Is it possible to programmatically compile java source code in memory only?

Question

I have found many references explaining how to programmatically compile a Java class using the JavaCompiler class:

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
int result = compiler.run(null, null, null, "a_file_name");

However, I would like to know if there is an open source library that let me compile source code generated programmatically (therefore without a src file being involved) and generate some byte code in an output stream (without generating a class file in the file system).

For example, I am looking for being able to write something like this:

InputStream input = generateSourceCode();
OutputStream output = getByteCode(input);
doCoolStuffWithByteCode(output);

Thanks for any help.

Answer 1

To start, look at the JavaCompiler API. Basically:

1. Create the Java class in a string.
2. Put the string into class that extends SimpleJavaFileObject.
3. Compile using a JavaCompiler instance.

Finally, call the methods the new class.

---

Here is an example that works with JDK6+:

import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.lang.reflect.InvocationTargetException;
import java.net.URI;
import java.util.Arrays;

import javax.tools.Diagnostic;
import javax.tools.DiagnosticCollector;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.SimpleJavaFileObject;
import javax.tools.ToolProvider;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.JavaFileObject.Kind;

public class CompileSourceInMemory {
  public static void main(String args[]) throws IOException {
    JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
    DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();

    StringWriter writer = new StringWriter();
    PrintWriter out = new PrintWriter(writer);
    out.println("public class HelloWorld {");
    out.println("  public static void main(String args[]) {");
    out.println("    System.out.println(\"This is in another java file\");");    
    out.println("  }");
    out.println("}");
    out.close();
    JavaFileObject file = new JavaSourceFromString("HelloWorld", writer.toString());

    Iterable<? extends JavaFileObject> compilationUnits = Arrays.asList(file);
    CompilationTask task = compiler.getTask(null, null, diagnostics, null, null, compilationUnits);

    boolean success = task.call();
    for (Diagnostic diagnostic : diagnostics.getDiagnostics()) {
      System.out.println(diagnostic.getCode());
      System.out.println(diagnostic.getKind());
      System.out.println(diagnostic.getPosition());
      System.out.println(diagnostic.getStartPosition());
      System.out.println(diagnostic.getEndPosition());
      System.out.println(diagnostic.getSource());
      System.out.println(diagnostic.getMessage(null));

    }
    System.out.println("Success: " + success);

    if (success) {
      try {
        Class.forName("HelloWorld").getDeclaredMethod("main", new Class[] { String[].class })
            .invoke(null, new Object[] { null });
      } catch (ClassNotFoundException e) {
        System.err.println("Class not found: " + e);
      } catch (NoSuchMethodException e) {
        System.err.println("No such method: " + e);
      } catch (IllegalAccessException e) {
        System.err.println("Illegal access: " + e);
      } catch (InvocationTargetException e) {
        System.err.println("Invocation target: " + e);
      }
    }
  }
}

class JavaSourceFromString extends SimpleJavaFileObject {
  final String code;

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

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

Answer 2

JavaDocs are your friend:

http://download.oracle.com/javase/6/docs/api/javax/tools/JavaCompiler.html

Look at the last section that refers to the SimpleJavaFileObject; it shows you how to use it in conjunction with code that is stored in a String

Answer 3

We gave a talk about this use case in JavaOne 2016 (the question is kind of old, but there seems to be some interest still).

There is a repository with examples of practical code generation using javac in-memory.

Specifically look at SimpleJavaCompiler for an example on how to do this in memory that deals with thread safety (we use it in the context of a server) for a single class. It could easily be adapted for a multi-class scenario.

There are also classes to deal with class loading and code generation (scoping of variables, generating unique names, name shadowing, etc.).