freeMemory() - Getting JVM Free Memory Information

Java Tutorials - Herong's Tutorial Examples

∟System Properties and Runtime Object Methods

∟freeMemory() - Getting JVM Free Memory Information

This section provides a tutorial example on how to use freeMemory(), totalMemory(), and maxMemory() of the Runtime class to get JVM total and free memory information.

There a couple of values used in Java related to memory management:

Free Memory: Number of bytes available for the application program.
Total Memory: Number of total bytes allocated to the application program.
Maximum Memory: Maximum number of bytes that can be allocated to the application program.

There is one method in the Runtime class that you can use to get the each of those values. Here is a program showing you how to use those methods properly:

/* FreeMemory.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
public class FreeMemory {
   private static long[] fm = new long[5];
   private static long[] tm = new long[5];
   private static long[] mm = new long[5];
   private static long[] um = new long[5];
   public static void main(String[] a) {
      Runtime rt = Runtime.getRuntime();
      getMemoryInfo(rt,0);
      getMemoryInfo(rt,1);
      getMemoryInfo(rt,2);
      getMemoryInfo(rt,3);
      getMemoryInfo(rt,4);
      printMemoryInfo(rt,0);
      printMemoryInfo(rt,1);
      printMemoryInfo(rt,2);
      printMemoryInfo(rt,3);
      printMemoryInfo(rt,4);
      for (int i=0; i<5; i++) {
         System.out.println("Get memory info - step = "+i);
         System.out.println("   Free memory = "+fm[i]);
         System.out.println("   Total memory = "+tm[i]);
         System.out.println("   Maximum memory = "+mm[i]);
         System.out.println("   Memory used = "+um[i]);
      }
   }
   public static void getMemoryInfo(Runtime rt, int i) {
      fm[i] = rt.freeMemory();
      tm[i] = rt.totalMemory();
      mm[i] = rt.maxMemory();
      um[i] = tm[i]-fm[i];
   }
   public static void printMemoryInfo(Runtime rt, int i) {
      long fm = rt.freeMemory();
      long tm = rt.totalMemory();
      long mm = rt.maxMemory();
      System.out.println("Print memory info - step = "+i);
      System.out.println("   Free memory = "+fm);
      System.out.println("   Total memory = "+tm);
      System.out.println("   Maximum memory = "+mm);
      System.out.println("   Memory used = "+(tm-fm));
   }
}

This program seems to be confusing:

Why do I need two different methods to retrieve the memory information?
Why are the methods called repeatedly 5 times?
Why are the printing statements for the getMemoryInfo() method not following the method call statements immediately?

You need to read the output of this program to answer those questions:

Print memory info - step = 0
   Free memory = 1777568
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 254048
Print memory info - step = 1
   Free memory = 1774712
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 256904
Print memory info - step = 2
   Free memory = 1772496
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 259120
Print memory info - step = 3
   Free memory = 1770280
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 261336
Print memory info - step = 4
   Free memory = 1768064
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 263552
Get memory info - step = 0
   Free memory = 1777568
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 254048
Get memory info - step = 1
   Free memory = 1777568
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 254048
Get memory info - step = 2
   Free memory = 1777568
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 254048
Get memory info - step = 3
   Free memory = 1777568
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 254048
Get memory info - step = 4
   Free memory = 1777568
   Total memory = 2031616
   Maximum memory = 134217728
   Memory used = 254048

Note that:

The difference between the getMemoryInfo() method and printMemoryInfo() method is that the getMemoryInfo() method saves the memory usage into arrays, while printMemoryInfo() method prints the memory usage values on the console. This is to show the impact of the printing statements, System.out.println(), on the memory usage.
Calling the methods repeatedly is to see if there are any thread other than the application program thread running that might affect the memory usage. As you can see from the output, the memory usage was un-changed during the 5 calls to getMemoryInfo(). However, after each call to printMemoryInfo(), the free memory decreased by more than 2000 bytes. This suggests that the System.out.println() method was leaving dead objects in memory for the garbage collector to remove them.
Calling the getMemoryInfo() at the beginning and putting the related printing statements at the end is to avoid any impact of printing statements on the printMemoryInfo() calls.