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 adversaries with a potentially exploitable denial of service attack, and consequently are forbidden.
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 leaks one {{vector}} 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 (1)
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 (2)
Prefer the use of standard language semantics where possible, as shown in this compliant solution.
{code:bgColor=#ccccff}
public void useVector(int count) {
for (int n = 0; n < count; n++) {
vector.add(Integer.toString(n));
}
// ...
while (!vector.isEmpty()){
vector.removeElementAt(vector.size() - 1);
}
}
{code}
h2. Compliant Solution (3)
An alternative way of clearing the {{vector}} is to use the {{vector.clear()}} method.
{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.
{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_, 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 will never executed in the event of an exception thrown by the {{reader.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 the {{pop}}-ed 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 even after the element is pop'ed. Such _obsolete references_ remain live, and consequently cannot be garbage collected.
h2. Compliant Solution (Assign {{null}} to Elements of Data Structures)
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 include individual element-objects in its list of objects to free. A {{NullPointerException}} results on subsequent attempts to access the particular object.
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-sized 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 variant of obsolete objects is unintentional retention of objects in {{Collections}} such as {{Map}}s. 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}
Although the metadata is {{transient}} by nature, it persists after the relevant socket is closed, until {{removeTempConnection()}} is invoked. 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 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 they 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}
Soft references are preferred over weak references for applications such as caching because weak references are garbage collected more aggressively, to the point that they become unsuitable.
h2. Risk Assessment
Memory leaks in Java applications may be exploited to cause denial of service.
|| GuidelineRule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| MSC06-J | low | unlikely | high | {color:green}{*}P1{*}{color} | {color:green}{*}L3{*}{color} |
h3. Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+MSC01-J].
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"
\[[Gupta 2005|AA. Bibliography#Gupts 05]\]
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 401|http://cwe.mitre.org/data/definitions/401.html] "Improper Release of Memory Before Removing Last Reference ('Memory Leak')"
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[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|MSC05-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!|MSC07-J. Minimize the scope of the SuppressWarnings annotation]
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