JavaCompiler from JDK 1.6: how to write class bytes directly to byte[] array?

Question

So I recently learned of the new JavaCompiler API available in JDK 1.6. This makes it very simple to compile a String to a .class file directly from running code:

String className = "Foo";
String sourceCode = "...";

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

List<JavaSourceFromString> unitsToCompile = new ArrayList<JavaSourceFromString>() 
    {{ 
         add(new JavaSourceFromString(className, sourceCode)); 
    }};

StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);
compiler.getTask(null, fileManager, null, null, null, unitsToCompile).call();
fileManager.close();    

ByteArrayOutputStream bos = new ByteArrayOutputStream();
FileInputStream fis = new FileInputStream(className + ".class");
IOUtils.copyStream(fis, bos);

return bos.toByteArray();

You can grab the source to JavaSourceFromString from the Javadoc.

This will very handily compile sourceCode to Foo.class in the current working directory.

My question is: is it possible to compile straight to a byte[] array, and avoid the messiness of dealing with File I/O altogether?

Answer 1

Maybe you could create your own javax.tools.JavaFileManager implementing class where you would return your own implementation of javax.tools.FileObject which would then write it out to memory instead to disk. So for your subclass of javax.tools.FileObject Writer openWriter() throws IOException method you would return a java.io.StringWriter. All the methods should be converted to their String counterparts.

Answer 2

The reason that there is no standard API to write bytecodes to a byte array is that compiling a single Java source file may result in multiple bytecode files. For example, any source file with nested / inner / anonymous classes will result in multiple bytecode files.

If you roll your own JavaFileManager, you will need to deal with this situation.