How to compile and run java source code in memory

Question

This question already has an answer here:

• Compile code fully in memory with javax.tools.JavaCompiler [duplicate] 7 answers

I want to treat a String as a Java file then compile and run it. In other words, use Java as a script language.

To get better performance, we should avoid writing .class files to disk.

Answer 1

This answer is from one of my blogs, Compile and Run Java Source Code in Memory.

Here are the three source code files.

MemoryJavaCompiler.java

package me.soulmachine.compiler;

import java.io.IOException;
import java.io.PrintWriter;
import java.io.Writer;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import javax.tools.*;

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

    public MemoryJavaCompiler() {
        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);
    }

    /**
     * Compile a single static method.
     */
    public Method compileStaticMethod(final String methodName, final String className,
        final String source)
        throws ClassNotFoundException {
        final Map<String, byte[]> classBytes = compile(className + ".java", source);
        final MemoryClassLoader classLoader = new MemoryClassLoader(classBytes);
        final Class clazz = classLoader.loadClass(className);
        final Method[] methods = clazz.getDeclaredMethods();
        for (final Method method : methods) {
            if (method.getName().equals(methodName)) {
                if (!method.isAccessible()) method.setAccessible(true);
                return method;
            }
        }
        throw new NoSuchMethodError(methodName);
    }


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


    /**
     * compile given String source and return bytecodes as a Map.
     *
     * @param fileName source fileName to be used for error messages etc.
     * @param source Java source as String
     * @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(String fileName, String source,
        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> compUnits = new ArrayList<JavaFileObject>(1);
        compUnits.add(fileManager.makeStringSource(fileName, source));

        return compile(compUnits, 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
        javax.tools.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;
    }
}

MemoryJavaFileManager.java

package me.soulmachine.compiler;


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 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);
        classBytes = new HashMap<>();
    }

    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");
            }
        }
    }
}

MemoryClassLoader.java

package me.soulmachine.compiler;


import java.io.File;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.StringTokenizer;

/**
 * ClassLoader that loads .class bytes from memory.
 */
final class MemoryClassLoader extends URLClassLoader {
    private Map<String, byte[]> classBytes;

    public MemoryClassLoader(Map<String, byte[]> classBytes,
        String classPath, ClassLoader parent) {
        super(toURLs(classPath), parent);
        this.classBytes = classBytes;
    }

    public MemoryClassLoader(Map<String, byte[]> classBytes, String classPath) {
        this(classBytes, classPath, ClassLoader.getSystemClassLoader());
    }

    public MemoryClassLoader(Map<String, byte[]> classBytes) {
        this(classBytes, null, ClassLoader.getSystemClassLoader());
    }

    public Class load(String className) throws ClassNotFoundException {
        return loadClass(className);
    }

    public Iterable<Class> loadAll() throws ClassNotFoundException {
        List<Class> classes = new ArrayList<Class>(classBytes.size());
        for (String name : classBytes.keySet()) {
            classes.add(loadClass(name));
        }
        return classes;
    }

    protected Class findClass(String className) throws ClassNotFoundException {
        byte[] buf = classBytes.get(className);
        if (buf != null) {
            // clear the bytes in map -- we don't need it anymore
            classBytes.put(className, null);
            return defineClass(className, buf, 0, buf.length);
        } else {
            return super.findClass(className);
        }
    }

    private static URL[] toURLs(String classPath) {
        if (classPath == null) {
            return new URL[0];
        }

        List<URL> list = new ArrayList<URL>();
        StringTokenizer st = new StringTokenizer(classPath, File.pathSeparator);
        while (st.hasMoreTokens()) {
            String token = st.nextToken();
            File file = new File(token);
            if (file.exists()) {
                try {
                    list.add(file.toURI().toURL());
                } catch (MalformedURLException mue) {}
            } else {
                try {
                    list.add(new URL(token));
                } catch (MalformedURLException mue) {}
            }
        }
        URL[] res = new URL[list.size()];
        list.toArray(res);
        return res;
    }
}

Explanations:

1. In order to represent a Java source file in memory instead of disk, I defined a StringInputBuffer class in the MemoryJavaFileManager.java.
2. To save the compiled .class files in memory, I implemented a class MemoryJavaFileManager. The main idea is to override the function getJavaFileForOutput() to store bytecodes into a map.
3. To load the bytecodes in memory, I have to implement a customized classloader MemoryClassLoader, which reads bytecodes in the map and turn them into classes.

Here is a unite test.

package me.soulmachine.compiler;


import org.junit.Test;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;

import static org.junit.Assert.assertEquals;

public class MemoryJavaCompilerTest {
    private final static MemoryJavaCompiler compiler = new MemoryJavaCompiler();

    @Test public void compileStaticMethodTest()
        throws ClassNotFoundException, InvocationTargetException, IllegalAccessException {
        final String source = "public final class Solution {\n"
            + "public static String greeting(String name) {\n"
            + "\treturn \"Hello \" + name;\n" + "}\n}\n";
        final Method greeting = compiler.compileStaticMethod("greeting", "Solution", source);
        final Object result = greeting.invoke(null, "soulmachine");
        assertEquals("Hello soulmachine", result.toString());
    }
}

Reference

1. JavaCompiler.java from Cloudera Morphlines
2. How to create an object from a string in Java (how to eval a string)?
3. InMemoryJavaCompiler
4. Java-Runtime-Compiler
5. 动态的Java - 无废话JavaCompilerAPI中文指南