Re: updating an ImageIcon's backing BufferedImage content from multiple threads

From:

Knute Johnson <nospam@rabbitbrush.frazmtn.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 05 Apr 2009 13:09:08 -0700

Message-ID:

<49d90fe3$0$19545$b9f67a60@news.newsdemon.com>

cbossens73@yahoo.fr wrote:

Hi Knute,
I'm talking about modifying a unique BufferedImage's
pixels using setRGB(...) method from different threads
and what the guarantees are regarding the pixels when
they're modified from different threads.

I guess that in other words I'd like to know if it's
possible to have the *same* BufferedImage accessed from
different threads and observed in different state.

Time-0, Thread-1 setRGB(0,0,0xFFFFFFFF)
Time-1, Thread-2 getRGB(0,0)

Is there any possibility that Thread-2 would *not* be
getting 0xFFFFFFFF?

Or is there any JVM/spec guarantee that I'll always be
reading 0xFFFFFFFF?

I (probably) cannot afford to create new BufferedImage
all the time for they're translucent BufferedImage
covering most of the screen using JNA/WindowUtils to
provide per-pixel translucency and "click-trough" (a click
on a fully transparent pixel is not caught by the JFrame
but directly relayed to the underlying app).

So I (think I) need to modify a BufferedImage and I'd like
to know if this can be done from several threads or not.

The underlying data of the BufferedImage is an array wrapped in a
DataBuffer. The docs do not show any of the access methods as
synchronized so I think it is safe to conclude that if you modify pixels
with BufferedImage.setRGB() in one thread, that they will not
necessarily be guaranteed to be visible to another thread, without some
sort of synchronization.

However if you draw on a BufferedImage in one thread and display it in
the another (e.g. the EDT), I have never seen it fail to work even
without synchronization.

So I think the answer to your question is that it will work fine but I'm
not exactly sure why.

--

Knute Johnson
email s/nospam/knute2009/

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