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 failure. Memory leaks afford attackers with a potentially exploitable denial of service attack, and consequentlymust arebe forbiddenavoided.
Excessive memory leaks can lead to memory exhaustion and denial of service. For more information, see [MSC07-J. Do not assume infinite heap space].
h2. Noncompliant Code Example (Off-By-One Programming Error)
This noncompliant code example shows a leaking {{Vector}} object. The condition for removing the {{vector}} element is mistakenly written as {{n > 0}} instead of {{n >= 0}}. Consequently, the method grows the vector by one element per invocation, and quickly exhausts the available heap space.
{code:bgColor=#FFCCCC}
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++;
}
}
}
{code}
h2. Compliant Solution ({{>=}})
This compliant solution corrects the mistake by changing the loop condition to {{n >= 0}}.
{code:bgColor=#ccccff}
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--) {
vector.removeElementAt(n);
}
}
{code}
h2. Compliant Solution ({{clear()}})
Prefer the use of standard language semantics where possible. This compliant solution uses the {{vector.clear()}} method, which removes all elements.
{code:bgColor=#ccccff}
public void useVector(int count) {
for (int n = 0; n < count; n++) {
vector.add(Integer.toString(n));
}
// ...
vector.clear(); // Clear the vector
}
{code}
Use the {{vector.subList(fromIndex, toIndex).clear()}} method to remove a subrange of elements from the {{vector}}. Note that the {{fromIndex}} and the {{toIndex}} can both be {{0}} as the {{count}} variable is {{1}} on each iteration \[[API 2006|AA. Bibliography#API 06]\].
h2. Noncompliant Code Example (Non-Local Instance Field)
This noncompliant code example declares and allocates a {{HashMap}} instance field that is used only in the {{doSomething()}} method.
{code:bgColor=#FFCCCC}
public class Storer {
private HashMap<Integer,String> hm = new HashMap<Integer, String>();
private void doSomething() {
hm.put(1, "java"); // hm is used only here and never referenced again
// ...
}
}
{code}
Programmers may be surprised that the {{HashMap}} will persist for the entire lifetime of the {{Storer}} instance.
h2. Compliant Solution (Reduce Scope of Instance Field)
This compliant solution declares the {{HashMap}} as a local variable within the {{doSomething()}} method, eliminating the memory link.
{code:bgColor=#ccccff}
public class Storer {
private void doSomething() {
HashMap<Integer,String> hm = new HashMap<Integer,String>();
hm.put(1,"java");
// ...
}
}
{code}
Localizing or confining the instance field to a narrower scope simplifies garbage collection; today's generational garbage collectors perform well with short-lived objects.
h2. Noncompliant Code Example (Lapsed Listener)
This noncompliant code example, known as the _Lapsed Listener_ ([Goetz 2005|AA. Bibliography#Goetz 05]), 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 will never be used again.
{code:bgColor=#FFCCCC}
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
}
}
{code}
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.
h2. Noncompliant Code Example (Exception Before Remove)
{mc} Bloch 08 says: The best way to ensure that callbacks are garbage collected promptly is to store only weak references to them, for instance, by storing them only as keys in a WeakHashMap. {mc}
This noncompliant code example attempts to remove the reader through use of the {{removeActionListener()}} method.
{code:bgColor=#FFCCCC}
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
}
{code}
However, the {{removeActionListener()}} statement is never executed if an exception is thrown by the {{readSomething()}} method.
h2. Compliant Solution ({{finally}} Block)
This compliant solution uses a {{finally}} block to ensure that the {{reader}} object's reference is unregistered.
{code:bgColor=#ccccff}
Reader reader = new Reader();
button.addActionListener(reader);
try {
reader.readSomething();
} catch (IOException e) {
// Handle exception
} finally {
button.removeActionListener(reader); // Always executed
}
{code}
h2. Noncompliant Code Example (Member Object Leaks)
This noncompliant code example implements a stack data structure \[[Bloch 2008|AA. Bibliography#Bloch 08]\] that continues to hold references to elements after thay have been popped from the stack.
{code:bgColor=#FFCCCC}
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);
}
}
}
{code}
The object references are retained on the stack even after the element is popped. Such _obsolete references_ remain live, and consequently cannot be garbage collected.
h2. Compliant Solution ({{null}})
This compliant solution assigns {{null}} values to all obsolete references.
{code:bgColor=#ccccff}
public Object pop() {
if (size == 0)
throw new EmptyStackException(); // Ensures object consistency
Object result = elements[--size];
elements[size] = null; // Eliminate obsolete reference
return result;
}
{code}
The garbage collector can then include individual objects formerly on the stack in its list of objects to free.
Although these examples appear trivial, and may fail to model production scenarios, _obsolete references_ remain common 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 or local variables is unnecessary, because the garbage collector handles these cases automatically \[[Commes 2007|AA. Bibliography#Commes 07]\].
h2. Noncompliant Code Example (Strong References)
A common variation of the obsolete object fallacy is unintentional retention of objects in collections such as maps. In this noncompliant code example, a server maintains temporary metadata about all committed secure connections.
{code:bgColor=#FFCCCC}
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);
}
}
{code}
It is possible to close a socket without removing it from this map. Consequently, this map can 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, nulling out original objects or referents (Socket connections) is unwieldy.
h2. Compliant Solution (Weak References)
This compliant solution uses _weak references_ to allow timely garbage collection.
{code:bgColor=#ccccff}
// ...
private Map<SSLSocket, InetAddress> m = Collections.synchronizedMap(new WeakHashMap<SSLSocket, InetAddress>());
{code}
Strong references prevent the garbage collector from reclaiming objects that are stored compositely, such as in a {{Map}}. According to the Java API \[[API 2006|AA. Bibliography#API 06]\], 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.
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 removing any entry that corresponds to a {{null}} return value (polling). Use of reference queues is a more efficient method \[[Goetz 2005b|AA. Bibliography#Goetz 05b]\].
h2. Compliant Solution (Reference Queue)
Reference queues provide a way to receive notifications when a referent is garbage collected. When the referent is eventually 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 GC clears the reference that referred to the referent, 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 hashmap is also garbage collected. Alternatively, this two-step procedure can be carried out manually by using the following code:
{code:bgColor=#ccccff}
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);
while ((wr = (WeakReference) queue.poll()) != null) { // poll for dead entries before adding more
m.remove(wr); // Removes the WeakReference object and the value (not the referent)
}
m.put(wr, ip);
}
public void removeTempConnection(SSLSocket sock) {
m.remove(sock);
}
}
{code}
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.
h2. Compliant Solution (Soft References)
Use of soft references is also permitted. Soft references guarantee that the referent will be reclaimed before an {{OutOfMemoryError}} results, and also that the referent will remain live until memory begins to run out.
{code:bgColor=#ccccff}
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) {
m.remove(sr); // Removes the WeakReference object and the value (not the referent)
}
m.put(sr, ip);
}
public void removeTempConnection(SSLSocket sock) {
m.remove(sock);
}
}
{code}
Weak references are garbage-collected more aggressively than soft references. Consequently weak references should be preferred in applications where efficient memory usage is critical, and soft references should be preferred in applications that rely heavily on caching.
h2. Risk Assessment
Memory leaks in Java applications may be exploited to cause denial of service.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| MSC05-J | low | unlikely | high | {color:green}{*}P1{*}{color} | {color:green}{*}L3{*}{color} |
h2. Related Guidelines
| [ISO/IEC TR 24772:2010|http://www.aitcnet.org/isai/] | "Memory Leak \[XYL\]" |
| [MITRE CWE|http://cwe.mitre.org/] | [CWE ID 401|http://cwe.mitre.org/data/definitions/401.html], "Improper Release of Memory Before Removing Last Reference ('Memory Leak')" |
h2. Bibliography
| \[[API 2006|AA. Bibliography#API 06]\] | Class Vector, Class WeakReference |
| \[[Bloch 2008|AA. Bibliography#Bloch 08]\] | Item 6: Eliminate obsolete object references |
| \[[Commes 2007|AA. Bibliography#Commes 07]\] | Memory Leak Avoidance |
| \[[Goetz 2005|AA. Bibliography#Goetz 05]\] | Lapsed listeners |
| \[[Goetz 2005b|AA. Bibliography#Goetz 05b]\] | "Memory leaks with global Maps" and "Reference queues" |
| \[[Gupta 2005|AA. Bibliography#Gupts 05]\] | |
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[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|jg:MSC04-J. Store passwords using a hash function] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|49. Miscellaneous (MSC)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|MSC06-J. Do not place a semicolon on the same line as an if, for, or while statement]
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