How many bytes can I store here, sir?

Every operating system has its own special procedure to access memory information and statistics. In Linux one can read /proc filesystem, in nearly every OS one can fork a subprocess to execute a system command to find out memory information or call a system dependent API.

But since Java has the motto of write once run everywhere, how can one access memory information in Java?

Memory belonging to JVM process

Reading process memory usage and free process memory is already available using JDK public API. There are a few ways of achieving this;

Runtime runtime = Runtime.getRuntime();
// maximum memory this JVM process can allocate
runtime.maxMemory();
// total allocated memory by JVM process
runtime.totalMemory();
// total free space in allocated memory
runtime.freeMemory();
MemoryMXBean memoryMXBean = ManagementFactory.getMemoryMXBean();

MemoryUsage heapMemoryUsage = memoryMXBean.getHeapMemoryUsage();
// total allocated *heap* memory by JVM process
heapMemoryUsage.getCommitted();
// total used *heap* memory
heapMemoryUsage.getUsed();
// maximum *heap* memory this JVM process can allocate
heapMemoryUsage.getMax();

// also there's another *MemoryUsage* bean to access
// non-heap memory allocated by JVM (See DirectByteBuffer)
MemoryUsage nonHeapMemoryUsage = memoryMXBean.getNonHeapMemoryUsage();
...

Physical And Swap Memory

There's no public API to access physical memory information (at least I don't know yet :). But you can guess that there should be an internal method hiding somewhere inside JDK. No need to go far, answer is in com.sun.management package.

com.sun.management.OperatingSystemMXBean bean has many useful methods to access physical and swap memory information (in contrast to not-very-useful java.lang.management.OperatingSystemMXBean).

com.sun.management.OperatingSystemMXBean operatingSystemMXBean
        = (com.sun.management.OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();

// total physical memory available
operatingSystemMXBean.getTotalPhysicalMemorySize();
// free physical memory available
operatingSystemMXBean.getFreePhysicalMemorySize();
// total swap space available
operatingSystemMXBean.getTotalSwapSpaceSize();
// free swap space available
operatingSystemMXBean.getFreeSwapSpaceSize();

Biggest problem in your head should be what if this class is not available in my environment. First solution pops out from our minds is using reflection. But there's also a standard-ish way of accessing this data: querying MBeanServer

public static long getTotalPhysicalMemorySize() {
    return queryPhysicalMemory("TotalPhysicalMemorySize");
}

public static long getFreePhysicalMemorySize() {
    return queryPhysicalMemory("FreePhysicalMemorySize");
}

public static long getTotalSwapSpaceSize() {
    return queryPhysicalMemory("TotalSwapSpaceSize");
}

public static long getFreeSwapSpaceSize() {
    return queryPhysicalMemory("FreeSwapSpaceSize");
}

private static long queryMBeanServer(String type) {
    MBeanServer mBean = ManagementFactory.getPlatformMBeanServer();
    try {
        ObjectName name = new ObjectName("java.lang", "type",
            "OperatingSystem");
        Object attribute = mBean.getAttribute(name, type);
        if (attribute != null) {
            return Long.parseLong(attribute.toString());
        }
    } catch (Exception ignored) {
        // means this information is not available
    }
    return -1L;
}

Go fill all emty space! It's yours!

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