Instead of initializing a member object using a constructor, sometimes a technique called lazy initialization is used to defer the construction of the member object until an instance is actually required. Lazy initialization also helps in breaking harmful circularities in class and instance initialization, and performing other optimizations \[[Bloch 05|AA. Java References#Bloch 05]\].
A class or an instance method is used for lazy initialization, depending on whether the member object is {{static}} or not. The method checks whether the instance has already been created and if not, creates it. If the instance already exists, it simply returns it. This is shown below:
{code:bgColor=#ccccff}
// Correct single threaded version using lazy initialization
final class Foo {
private Helper helper = null;
public Helper getHelper() {
if (helper == null) {
helper = new Helper();
}
return helper;
}
// ...
}
{code}
In a multithreading scenario, the initialization must be synchronized so that two or more threads do not create multiple instances of the member object. The code shown below is safe for execution in a multithreaded environment, albeit slower than the previous, single threaded code example.
{code:bgColor=#ccccff}
// Correct multithreaded version using synchronization
final class Foo {
private Helper helper = null;
public synchronized Helper getHelper() {
if (helper == null) {
helper = new Helper();
}
return helper;
}
// ...
}
{code}
The double checked locking (DCL) idiom is used to provide lazy initialization in multithreaded code. In a multithreading scenario, traditional lazy initialization is supplemented by reducing the cost of synchronization for each method access by limiting the synchronization to the case where the instance is required to be created and forgoing it when retrieving an already created instance.
The double-checked locking pattern uses block synchronization instead of method synchronization. It strives to make the previous code example faster by installing aan additional {{null}} check before attempting to synchronizesynchronization. This makes a potentially expensive synchronization necessary only for initialization, and dispensable for the common case of retrieving the value of {{helper}}. The noncompliant code example shows the originally proposed DCL pattern.
h2. Noncompliant Code Example
This noncompliant code example uses the incorrect form of the double checked locking idiom.
{code:bgColor=#FFCCCC}
// "Double-Checked Locking" idiom
final class Foo {
private Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized (this) {
if (helper == null) {
helper = new Helper();
}
}
}
return helper;
}
// Other methods and members...
}
{code}
According to the Java Memory Model (discussion reference) \[[Pugh 04|AA. Java References#Pugh 04]\]:
{quote}
... writes that initialize the {{Helper}} object and the write to the {{helper}} field can be done or perceived out of order. As a result, a thread which invokes {{getHelper()}} could see a non-null reference to a {{helper}} object, but see the default values for fields of the {{helper}} object, rather than the values set in the constructor.
Even if the compiler does not reorder those writes, on a multiprocessor the processor or the memory system may reorder those writes, as perceived by a thread running on another processor.
{quote}
This makes the originally proposed double-checked locking pattern insecure. The guideline [CON26-J. Do not publish partially initialized objects] further discusses the possibility of a non-null reference that refers to a partially initialized object.
h2. Compliant Solution ({{volatile}})
This compliant solution declares the {{Helper}} object as {{volatile}} and consequently, uses the correct form of the double-checked locking idiom.
{code:bgColor=#ccccff}
// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
final class Foo {
private volatile Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized (this) {
if (helper == null) {
helper = new Helper(); // If the helper is null, create a new instance
}
}
}
return helper; // If helper is non-null, return its instance
}
}
{code}
If a thread initializes the {{Helper}} object, a [happens-before relationship|BB. Definitions#happens-before order] is established between this thread and another that retrieves and returns the instance. \[[Pugh 04|AA. Java References#Pugh 04]\] and \[[Manson 04|AA. Java References#Manson 04]\]
"Today, the double-check idiom is the technique of choice for lazily initializing an instance field. While you can apply the double-check idiom to {{static}} fields as well, there is no reason to do so: the lazy initialization holder class idiom is a better choice." \[[Bloch 08h2. Compliant Solution (static initialization)
This compliant solution initializes the {{helper}} field in the declaration of the {{static}} variable \[[Manson 06|AA. Java References#BlochReferences#Manson 0806]\].
h2. Compliant Solution (static initialization)
This compliant solution initializes the {{helper}} field in the declaration of the {{static}} variable \[[Manson 06|AA. Java References#Manson 06]\].
{code:bgColor=#ccccff}
final class Foo {
private static final Helper helper = new Helper();
public static Helper getHelper() {
return helper;
}
{code:bgColor=#ccccff}
final class Foo {
private static final Helper helper = new Helper();
public static Helper getHelper() {
return helper;
}
}
{code}
Variables that are declared {{static}} and initialized at declaration or from a static initializer, are guaranteed to be fully constructed before being made visible to other threads. This approach should not be confused with eager initialization because in this case, the Java Language Specification guarantees lazy initialization of the class when it is first used \[[JLS 05 before being made visible to other threads. This approach should not be confused with eager initialization because in this case, the Java Language Specification guarantees lazy initialization of the class when it is first used \[[JLS 05|AA. Java References#JLS 05]\].
"Today, the double-check idiom is the technique of choice for lazily initializing an instance field. While you can apply the double-check idiom to {{static}} fields as well, there is no reason to do so: the lazy initialization holder class idiom is a better choice." \[[Bloch 08|AA. Java References#JLSReferences#Bloch 0508]\].
h2. Compliant Solution (initialize-on-demand holder class idiom)
This compliant solution uses the initialize-on-demand holder class idiom that implicitly incorporates lazy initialization. It uses a {{static}} variable as suggested in the previous compliant solution and declares it within a {{static}} inner class, {{Holder}}.
{code:bgColor=#ccccff}
final class Foo {
// Lazy initialization
private static class Holder {
static Helper helper = new Helper();
}
public static Helper getInstance() {
return Holder.helper;
}
}
{code}
Initialization of the {{Holder}} class is deferred until the {{getInstance()}} method is called, following which the {{helper}} field is initialized. The only limitation of this method is that it works only for {{static}} fields and not for instance fields \[[Bloch 01|AA. Java References#Bloch 01]\]. This idiom is a better choice than the double checked locking idiom for lazily initializing {{static}} fields \[[Bloch 08|AA. Java References#Bloch 08]\].
h2. Compliant Solution ({{ThreadLocal}} storage)
This compliant solution (originally suggested by Alexander Terekhov \[[Pugh 04|AA. Java References#Pugh 04]\]) uses a {{ThreadLocal}} object to lazily create a {{Helper}} instance.
{code:bgColor=#ccccff}
class Foo {
// If perThreadInstance.get() returns a non-null value, this thread
// has done synchronization needed to see initialization of helper
private final ThreadLocalThreadLocal<Object> perThreadInstance = new ThreadLocalThreadLocal<Object>();
private Helper helper = null;
public Helper getHelper() {
if (perThreadInstance.get() == null) {
createHelper();
}
return helper;
}
private final synchronized void createHelper() {
synchronized(this) {
if (helper == null) {
helper = new Helper();
}
// Any non-null value would do as the argument here
perThreadInstance.set(perThreadInstancethis);
}
}
}
{code}
h2. Compliant Solution ({{java.util.concurrent}} utilities)
This compliant solution uses an {{AtomicReference}} wrapper around the {{Helper}} object. It uses the standard {{compareAndSet}} (CAS) functionality to set a newly created {{Helper}} object if {{helperRef}} is {{null}}. Otherwise, it simply returns the already created instance. (Tom Hawtin, [JMM Mailing List|https://mailman.cs.umd.edu/mailman/private/javamemorymodel-discussion/2006-December/000067.html])
{code:bgColor=#ccccff}
// Uses atomic utilities
final class Foo {
private final AtomicReference<Helper> helperRef =
new AtomicReference<Helper>();
public Helper getHelper() {
Helper helper = helperRef.get();
if (helper != null) {
return helper;
}
Helper newHelper = new Helper();
return helperRef.compareAndSet(null, newHelper) ?
newHelper :
helperRef.get();
}
}
{code}
While this code ensures that only one {{Helper}} object is prevented from being garbage collected, it allows multiple {{Helper}} objects to be created. If constructing multiple {{Helper}} objects is infeasible or expensive, this solution may not be inappropriateappropriate.
{mc} Ok, so this won't work with weak refs because the refs are still strong. Even if not, helper may be mistakenly assigned null. ~DM
For example, if there is only a limited amount of memory available, this approach may cause an {{OutOfMemoryError}} unless a weak reference is used to hold {{Helper}}.
{mc}
h2. Compliant Solution ({{immutable}})
In this compliant solution the {{Foo}} class is unchanged, but the {{Helper}} class is made [immutable|BB. Definitions#immutable]. Consequently, the {{Helper}} class is guaranteed to be fully constructed before becoming visible. The object must be truly immutable; it is not sufficient for the program to refrain from modifying the object.
{code:bgColor=#CCCCFF}
public final class Helper {
private final int n;
public Helper(int n) {
this.n = n;
}
// Other fields and methods, all fields are final
}
final class Foo {
private Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized (this) {
if (helper == null) {
helper = new Helper(); // If the helper is null, create a new instance
}
}
}
return helper; // If helper is non-null, return its instance
}
}
{code}
Note that if class {{Foo}} were mutable, the {{Helper}} field would need to be declared {{volatile}} as recommended in [CON09-J. Ensure visibility of shared references to immutable objects]. Also, the {{getHelper()}} method is an instance method and the accessibility of the {{helper}} field is {{private}}. This allows safe publication of the {{Helper}} object, in that, a thread cannot observe a partially initialized {{Foo}} object ([CON26-J. Do not publish partially initialized objects]). The class {{Helper}} is also compliant with [CON26-J. Do not publish partially initialized objects] and consequently, it cannot be observed to be in a partially initialized state.
h2. Exceptions
*EX1:* Explicitly synchronized code (that uses method synchronization or proper block synchronization, enclosing all initialization statements) does not require the use of double-checked locking.
*EX2:* "Although the \[noncompliant form of the\] double-checked locking idiom cannot be used for references to objects, it can work for 32-bit primitive values (e.g., int's or float's). Note that it does not work for long's or double's, since unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic." \[[Pugh 04|AA. Java References#Pugh 04]\]. (See [CON25-J. Ensure atomicity when reading and writing 64-bit values] for more information.)
h2. Risk Assessment
Using incorrect forms of the double checked locking idiom can lead to synchronization issues.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON22- J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |
h3. Automated Detection
TODO
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+CON43-J].
h2. References
\[[API 06|AA. Java References#API 06]\]
\[[JLS 05|AA. Java References#JLS 05]\] 12.4 "Initialization of Classes and Interfaces"
\[[Pugh 04|AA. Java References#Pugh 04]\]
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 48: "Synchronize access to shared mutable data"
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 71: "Use lazy initialization judiciously"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"
----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON21-J. Use thread pools to enable graceful degradation of service during traffic bursts] [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)] [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON23-J. Address the shortcomings of the Singleton design pattern]
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