创建型模式-单例

单例模式
结构图


模式说明
1.单例模式一般是无状态类。

2.保证一个类仅有一个实例，并提供一个访问它的全局访问点。

客户端

public static void main(String[] args) {
		Singleton instance = Singleton.getInstance();
		ConcurrentSingleton inst1 = ConcurrentSingleton.getInstance();
		DoubleCheckSingleton inst2 = DoubleCheckSingleton.getInstance();
		StaticInitializationSingleton inst3 = StaticInitializationSingleton
				.getInstance();
}

类设计

public class ConcurrentSingleton {


	private static ConcurrentSingleton instance;
	private static Object lock = new Object();


	private ConcurrentSingleton() {
	}


	public static ConcurrentSingleton getInstance() {
		synchronized (lock) {
			if (instance == null) {
				instance = new ConcurrentSingleton();
			}
		}
		return instance;
	}


}


public class DoubleCheckSingleton {
	private static DoubleCheckSingleton instance;
	private static Object lock = new Object();


	private DoubleCheckSingleton() {
	}


	public static DoubleCheckSingleton getInstance() {
		if (instance == null) {
			synchronized (lock) {
				if (instance == null) {
					instance = new DoubleCheckSingleton();
				}
			}
		}
		return instance;
	}
}


public class StaticInitializationSingleton {
	private final static StaticInitializationSingleton instance = new StaticInitializationSingleton();


	private StaticInitializationSingleton() {
	}


	public static StaticInitializationSingleton getInstance() {
		return instance;
	}
}