The singleton design pattern's intent is succinctly described by the seminal work of Gamma et al. [Gamma 1995]:
Ensure a class only has one instance, and provide a global point of access to it.
Because there is only one Singleton instance, "any instance fields of a Singleton will occur only once per class, just like static fields. Singletons often control access to resources such as database connections or sockets" [Fox 2001]. Other applications of singletons involve maintaining performance statistics, system monitoring and logging, implementing printer spoolers, or even ensuring that only one audio file plays at a time. Classes that contain only static methods are good candidates for the Singleton pattern.
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This design pattern is often implemented incorrectly. Refer to rule LCK10-J. Do not use incorrect forms Use a correct form of the double-checked locking idiom for more details on the correct use of the double-checked locking idiom.
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This is known as the initialize-on-demand holder class idiom. Refer to rule LCK10-J. Do not use incorrect forms Use a correct form of the double-checked locking idiom for more information.
Noncompliant Code Example (Serializable)
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"If a singleton contains a nontransient object reference field, the contents of this field will be deserialized before the singletonâs readResolve method is run. This allows a carefully crafted stream to 'steal' a reference to the originally deserialized singleton at the time the contents of the object reference field are deserialized" [Bloch 2008].
Compliant Solution (Enumeration Types)
Stateful singleton classes must be nonserializable. As a precautionary measure, classes that are serializable must not save a reference to a singleton object in their nontransient or nonstatic instance variables. This prevents the singleton from being indirectly serialized.
Bloch [Bloch 2008] suggests the use of an enumeration type as a replacement for traditional implementations when serializable singletons are indispensable.
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When the singleton class must indirectly implement the Cloneable interface through inheritance, the object's clone() method must be overridden with one that throws a CloneNotSupportedException exception [Daconta 2003].
| Code Block | ||
|---|---|---|
| ||
class MySingleton implements Cloneable {
private static MySingleton instance;
private MySingleton() {
// private constructor prevents instantiation by untrusted callers
}
// Lazy initialization
public static synchronized MySingleton getInstance() {
if (instance == null) {
instance = new MySingleton();
}
return instance;
}
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
}
|
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This compliant solution demonstrates this technique. It prints a consistent hash code across all scopes. It uses the ObjectPreserver class [Grand 2002] described in rule TSM02-J. Do not use background threads during class initialization.
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Bibliography
Item 3. Enforce the singleton property with a private constructor or an enum type; and Item 77. For instance control, prefer enum types to | |
Item 15. Avoiding singleton pitfalls | |
9.10 Enforcing the Singleton Pattern | |
[Fox 2001] | |
Singleton | |
Chapter 5, Creational Patterns, Singleton | |
[JLS 2005] |
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