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    <p>Um, lots.</p> <p>There are several parameters here. The specific VM, plus there are usually run-time parameters on the VM as well. That's somewhat driven by the operating system: what support does the underlying OS have for threads and what limitations does it put on them? If the VM actually uses OS-level threads at all, the good old red thread/green thread thing.</p> <p>What "support" means is another question. If you write a Java program that is just something like</p> <pre><code> class DieLikeADog { public static void main(String[] argv){ for(;;){ new Thread(new SomeRunaable).start(); } } } </code></pre> <p>(and don't complain about little syntax details, I'm on my first cup of coffee) then you should certainly expect to get hundreds or thousands of threads running. But <em>creating</em> a Thread is relatively expensive, and scheduler overhead can get intense; it's unclear that you could have those threads do anything useful.</p> <h3>Update</h3> <p>Okay, couldn't resist. Here's my little test program, with a couple embellishments:</p> <pre><code>public class DieLikeADog { private static Object s = new Object(); private static int count = 0; public static void main(String[] argv){ for(;;){ new Thread(new Runnable(){ public void run(){ synchronized(s){ count += 1; System.err.println("New thread #"+count); } for(;;){ try { Thread.sleep(1000); } catch (Exception e){ System.err.println(e); } } } }).start(); } } } </code></pre> <p>On OS/X 10.5.6 on Intel, and Java <s>6</s> 5 (see comments), here's what I got</p> <pre> New thread #2547 New thread #2548 New thread #2549 Can't create thread: 5 New thread #2550 Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread at java.lang.Thread.start0(Native Method) at java.lang.Thread.start(Thread.java:592) at DieLikeADog.main(DieLikeADog.java:6) </pre>
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    1. POHow many threads can a Java VM support?
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    1. USCharlie Martin
    1. USCharlie Martin
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    1. COHow much memory did you start the JVM with? That matters.
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      1. USEddie
    2. COthe default. Questioner has a test program now, and I want another cup of coffee.
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      1. USCharlie Martin
    3. COOkay, you tempted me, @Eddie, but messing about with -Xmx and -Xss just lead to a lot of combinations that don't work well. I need an undergrad who wants a research project.
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