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. For more information, see rule [MSC05-J. Do not exhaust heap space].
h2. 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.
{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}
h2. 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.
{code:bgColor=#FFCCCC}
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");
// ...
}
}
{code}
Programmers may be surprised that the {{HashMap}} persists 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. 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.
{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_ \[[Goetz 2005|AA. References#Goetz 05]\], 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 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.
{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}
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 of code
// Dereferenced, but control flow can change
button.removeActionListener(reader);
} catch (IOException e) {
// Forward to handler
}
{code}
If an exception is thrown by the {{readSomthingreadSomething()}} method, the {{removeActionListener()}} statement is never executed.
h2. Compliant Solution ({{finally}} Block)
This compliant solution uses a {{finally}} block to ensure that the {{reader}} object's reference is removed.
{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 \[[Bloch 2008|AA. References#Bloch 08]\] that continues to hold references to elements after they have been popped off the stack.
{code:bgColor=#FFCCCC}
ppublic 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_ cause objects to remain live; consequently, the objects cannot be garbage-collected.
h2. Compliant Solution ({{null}})
This compliant solution assigns {{null}} 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 referenced from the stack in its list of objects to free.
Although these examples appear trivial and 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 \[[Commes 2007|AA. References#Commes 07]\].
h2. 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.
{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 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.
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 inside container objects, such as in a {{Map}}. According to the Java API \[[API 2006|AA. References#API 06]\], 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 \[[Goetz 2005b|AA. References#Goetz 05b]\].
h2. 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:
{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);
// 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);
}
}
{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}} occurs 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) {
// 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);
}
}
{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 in a DoS attack.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| MSC04-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-401|http://cwe.mitre.org/data/definitions/401.html]. Improper release of memory before removing last reference ("memory leak") |
h2. Bibliography
| \[[API 2006|AA. References#API 06]\] | Class {{Vector}}, Class {{WeakReference}} |
| \[[Bloch 2008|AA. References#Bloch 08]\] | Item 6. Eliminate obsolete object references |
| \[[Commes 2007|AA. References#Commes 07]\] | Memory Leak Avoidance |
| \[[Goetz 2005|AA. References#Goetz 05]\] | Lapsed Listeners |
| \[[Goetz 2005b|AA. References#Goetz 05b]\] | Memory Leaks with Global Maps; Reference Queues |
| \[[Gupta 2005|AA. References#Gupts 05]\] | |
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[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|MSC03-J. Never hard code sensitive information] [!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!|MSC05-J. Do not exhaust heap space]
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