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Programming errors can prevent garbage collection of objects that are no longer relevant to program operation. The garbage collector collects only unreachable objects; consequently, the presence of reachable objects that remain unused indicates memory mismanagement. Consumption of all available heap space can cause an OutOfMemoryError, which usually results in program termination.

Excessive memory leaks can lead to memory exhaustion and denial of service (DoS) and must be avoided (see MSC05-J. Do not exhaust heap space for more information).

Noncompliant Code Example (Off-by-One Programming Error)

The vector object in the noncompliant code example leaks memory. The condition for removing the vector element is mistakenly written as n > 0 instead of n >= 0. Consequently, the method fails to remove one element per invocation and quickly exhausts the available heap space.

public class Leak {
  static Vector vector = new Vector();

  public void useVector(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.useVector(1);
      i++;
    }
  }
}

Compliant Solution (>=)

This compliant solution corrects the mistake by changing the loop condition to n >= 0. It also wraps the cleanup code in a finally block so that it still executes even if the interim code throws an exception.

public void useVector(int count) {
  int n = 0;
  try {
    for (; n < count; n++) {
      vector.add(Integer.toString(n));
    }
    // ...
  } finally {
    for (n = n - 1; n >= 0; n--) {
      vector.removeElementAt(n);
    }
  }
}	

Compliant Solution (clear())

Prefer the use of standard language semantics where possible. This compliant solution uses the vector.clear() method, which removes all elements:

public void useVector(int count) { 	
  try {
    for (int n = 0; n < count; n++) {
      vector.add(Integer.toString(n));
    }
    // ...
  } finally {
    vector.clear(); // Clear the vector
  }
}

Noncompliant Code Example (Nonlocal Instance Field)

This noncompliant code example declares and allocates a HashMap instance field that is used only in the doSomething() method:

public class Storer {
  private HashMap<Integer,String> hm = new HashMap<Integer, String>();
  
  private void doSomething() {
    // hm is used only here and never referenced again
    hm.put(1, "java");
    // ...
  }
}

Programmers may be surprised that the HashMap persists for the entire lifetime of the Storer instance.

Compliant Solution (Reduce Scope of Instance Field)

This compliant solution declares the HashMap as a local variable within the doSomething() method. The hm local variable is eliminated after the method returns. When the local variable holds the only reference to the HashMap, the garbage collector can reclaim its associated storage.

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

Localizing or confining the instance field to a narrower scope simplifies garbage collection; today's generational garbage collectors perform well with short-lived objects.

Noncompliant Code Example (Lapsed Listener)

This noncompliant code example, known as the Lapsed Listener [Goetz 2005a], demonstrates unintentional object retention. The button continues to hold a reference of the reader object after completion of the readSomething() method, even though the reader object is never used again. Consequently, the garbage collector cannot collect the reader object. A similar problem occurs with inner classes because they hold an implicit reference to the enclosing 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 (Exception before Remove)

This noncompliant code example attempts to remove the reader through use of the removeActionListener() method:

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

If an exception is thrown by the readSomething() method, the removeActionListener() statement is never executed.

Compliant Solution (finally Block)

This compliant solution uses a finally block to ensure that the reader object's reference is removed:

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

Noncompliant Code Example (Member Object Leaks)

This noncompliant code example implements a stack data structure [Bloch 2008] that continues to hold references to elements after they have been popped off the stack:

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);
    }
  }
}

The object references are retained on the stack even after the element is popped. Such obsolete references cause objects to remain live; consequently, the objects cannot be garbage-collected.

Compliant Solution (null)

This compliant solution assigns null to all obsolete references:

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

The garbage collector can then include individual objects formerly referenced from the stack in its list of objects to free.

Although these examples appear trivial and do not represent significant problems in production code, obsolete references remain a concern when dealing with data structures such as hash tables containing many large records. It is prudent to assign null to array-like custom data structures; doing so with individual objects references or local variables is unnecessary because the garbage collector handles these cases automatically [Commes 2007].

Noncompliant Code Example (Strong References)

A common variation of the obsolete object fallacy is the unintentional retention of objects in collections such as maps. In this noncompliant code example, a server maintains temporary metadata about all committed secure connections:

class HashMetaData {
  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);  
  }	
}

It is possible to close a socket without removing it from this map. Consequently, this map may contain dead sockets until removeTempConnection() is invoked on them. In the absence of notification logic, it is impossible to determine when to call removeTempConnection(). Moreover, nullifying original objects or referents (Socket connections) is unwieldy.

Compliant Solution (Weak References)

This compliant solution uses weak references to allow timely garbage collection:

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

Strong references prevent the garbage collector from reclaiming objects that are stored inside container objects, such as in a Map. According to the Java API [API 2014], weak reference objects "do not prevent their referents from being made finalizable, finalized, and then reclaimed."

Keys held in WeakHashMap objects are referenced through weak references. Objects become eligible for garbage collection when they lack strong references. Consequently, use of weak references allows the code to refer to the referent without delaying garbage collection of the referent. This approach is suitable only when the lifetime of the object is required to be the same as the lifetime of the key.

Simply facilitating garbage collection of unneeded objects through use of weak references is insufficient. Programs must also prune the data structure so that additional live entries can be accommodated. One pruning technique is to call the get() method of WeakHashMap and remove any entry that corresponds to a null return value (polling). Use of reference queues is a more efficient method [Goetz 2005b].

Compliant Solution (Reference Queue)

Reference queues provide notifications when a referent is garbage-collected. When the referent is garbage-collected, the HashMap continues to strongly reference both the WeakReference object and the corresponding map value (for each entry in the HashMap).

When the garbage collector clears the reference to an object, it adds the corresponding WeakReference object to the reference queue. The WeakReference object remains in the reference queue until some operation is performed on the queue (such as a put() or remove()). After such an operation, the WeakReference object in the hash map is also garbage-collected. Alternatively, this two-step procedure can be carried out manually by using the following code:

class HashMetaData {
  private Map<WeakReference<SSLSocket>, InetAddress> m = 
      Collections.synchronizedMap(
        new HashMap<WeakReference<SSLSocket>, InetAddress>());
  ReferenceQueue queue = new ReferenceQueue();
  
  public void storeTempConnection(SSLSocket sock, InetAddress ip) {
    WeakReference<SSLSocket> wr = new WeakReference<SSLSocket>(sock, queue);

    // Poll for dead entries before adding more 
    while ((wr = (WeakReference) queue.poll()) != null) {
      // Removes the WeakReference object and the value (not the referent) 
      m.remove(wr); 
    }  
    m.put(wr, ip);
  }

  public void removeTempConnection(SSLSocket sock) {
    m.remove(sock);  
  }
}

Note that the two-argument constructor of WeakReference takes a Queue argument and must be used to perform direct queue processing. Dead entries should be pruned prior to insertion.

Compliant Solution (Soft References)

Use of soft references is also permitted. Soft references guarantee that the referent will be reclaimed before an OutOfMemoryError occurs and also that the referent will remain live until memory begins to run out.

class HashMetaData {
  private Map<SoftReference<SSLSocket>, InetAddress> m = 
        Collections.synchronizedMap(
        new HashMap<SoftReference<SSLSocket>, InetAddress>());
  ReferenceQueue queue = new ReferenceQueue();

  public void storeTempConnection(SSLSocket sock, InetAddress ip) {
    SoftReference<SSLSocket> sr = new SoftReference<SSLSocket>(sock, queue);
    while ((sr = (SoftReference) queue.poll()) != null) {
      // Removes the WeakReference object and the value (not the referent)
      m.remove(sr); 
    }  
    m.put(sr, ip);
  }

  public void removeTempConnection(SSLSocket sock) {
    m.remove(sock);  
  }	
}

Weak references are garbage-collected more aggressively than soft references. Consequently, weak references should be preferred in applications in which efficient memory usage is critical, and soft references should be preferred in applications that rely heavily on caching.

Risk Assessment

Memory leaks in Java applications may be exploited in a DoS attack.

Automated Detection

Related Guidelines

Bibliography