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{quote}
Ensure a class only has one instance, and provide a global point of access to it.
{quote}

"Since 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." \[[Fox 01|AA. Java References#Fox 01]\]. Other applications of singletons involve maintaining performance statistics, system monitoring and logging, implementing printer spoolers or as simple as ensuring that only one audio file plays at a time. 

A typical implementation of the Singleton pattern in Java is the creation of a single instance of the

 singleton class that encloses a {{private static}} instance field.

 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.

h2.

 Noncompliant Code

}
    return INSTANCE;
  }
}
{code}

This design pattern is often implemented incorrectly. Refer to [CON22-J. Do not use incorrect forms of the double-checked locking idiom] for more details on the double-checked locking idiom.

h2. Noncompliant Code Example (Serializable singleton)

This noncompliant code example implements the {{java.io.Serializable}} interface which allows the class to be serializable. Deserialization of the class implies that multiple instances of the singleton can be created.

{code:bgColor=#FFcccc}
class MySingleton implements Serializable {
  private static final long serialVersionUID = 6825273283542226860L;
  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;
  }
}
{code}

A singleton's constructor cannot enforce the requirement that the number of instances be limited to one by installing any checks because serialization provides a mechanism to bypass the object's constructor.

h2. Compliant Solution (1) ({{readResolve}} method)

It is recommended that stateful singleton classes be made non-serializable. As a precautionary measure, classes that are serializable must never save a reference to a singleton object in their instance variables. This prevents the singleton from being indirectly serialized. The {{getInstance}} method should be used instead, whenever access to the object is required.

If making a singleton class serializable is indispensable, ensure that only one instance of the class exists by adding a {{readResolve()}} method which can be made to return the original instance. The phantom instance obtained after deserialization is left to the judgment of the garbage collector. \[[Bloch 08|AA. Java References#Bloch 08]\]

{code:bgColor=#ccccff}
private Object readResolve() {
  return INSTANCE;
}
{code} 

If the serializable singleton class has any other instance fields, they must be declared {{transient}} to be compliant (described later in the nontransient istance fields noncompliant code example). 

h2. Compliant Solution (2) ({{enum}} types)

Bloch \[[Bloch 08|AA. Java References#Bloch 08]\] suggests the use of an {{enum}} type as a replacement for traditional implementations. 

{code:bgColor=#ccccff}
public enum MySingleton {
  INSTANCE;
  // Other methods
}
{code}

Functionally, this approach is equivalent to commonplace implementations and is safer. It ensures that only one instance of the object exists at any instant and also provides the serialization property as {{java.lang.Enum<E>}} extends {{Serializable}}.


h2. Noncompliant Code Example (nontransient instance fields)

This serializable noncompliant code example uses a nontransient instance field {{str}}.

{code:bgColor=#FFcccc}
class MySingleton implements Serializable {
  private static final long serialVersionUID = 2787342337386756967L;
  private static MySingleton INSTANCE;
  private String[] str = {"one", "two", "three"}; // nontransient instance field
                 
  private MySingleton() {
    // private constructor prevents instantiation by untrusted callers
  }

  public void displayStr() {
    System.out.println(Arrays.toString(str));
  }
 
  private Object readResolve() {
    return INSTANCE;
  }
}
{code}

"If a singleton contains a nontransient object reference field, the contents of this field will be deserialized before the singleton’s {{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." \[[Bloch 08|AA. Java References#Bloch 08]\].

h2. Compliant Solution (1) (transient fields)

This compliant solution declares the {{str}} instance field {{transient}} so that it is not serialized.

{code:bgColor=#ccccff}
class MySingleton implements Serializable {
  // ...
  private transient String[] str = {"one", "two", "three"}; // nontransient field
  // ...
}
{code}

h2. Compliant Solution (2) ({{enum}} types, non-transient fields)

This compliant solution uses the {{enum}} type to ensure that only one instance of the singleton exists at any time.

{code:bgColor=#ccccff}
public enum MySingleton {
  INSTANCE;
  private String[] str = {"one", "two", "three"}; // nontransient field
     
  public void displayStr() {
    System.out.println(Arrays.toString(str));
  }	 
}
{code}

h2. Noncompliant Code Example (Cloneable singleton)

This noncompliant code example shows a singleton that implements the {{java.lang.Cloneable}} interface.

{code:bgColor=#FFcccc}
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;
  }
}
{code}

h2. Compliant Solution (override {{clone}} method)

It is also possible to create a copy of the singleton by cloning it using the object's {{clone()}} method if the singleton class implements {{java.lang.Cloneable}} directly or through inheritance. To address this issue, do not make the class cloneable by not implementing the interface or avoiding to derive from a class that already implements it. 

If the singleton class indirectly implements the {{Cloneable}} interface through inheritance, override the object's {{clone()}} method and throw a {{CloneNotSupportedException}} exception from within it. \[[Daconta 03|AA. Java References#Daconta 03]\]

{code:bgColor=#ccccff}
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();
  }
}
{code}

See [MSC05-J. Make sensitive classes noncloneable] for more details about restricting the {{clone()}} method.


h2. 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.  
{mc} what kind of an NCE do we show here? {mc}

h2. 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 \[[Patterns 02|AA. Java References#Patterns 02]\]). 

{code:bgColor=#ccccff}
public class ObjectPreserver implements Runnable {
  private static ObjectPreserver lifeLine = new ObjectPreserver();
  
  // Neither this class, nor HashSet will be garbage collected.
  // References from HashSet to other objects will also exhibit this property
  private static HashSet protectedSet = new HashSet();
 
  private ObjectPreserver() {
    new Thread(this).start();  // keeps the reference alive
  }
 
  public synchronized void run(){
    try {
      wait();
    } catch(InterruptedException e) { /* Forward to handler */ }
  }

  // Objects passed to this method will be preserved until
  // the unpreserveObject method is called
  public static void preserveObject(Object o) {
    protectedSet.add(o);
  }

  // Unprotect the objects so that they can be garbage collected
  public static void unpreserveObject(Object o) {
    protectedSet.remove(o);
  }
}
{code} 

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. \[[Daconta 03|AA. Java References#Daconta 03]\]

h2. 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.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON23- J | low | unlikely | medium | {color:green}{*}P2{*}{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+CON32-J].


h2. 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/]&nbsp;
\[[Daconta 03|AA. Java References#Daconta 03]\] 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" and Item 77: "For instance control, prefer enum types to readResolve"
\[[MITRE 09|AA. Java References#MITRE 09]\] [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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[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON22-J. Do not use incorrect forms of the double-checked locking idiom]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON24-J. Use a unique channel to acquire locks on any file]