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=awt Enable single-threading of OpenGL work on AWT event dispatch thread (current default on all
platforms, and also the default behavior older releases)
-Dopengl.1thread=worker Enable single-threading of OpenGL work on newly-created worker thread (not suitable for Mac
OS X or X11 platforms, and risky on Windows in applet environments)
*/
public class Threading {
/** 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() {
ThreadingImpl.disableSingleThreading();
}
/** Indicates whether OpenGL work is being automatically forced to a
single thread in this implementation. */
public static boolean isSingleThreaded() {
return ThreadingImpl.isSingleThreaded();
}
/** 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 {
return ThreadingImpl.isOpenGLThread();
}
/** 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 {
ThreadingImpl.invokeOnOpenGLThread(r);
}
}