Heap Expansion Demonstration

JVM Tutorials - Herong's Tutorial Examples

∟Heap Expansion Demonstration

This section provides a tutorial example to show the expansion of Heap when more objects are allocated. When Heap runs out of space, JVM throws the OutOfMemoryError exception.

Here are my understandings of the Heap data area, based readings available on the Internet:

The Heap data area is shared by all threads.
The Heap contains all data objects including both class instances and arrays, except those allocated explicitly in Method Area and Direct Memory area.
The Heap will be dynamically expanded when more data objects are allocated.
The totalMemory() method of the current Runtime object returns the total Heap space currently reserved by the JVM.
The freeMemory() method of the current Runtime object returns the free Heap space currently available for new object allocations.
JVM garbage collector will automatically remove objects that are not used any more.
In HotSpot JVM, the size of Method Area can be controlled by "-Xms (Minimum size)" and "-Xmx (Maximum size)" options.

To validate some of these understandings and demonstrate the Heap usage and expansion, I wrote the following sample program, HeapMemoryCrash.java, forcing the JVM run out of memory on the Heap:

/* HeapMemoryCrash.java
 * Copyright (c) 2014, HerongYang.com, All Rights Reserved.
 */
class HeapMemoryCrash {
   public static void main(String[] a) {
      int max = 1000;
      Object[] arr = new Object[max];
      heapCheck();
      for (int n=0; n<max; n++) {
         arr[n] = new long[10*1024*128];
         System.out.println((n+1)+": "
            +((n+1)*10)+" MB of objects created.");
         heapCheck();
      }
   }
   public static void heapCheck() {
      Runtime rt = Runtime.getRuntime();
      rt.gc();
      long total = rt.totalMemory();
      long free = rt.freeMemory();
      long used = total - free;
      java.io.Console con = System.console();
      con.format("Total memory: %s%n",total);
      con.format(" Free memory: %s%n",free);
      con.format(" Used memory: %s%n",used);
      String str = con.readLine("Press ENTER key to continue: ");
   }
}

This program is designed to:

Run a loop with 1000 iterations.
At each iteration, an array object of long[] is created with 10*1024*128 elements. Since each element is 8 bytes long, the total storage needed for this object is 10 MB.
The reference to each array object is stored in another array to keep it "used" so the garbage collector will not remove it.
totalMemory() and freeMemory() methods are used to report the current usage of the Heap.

Compile and run with HotSpot JVM 1.8:

C:\herong>\progra~1\java\jdk1.8.0\bin\javac HeapMemoryCrash.java

C:\herong>\progra~1\java\jdk1.8.0\bin\java HeapMemoryCrash
Total memory: 16318464
 Free memory: 16059952
 Used memory: 258512
Press ENTER key to continue:
1: 10 MB of objects created.
Total memory: 26152960
 Free memory: 15377464
 Used memory: 10775496
Press ENTER key to continue:
2: 20 MB of objects created.
Total memory: 51429376
 Free memory: 30168320
 Used memory: 21261056
Press ENTER key to continue:
3: 30 MB of objects created.
Total memory: 76767232
 Free memory: 45020400
 Used memory: 31746832
Press ENTER key to continue:
...
23: 230 MB of objects created.
Total memory: 259522560
 Free memory: 18080840
 Used memory: 241441720
Press ENTER key to continue:
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
   at HeapMemoryCrash.main(HeapMemoryCrash.java:10)

The output confirms that:

Array objects are allocated in the Heap.
Used Heap memory reported by the difference of totalMemory() and freeMemory() do match the total size of objects in the Heap.
The default Heap size limit is about 256 MB.
When Heap size expands to the specified limit, HotSpot throws java.lang.OutOfMemoryError on "Java heap space".

See other parts of the book for more tutorials on the Heap data area.

Last update: 2014.

Table of Contents

About This Book

Downloading and Installing JDK 1.8.0 on Windows

Downloading and Installing JDK 1.7.0 on Windows

java.lang.Runtime Class - The JVM Instance

java.lang.System Class - The Operating System

ClassLoader Class - Class Loaders

Class Class - Class Reflections

Sun's JVM - Java HotSpot VM

JRockit JVM 28.2.7 by Oracle Corporation