LWJGL context instantiation problem

[Icyene]

Hello!

I've wanted to create a simple tile-based game for a while, and recently I've had the time to do it. I originally wrote said game using Java2D, and then decided for performance reasons to switch to LWJGL. Since then, I've run across a problem which strikes me as exceedingly newbish, yet which I seem to be unable to solve.

I am trying to initialize a 480x320p frame, and paint an image of the same dimensions on it using LWJGL.

I am utilizing the following code:

public class TestApplet extends Applet {

    /**
     * The Canvas where the LWJGL Display is added
     */
    Canvas display_parent;

    /**
     * Thread which runs the main game loop
     */
    Thread gameThread;

    int width = 480;
    int height = 320;

    public TestApplet() {
        init();
    }

    /**
     * Once the Canvas is created its add notify method will call this method to
     * start the LWJGL Display and game loop in another thread.
     */
      public void startLWJGL() {
        try {
            Display.setParent(display_parent);

            // get modes
            Display.setTitle("Test");
            Display.setFullscreen(false);
            Display.create();
            // enable textures since we're going to use these for our sprites
            glEnable(GL_TEXTURE_2D);

            // disable the OpenGL depth test since we're rendering 2D graphics
            glDisable(GL_DEPTH_TEST);

            glMatrixMode(GL_PROJECTION);
            glLoadIdentity();

            glOrtho(0, width, height, 0, -1, 1);
            glMatrixMode(GL_MODELVIEW);
            glLoadIdentity();
            glViewport(0, 0, width, height);
        } catch (LWJGLException e) {
            e.printStackTrace();
        }

        gameThread = new Thread() {
            public void run() {
                glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
                new TextureLoader().getSprite("Test.png").draw(0, 0);
            }
        };

        gameThread.start();
    }


    /**
     * Tell game loop to stop running, after which the LWJGL Display will be destoryed.
     * The main thread will wait for the Display.destroy() to complete
     */
    private void stopLWJGL() {
        // TODO some stopping code here
    }

    /**
     * Applet Destroy method will remove the canvas, before canvas is destroyed it will notify
     * stopLWJGL() to stop main game loop and to destroy the Display
     */
    public void destroy() {
        remove(display_parent);
        super.destroy();
        System.out.println("Clear up");
    }

    /**
     * initialise applet by adding a canvas to it, this canvas will start the LWJGL Display and game loop
     * in another thread. It will also stop the game loop and destroy the display on canvas removal when
     * applet is destroyed.
     */
    public void init() {
        setLayout(new BorderLayout());
        try {
            display_parent = new Canvas() {
                public void addNotify() {
                    super.addNotify();
                    startLWJGL();
                }

                public void removeNotify() {
                    stopLWJGL();
                    super.removeNotify();
                }
            };
            display_parent.setSize(getWidth(), getHeight());
            add(display_parent);
            display_parent.setFocusable(true);
            display_parent.requestFocus();
            display_parent.setIgnoreRepaint(true);
            setVisible(true);
        } catch (Exception e) {
            System.err.println(e);
            throw new RuntimeException("Unable to create display");
        }
    }
}

Essentially, it is the same as the example on LWJGL's Github example folder, yet with no logic but painting a screen constantly.

And yet, whenever I attempt to run the above snippet, I am hit with the following exceptions:

Exception in thread "Thread-5" java.lang.NullPointerException
	at org.lwjgl.opengl.GL11.glClear(GL11.java:586)
	at amber.TestApplet$1.run(TestApplet.java:69)
java.lang.RuntimeException: Unable to create display
	at amber.TestApplet.init(TestApplet.java:135)
	at sun.applet.AppletPanel.run(AppletPanel.java:424)
	at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.IllegalStateException: From thread Thread[thread applet-amber.TestApplet,4,file:/a/path/-threadGroup]: Thread[Thread-3,4,] already has the context current
	at org.lwjgl.opengl.ContextGL.checkAccess(ContextGL.java:184)
	at org.lwjgl.opengl.ContextGL.makeCurrent(ContextGL.java:189)
	at org.lwjgl.opengl.DrawableGL.makeCurrent(DrawableGL.java:110)
	at org.lwjgl.opengl.Display.makeCurrent(Display.java:731)
	at org.lwjgl.opengl.Display.makeCurrentAndSetSwapInterval(Display.java:1050)
	at org.lwjgl.opengl.Display.setParent(Display.java:478)
	at amber.TestApplet.startLWJGL(TestApplet.java:43)
	at amber.TestApplet$2.addNotify(TestApplet.java:120)
	at java.awt.Container.addImpl(Container.java:1068)
	at java.awt.Container.add(Container.java:365)
	at amber.TestApplet.init(TestApplet.java:129)
	... 2 more

Googling for a solution has been useless so far: all the results point to an error involving LWJGL which the game MineCraft encountered at some point in time.

Needless to say, I am very confused and I would greatly appreciate any tidbit of light that could be shed on this beginner issue of mine.

[Cornix]

lwjgl has some problems with multi-threading since the openGl library is static.
Judging from this exception:

java.lang.IllegalStateException: From thread Thread[thread applet-amber.TestApplet,4,file:/a/path/-threadGroup]: Thread[Thread-3,4,] already has the context current

I'd say this is your problem.
Try it without creating a new thread in your code.

[quew8]

I've never really understood the benefits of running your main game loop on anything but the main thread.

[Icyene]

That seemed to fix that issue, thanks!