双重检验锁

public class PersonMassManger {
private volatile static PersonMassManger mInstance;
private Context context;
private PersonMassDao personMassDao;

private PersonMassManger(Context context) {
    this.context = context;
    getPersonMassDao();
}

public static PersonMassManger getInstent(Context context) {
    if (mInstance == null) {
        synchronized (PersonMassManger.class) {
            if (mInstance == null) {
                mInstance = new PersonMassManger(context);
            }
            return mInstance;
        }
    }
    return mInstance;
}

}