单例模式

Double Check Lock(双重检查锁):

public class kongjiaoshi {
    private static kongjiaoshi kongjiaoshi = null;
    public static kongjiaoshi getInstance() {
        if (kongjiaoshi == null) {
            synchronized (com.example.liang.siruanbei.utils.kongjiaoshi.class){//注意这里括号里面写的是.class，而不是.this
                if (kongjiaoshi == null) {
                    kongjiaoshi = new kongjiaoshi();
                }
            }

        }
        return kongjiaoshi;
    }