Runtime Class in Java

The Runtime class, in java.lang, represents the running JVM itself. Every Java program has exactly one Runtime object, obtained with Runtime.getRuntime(). It is used to ask how many processors are available, how much memory the heap has and uses, and to control the JVM: requesting garbage collection, adding shutdown hooks, running external programs and exiting. In this lesson, you will learn its methods and how to measure memory with it.

Methods of Runtime

How the Memory Numbers Relate

used  = totalMemory() - freeMemory()
maxMemory() >= totalMemory() >= used

The heap can grow from totalMemory() up to maxMemory() as needed.

Example: Measuring Memory Use

Let's see an example that reports processors and memory, and measures how much heap one million Integer objects take.

public class RuntimeDemo {
    static long usedMb(Runtime rt) {
        return (rt.totalMemory() - rt.freeMemory()) / (1024 * 1024);
    }
 
    public static void main(String[] args) {
        Runtime rt = Runtime.getRuntime();
        System.out.println("Java version: " + Runtime.version().feature());
        System.out.println("Processors at least 1? "
            + (rt.availableProcessors() >= 1));
        System.out.println("max >= total >= used? "
            + (rt.maxMemory() >= rt.totalMemory()
               && rt.totalMemory() >= rt.totalMemory() - rt.freeMemory()));
 
        rt.gc();
        long before = usedMb(rt);
        Integer[] boxed = new Integer[1_000_000];
        for (int i = 0; i < boxed.length; i++) {
            boxed[i] = i + 1000;
        }
        rt.gc();
        long after = usedMb(rt);
        System.out.println("One million Integers used about 15 to 25 MB? "
            + (after - before >= 15 && after - before <= 25));
        System.out.println("Kept: " + boxed.length);
    }
}

Java version: 25
Processors at least 1? true
max >= total >= used? true
One million Integers used about 15 to 25 MB? true
Kept: 1000000

Explanation: Runtime.version().feature() gives the major version, 25. Memory is measured as totalMemory() - freeMemory() after requesting a GC, so leftover garbage doesn't count. One million Integer objects use about 20 MB: 16 bytes per object plus 4 bytes per array slot, as explained in "Object Memory Layout in Java". The exact numbers vary, so the program checks a range.

More Detailed Metrics

Common Mistakes

Reading freeMemory() as "memory left before OutOfMemoryError". The heap can still grow up to maxMemory().

Using Runtime.exec with a single command string. It splits on spaces unpredictably; use ProcessBuilder.

Calling halt instead of exit. halt skips shutdown hooks and cleanup.

Practice

1. Easy: Print maxMemory() in MB, then run the program with -Xmx256m and check that it changes.
2. Harder: Measure the memory used by one million ints in an int[] and compare it with the Integer[] above.

Key Takeaways