单例模式--饿汉（按需创建）、懒汉（多线程以及多线程下改进）

代码注释有介绍，（排版过于难受，下次用md排完再传上来）

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:15
 * @description: 懒汉模式
 */
public class SingleTon1 {
    public SingleTon1(){}
    private static SingleTon1 instance1 = null;

    public static SingleTon1 getInstance1(){
        if (instance1==null){
            instance1 = new SingleTon1();
        }
        return instance1;
    }
}

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:21
 * @description: 饿汉模式
 */
public class SingleTon2
{
    private SingleTon2(){}
    private static final SingleTon2 instance2 = new SingleTon2();

    public static SingleTon2 getInstance2(){
        return instance2;
    }
}

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:32
 * @description: 解决多线程问题(在创建单例时加锁)
 */
public class SingleTon3 {
    private SingleTon3(){}
    private static SingleTon3 instance3 = null;
    public static synchronized SingleTon3 getInstance3(){
        if (instance3 == null){
            instance3 = new SingleTon3();
        }
        return instance3;
    }
}

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:36
 * @description: 在方法上加锁效率低下；可以先判断是否有创建过单例，如果没有在加锁创建
 */
public class SingleTon4 {

    private SingleTon4(){}

    private static final Object myLocker = new Object();

    private static SingleTon4 instance4 = null;

    public static SingleTon4 getInstance4(){
        if (instance4 == null){
            synchronized (myLocker){
                instance4 = new SingleTon4();
            }
        }
        return instance4;
    }
}

SingleTon按需创建：解决饿汉式过早创建单例的问题；当需要创建式，再通过静态内部类取创建。

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:52
 * @description: 解决SingleTon2;创建过早问题
 */
public class SingleTon5 {
    SingleTon5() {
    }
    private static class Nested {
        private final static SingleTon5 INSTANCE5 = new SingleTon5();
    }
    public static SingleTon5 getInstance(){
        return Nested.INSTANCE5;
    }

}

这是SingleTon1（懒汉式）时会出现的问题。