单例模式 静态内部类方式

public class FolderV3Singleton {

    private FolderV3Singleton() {
    }

    private static class DeviceHolder {
        private static FolderV3Singleton ourInstance = new FolderV3Singleton();
    }

    public static FolderV3Singleton getInstance() {
        return DeviceHolder.ourInstance;
    }

    public List<String> getFolderList() {
        return folderList;
    }

    public void setFolderList(List<String> folderList) {
        this.folderList = folderList;
    }

    private List<String> folderList = null;
}