Use this code:

// Get current size of heap in bytes.
long heapSize = Runtime.getRuntime().totalMemory();

// Get maximum size of heap in bytes. The heap cannot grow beyond this size.
// Any attempt will result in an OutOfMemoryException.
long heapMaxSize = Runtime.getRuntime().maxMemory();

// Get amount of free memory within the heap in bytes. This size will 
// increase after garbage collection and decrease as new objects are created.
long heapFreeSize = Runtime.getRuntime().freeMemory();

It was useful to me to know it.

You could use JMX to collect the usage of heap memory at runtime.

Code Example:

import java.lang.management.ManagementFactory;
import java.lang.management.MemoryPoolMXBean;
import java.lang.management.MemoryType;
import java.lang.management.MemoryUsage;

for (MemoryPoolMXBean mpBean: ManagementFactory.getMemoryPoolMXBeans()) {
    if (mpBean.getType() == MemoryType.HEAP) {
        System.out.printf(
            "Name: %s: %s\n",
            mpBean.getName(), mpBean.getUsage()
        );
    }
}

Output Example:

Name: Eden Space: init = 6619136(6464K) used = 3754304(3666K) committed = 6619136(6464K) max = 186253312(181888K)
Name: Survivor Space: init = 786432(768K) used = 0(0K) committed = 786432(768K) max = 23265280(22720K)
Name: Tenured Gen: init = 16449536(16064K) used = 0(0K) committed = 16449536(16064K) max = 465567744(454656K)

If your have question about "Eden Space" or "Survivor Space", check out How is the java memory pool divided

Each Java process has a pid, which you first need to find with the jps command.

Once you have the pid, you can use jstat -gc [insert-pid-here] to find statistics of the behavior of the garbage collected heap.

• jstat -gccapacity [insert-pid-here] will present information about memory pool generation and space capabilities.

• jstat -gcutil [insert-pid-here] will present the utilization of each generation as a percentage of its capacity. Useful to get an at a glance view of usage.

See jstat docs on Oracle's site.

You can use jstat, like :

 jstat -gc pid

Full docs here : http://docs.oracle.com/javase/7/docs/technotes/tools/share/jstat.html

Use jps and jstack to learn more about Java processes. jstat can also be useful for Java statistics monitoring.

There is a Runtime.maxMemory, which the documentation says will return the maximum amount of memory the Java Virtual Machine will attempt to use, but it is not specific whether it is the maximum heap memory set when launching the JVM.

Just as a test, I wrote the following program:

class MaxMemory {
    public static void main(String[] args) {
        System.out.println(Runtime.getRuntime().maxMemory());
    }
}

The results of the execution was the following:

C:\coobird\>java -Xmx64m MaxMemory
66650112

C:\coobird\>java -Xmx128m MaxMemory
133234688

This is using Java SE 6 version 1.6.0_12 on Windows.

Although the values are close to 64 MB (67,108,864 bytes) and 128 MB (134,217,728 bytes), they aren't exactly so.