How to check the memory used by a program in Java?

For a long-running java code, which makes heavy use of dynamic memory, we may end up with Out-Of-Memory errors due to a memory shortage of the heap space.

In the below program, we can test the free java heap space used by a program. If the heap space is used more than 90 percent, then the garbage collector is explicitly called. The System.gc() call is blocking the calling thread until the garbage collector has completed. Therefore, this code can be executed in a separate thread.

Example

public class GCTest {
   public void runGC() {
      Runtime runtime = Runtime.getRuntime();
      long memoryMax = runtime.maxMemory();
      long memoryUsed = runtime.totalMemory() - runtime.freeMemory();
      double memoryUsedPercent = (memoryUsed * 100.0) / memoryMax;
      System.out.println("memoryUsedPercent: " + memoryUsedPercent);
      if (memoryUsedPercent > 90.0)
         System.gc();
   }
   public static void main(String args[]) {
      GCTest test = new GCTest();
         test.runGC();
   }
}

Output

memoryUsedPercent: 0.07187129404943532