Incremental Upload Processing Using FileEntryCallback

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Incremental Upload Processing Using FileEntryCallback

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The all-new ACE FileEntry component introduces in-memory file processing. This feature is designed to provide for antivirus scanning, checksum verification or a scenario where the upload doesn't need to be saved to disk, and doing so immediately is excessive IO.

The functionality requires that a listener bean implement FileEntryCallback to handle the incrementally uploaded bytes.
Instances of FileEntryCallback must implement:

/* Notification for upload handler when a file begins uploading.
 * fileInfo - information known about the file, before downloading the contents */
public void begin(FileEntryResults.FileInfo fileInfo);

/* We're working with chunks of bytes, as we receive them... */
public void write(byte[] buffer, int offset, int length);
public void write(int data);

/* Notification for upload handler that the file is finished. */
public void end(FileEntryResults.FileInfo fileInfo);

This example demos in-memory processing to provide an MD5 hash for an uploaded file.




Implementing FileEntryCallback


Creating our handler is the most important step of this tutorial. Everything else is just wiring up the dependant parts.

Check fileInfo.getStatus() to determine if the file has pre-failed uploading, due to too many files uploaded, an invalid file extension, or content type.
  • The Class Properties

    The MessageDigest class is an Apache Commons class for using one-way hash algorithms like MD5 and SHA.

    FileEntryMD5Callback.java
    private MessageDigest digest;
    
  • The begin Method

    The begin method is where your callback should request any resources it will require.
    The example sets up a MessageDigest object.

    FileEntryMD5Callback.java
    // Set up a instance of a MD5 block-encoder
    public void begin(FileEntryResults.FileInfo fileInfo) {
        try { digest = MessageDigest.getInstance("MD5"); }
        catch (NoSuchAlgorithmException e) {
            FacesUtils.addErrorMessage("MD5 Algorithm Not Available.");
    }}
  • The write Methods

    These methods will be passed portions of file data. If there is a chance the file will eventually be saved, the byte input should be cached here.
    This example immediately computes the data as part of our hash.

    FileEntryMD5Callback.java
    // Hash a block of bytes
    public void write(byte[] bytes, int offset, int length) {
        digest.update(bytes, offset, length);
    }
    // Hash a single byte
    public void write(int i) { digest.update((byte) i); }  
    
  • The end Method

    This method triggers when a file upload is complete, either successfully or with errors.
    If you want to invalidate a successful upload, perhaps it's over quota, then this method should handle the case. It may massage the result (raising some prompts and accepting something), or possibly fail the upload for good by calling FileInfo.updateStatus(...) with the status "FileEntryStatuses.INVALID".
    The example below either prints success, or when handling an error, prints the default status FacesMessage.

    FileEntryMD5Callback.java
    // When FileEntryCallback ends for a file:
    public void end(FileEntryResults.FileInfo fileEntryInfo) {
        // If the file upload was completed properly
        if (fileEntryInfo.getStatus().isSuccess()) {
            // Implement some application-layer checks on entire file if needed
            // etc.
            // If those went fine add a success message.
            FacesUtils.addInfoMessage(TutorialMessageUtils.getMessage("content.callback.result.message") + " " + getHash());
        } else {
            // Implement error handling given for specific status cases,
            // or you could use your own general fail message.
            FacesContext c = FacesContext.getCurrentInstance();
            UIComponent comp = c.getViewRoot().findComponent("fileEntry");
            clientId = comp.getClientId();
            c.addMessage(null, fileEntryInfo.getStatus().getFacesMessage(c, comp, fileEntryInfo));
        }
    }  
    

Adding the FileEntry Component to a Facelet Page


Nothing special is required in the facelet of this tutorial. The sample below just has some simple styling and an EL reference to the callback bean.

main.xhtml
<ace:fileEntry id="fileEntry" callback="#{fileMD5EncodingCallback}" styleClass="ib" />
<h:commandButton styleClass="ib" value="Get MD5 Checksum" />
<div class="messages-holder"><h:messages styleClass="messages" /></div>
<div style="clear:both; height:0px;">&#160;</div>

That concludes this overview of the new in-memory processing feature. If you're interested in further details of how this example works, take a look at the complete source code below; or sign up for ICEfaces training for a complete guide to this example and every feature of ICEfaces!

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