CUBA Platform. Developer's Manual
4.5.2.1. Components

Declare the component in an XML screen descriptor:

<multiUpload id="multiUploadField" caption="msg://upload"/>

In the screen controller, inject the component itself, and the FileUploadingAPI and DataSupplier interfaces. Then, in init() add a listener to the component, which will react to successful upload events or errors:

@Inject
protected FileMultiUploadField multiUploadField;

@Inject
protected FileUploadingAPI fileUploading;

@Inject
protected DataSupplier dataSupplier;

@Override
public void init(Map<String, Object> params) {
    multiUploadField.addListener(new FileMultiUploadField.UploadListener() {
        @Override
        public void queueUploadComplete() {
            Map<UUID, String> uploadMap = multiUploadField.getUploadsMap();
            for (Map.Entry<UUID, String> entry : uploadMap.entrySet()) {
                UUID fileId = entry.getKey();
                String fileName = entry.getValue();
                FileDescriptor fd = fileUploading.getFileDescriptor(fileId, fileName);
                // save file to FileStorage
                try {
                    fileUploading.putFileIntoStorage(fileId, fd);
                } catch (FileStorageException e) {
                    new RuntimeException(e);
                }
                // save file descriptor to database
                dataSupplier.commit(fd, null);
            }
            multiUploadField.getUploadsMap().clear();
        }
    });
}

The component will invoke queueUploadComplete() after uploading all selected files to a temporary storage of the client tier. At this point, by invoking getUploadsMap() you can get a map of temporary storage file identifiers to file names. Then, for each file, a corresponding FileDescriptor object is created based on this data. com.haulmont.cuba.core.entity.FileDescriptor (do not confuse with java.io.FileDescriptor) is a persistent entity, which uniquely identifies an uploaded file and then is used to download the file from the system.

FileUploadingAPI.putFileIntoStorage() is used to move the uploaded file from the temporary client storage to FileStorage. Parameters of this method are temporary storage file identifier and the FileDescriptor object.

After uploading the file to FileStorage, the FileDescriptor instance is saved in a database by invoking DataSupplier.commit(). The saved instance returned by this method can be set to an attribute of an entity related to this file. In this case, FileDescriptor is simply stored in the database and gives access to the file through the Administration -> External Files screen.

After processing, the list of files should be cleared via the clearUploads() method in order to prepare for further uploads.

Maximum upload size is determined by the cuba.client.maxUploadSizeMb application property and is equal to 20Mb by default. If a user selects a file of a larger size, a corresponding message will be displayed and the upload will be interrupted.