《23种设计模式（Java版）》| 单例模式（内附源码案例）。

//饿汉式（静态变量）
class Singleton{
	//1.构造器私有化 
	private Singleton() {
		
	}
	//2.在本类内部创建对象实例
	private final static Singleton instance=new Singleton();
	
	//3.提供一个公有的静态方法，返回实例对象
	public static Singleton getInstance() {
		return instance;
	}
}

//饿汉式（静态代码块）
class Singleton{
	//1.构造器私有化 
	private Singleton() {
		
	}
	//2.在本类内部创建对象实例
	private static Singleton instance;
	
	static {
		///在静态代码块中，创建单例对象
		instance=new Singleton();
	}
	
	//3.提供一个公有的静态方法，返回实例对象
	public static Singleton getInstance() {
		return instance;
	}
}

// 懒汉式(线程不安全)
class Singleton{
	private static Singleton instance;
	
	private Singleton() {
	
	}
	
	//提供一个静态的公有方法，当方法使用时，才去创建对象
	public static Singleton getInstance() {
		if(instance==null) {
			instance=new Singleton();
		}
		return instance;
	}
}

//懒汉式(线程程同步)
class Singleton{
	private static Singleton instance;
	
	private Singleton() {}
	
	//提供一个静态的公有方法，加入同步机制，解决线程安全
	public static synchronized Singleton getInstance() {
		if(instance==null) {
			instance=new Singleton();
		}
		return instance;
	}
}

//懒汉式(线程安全，同步代码块)
class Singleton{
	private static Singleton instance;
	
	private Singleton() {}
	
	//提供一个静态的公有方法，加入同步机制，解决线程安全
	public static synchronized Singleton getInstance() {
		if(instance==null) {
			synchronized(Singleton.class){
				instance=new Singleton();
			}
		}
		return instance;
	}
}

//双重检查
class Singleton{
	private static volatile Singleton instance;
	
	private Singleton() {}
	
	//提供一个静态的公有方法，加入双重检查，解决线程安全，同时解决懒加载
	public static  Singleton getInstance() {
		if(instance==null) {
			synchronized (Singleton.class) {
				if(instance==null) {
					instance=new Singleton();
				}
			}
		}
		return instance;
	}
}

//静态内部类 推荐使用
class Singleton{
	private static volatile Singleton instance;
	
	//构造器私有化
	private Singleton() {}
	
	//写静态内部类，该类中有一个静态的属性Singleton
	private static class SingletonInstance {
		private static final Singleton instance=new Singleton();
	}
	
	//提供一个静态的公有方法,直接返回静态内部类的属性
	public static  Singleton getInstance() {
	
		return SingletonInstance.instance;
	}
}

//枚举实现
enum Single{
	
	INSTANCE;
	public void sayOK() {
		System.out.println("OK");
	}
}

public class Runtime {
	//类内部创建对象实例化
    private static Runtime currentRuntime = new Runtime();

    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance
     * methods and must be invoked with respect to the current runtime object.
     *
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
     //提供静态方法返回实例对象
    public static Runtime getRuntime() {
        return currentRuntime;
    }
	//构造器私有化
    /** Don't let anyone else instantiate this class */
    private Runtime() {}
  }