Why dose max heap size change?

[=?GB2312?B?stzQ8bar?=]

Hi all,

I am doing some monitor jobs for JVM with JMX, and has been stuck on a
question about max heap size.

The javadoc of the field *max* of class *java.lang.management.HeapUsage*says,

      "represents the maximum amount of memory (in bytes) that can be used
for memory management. Its value may be undefined.

       *The maximum amount of memory may change over time if defined*. "

So, my question is,

       Why dose max heap size change, if I defined it with Xmx?

Any response will be appreciated.

Best Regards.

caoxudong
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