一．单件模式一般实现

public class Singleton {	private static Singleton uniqueInstance;	private Singleton() {}	public static Singleton getInstance() {		if (uniqueInstance == null) {			uniqueInstance = new Singleton();		}		return uniqueInstance;	}}

public class Singleton {	private static Singleton uniqueInstance;	private Singleton() {}	public synchronized static Singleton getInstance() {		if (uniqueInstance == null) {			uniqueInstance = new Singleton();		}		return uniqueInstance;	}}

 问题：性能降低

A． 如何getInstance()的性能对应用程序不是关键，就什么都不要做．同步可能使应用程序的执行效率降低100倍，但如果此方法不是被频繁的调用，则不用修改．因为同步的getInstance()方法既简单又有效．

B．使用＂急切＂创建实例，而不用延迟化的实例的方法

public class Singleton {	private static Singleton uniqueInstance = new Singleton();	public synchronized Singleton getInstance() {		return uniqueInstance;	}}

 C．使用＂双重检查加锁＂，尽量减少使用同步：如果性能是你关心的，此方法可大大减少时间消耗

public class Singleton {	private static volatile Singleton uniqueInstance;	public static Singleton getInstance() {		if (uniqueInstance == null) {			synchronized (Singleton.class) {				if (uniqueInstance == null) {					uniqueInstance = new Singleton();				}			}		}		return uniqueInstance;	}}