A Java ClassLoader that can load classes from Java source file. This ClassLoader can't compile recursively.

DynamicCompileClassLoader.java

package test;

import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;

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

public class DynamicCompileClassLoader extends URLClassLoader {
    private String sourceInputFolder;
    private String classOutputFolder;
    private JavaCompiler compiler;

    private DynamicCompileClassLoader(URL[] urls, String sourceInputFolder,
            String classOutputFolder, ClassLoader classLoader) {
        super(urls, classLoader);
        this.sourceInputFolder = sourceInputFolder;
        this.classOutputFolder = classOutputFolder;
        this.compiler = ToolProvider.getSystemJavaCompiler();
    }

    public static DynamicCompileClassLoader create(String sourceInputFolder,
            String classOutputFolder, ClassLoader classLoader) {
        File outputDir = new File(classOutputFolder);
        if (!outputDir.exists()) {
            outputDir.mkdirs();
        }
        URL url = null;
        try {
            url = outputDir.toURI().toURL();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }
        return new DynamicCompileClassLoader(new URL[] {url}, sourceInputFolder, classOutputFolder,
                classLoader);
    }


    public Class<?> findClass(String qualifiedClassName) throws ClassNotFoundException {
        String filepath = Paths.get("", qualifiedClassName.split("\\.")).toString();
        Path path = Paths.get(sourceInputFolder, filepath + ".java");
        File sourceFile = path.toFile();
        if (!sourceFile.exists()) {
            return super.findClass(qualifiedClassName);
        }
        File classFile = Paths.get(classOutputFolder, filepath + ".class").toFile();
        if (classFile.exists()) {
            if (!sourceFile.exists() || classFile.lastModified() > sourceFile.lastModified()) {
                return super.findClass(qualifiedClassName);
            }
        }
        compileClassFile(sourceFile);
        return super.findClass(qualifiedClassName);
    }

    private void compileClassFile(File sourceFile) {
        Iterable<String> options = Arrays.asList("-d", classOutputFolder);
        try (StandardJavaFileManager fileManager =
                compiler.getStandardFileManager(null, null, null)) {
            Iterable<? extends JavaFileObject> fileObjects =
                    fileManager.getJavaFileObjects(sourceFile);
            JavaCompiler.CompilationTask task =
                    compiler.getTask(null, null, null, options, null, fileObjects);
            Boolean result = task.call();
            if (result == null || !result.booleanValue()) {
                throw new RuntimeException(
                        "Compilation failed. class file path:" + sourceFile.getPath());
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }

    }

    public static void main(String[] args)
            throws ClassNotFoundException, InstantiationException, IllegalAccessException {
        DynamicCompileClassLoader dcc = DynamicCompileClassLoader.create("demo", "demo_class",
                DynamicCompileClassLoader.class.getClassLoader());
        Class<?> class1 = dcc.loadClass("Test1");
        System.out.println(class1.getName());
        Class<?> class2 = dcc.loadClass("Test2");
        System.out.println(class2.getName());
    }

}

Test1.java

public class Test1 {
    public static void main(String[] args) {
        System.out.println("Test1 compiled.");
    }
}

Test2.java

public class Test2 extends Test1{
    
    public void foo(){
        System.out.println("bar");
    }
    
    public static void main(String[] args) {
        System.out.println("Test2 compiled.");
    }
}