MSC06-J. Avoid memory leaks

Even though Java supports memory management through garbage collection, there are innumerable possibilities of introducing memory leaks as a result of programming errors. Furthermore, the garbage collector collects only the unreachable objects and not those that are still reachable. The presence of reachable objects that remain unused indicates memory mismanagement.

Depending on program scale and available memory, one of the least desirable errors, the OutOfMemoryError manifests itself when the heap space runs out. This usually results in program failure.

Noncompliant Code Example

This noncompliant code example shows a leaking Vector object. The memory leak quickly exhausts the heap space as the condition for removing the vector element is mistakenly written as n > 0 instead of n >= 0. As a result, in every iteration, the method leaks one vector element.

public class Leak {
  static Vector vector = new Vector();
  public void leakingVector(int count) { 	
    for (int n = 0; n < count; n++) {
      vector.add(Integer.toString(n));
    }
    for (int n = count - 1; n > 0; n--) { // Free the memory
      vector.removeElementAt(n);
    }	
  }

  public static void main(String[] args) throws IOException {
    Leak le = new Leak();
    int i = 1;
    while(true) {
      System.out.println("Iteration: " + i);
      le.leakingVector(1);
      i++;
    }
  }
}

Compliant Solution (1)

This compliant solution corrects the mistake by changing the loop condition to n >= 0.

for (int n = count - 1; n >= 0; n--) {
  vector.removeElementAt(n);
}	

Compliant Solution (2)

Prefer the use of standard language semantics where possible, as shown in this compliant solution.

while (!vector.isEmpty()){
  vector.removeElementAt(vector.size() - 1);	        	    
}

Compliant Solution (3)

An alternative way of clearing the vector is to use the vector.clear() method. Likewise, if a range of elements has to be released from the vector, vector.subList(fromIndex, toIndex).clear() can be used. In the context of this compliant solution, the fromIndex and the toIndex can both be 0 as the count variable is 1 on each iteration. [[API 06]]

vector.clear();

Noncompliant Code Example

This noncompliant code example creates a HashMap instance field within the class body but uses it only in the doSomething() method. It is not obvious that it will continue to persist as long as the class BadScope's instance is alive.

public class BadScope {
  private HashMap<Integer,String> hm = new HashMap<Integer,String>();
  
  private void doSomething() {
    hm.put(1,"java");  // hm is used only here
  }
}

Compliant Solution

Localizing or confining the instance field to a narrower scope gives the garbage collector a better chance of succeeding at collecting the object in a timely manner. Short-lived objects are always collected quickly by generational garbage collectors.

public class GoodScope {
  private void doSomething() {
    HashMap<Integer,String> hm = new HashMap<Integer,String>();
    hm.put(1,"java");
  }
}

Noncompliant Code Example

This noncompliant code example demonstrates unintentional object retention and is commonly called the Lapsed Listener. The button continues to hold a reference of the reader object even after completion of the readSomething() method. As a result, the garbage collector does not collect the reader object. A similar problem occurs with inner classes as they hold an implicit reference to the outer class.

public class LapseEvent extends JApplet   {
  JButton button;
  public void init() {
    button = new JButton("Click Me");
    getContentPane().add(button, BorderLayout.CENTER);
    Reader reader = new Reader();
    button.addActionListener(reader);
    try {
      reader.readSomething();
    } catch (IOException e) { 
      // Handle exception 
    }		 
  }
}

class Reader implements ActionListener{
  public void actionPerformed(ActionEvent e)  {
    Toolkit.getDefaultToolkit().beep();
  }
  public void readSomething() throws IOException {
    // Read from file
  }
}

Noncompliant Code Example

To resolve this problem, a matching pair of the removeActionListener() should be used, as shown below. Unfortunately, this is not the panacea because an exception in the reader.readSomething() method can change the control flow in such a way that the removeActionListener statement is never executed.

Reader reader = new Reader();
button.addActionListener(reader);
try {
  reader.readSomething();  // Can skip next line
  button.removeActionListener(reader);  // Dereferenced, but control flow can change
} catch (IOException e) { 
  // Forward to handler 
}		 

Compliant Solution

This compliant solution uses the finally block to ensure that the reader object's reference is unregistered.

Reader reader = new Reader();
button.addActionListener(reader);
try {
  reader.readSomething();
} catch (IOException e) { 
  // Handle exception 
} finally {
  button.removeActionListener(reader);  // Always gets executed
}

Noncompliant Code Example

This noncompliant code example implements a stack data structure [[Bloch 08]]. The main issue is that it does not allow the garbage collector to deallocate memory after the pop operation. The object references are retained even after the element is pop'ed. Such obsolete references are not garbage collected automatically. This can get even more deceitful because none of the objects referenced by the offending object get garbage collected.

public class Stack {
  private Object[] elements;
  private int size = 0;

  public Stack(int initialCapacity) {
    this.elements = new Object[initialCapacity];
  }

  public void push(Object e) {
    ensureCapacity();
    elements[size++] = e;
  }

  public Object pop() { // This method causes memory leaks
    if (size == 0) {
      throw new EmptyStackException();
    }
    return elements[--size];
  }
  /**
  * Ensure space for at least one more element, roughly
  * doubling the capacity each time the array needs to grow.
  */
  private void ensureCapacity() {
    if (elements.length == size) {
      Object[] oldElements = elements;
      elements = new Object[2 * elements.length + 1];
      System.arraycopy(oldElements, 0, elements, 0, size);
    }
  }
}

Compliant Solution

This compliant solution assigns null values to all obsolete references. The garbage collector can now include individual element-objects in its list of objects to free. A NullPointerException results on subsequent attempts to access the particular object.

public Object pop() {
  if (size==0)
    throw new EmptyStackException(); // Ensures object consistency
  Object result = elements[--size];
  elements[size] = null; // Eliminate obsolete reference
  return result;
} 

While these examples may not model production scenarios, it is not uncommon to uncover obsolete references when dealing with data structures such as hash tables that contain many large-sized records. It is prudent to assign null to array-like custom data structures, however, doing so with individual objects or local variables has no specific advantages. The garbage collector is sufficiently equipped to handle these cases. [[Commes 07]]

Noncompliant Code Example

A common variant of the aforementioned noncompliant code example is the unintentional retention of objects when using a Map or a similar Collections object. In this noncompliant code example, a server maintains temporary metadata about all secure connections it commits to. Although the metadata is designed to be transient by nature, it persists even after the particular socket is closed. Unless some notification logic is installed, it is impossible to determine the best time to eliminate the object reference. Likewise, nulling out original objects or referents (Socket connections) by itself, proves to be unwieldy.

class HashMemLeak {
  private Map<SSLSocket, InetAddress> m = Collections.synchronizedMap(new HashMap<SSLSocket, InetAddress>());
  public void storeTempConnection(SSLSocket sock, InetAddress ip) {
	m.put(sock, ip);  
  }
  public void removeTempConnection(SSLSocket sock) {
	m.remove(sock);  
  }	
}

Compliant Solution

This compliant solution uses weak references to ameliorate the issue. Strong references typically used in code, do not allow the garbage collector to reclaim the objects that are stored compositely, such as in a Map. According to the Java API [[API 06]], weak reference objects: "... do not prevent their referents from being made finalizable, finalized, and then reclaimed".

A referent is the object that is being referred to. As soon as any strong references to the object are found to have phased out, the garbage collector reclaims the referent. With WeakHashMap, the map's key is weakly referred to and as a result determines whether the corresponding referents are ready to be collected.

// ...
private Map<SSLSocket, InetAddress> m = Collections.synchronizedMap(new WeakHashMap<SSLSocket, InetAddress>());
// ...

It is not enough to facilitate the collection of unneeded objects with weak references. It is critical to prune the data structure so that more entries can be accommodated in the newly created space. This can be achieved by calling the get() method of WeakHashMap and removing the entry that corresponds to the null return value (polling). A more efficient method is to use a reference queue. [[Goetz 05b]]

Compliant Solution

If the referent is assigned the value null, it is eventually garbage collected. However, the HashMap continues to strongly reference the WeakReference object and the corresponding value (for each entry in the HashMap). As soon as the GC clears the reference (which referred to the referent), it adds the corresponding WeakReference object to the reference queue. It remains there unless some operation is performed on the queue (such as a put() or remove()). After such an operation, the WeakReference object in the hashmap is also collected. Alternatively, this two-step procedure can be carried out manually by using the following code:

ReferenceQueue queue = new ReferenceQueue();
WeakReference wr = new WeakReference(key, queue); // Two-arg constructor, key = 'sock'
hashmap.put(wr, value);
while ((wr = (WeakReference) queue.poll()) != null) {
  hashmap.remove(wr); // Removes the WeakReference object and the value (not the referent)
}

Note that the two-argument constructor of WeakReference takes a Queue argument and must be used to perform direct queue processing.

Compliant Solution

It is also permissible to use soft references because they guarantee that the referent will be reclaimed before an OutOfMemoryError results but no sooner than the time when memory begins to run out.

Risk Assessment

Memory leaks in Java applications may be exploited to cause denial of service.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]] Class Vector, Class WeakReference
[[Gupta 05]]
[[Bloch 08]] Item 6: Eliminate obsolete object references
[[Commes 07]] Memory Leak Avoidance
[[Goetz 05]] Lapsed listeners
[[Goetz 05b]] "Memory leaks with global Maps" and "Reference queues"
[[MITRE 09]] CWE ID 401 "Failure to Release Memory Before Removing Last Reference (aka 'Memory Leak')"

MSC07-J. Eliminate class initialization cycles      49. Miscellaneous (MSC)      MSC08-J. Avoid cyclic dependencies between packages