com.lasmanis.javapatterns

Class Singleton

java.lang.Object

com.lasmanis.javapatterns.Singleton

public class Singleton
extends Object

Basic singleton implementation.

This class provide the implementation of a thread-safe, generic singleton pattern. instance(java.lang.Class) is a thread-safe operation for both retreval and creation. Derived classes are responsible for initializing themselves in a thread-safe manner. If initialization is required, the code involved is contained in a synchronized block tied to the Singleton class.

Deriving classes should declare a non-public default constructor. This class assumes the existing of a default constructor (ie, takes no parameters) and uses reflection to locate the constructor if an instance of a particular derived class has not yet been instantiated. For safety reasons, derived classes should not declare any public constructors.

This class provides no facilities for calling a non-default constructor and initialization of the created instance is left to the derived class.

The expect form of a derived class looks is as follows:

 
public class IntegerContainer
    extends Singleton
{
    // instance local data
    public int a;

    // private constructor for initialization
    private IntegerContainer()
    {
        a = 1;
    }

    // convenience method
    public static IntegerContainer instance()
          throws InstantiationException,
              IllegalAccessException,
              NoSuchMethodException,
              IllegalArgumentException,
              InvocationTargetException,
              ExceptionInInitializerError,
              SecurityException
    {
        return IntegerContainer.instance(IntegerContainer.class);
    }
}
 
 

The addition of the simplified instance() is purely for the convenience of the programmer. It simplifes the invocation and reduces accidental type mis-matches. With this additional method, getting an instance is as simple as :

 
 IntegerContainer i = IntegerContainer.instance();
 
 

Author:

mpl

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	Singleton() Constructor.

Method Summary

Modifier and Type	Method and Description
static Map<Class<? extends Singleton>,Singleton>	getInstances() retrieve the instances backing store.
static <U extends Singleton> U	instance(Class<U> c) Access the instance backing the singleton

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Singleton

protected Singleton()

Constructor.

Scope is protected for instantiation control.

Method Detail

instance

public static <U extends Singleton> U instance(Class<U> c)
                                        throws InstantiationException,
                                               IllegalAccessException,
                                               NoSuchMethodException,
                                               IllegalArgumentException,
                                               InvocationTargetException,
                                               ExceptionInInitializerError,
                                               SecurityException,
                                               NullPointerException

Access the instance backing the singleton

This method returns an existing derived Singleton instance or allocates a new instance. This method uses reflection, specifically Constructor.newInstance(java.lang.Object[]) and Class.getDeclaredConstructor(java.lang.Class[]) to accomplish this. The method also assumes the existence of the default constructor taking zero parameters and will throw NoSuchMethodException if the default constructor is not found.

This is a thread-safe operation.

Type Parameters:

U - the type parameter of the derived object object.

Parameters:

c - the Class object corresponding to U.

Returns:

the derived Singleton object.

Throws:

InstantiationException - if the class that declares the underlying constructor represents an abstract class.

IllegalAccessException - if this Constructor object enforces Java language access control and the underlying constructor is inaccessible.

NoSuchMethodException - if the default constructor is not found.

IllegalArgumentException - if the number of actual and formal parameters differ; if an unwrapping conversion for primitive arguments fails; or if, after possible unwrapping, a parameter value cannot be converted to the corresponding formal parameter type by a method invocation conversion; if this constructor pertains to an enum type.

InvocationTargetException - if the underlying constructor throws an exception.

ExceptionInInitializerError - if the initialization provoked by this method fails.

SecurityException - if the security manager is present and prevents access. See Class.getDeclaredConstructor(java.lang.Class[])

NullPointerException - if c is null.

getInstances

public static Map<Class<? extends Singleton>,Singleton> getInstances()

retrieve the instances backing store.

Returns:

the map of instances