Episode 25. Reflection and vampire classes, and compiling Java from within Java.

We have heard the word "Reflection" thrown around, what does it mean? it is a new Twilight series? is it about Vampires? In all, we shed sunlight into what reflection is (and more importantly why in the world you want to use it). And also cover a technique to compile and load programs within your program. Javascript guys had access to this by doing eval("your program here"), and while Java doesn't have an eval function, there are ways of achieving similar results (and very specific reasons to do this crazy technique. Mostly performance)

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Nimbus code to get the dang CacheMode (considered a private variable)

public class ButtonPainter extends AbstractRegionPainter {
    public ButtonPainter() {

        Class<?> c = null;
        PaintContext ctx = new PaintContext(new Insets(0,0,0,0), new Dimension(100,100), false, null, Double.POSITIVE_INFINITY, 2.0 );

        try {
            c = Class.forName("javax.swing.plaf.nimbus.AbstractRegionPainter$PaintContext$CacheMode");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        if (c != null) {
            Object cacheMode = c.getEnumConstants()[2];   // NINE_SQUARE_SCALE
            for (Field field : ctx.getClass().getDeclaredFields()) {
                if (c.getName().equals(field.getType().getName())) {        // if Field is the CacheMode
                    try {
                        // the following lines would not be necessary for example if
                        // AbstractRegionPainter.cacheMode were protected or public.

                        field.setAccessible(true);                          // make it accessible so that we can set it
                        field.set(ctx, cacheMode);                          // set the cachemode
                        // this is equivalent as sayin "ctx.cacheMode = CacheMode.NINE_SQUARE_SCALE" if it were public/protected
                        break;
                    } catch (IllegalAccessException e) {
                        e.printStackTrace();
                    }
                }
            }
        }

    }

    @Override
    protected PaintContext getPaintContext() {
        return null;  //To change body of implemented methods use File | Settings | File Templates.
    }

    @Override
    protected void doPaint(Graphics2D g, JComponent c, int width, int height, Object[] extendedCacheKeys) {
        //To change body of implemented methods use File | Settings | File Templates.
    }
}

Code to get instance from a source file

    public static Object getInstanceForSource(String className, String sourceForRule) {
        String filename = "tmp/" + className + ".java";
        File file = new File(filename);
        writeFile(filename, sourceForRule);    
        String classPath = System.getProperty("java.class.path");
        String[] args = new String[]{
                "-classpath", classPath,
                filename
        };


        StringWriter compilerOutput = new StringWriter();
        int status = com.sun.tools.javac.Main.compile(args,new PrintWriter(compilerOutput));
        additionalInfo.value = compilerOutput.toString();
        switch (status) {
            case 0:  // OK
                // Make the class file temporary as well
                File classFile = new File("./tmp/");
                try {
                    // Try to access the class and run its main method
                    URLClassLoader loader = new URLClassLoader(new URL[] {classFile.toURI().toURL()});
                    Class clazz = loader.loadClass(className);
                    return clazz.newInstance();
                } catch (Exception ex) {
                    additionalInfo.value = "Exception in main: " + Utilities.exceptionToString(ex)+"\n"+additionalInfo.value;
                }
                break;
            case 1:
                System.out.println ("Status: Error" +"\n"+additionalInfo.value);
                break;
            case 2:
                System.out.println ("Status: CMDERR" +"\n"+additionalInfo.value);
                break;
            case 3:
                System.out.println ("Status: SYSERR" +"\n"+additionalInfo.value);
                break;
            case 4:
                System.out.println ("Status: ABNORMAL" +"\n"+additionalInfo.value);
                break;
            default:
                System.out.println ("Status: UNKNOWN" +"\n"+additionalInfo.value);
                break;
        }
        return null;
    }

    public static void writeFile(String fileName, String content) {
        Writer writer;
        File file = new File(fileName);
        try {
            writer = new BufferedWriter(new FileWriter(file));
            writer.write(content);
            writer.close();
        } catch (IOException e) {
           System.out.println ("I/O exception "+e);
        }
    }

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Reflection 'trail' http://docs.oracle.com/javase/tutorial/reflect/index.html
Create dynamic applications with javax.tools http://www.ibm.com/developerworks/java/library/j-jcomp/index.html
URLClassLoaders (loads .class files generated http://docs.oracle.com/javase/6/docs/api/java/net/URLClassLoader.html
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