* creates itemUIs, which show items' information and upload status.
* UploadItemUIs created for every file items.
*/
private void updateComponents() {
int len = this.model.getSize();
CustomBoxLayout layout = new CustomBoxLayout();
this.jPanel2.setLayout(layout);
for (int i = 0; i < len; i++) {
UploadItemUI item = new UploadItemUI();
item.setConfigHolder(configHolder);