ClassLoading

package com.stackoverflow;
import java.lang.reflect.Method;
import java.net.URI;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.List;

import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.StandardLocation;
import javax.tools.ToolProvider;

public class Test1 {

    private static final Path TEMP_PATH = Paths.get("/tmp/");

    public static void main(String[] args) throws Exception {
        String thisPackage = Test1.class.getPackage().getName();
        String className = thisPackage + ".Formula";
        String body = "package " + thisPackage + ";   "
                    + "public class Formula {         "
                    + "    double calculateFails() {  "
                    + "        return 42;             "
                    + "    }                          "
                    + "}                              ";

        compile(className, body, TEMP_PATH);

        loadClass(className, TEMP_PATH);

    }

    private static void loadClass(String className, Path path) throws Exception {
        // com/stackoverflow/Test1.class
        String idem = Test1.class.getCanonicalName().replace(".", "/") + ".class";
        // The URL
        String classFile = Test1.class.getClassLoader().getResource(idem).toString();
        // Remove the class name
        String repository = classFile.substring(0, classFile.length() - idem.length());

        URL location = new URL(repository);
        URLClassLoader loader = new URLClassLoader(new URL[]{path.toUri().toURL(), location}, null);

        Class<?> formulaClass = loader.loadClass(className);

        Class<Test1> test1 = (Class<Test1>) loader.loadClass(Test1.class.getCanonicalName());

        assert test1.getClassLoader().equals(formulaClass.getClassLoader()) ;

        Method compute = test1.getDeclaredMethod("compute", Class.class);
        compute.invoke(test1, formulaClass);

        loader.close();
    }

    public static void compute(Class<?> formulaClass) throws Exception {
        Method calculateFails = formulaClass.getDeclaredMethod("calculateFails");
        //next line throws exception:
        double valueFails = (Double) calculateFails.invoke(formulaClass.newInstance());
        System.out.println("HOORAY! valueFails = " + valueFails);
    }

    private static void compile(String className, String body, Path path) throws Exception {
        List<JavaSourceFromString> sourceCode = Arrays.asList(new JavaSourceFromString(className, body));

        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);
        fileManager.setLocation(StandardLocation.CLASS_OUTPUT, Arrays.asList(path.toFile()));
        boolean ok = compiler.getTask(null, fileManager, null, null, null, sourceCode).call();

        System.out.println("compilation ok = " + ok);
    }

    public static class JavaSourceFromString extends SimpleJavaFileObject {
        final String code;

        JavaSourceFromString(String name, String code) {
            super(URI.create("string:///" + name.replace('.', '/') + JavaFileObject.Kind.SOURCE.extension),
                    JavaFileObject.Kind.SOURCE);
            this.code = code;
        }

        @Override
        public CharSequence getCharContent(boolean ignoreEncodingErrors) {
            return code;
        }
    }
}