How do I determine MaxDirectMemorySize on a running JVM?

Question

I have an application which uses DirectByteBuffers to store data, but I'd like to know what MaxDirectMemorySize is so I don't accidentally exceed it.

Without configuring this manually, how can I figure out, from within the program, what MaxDirectMemorySize is?

Answer 1

The accepted answer only works if the option is explicitly specified on the command line. As of Java 6, you can access the option directly using the HotSpotDiagnosticMXBean. The following Java 7 code can read it conveniently:

final HotSpotDiagnosticMXBean hsdiag = ManagementFactory
        .getPlatformMXBean(HotSpotDiagnosticMXBean.class);
if (hsdiag != null) {
    System.out.println(hsdiag.getVMOption("MaxDirectMemorySize"));
}

Note that this may return a value of zero, meaning to use the default setting, which is equal to Runtime.getRuntime().maxMemory(). For example, with Oracle JDK 7u71 64-bit on Windows 7, this returns 3,690,987,520.

Alternatively, if you're willing to resort to accessing the sun.misc package, it's available directly by calling sun.misc.VM.maxDirectMemory().

Answer 2

Yuu can get ALL JVM parameters with...

RuntimeMXBean RuntimemxBean = ManagementFactory.getRuntimeMXBean();
List<String> args=RuntimemxBean.getInputArguments();

for(int i=0;i<args.size();i++) {
    System.out.println(args.get(i)); 
}