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• A memory leak
• An infinite loop
• The program requires more memory than is present by default in the heap
• Incorrect implementation of common data structures (such as hash tables, vectors, etc.)

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Noncompliant Code Example 1

This example uses an unlimited amount of memory, due to which the program can easily exhaust the heap.

A heap error will be generated if the heap is continued continues to be accessed even if there is no memory left in the heap.

public class ShowHeapError {

    Vector<String> names = new Vector<String>();
    String newName=null;
    InputStreamReader input = new InputStreamReader(System.in);
    BufferedReader reader = new BufferedReader(input);

    public void addNames(){
    	do{
    		//adding unknown number of records to a list
    		//the user can enter as much data as he wants and exhaust the heap
    		System.out.print(" To quit, enter \"quit\"\nEnter record: ");
          	try {
          		newName = reader.readLine();
          		if(!newName.equalsIgnoreCase("quit")){
          			//names are continued to be added without bothering about the size on the heap
          			names.addElement(newName);
          		}
   			} catch (IOException e) {
   			}
            System.out.println(newName);

    	}
        while (!newName.equalsIgnoreCase("quit"));
    }

    public static void main(String[] args) {
         ShowHeapError demo = new ShowHeapError();
         demo.addNames();
    }
}

Compliant

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Solution 1

To handle this problem, where the data structure size is so big that the heap gets exhausted, the user should consider using databases, where the record will get written on to the disk. Hence, this structure will never outgrow the heap.

In the above example, the user can re-use reuse a single Long long variable where the input gets stored and write that value into a simple database , containing a table User with a field userID and any other required fields. This will prevent the heap to get from getting exhausted.

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Noncompliant Code Example 2

In this example, the program needs more memory on the heap than the default. In a server-class machine running either VM (client or server) with a parallel garbage collector, the default initial and maximum heap sizes are as follows for J2SE 5.0 \[1\]:

• initial heap size:

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• larger of 1/64th of the machine's physical memory on the machine or some reasonable minimum

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• maximum heap size:

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• smaller of 1/4th of the physical memory or 1GB
  

  Code Block
  bgColor #FFcccc
  public class ShowHeapError { /*assuming the heap size as 512mB (calculated as 1/4th of 2 GB RAM = 512mB) * Considering long values being entered (64 bits each, the max number of elements * would be 5122mB/64bits = 67108864) */ Vector<Long> names = new Vector<Long>(67108865); long newID=0L; int count = 67108865; int i = 0; InputStreamReader input = new InputStreamReader(System.in); Scanner reader = new Scanner(input); public void addNames(){ do{ /*adding unknown number of records to a list * the user can enter more number of IDs than what the heap can support and exhaust the heap * assume that the record ID is a 64 bit long value */ System.out.print(" To quit, enter -1\nEnter recordID: "); newID = reader.nextLong(); //names are continued to be added without bothering about the size on the heap names.addElement(newID); System.out.println(newID); i++; }while (i<count || newID!=-1); } public static void main(String[] args) { ShowHeapError demo = new ShowHeapError(); demo.addNames(); } }

Compliant

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Solution 2

This exception can be avoided by either making sure that there are no infinite loops or memory leaks. If the programmer knows that the application would require a lot of memory then, he can increase the heap size in Java using the following runruntime time parameters \[2\]:

java -Xms<initial heap size> -Xmx<maximum heap size>

for For example:

java -Xms128m -Xmx512m ShowHeapError

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It is difficult identifying a heap exhaustion, since static analysis tools would not be able to pin point pinpoint anything and the heap size could be different for different machines.

In the case of the heap size is being increased through the command line, then the assessment would be as follows:

In the case of the database solution is being used, the cost would increase to high due to the coding of a disk-based solution.

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