Re: JavaScript not being blocked/synched by Applet init()

Mark Space <>
Mon, 11 May 2009 19:19:33 -0700
Richard Maher wrote:
 > "Richard Maher" <> wrote in message
 > news:gu56jd$he8$
 >> PS. Anyone got a link to this "forthcoming new LiveConnect
 > I found this think for those who will also be looking. (I found the bug
 > report to be less ambiguous in its terminology, but anyway here's the

While direct from Sun, I'm pretty sure it's incorrect:

"The JavaScript engines in all of today's web browsers are conceptually
single threaded with respect to the web page. The Java language,
however, is inherently multithreaded. If multiple Java threads attempt
to call into the JavaScript engine, only one will be allowed through at
any given time. The rest of the threads will wait for one of two
situations to occur: either the initial Java-to-JavaScript call
completes, or a JavaScript-to-Java call is made. In either of these
occurrences, the JavaScript engine once again becomes "available" for
Java-to-JavaScript calls. If multiple Java threads were attempting to
make concurrent calls to the JavaScript engine, an arbitrary one will be
selected for its call to proceed."

JavaScript IS NOT single threaded. It can receive events from outside
sources (hello XMLHttpRequest) and will spawn additional threads of
execution. Since there's no synchronization method in JavaScript, this
can be a huge hassle. There's an algorithm available to synchronize
threads using only shared memory. Read on:


Anyway, with regard to Java, I think the best policy is to assume your
JApplet is being accessed in a multithreaded environment and program
defensively. I'd make all methods visible to JavaScript "synchronized",
including redefining all JApplet methods just to make them synchronized.

(I refuse to redefine the entire JComponent hierarchy, which is huge.
This is one instance where one must just pray that these methods are
never invoked. I don't think there's any way to synchronize a parent
classes methods. Although if you know a specific method will be call
(setSize() is common) synchronize that.)

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