一，懶漢式

優點：執行緒安全

缺點：類載入時就初始化了，沒有實現懶載入。

二，餓漢式（執行緒不安全的形式）

優點：懶載入

缺點：執行緒不安全

public class SingleTonSecond {

    private static 
 
SingleTonSecond singleTonSecond;

    private SingleTonSecond() {

    }

    public static SingleTonSecond getInstance() {
        if (null == singleTonSecond) {
            singleTonSecond = new SingleTonSecond();
        }
        return singleTonSecond;
    }
}

三，懶漢式（執行緒安全之同步方法）

優點：執行緒安全，懶載入

    缺點：每次獲取單例都要鎖定整個方法，效能較差

public class SingleTonThird {

    private static SingleTonThird singleTonThird;

    private SingleTonThird(){

    }
    public synchronized static SingleTonThird getInstance(){
        if(null==singleTonThird){
            singleTonThird=new SingleTonThird();
        }
        return singleTonThird
 
;
    }

}

四，懶漢式（執行緒安全之同步程式碼塊）------推薦

    優點：執行緒安全，只在第一次獲取單例時鎖定了單例物件。

推薦!!!!!!!!!

public class SingleTonFourth {

    private volatile static SingleTonFourth singleTonFourth;

    private SingleTonFourth(){

    }
    public static SingleTonFourth getInstance(){
        if(null==singleTonFourth){
            synchronized (singleTonFourth){
                if (null==singleTonFourth){
                    singleTonFourth=new SingleTonFourth();
                }
            }
        }
        return singleTonFourth;
    }
}

五，靜態內部類方式-----------推薦!!!!!!!!

    優點：執行緒安全，懶載入，效率高靜態內部類的凡是在單例類在被JVM載入到記憶體時並不會立即例項化，而是在第一次呼叫

的getInstance（）方法時才會將SingleTonFifthInner類載入到記憶體，從而例項化SingleTonFifthInner的靜態屬性 

SINGLETON，才開始載入例項化SINGLETON。並且在例項化SINGLETON的時候，JVM幫助我們保證了執行緒的安全性。

public class SingleTonFifth {

    private SingleTonFifth() {

    }

    public static SingleTonFifth getInstance() {
        return SingleTonFifthInner.SINGLETON;
    }

    private static class SingleTonFifthInner {
        private static final SingleTonFifth SINGLETON = new SingleTonFifth();
    }

}

六，列舉型別實現單例模式

public class SingleTonSixth {

    private SingleTonSixth(){

    }

    public static SingleTonSixth getInstance(){
        return SingleTonEnum.ISNTANCE.getInstancee();
    }

    private static enum SingleTonEnum{
        ISNTANCE;

        private  SingleTonSixth singleTonSixth;

        private  SingleTonEnum(){
            singleTonSixth=new SingleTonSixth();
        }

        private  SingleTonSixth getInstancee(){
            return singleTonSixth;
        }

    }
}