设计模式之单件模式

单件模式定义：确保一个类只有一个实例，并提供一个全局访问点。

简单定义：确保有且只有一个对象被创建。

1、单件模式最简单的实现（多线程下可能出现多个实例，非线程安全）：

package headfirst.singleton.classic;

// NOTE: This is not thread safe!

public class Singleton {
	private static Singleton uniqueInstance;
 
	// other useful instance variables here
 
	private Singleton() {}
 
	public static Singleton getInstance() {
		if (uniqueInstance == null) {
			uniqueInstance = new Singleton();
		}
		return uniqueInstance;
	}
 
	// other useful methods here
}

2、线程安全，但效率不高的实现：

package headfirst.singleton.threadsafe;

public class Singleton {
	private static Singleton uniqueInstance;
 
	// other useful instance variables here
 
	private Singleton() {}
 
	public static synchronized Singleton getInstance() {
		if (uniqueInstance == null) {
			uniqueInstance = new Singleton();
		}
		return uniqueInstance;
	}
 
	// other useful methods here
}

3、一种改进（双重判断）：

package headfirst.singleton.dcl;

//
// Danger!  This implementation of Singleton not
// guaranteed to work prior to Java 5
//

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

4、另一种改进（过早初始化）：

package headfirst.singleton.stat;

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

5、利用内部类解决过早初始化问题：

package headfirst.singleton.subclass;

public class Singleton {

	private Singleton() {

	}

	public static Singleton getInstance() {
		return Nested.instance;
	}

	static class Nested {
		static Singleton instance = new Singleton();
	}
}