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The instance can be created using lazy initialization, which means that the instance is not created when the class loads but when it is first used.

Noncompliant Code Example

When the getter method is called by two (or more) threads or classes simultaneously, multiple instances of the Singleton class might result if access is not synchronized.

class MySingleton {
  private static MySingleton _instance;

  private MySingleton() {
    // construct object 
    // private constructor prevents instantiation by outside callers
  }

  // lazy initialization
  // error, no synchronization on method access

  public static MySingleton getInstance() {
    if (_instance == null) {
     _instance = new MySingleton();
    }
    return _instance;
  }

  // Remainder of class definition 
}

Noncompliant Code Example

Multiple instances can be created even if a synchronized(this) block is added to the constructor call.

// Also an error, synchronization does not prevent
// two calls of constructor.
public static MySingleton getInstance() {
 if (_instance == null) {
   synchronized (MySingleton.class) {
     _instance = new MySingleton();
   }
 }
 return _instance;
}

Compliant Solution

To avoid these issues, make getInstance() a synchronized method.

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Applying a static modifier to the getInstance() method (which returns the Singleton) allows the method to be accessed subsequently (after the initial call) without creating a new object.

Noncompliant Code Example

Another solution for Singletons to be thread-safe is double-checked locking. Unfortunately, it is not guaranteed to work because compiler optimizations can force the assignment of the new Singleton object before all its fields are initialized.

// double-checked locking
public static MySingleton getInstance() {
 if (_instance == null) {
   synchronized (MySingleton.class) {
     if (_instance == null) {
        _instance = new MySingleton();
     }
   }
 }
}

Noncompliant Code Example

The other range of Singleton related subtleties involve object serialization and cloning. Serialization allows objects to be constructed without invoking the constructor and in turn allows object replication. It is also possible to create a copy of the Singleton object by cloning it using the object's clone method whenever the Singleton class implements Cloneable directly or through inheritance. Both these conditions violate the Singleton Design Pattern's guarantees.

Compliant Solution

It is recommended that stateful Singleton classes be made non-serializable. As a precautionary measure, (serializable) classes must never save a reference to a singleton object in its instance variables. The getInstance method should be used instead, whenever access to the object is required.

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See MSC32-J. Make sensitive classes noncloneable for more details about restricting the clone() method.

Noncompliant Code Example

When the utility of a class is over, it is free to be garbage collected. A dynamic reference can however, cause another instance of the Singleton class to be returned. This behavior can be troublesome when the program needs to maintain only one instance throughout its lifetime.

Compliant Solution

This compliant solution takes into account the garbage collection issue described above. A class is not garbage collected until the {{ClassLoader}} object used to load it itself becomes eligible for garbage collection. An easier scheme to prevent the garbage collection is to ensure that there is a direct or indirect reference from a live thread to the singleton object that needs to be preserved. This compliant solution demonstrates this method (adopted from \[[Patterns 02|AA. Java References#Patterns 02]\]). 

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To be fully compliant, it must be ensured that the class obeys the _Singleton_ pattern's design contract. It is unreasonable to use the class for anything else, for example, as a method to share global state. \[[Daconta 03|AA. Java References#Daconta 03]\]

Risk Assessment

Using lazy initialization in a Singleton without synchronizing the getInstance() method may lead to creation of multiple instances and can as a result, violate the expected contract.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html]
\[[Fox 01|AA. Java References#Fox 01]\] [When is a Singleton not a Singleton?|http://java.sun.com/developer/technicalArticles/Programming/singletons/] 
\[[Daconta 03|AA. Java References#Daconta 03]\] Item 15: Avoiding Singleton Pitfalls;
\[[Darwin 04|AA. Java References#Darwin 04]\] 9.10 Enforcing the Singleton Pattern
\[[Gamma 95|AA. Java References#Gamma 95]\] Singleton
\[[Patterns 02|AA. Java References#Patterns 02]\] Chapter 5, Creational Patterns, Singleton
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 3: "Enforce the singleton property with a private constructor or an enum type"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 543|http://cwe.mitre.org/data/definitions/543.html] "Use of Singleton Pattern in a Non-thread-safe Manner"

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