OpenGL is specified as a thread-safe API, but in practice there are multithreading-related issues on most, if not all, of the platforms which support it. For example, some OpenGL implementations do not behave well when one context is made current first on one thread, released, and then made current on a second thread, although this is legal according to the OpenGL specification. On other platforms there are other problems.
Due to these limitations, and due to the inherent multithreading in the Java platform (in particular, in the Abstract Window Toolkit), it is often necessary to limit the multithreading occurring in the typical application using the OpenGL API.
In the current reference implementation, for instance, multithreading has been limited by forcing all OpenGL-related work for GLAutoDrawables on to a single thread. In other words, if an application uses only the GLAutoDrawable and GLEventListener callback mechanism, it is guaranteed to have the most correct single-threaded behavior on all platforms.
Applications using the GLContext makeCurrent/release API directly will inherently break this single-threaded model, as these methods require that the OpenGL context be made current on the current thread immediately. For applications wishing to integrate better with an implementation that uses the single-threaded model, this class provides public access to the mechanism used by the implementation.
Users can execute Runnables on the internal thread used for performing OpenGL work, and query whether the current thread is already this thread. Using these mechanisms the user can move work from the current thread on to the internal OpenGL thread if desired.
This class also provides mechanisms for querying whether this internal serialization of OpenGL work is in effect, and a programmatic way of disabling it. In the current reference implementation it is enabled by default, although it could be disabled in the future if OpenGL drivers become more robust on all platforms.
In addition to specifying programmatically whether the single
thread for OpenGL work is enabled, users may switch it on and off
using the system property opengl.1thread. Valid values
for this system property are:
-Dopengl.1thread=false Disable single-threading of OpenGL work
-Dopengl.1thread=true Enable single-threading of OpenGL work (default -- on a newly-created worker thread)
-Dopengl.1thread=auto Select default single-threading behavior (currently on)
-Dopengl.1thread=worker Enable single-threading of OpenGL work on newly-created worker thread (default)
-Dopengl.1thread=awt Enable single-threading of OpenGL work on AWT event dispatch thread (the default behavior in older releases)
*/
public class Threading {
private static boolean singleThreaded = true;
private static final int AWT = 1;
private static final int WORKER = 2;
private static int mode = WORKER;
static {
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
String workaround = System.getProperty("opengl.1thread");
if (workaround != null) {
workaround = workaround.toLowerCase();
if (workaround.equals("true") ||
workaround.equals("auto") ||
workaround.equals("worker")) {
// Nothing to do; default = singleThreaded, mode = WORKER
} else if (workaround.equals("awt")) {
singleThreaded = true;
mode = AWT;
} else {
singleThreaded = false;
}
}
printWorkaroundNotice();
return null;
}
});
}
/** No reason to ever instantiate this class */
private Threading() {}
/** If an implementation of the javax.media.opengl APIs offers a
multithreading option but the default behavior is single-threading,
this API provides a mechanism for end users to disable single-threading
in this implementation. Users are strongly discouraged from
calling this method unless they are aware of all of the
consequences and are prepared to enforce some amount of
threading restrictions in their applications. Disabling
single-threading, for example, may have unintended consequences
on GLAutoDrawable implementations such as GLCanvas, GLJPanel and
GLPbuffer. Currently there is no supported way to re-enable it
once disabled, partly to discourage careless use of this
method. This method should be called as early as possible in an
application. */
public static void disableSingleThreading() {
singleThreaded = false;
if (Debug.verbose()) {
System.err.println("Application forced disabling of single-threading of javax.media.opengl implementation");
}
}
/** Indicates whether OpenGL work is being automatically forced to a
single thread in this implementation. */
public static boolean isSingleThreaded() {
return singleThreaded;
}
/** Indicates whether the current thread is the single thread on
which this implementation of the javax.media.opengl APIs
performs all of its OpenGL-related work. This method should only
be called if the single-thread model is in effect. */
public static boolean isOpenGLThread() throws GLException {
if (!isSingleThreaded()) {
throw new GLException("Should only call this in single-threaded mode");
}
switch (mode) {
case AWT:
if (Java2D.isOGLPipelineActive()) {
// FIXME: ideally only the QFT would be considered to be the
// "OpenGL thread", but we can not currently run all of JOGL's
// OpenGL work on that thread. For now, run the GLJPanel's
// Java2D/JOGL bridge on the QFT but everything else on the
// EDT, except when we're already on the QFT.
return (Java2D.isQueueFlusherThread() ||
EventQueue.isDispatchThread());
} else {
return EventQueue.isDispatchThread();
}
case WORKER:
if (Java2D.isOGLPipelineActive()) {
// FIXME: ideally only the QFT would be considered to be the
// "OpenGL thread", but we can not currently run all of JOGL's
// OpenGL work on that thread. For now, run the GLJPanel's
// Java2D/JOGL bridge on the QFT but everything else on the
// worker thread, except when we're already on the QFT.
return (Java2D.isQueueFlusherThread() ||
GLWorkerThread.isWorkerThread());
} else {
return GLWorkerThread.isWorkerThread();
}
default:
throw new InternalError("Illegal single-threading mode " + mode);
}
}
/** Executes the passed Runnable on the single thread used for all
OpenGL work in this javax.media.opengl API implementation. It is
not specified exactly which thread is used for this
purpose. This method should only be called if the single-thread
model is in use and if the current thread is not the OpenGL
thread (i.e., if isOpenGLThread() returns
false). It is up to the end user to check to see whether the
current thread is the OpenGL thread and either execute the
Runnable directly or perform the work inside it. */
public static void invokeOnOpenGLThread(Runnable r) throws GLException {
if (!isSingleThreaded()) {
throw new GLException ("Should only call this in single-threaded mode");
}
if (isOpenGLThread()) {
throw new GLException ("Should only call this from other threads than the OpenGL thread");
}
switch (mode) {
case AWT:
// FIXME: ideally should run all OpenGL work on the Java2D QFT
// thread when it's enabled, but there are issues with this when
// the GLJPanel is not using the Java2D bridge; would like to run
// its OpenGL work on the QFT, but do the image drawing from the
// EDT. Other issues still remain with the GLCanvas as well.
// if (Java2D.isOGLPipelineActive()) {
// Java2D.invokeWithOGLContextCurrent(null, r);
// } else {
try {
EventQueue.invokeAndWait(r);
} catch (InvocationTargetException e) {
throw new GLException(e.getTargetException());
} catch (InterruptedException e) {
throw new GLException(e);
}
// }
break;
case WORKER:
if (!GLWorkerThread.isStarted()) {
synchronized (GLWorkerThread.class) {
if (!GLWorkerThread.isStarted()) {
GLWorkerThread.start();
}
}
}
try {
GLWorkerThread.invokeAndWait(r);
} catch (InvocationTargetException e) {
throw new GLException(e.getTargetException());
} catch (InterruptedException e) {
throw new GLException(e);
}
break;
default:
throw new InternalError("Illegal single-threading mode " + mode);
}
}
private static void printWorkaroundNotice() {
if (singleThreaded && Debug.verbose()) {
System.err.println("Using " +
(mode == AWT ? "AWT" : "OpenGL worker") +
" thread for performing OpenGL work in javax.media.opengl implementation");
}
}
}