The double checked locking idiom is sometimes used to provide lazy initialization in multithreaded code. In a multi-threading scenario, lazy initialization refers to reducing the cost of synchronization on each method access by deferring the synchronization to the moment when the object is actually initialized.
The code shown below is correctly synchronized, albeit slower. The double-checked locking pattern strives to make it faster.
{code:bgColor=#ccccff}
// Correct multithreaded version using synchronization
class Foo {
private Helper helper = null;
public synchronized Helper getHelper() {
if (helper == null) {
helper = new Helper();
}
return helper;
}
// Other functions and members...
}
{code}
This code ensures that in a multithreaded context, only one instance of the {{Helper}} object can exist at a particular time. The double checked locking idiom eliminates the need to synchronize every time the {{getHelper()}} method is invoked, to achieve performance gains. If implemented incorrectly, it may offer no such benefits and lead to erroneous or ineffective synchronization.
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}
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
class Foo {
private Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized(this) {
if (helper == null)
helper = new Helper();
}
}
return helper;
}
// other functions and members...
}
{code}
h2. Compliant Solution (volatile)
This compliant solution declares the {{Helper}} object as {{volatile}}.
{code:bgColor=#ccccff}
// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
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}
JDK 5.0 allows a write of a {{volatile}} variable to be reordered with respect to a previous read or write. A read of a {{volatile}} variable cannot be reordered with respect to any following read or write. Because of this, the double checked locking idiom can work when {{helper}} is declared {{volatile}}. If a thread initializes the {{Helper}} object, a happens-before relationship is established between this thread and another that retrieves and returns the instance. \[[Pugh 04|AA. Java References#Pugh 04]\] and \[[Manson 04|AA. Java References#Manson 04]\]
h2. Compliant Solution (static eager initialization)
This compliant solution eagerly initializes the {{helper}} in the declaration of the {{static}} variable \[[Pugh 04|AA. Java References#Pugh 04]\] (sic).
{mc}
Please check: \[[Pugh 04|AA. Java References#Pugh 04]\] lists this as lazy initialization but it seemsis more likeactually eager initialization and so the (sic).
{mc}
{code:bgColor=#ccccff}
class Foo {
static final Helper helper = new Helper();
private Helper() {
// ...
}
public static Helper getHelper() {
return helper;
}
}
{code}
Variables declared {{static}} are guaranteed to be initialized and made visible to other threads immediately. Static initializers also exhibit these properties.
h2. Compliant Solution (static lazy initialization)
This compliant solution incorporates lazy initialization which makes it more productive. It also uses a {{static}} variable as suggested in the previous compliant solution. The variable is declared within a {{static}} inner, {{Holder}} class.
{code:bgColor=#ccccff}
class Foo {
// Lazy initialization
private static class Holder {
static Helper helper = new Helper();
}
public static Helper getInstance() {
return Holder.helper;
}
}
{code}
This idiom is called the initialize-on-demand holder class idiom. Initialization of the {{Holder}} class is deferred until the {{getInstance()}} method is called, following which the {{helper}} is initialized. The only limitation of this method is that it works only for {{static}} fields and not instance fields. \[[Bloch 01|AA. Java References#Bloch 01]\]
h2. Exceptions
*EX1:* Explicitly synchronized code does not require the use of double-checked locking.
*EX2:* "Although 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." \[[Pugh 04|AA. Java References#Pugh 04]\]
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
\[[API 06|AA. Java References#API 06]\]
\[[Pugh 04|AA. Java References#Pugh 04]\]
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 48: "Synchronize access to shared mutable data"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"
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[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON21-J. Facilitate thread reuse by using Thread Pools] [!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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