Compiling Java code dynamically and running it in Espresso.

DynamicJava.java

import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.Value;


/**
 * Some code taken from: https://www.soulmachine.me/blog/2015/07/22/compile-and-run-java-source-code-in-memory/
 */
public class DynamicJava {

    static final String FILE_NAME = "Solution.java";    
    static final String SOURCE =
        "public final class Solution {\n" +
        "   public static String greet(String name) {\n" +
        "       return \"Hello \" + name;\n" +
        "   }\n" +
        "}\n";


    static Map<String, byte[]> compileInTheHost(String fileName, String source) {
        final InMemoryJavaCompiler compiler = new InMemoryJavaCompiler();
        final Map<String, byte[]> classes = compiler.compile(fileName, source);
        return classes;
    }

    static Value compileInTheGuest(Context context, String fileName, String source) {
        Value bindings = context.getBindings("java");
        Value InMemoryJavaCompiler_klass = bindings.getMember(InMemoryJavaCompiler.class.getName());
        Value compiler = InMemoryJavaCompiler_klass.newInstance();
        Value classes = compiler.invokeMember("compile", fileName, source);
        return classes;
    }

    static void testInTheHost(Map<String, byte[]> classes) {
        MemoryClassLoader loader = new MemoryClassLoader(classes);
        String result;
        try {            
            Class<?> Solution_class = loader.loadClass("Solution");
            Method greet = Solution_class.getDeclaredMethod("greet", String.class);
            result = (String) greet.invoke(null, "host");
        } catch (Exception e) {
            throw new RuntimeException(e);
        }        
        System.out.println("(testInTheHost) result: " + result);
    }

    static Value toHost(Context context, Map<String, byte[]> classes) {        
        Value bindings = context.getBindings("java");
        Value ByteArray_klass = bindings.getMember(byte[].class.getName());
        Value HashMap_klass = bindings.getMember(HashMap.class.getName());

        Value guestClasses = HashMap_klass.newInstance();
        for (Map.Entry<String, byte[]> entry : classes.entrySet()) {
            String key = entry.getKey();
            byte[] value = entry.getValue();    
            Value guestValue = ByteArray_klass.newInstance(value.length);
            for (int i = 0; i < value.length; ++i) {
                guestValue.setArrayElement(i, value[i]);
            }
            // key is automatically converted into a guest String.
            guestClasses.invokeMember("put", key, guestValue);
        }

        return guestClasses;
    }    

    static void testInTheGuest(Context context, Value classes) {
        Value bindings = context.getBindings("java");
        Value MemoryClassLoader_klass = bindings.getMember(MemoryClassLoader.class.getName());
        Value loader = MemoryClassLoader_klass.newInstance(classes);
        Value Solution = loader.invokeMember("loadClass", "Solution");
        Value result = Solution.getMember("static").invokeMember("greet", "Espresso");
        System.out.println("(testInTheGuest) result: " + result.asString());
    }
    public static void main(String[] args) {
        try (Context context = Context.newBuilder("java")
            .allowAllAccess(true)
            // To expose MemoryClassLoader to the guest.
            .option("java.Classpath", System.getProperty("java.class.path"))
            .build()) {

            // Compile and run in the host.
            Map<String, byte[]> classes_host = compileInTheHost(FILE_NAME, SOURCE);
            testInTheHost(classes_host);

            // Compile and run in the guest.
            Value classes_guest = compileInTheGuest(context, FILE_NAME, SOURCE);
            testInTheGuest(context, classes_guest);

            // Compile in the host (already done) and run in the guest.
            testInTheGuest(context, toHost(context, classes_host));
        }
    }
}

class MemoryClassLoader extends URLClassLoader {
    private final Map<String, byte[]> classBytes = new ConcurrentHashMap<String, byte[]>();

    public MemoryClassLoader(Map<String, byte[]> classBytes) {
        super(new URL[0], MemoryClassLoader.class.getClassLoader());
        this.classBytes.putAll(classBytes);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] buf = classBytes.get(name);
        if (buf == null) {
            return super.findClass(name);
        }
        classBytes.remove(name);
        return defineClass(name, buf, 0, buf.length);
    }
}

InMemoryJavaCompiler.java

import java.io.IOException;
import java.io.PrintWriter;
import java.io.Writer;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

import javax.tools.*;

/**
 * Simple interface to Java compiler using JSR 199 Compiler API.
 */
public class InMemoryJavaCompiler {
    private final javax.tools.JavaCompiler tool;
    private StandardJavaFileManager stdManager;

    public InMemoryJavaCompiler() {
        tool = ToolProvider.getSystemJavaCompiler();
        if (tool == null) {
            throw new RuntimeException("Could not get Java compiler. Please, ensure that JDK is used instead of JRE.");
        }
        stdManager = tool.getStandardFileManager(null, null, null);
    }

    public Map<String, byte[]> compile(String fileName, String source) {
        return compile(Collections.singletonMap(fileName, source), new PrintWriter(System.err), null, null);
    }

    /**
     * compile given String source and return bytecodes as a Map.
     *
     * @param sources filename -> source
     * @param err error writer where diagnostic messages are written
     * @param sourcePath location of additional .java source files
     * @param classPath location of additional .class files
     */
    private Map<String, byte[]> compile(Map<String, String> sources,
        Writer err, String sourcePath, String classPath) {
        // to collect errors, warnings etc.
        DiagnosticCollector<JavaFileObject> diagnostics =
            new DiagnosticCollector<JavaFileObject>();

        // create a new memory JavaFileManager
        MemoryJavaFileManager fileManager = new MemoryJavaFileManager(stdManager);

        // prepare the compilation unit
        List<JavaFileObject> compilationUnits = sources.entrySet().stream()
            .map(entry -> fileManager.makeStringSource(entry.getKey(), entry.getValue()))
            .collect(Collectors.toList());

        return compile(compilationUnits, fileManager, err, sourcePath, classPath);
    }

    private Map<String, byte[]> compile(final List<JavaFileObject> compUnits,
        final MemoryJavaFileManager fileManager,
        Writer err, String sourcePath, String classPath) {
        // to collect errors, warnings etc.
        DiagnosticCollector<JavaFileObject> diagnostics =
            new DiagnosticCollector<JavaFileObject>();

        // javac options
        List<String> options = new ArrayList<String>();
        options.add("-Xlint:all");
        //       options.add("-g:none");
        options.add("-deprecation");
        if (sourcePath != null) {
            options.add("-sourcepath");
            options.add(sourcePath);
        }

        if (classPath != null) {
            options.add("-classpath");
            options.add(classPath);
        }

        // create a compilation task
        JavaCompiler.CompilationTask task =
            tool.getTask(err, fileManager, diagnostics,
                options, null, compUnits);

        if (task.call() == false) {
            PrintWriter perr = new PrintWriter(err);
            for (Diagnostic diagnostic : diagnostics.getDiagnostics()) {
                perr.println(diagnostic);
            }
            perr.flush();
            return null;
        }

        Map<String, byte[]> classBytes = fileManager.getClassBytes();
        try {
            fileManager.close();
        } catch (IOException exp) {
        }

        return classBytes;
    }
}

MemoryFileManager.java

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.URI;
import java.nio.CharBuffer;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import javax.tools.FileObject;
import javax.tools.ForwardingJavaFileManager;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject;
import javax.tools.JavaFileObject.Kind;
import javax.tools.SimpleJavaFileObject;

/**
 * JavaFileManager that keeps compiled .class bytes in memory.
 */
@SuppressWarnings("unchecked")
final class MemoryJavaFileManager extends ForwardingJavaFileManager {

    /** Java source file extension. */
    private final static String EXT = ".java";

    private Map<String, byte[]> classBytes;

    public MemoryJavaFileManager(JavaFileManager fileManager) {
        super(fileManager);
        this.classBytes = new ConcurrentHashMap<>();
    }

    public Map<String, byte[]> getClassBytes() {
        return classBytes;
    }

    public void close() throws IOException {
        classBytes = null;
    }

    public void flush() throws IOException {
    }

    /**
     * A file object used to represent Java source coming from a string.
     */
    private static class StringInputBuffer extends SimpleJavaFileObject {
        final String code;

        StringInputBuffer(String fileName, String code) {
            super(toURI(fileName), Kind.SOURCE);
            this.code = code;
        }

        public CharBuffer getCharContent(boolean ignoreEncodingErrors) {
            return CharBuffer.wrap(code);
        }
    }

    /**
     * A file object that stores Java bytecode into the classBytes map.
     */
    private class ClassOutputBuffer extends SimpleJavaFileObject {
        private String name;

        ClassOutputBuffer(String name) {
            super(toURI(name), Kind.CLASS);
            this.name = name;
        }

        public OutputStream openOutputStream() {
            return new FilterOutputStream(new ByteArrayOutputStream()) {
                public void close() throws IOException {
                    out.close();
                    ByteArrayOutputStream bos = (ByteArrayOutputStream)out;
                    classBytes.put(name, bos.toByteArray());
                }
            };
        }
    }

    public JavaFileObject getJavaFileForOutput(JavaFileManager.Location location,
        String className,
        Kind kind,
        FileObject sibling) throws IOException {
        if (kind == Kind.CLASS) {
            return new ClassOutputBuffer(className);
        } else {
            return super.getJavaFileForOutput(location, className, kind, sibling);
        }
    }

    static JavaFileObject makeStringSource(String fileName, String code) {
        return new StringInputBuffer(fileName, code);
    }

    static URI toURI(String name) {
        File file = new File(name);
        if (file.exists()) {
            return file.toURI();
        } else {
            try {
                final StringBuilder newUri = new StringBuilder();
                newUri.append("mfm:///");
                newUri.append(name.replace('.', '/'));
                if(name.endsWith(EXT)) newUri.replace(newUri.length() - EXT.length(), newUri.length(), EXT);
                return URI.create(newUri.toString());
            } catch (Exception exp) {
                return URI.create("mfm:///com/sun/script/java/java_source");
            }
        }
    }
}

run.sh

# On Linux, with a GraalVM with Java on Truffle (Espresso) installed (gu install espresso).
javac *.java
LD_DEBUG=unused java DynamicJava