Into The Labyrinth – using the JavaCompiler API from Jython

The Plumber

After having neglected Java for years I began to re-examine it this month together with Jython and my initial reaction was a culture shock.

Java is infamous for being a “plumbing language” i.e. you have to subclass some classes or implement a few interfaces and then plug them into a framework. Alternatively you have to call framework methods that expect a bunch of interrelated objects as parameters. None of those class implementations is particularly complicated but you have to figure out how all the objects are related to each other and essentially deal with configurations and dependencies on object level. It is easy to mess things up with every additional point of failure. There is also a tendency for abstraction inversion: you have to undertake many concrete steps within a complex machinery to create a simple building block. Abstraction inversion is an indication of a system being overdesigned.

The topic of this article is Javas compiler API and it is a nice show case to highlight some differences which are not really situated on language level but in the way problems are approached. Take Pythons `compile` function for example. Pythons `compile` function has the following signature

`compile: (code_str, file_name, eval_mode) ` -> `code_object`

and is dead simple to use:

>>> compile("import EasyExtend.langlets", "<input>", "exec")
<code object <module> at 00EB0578, file "<input>", line 1>

If you need to read the source from a file you do just this

>>> compile(open("").read(), "input", "exec")
<code object <module> at 00EB0578, file "<input>", line 1>

If you want to store the resulting bytecode in an appropriate file you need to do a little more work

import os, struct, marshal, imp
def write_code(filename, code):
    f = open(filename, "wb")
        mtime = os.path.getmtime(filename)
        mtime = struct.pack('<i', mtime)
        MAGIC = imp.get_magic()
        f.write(MAGIC + mtime)
        marshal.dump(code, f)

This creates a Python bytecode file ( usually a ‘ pyc’ file ) and serializes the code object. Something like this could be implemented on the method level of the `code` object itself and a single call `code.write(filename)` would suffice to store the code.

How to compile Java dynamically?

Java is different. The Java Compiler API is specified in JSR 199. It contains 20 classes/interfaces and you have to figure out their interplay in order to compile a source string. Half of them are `JavaFile` or `JavaFileManager` classes/interfaces so what’s actually peripheral has moved to the center. The JavaCompiler API is new to Java 6 and it seems dynamic compilation didn’t work out of the box prior to release 6. I suppose one had to call `javac` from the command line or use a different compiler such as Janino.

There isn’t any easy way to get into the compiler API. The JSR 199 isn’t exactly a design document but a terse javadoc API documentation. Tutorials are rare and mostly superficial in that they cover the most simple use case only. A notable exception is an introduction written by Andrew Davison. It covers the most relevant use cases and more. Alternatively one might skim through the tests of the JDK 6. It contains expression evaluation code written by the JSR 199 implementor Peter von der Ahé. Once again tests are documentation. The Jython 2.5 code I’ll present follows Davisons article and is mostly a transcript of his Java code in the relevant parts.

The JavaCompiler API from Jython

Prerequisites: a JDK for Java 6 has to be installed first. The Java compiler tools are implemented in


and that path has to be added to the Java class path when Jython is invoked. Alternatively you can add the path to the` PYTHONPATH` inside of your application. I’ll chose the latter approach here:

import sys
sys.path.append(os.path.join(os.environ["JAVA_HOME"],"lib", "tools.jar"))

The environment variable `JAVA_HOME` has to be set to your JDK path – not to the JRE path. This might have to be changed. In case of my Windows notebook the path is

`JAVA_HOME = C:\Programme\Java\jdk1.6.0_13`.

Another configuration aspect concerns access to protected members of Java classes. By default this is disabled in Jython. We will need to access protected member functions once we override a Java class loader. For that reason one has to set following disrespectful flag in Jythons `registry` file = false

For more information see the Jython FAQ.

Now we are ready to import the compiler tools:

from import*
from import JavacTool
compiler = JavacTool.create()
assert compiler

The CompilationTask

After having created a compiler instance we now care about the `CompilationTask` which is defined in JSR 199. It is basically a `callable`, an interface which specifies a parameter-less `call()` method that returns a value of type `Boolean`. It is this `call`method that has to be overridden in subclasses and called for compilation. In case of the JavaCompiler framework one doesn’t have to override the `CompilationTask`explicitly but fetches it from the `compiler` object using the the `getTask` method:

JavaCompiler.CompilationTask getTask(Writer out,
                             JavaFileManager fileManager,
                             DiagnosticListener&lt;? super JavaFileObject&gt; diagnosticListener,
                             Iterable&lt;String&gt; options,
                             Iterable&lt;String&gt; classes,
                             Iterable&lt;? extends JavaFileObject&gt; compilationUnits)

The parameters need further examination but once we have understood how to create the objects required to fetch the `CompilationTask` the compilation is performed by


The meaning of the parameters of the `CompilationTask` is as follows:


  • out – a Writer for additional output from the compiler; use System.err if null
  • fileManager – a file manager; if null use the compiler’s standard filemanager
  • diagnosticListener – a diagnostic listener; if null use the compiler’s default method for reporting diagnostics
  • options – compiler options, null means no options
  • classes – class names (for annotation processing), null means no class names
  • compilationUnits – the compilation units to compile, null means no compilation units

For now we will ignore the `out` parameter as well as compiler options and class names passed to the annotation processors.

The `compilationUnits` holds the source code to be compiled. In case of Jython it will be a Python list containing a single `JavaFileObject`.

So a call of `getTask` will have the following shape



`DiagnosticCollectors` are used for error reporting. A `DiagnosticCollector` implements a `DiagnosticListener` interface. It is parametrized by some type `S` and holds a possibly empty list of `Diagnostic<S>` objects which can be fetched.

We only need to know as much about diagnostics to handle failure cases:

 if not
     msg = "\n  "+"\n  ".join(str(d) for d in diagnostics.getDiagnostics())
     raise JavaCompilationError(msg)

The `JavaCompilationError` is a custom Jython exception. The JavaCompiler API doesn’t raise an exception on compilation failure.


The `JavaFileObject` specifies a file abstraction. For our own purposes we need two of them: one that is readable and holds the source code ( a string ) and one that is writable and holds the bytecode ( an array of bytes ). Both derive from `SimpleJavaFileObject` which provides an implementation of the `JavaFileObject` interface.

from import URI
from import*
class StringJFO(SimpleJavaFileObject):
    JavaFileObject implemention used to hold the source code.
    def __init__(self, className, codestr):
        self.codestr = codestr
        super(StringJFO, self).__init__(URI(className),
    def getCharContent(self, errs):
        return self.codestr
class ByteArrayJFO(SimpleJavaFileObject):
    JavaFileObject implementation used to hold the byte code.
    def __init__(self, className, kind):
        super(ByteArrayJFO, self).__init__(URI(className), kind)
        self.baos = ByteArrayOutputStream()
    def openInputStream(self):
        return ByteArrayInputStream(self.getByteArray())
    def openOutputStream(self):
        return self.baos
    def getByteArray(self):
        return self.baos.toByteArray()

In case of the `ByteArrayJFO` a writable `ByteArrayOutputStream` is created that can be fetched by the framework using the `openInputStream` method.

An instance of `StringJFO` will become our `compilationUnit` and an instance of `ByteArrayJFO` will be returned by a still to be defined `FileManager`:

class ByteJavaFileManager(ForwardingJavaFileManager):
    def __init__(self, fileManager):
        super(ByteJavaFileManager, self).__init__(fileManager)
        self.code = None
    def getJavaFileForOutput(self, location, className, kind, sibling):
        self.code = ByteArrayJFO(className, kind)
        return self.code

A Java compiler in Jython

Now we can put all those things together and define a `compileClass` function.

def compileClass(className, codeStr, *flags):
    compiler = JavacTool.create()
    assert compiler, "Compiler not found"
    diagnostics = DiagnosticCollector()
    jfm = ByteJavaFileManager(compiler.getStandardFileManager(diagnostics,
    task = compiler.getTask(None,
                            [StringJFO(className+".java", codeStr)])
    if not
        e = "\n  "+"\n  ".join(str(d) for d in diagnostics.getDiagnostics())
        raise JavaCompilationError(e)
    return jfm.code

In order to initialize the `StringJFO` object we have to pass a file name which corresponds to the the class name we use. The return values of `compileClass` is an object of type `ByteArrayJFO` that holds the byte code.

Adding a ClassLoader

We are almost done. To complete our exercise we have to make the byte code executable which means we have to define a class loader and create an actual Java class. Since we operate from within Jython it will be immediately wrapped into a Python type.

from java.lang import ClassLoader
from java.lang import ClassNotFoundException
class ByteClassLoader(ClassLoader):
    def __init__(self, code):
        super(ByteClassLoader, self).__init__(ClassLoader.getClassLoader())
        self.code = code
    def findClass(self, className):
        code = self.code.getByteArray()
        cl = self.defineClass(className, code, 0, len(code))
        if cl is None:
            raise ClassNotFoundException(className)
            return cl
def createJavaClass(className, codeStr, *compilerflags):
    loader = ByteClassLoader(compileClass(className, codeStr, *compilerflags))
    return loader.loadClass(className)

To a very good end we have hidden the JavaCompiler API behind a single function with a tiny interface. There is not something Python has been needed for actually. The JavaCompiler API is a Swiss Army knife style API that requires a whole object tree to be created to achieve a simple effect.

Java classes from within Jython

Finally we want to demonstrate the value of our implementation showing a few simple examples. The `` module contains the complete code and can be downloaded here.

The first example is our “Hello Jython”:

codeStr = """
public class Foo {
    public static void main(String args[])
        System.out.println("Hello, "+args[0]);
Foo = createJavaClass("Foo", codeStr)
print Foo  # &lt;type 'Foo'&gt;
Foo.main(["Jython!"])  #  Hello, Jython!

In our next example we import some symbols from a Java library:

codeStr = '''
import java.lang.Math;
public class Prime {
    public static Boolean isPrime(double n)
        for(int d=2;d&lt;=Math.sqrt(n);d++)
            if(n%d == 0)
                return false;
        return true;
Prime = createJavaClass("Prime", codeStr)
Prime.isPrime(2)     # True
Prime.isPrime(9971)  # False
Prime.isPrime(9973)  # True

————— Update ! We. 2009-24-06 —————–

Right now it is not possible to derive a class from a dynamically compiled Java class. If `Foo` is a dynamically generated Java class and `Bar`is defined as

class Bar(Foo):

then a `ClassNotFoundException` is raised. The only possible workaround I see right now is to store the class to the disk and import it right after this. A possible adaption of the `createJavaClass` function looks like:

def createJavaClass(className,
                    todisk = True,
                    remove = True,
    Compiles and loads a new Java class.
    compiled = compileClass(className, codeStr, *compilerflags)
    if todisk:
        code = compiled.getByteArray()
        clsfile = open(className+".class", "wb")
        jclass = __import__(className)
        if remove:
        return jclass
        loader = ByteClassLoader(compileClass(className, codeStr, *compilerflags))
        return loader.loadClass(className)
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1 Response to Into The Labyrinth – using the JavaCompiler API from Jython

  1. Pingback: Trails of EasyExtend » Blog Archive » Stitches of a flea language - defining Java annotations in Jython - Projects and projections

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