//add an Integer property, final and not exposed
		javaClass.addProperty(Integer.class, "id").setMutable(true).setAccessible(false);
		//add a new property with class 'demo.JavaCodeGenerator.App', an Import line is added
		javaClass.addProperty("demo.JavaCodeGenerator.App", "app");

		//add a public constructor function, 
		javaClass.addMethod()
		  .setConstructor(true)
		  .setPublic()
		  .setBody("this.id = id;")
		  .addParameter(Integer.class, "id");
				
		//add another private function
		javaClass.addMethod()
		.setName("increaseId")
		.setPrivate()
		.setBody("this.id += step;")
		.addParameter("int", "step");
		
		//check if any syntax error
		if(javaClass.hasSyntaxErrors()){
			System.err.println("SyntaxError: "+javaClass.getSyntaxErrors());
		}
		
		//output to file
		String filePath = "src/main/java/demo/JavaCodeGenerator/roaster/PersonPojo.java";
		FileUtils.forceMkdir(new File("src/main/java/demo/JavaCodeGenerator/roaster"));
		FileUtils.writeStringToFile(new File(filePath), javaClass.toString());
	}

}

The output class looks as below:

package demo.JavaCodeGenerator.roaster;

import java.io.Serializable;
import java.util.Date;
import demo.JavaCodeGenerator.App;

public class PersonPojo extends Date implements Serializable {

    private static final long serialVersionUID = -1L;
    private String firstName;
    private Integer id;
    private App app;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public App getApp() {
        return app;
    }

    public void setApp(App app) {
        this.app = app;
    }

    public PersonPojo(java.lang.Integer id) {
        this.id = id;
    }

    private void increaseId(int step) {
        this.id += step;
    }
}