Check available heapSize programmatically?

问题:

I am using Eclipse. The problem is my application crashes if the allocated memory is less then 512MB. Now is there anyway to check the available memory for a program before starting heavy memory exhaustive processing? For example, I know we can check available JVM heap size as:

long heapSize = Runtime.getRuntime().totalMemory();
System.out.println("Heap Size = " + heapSize);

Problem is, this gives the JVM heap size. Even increasing it does not work using Eclipse. In Eclipse, if I change the VM arguments then it works. However the printout from above statements is always the same. Is there any command through which I can exactly know how much memory I am allocated for a particular application?

回答1:

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

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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)

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