实现单例的几种方式

/**
 * 饿汉式单例：在使用该类的静态成员时，无论有没有使用单例类，都会创建单例对象
 * 
 * @author wangpeng
 *
 */
public class Singleton {

	private Singleton() {
		// TODO Auto-generated constructor stub
		System.out.println("Singleton is create");
	}

	public static int staticMember = 0;

	private static Singleton instance = new Singleton();

	public static Singleton getInstance() {
		return instance;
	}

	public static void createString() {
		System.out.println("createString in Singleton");
	}

	public static void main(String[] args) {
		// 输出Singleton is create和createString in Singleton
		Singleton.staticMember = 1;
		// Singleton.createString();
	}
}

/**
 * 懒汉式单例：通过判断单例对象是否为null和加同步关键字，保证只在第一次使用单例类创建单例类，避免创建重量级单例类慢的问题。在高并发情况下，同步关键字的使用可能会降低性能
 * 
 * @author wangpeng
 *
 */
public class LazySingleton {

	private LazySingleton() {
		System.out.println("LazySingleton is create");
	}

	public static int staticMember = 0;

	private static LazySingleton instance = null;

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

	public static void createString() {
		System.out.println("createString in Singleton");
	}

	public static void main(String[] args) {
		// 只输出createString in Singleton
		LazySingleton.staticMember = 1;
		LazySingleton.createString();
	}
}

/**
 * 使用内部类来实现单例：（1）由于类加载是单线程的，故可以替代synchronized关键字解决并发问题。
 * （2）当单例类加载时，其内部类不会初始化。只当调用getInstance方法时，才会创建单例类
 * 
 * @author wangpeng
 *
 */
public class StaticSingleton {

	private StaticSingleton() {
		// TODO Auto-generated constructor stub
		System.out.println("StaticSingleton is create");
	}

	/**
	 * 持有单例对象的内部类
	 */
	private static class SingletonHolder {
		private static StaticSingleton instance = new StaticSingleton();
	}

	public static StaticSingleton getInstance() {
		return SingletonHolder.instance;
	}

	public static void createString() {
		System.out.println("createString in Singleton");
	}

	public static void main(String[] args) {
		StaticSingleton.getInstance();
	}
}