程式碼註釋有介紹，（排版過於難受，下次用md排完再傳上來）

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:15
 * @description: 懶漢模式
 */
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: 餓漢模式
 */
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: 解決多執行緒問題(在建立單例時加鎖)
 */
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: 在方法上加鎖效率低下；可以先判斷是否有建立過單例，如果沒有在加鎖建立
 */
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按需建立：解決餓漢式過早建立單例的問題；當需要建立式，再通過靜態內部類取建立。

package singleton;

/**
 * @author: ycz
 * @date: 2018/12/24 0024 22:52
 * @description: 解決SingleTon2;建立過早問題
 */
public class SingleTon5 {
    SingleTon5() {
    }
    private static class Nested {
        private final static SingleTon5 INSTANCE5 = new SingleTon5();
    }
    public static SingleTon5 getInstance(){
        return Nested.INSTANCE5;
    }

}

這是SingleTon1（懶漢式）時會出現的問題。