Java Exception: "java.lang.OutOfMemoryError: Java heap space"

JVM Tutorials - Herong's Tutorial Examples

∟Memory Management and Garbage Collectors

∟Java Exception: "java.lang.OutOfMemoryError: Java heap space"

This section provides a tutorial example on how JVM expands the Heap space to allocate more objects and leads to the 'java.lang.OutOfMemoryError: Java heap space' Java exception, if it is not allowed to expand any more.

First, let's use the following simple program, HeapMemoryCrash.java, to observe what will happen when the Heap runs out of memory:

/* 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. You can make it bigger, if you like.
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 referenced (or "used") so the JVM will not deallocate it from the Heap.
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:\>\progra~1\java\jdk1.8.0\bin\java -Xms20m -Xmx50m HeapMemoryCrash
Total memory: 20447232
 Free memory: 20188720
 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: 50724864
 Free memory: 29463808
 Used memory: 21261056
Press ENTER key to continue:
3: 30 MB of objects created.
Total memory: 50724864
 Free memory: 18978032
 Used memory: 31746832
Press ENTER key to continue:
4: 40 MB of objects created.
Total memory: 50724864
 Free memory: 8492256
 Used memory: 42232608
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:

The "-Xms20m" option works and the JVM reserved 20,447,232 bytes, about 20 MB at the startup time based on the first call of totalMemory().
The first heapCheck() call reported that 258,512 bytes of Heap space was used. At that time, my Java application only created 1 object of an array, Object[1000], which is about 8,000 bytes. So the JVM itself used about 250 KB space from the Heap.
The second heapCheck() call reported that 10,775,496 bytes of Heap space was used, an increase of 10,516,984 bytes. At that time, my Java application added 1 object of an array, long[10*1024*128], which is about 10,485,760 bytes matching the Heap used space increase amount with some array overhead usage. This confirms that array objects are allocated in the Heap.
The second heapCheck() call also reported that JVM expanded the total Heap space from 10 MB to 26 MB (26,152,960 bytes). This is allowed, because I set the maximum Heap size to be 50 MB with "-Xmx50m". But this expansion seems be unnecessary, because my Java application was asking to allocate an object of 10 MB, and there was enough Heap space, 20,188,720 bytes, without the expansion. This behavior shows that the JVM tends to reserve more Heap space than what is actually needed.
After 40 MB of objects allocated, my Java application used about 42 MB out of 50 MB from the Heap. There was only 8.5 MB space left in the Heap.
When my Java application asks to allocate another object with 10 MB, the JVM threw out an exception: "java.lang.OutOfMemoryError: Java heap space". This is expected, because the JVM can not expand the Heap space any more because of the "-Xmx50m" option.

Now rerun the example with a higher Heap size to see how much space the operating system allows you to give the JVM:

C:\herong>\progra~1\java\jdk1.8.0\bin\java -Xms1550m HeapMemoryCrash
Total memory: 1571160064
 Free memory: 1570902744
 Used memory: 257320
Press ENTER key to continue:
...

C:\herong>\progra~1\java\jdk1.8.0\bin\java -Xms1560m HeapMemoryCrash
Error occurred during initialization of VM
Could not reserve enough space for 1597440KB object heap

This tells me that my 32-bit Windows system only allows my specify up to 1,550 MB for the JVM heap space.

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