lazysingleton1.java

Java与模式 源代码

package com.javapatterns.singleton.answers;

/**
 * Only once instance of the class may be created during the execution of any given program. Instances of this class should
 * be aquired through the getInstance() method. Notice that there are no public constructors for this class.
 */

import java.util.Date;

public class LazySingleton1
{
    private static LazySingleton1 instance = null;

    // These fields help us see whether one or
    // more Singletons were created. There is usually
    // no need for them in a Singleton.
    private int instanceNumber;
    private Date creationDate;

    // Counts instantiations. Should always be  0 or 1
    private static int numberOfInstances;

    private LazySingleton1()
    {
        instanceNumber = ++numberOfInstances;
        creationDate = new Date();
    }

    public static LazySingleton1 getInstance()
    {
        if (instance == null)
        {
            synchronized(LazySingleton1.class)
            {
                if (instance == null)
                {
                    instance = new LazySingleton1();
                }
            }
        }
        return instance;
    }

    public String aboutMe()
    {
        return "Hi, I'm an instance of LazySingleton1.\n" +
        "My classloader is " + getClass().getClassLoader() + ".\n" +
        "My identity hash code " + System.identityHashCode(this) + "\n" +
        "So far, since the class was loaded, " + numberOfInstances + " instance" +
        (numberOfInstances == 1 ? "" : "s") +
        " of this Singleton class " + (numberOfInstances == 1 ? "has" : "have") +
        " been created.\nI'm instance number " + instanceNumber + ".\n" +
        "I was created at " + creationDate + "\n" +
        "If there's really only one instance of my class, " +
        "these items will be constant for all instances of LazySingleton1.";
    }
}