1、餓漢。線程安全，類裝載時就實例化。

public class SingleInstance {

    private SingleInstance(){}

    private static SingleInstance mSingleInstance = new SingleInstance();

    public static SingleInstance getInstance(){
        return mSingleInstance;
    }

}

2、懶漢。懶加載，效率較低。

public class SingleInstance {

    private SingleInstance(){}

    private static SingleInstance mSingleInstance;

    public static synchronized SingleInstance getInstance(){
        if (mSingleInstance == null) {
            mSingleInstance = new SingleInstance();
        }
        return mSingleInstance;
    }
}

3、雙重校驗鎖。使用volatile變量，輕量級的同步機制。

public class SingleInstance {

    private SingleInstance(){}

    private volatile static SingleInstance mSingleInstance;

    public static SingleInstance getInstance(){
         
        if (mSingleInstance == null) {
            synchronized (SingleInstance.class) {
                if (mSingleInstance == null) {
                    mSingleInstance = new SingleInstance();
                }
            }
        }
        return mSingleInstance;
    }
 }

4、靜態內部類

public class SingleInstance {

    private SingleInstance(){}

    public static SingleInstance getInstance(){
        return SingleHolder.instance;
    }

    private static class SingleHolder {
        private static SingleInstance instance = new SingleInstance();
    }

}

5、枚舉

public enum SingleInstance {
    INSTANCE;
}