单例模式的五种写法

下面给出单例模式的五种写法：

package java面试宝典;

public class Singleton1 {
	/*
	 * 推荐
	 */
	
	private static final Singleton1  instance=new Singleton1();
	public static Singleton1 getInstance(){
		return instance;
	}
	
}

package java面试宝典;

public class Singleton2 {
	/*
	 * 也不错
	 */
	private static class lazy{
		public static final Singleton2 instance=new Singleton2();
	}
	public static Singleton2 getInstance(){
		return lazy.instance;
	}
}

package java面试宝典;

public class Singleton3 {
	/*
	 * 线程安全，但是锁是重量级锁，效率低
	 */
	private static Singleton3 instance=null;
	public static synchronized Singleton3 getInstance(){
		if(instance==null){
			instance=new Singleton3();
		}
		return instance;
	}
}

package java面试宝典;

public class Singleton4 {
	/*
	 * 双重判断加锁，效率比第三个更高，
	 */
	private static Singleton4 instance=null;
	public static Singleton4 getInstance(){
		if(instance==null){
			synchronized(Singleton4.class){
				if(instance==null){
					instance=new Singleton4();
				}
			}
		}
		return instance;
	}
	
}

package java面试宝典;

public class Singleton5 {
	/*
	 * 非线程安全，不好
	 */
	private static Singleton5 instance=null;
	public static Singleton5 getInstance(){
		if(instance==null){
			instance=new Singleton5();
		}
		return instance;
	}
}