单例模式------确保一个类只有一个实例，并提供一个全局访问点。

类图：

1、懒汉式

/**

* 懒汉式

* @author 码农

*

*/

public class LazySingleton {

private static LazySingleton uniqueInstance;

private LazySingleton(){}

public static LazySingleton getInstance(){

if(uniqueInstance==null){

uniqueInstance=new  LazySingleton();

}

return uniqueInstance;

}

}

2、饿汉式

/**

* 饿汉式

* @author 码农

*

*/

public class HungrySingleton {

private static HungrySingleton uniqueInstance=new HungrySingleton();

private HungrySingleton(){}

public static HungrySingleton getInstance(){

return uniqueInstance;

}

}

3、线程安全模式

/**

* 线程安全模式

* @author 码农

*

*/

public class SynchronizedSingleton {

private static SynchronizedSingleton uniqueInstance;

private SynchronizedSingleton(){}

public static synchronized SynchronizedSingleton getInstance(){

if(uniqueInstance==null){

uniqueInstance=new SynchronizedSingleton();

}

return uniqueInstance;

}

}

4、双重枷锁模式

/**

* 双重加锁模式

* @author zyzhmily

*

*/

public class DoubleLockSingleton {

private volatile static DoubleLockSingleton uniqueInstance;

private DoubleLockSingleton(){}

      public static DoubleLockSingleton getInstance(){

    if(uniqueInstance==null){

    synchronized(DoubleLockSingleton.class){

    if(uniqueInstance==null){

    uniqueInstance=new DoubleLockSingleton();

    }

    }

    }

    return uniqueInstance;

}

}