Encrypting already existing files in AWS S3 using the AWS Java API

In my last post I covered how to server-side encrypt files in S3 using the AWS Java API. Unfortunately, if you didn’t turn on encryption from the very first day when uploading to S3, you may have some files that are not encrypted. This post will cover an easy block of Java code which you can use to server-side encrypt any existing files that aren’t already, using the AWS Java API.

In summary, you need to loop through all existing files in a bucket, and see which one is not encrypted. And if not encrypted, you set the metadata to turn on server-side encryption, and have to save the file again in S3. Note: this may change the timestamps on your files, but this is essentially the only way through the API to save the metadata for a file to turn on encryption.

Here is the code:

public S3EncryptionMigrator(String bucketName) {
 Logger.getLogger("com.amazonaws.http.AmazonHttpClient").setLevel(Level.OFF); //AWS API outputs too much information, totally flodding the console. Turn it off

 AmazonS3Client amazonS3Client = new AmazonS3Client(...);

 ObjectListing objectListing = amazonS3Client.listObjects(bucketName);
 List s3ObjectSummaries = objectListing.getObjectSummaries();
 while (objectListing.isTruncated()) {
  objectListing = amazonS3Client.listNextBatchOfObjects(objectListing);

 for (S3ObjectSummary s3ObjectSummary: s3ObjectSummaries) {
  String s3ObjectKey = s3ObjectSummary.getKey();
  S3Object unecryptedS3Object = amazonS3Client.getObject(bucketName, s3ObjectKey);
  ObjectMetadata meta = unecryptedS3Object.getObjectMetadata();
  String currentSSEAlgorithm = meta.getSSEAlgorithm();
  if (currentSSEAlgorithm != null && currentSSEAlgorithm.equals(ObjectMetadata.AES_256_SERVER_SIDE_ENCRYPTION))
   continue; //Already encrypted, skip
  meta.setSSEAlgorithm(ObjectMetadata.AES_256_SERVER_SIDE_ENCRYPTION); //set encryption
  CopyObjectRequest copyObjectRequest = new CopyObjectRequest(bucketName, s3ObjectKey, bucketName, s3ObjectKey);
  amazonS3Client.copyObject(copyObjectRequest); //Save the file
  System.out.println(">> '" + s3ObjectKey + "' encrypted.");

Let’s examine the code. First you instantiate AmazonS3Client with the correct credentials. This should be tailored to your S3 authentication setup.  You start by getting a list of all files in a bucket. Note that you have to loop through objectListing.getObjectSummaries() because only 1000 results are returned at a time. In case you have more than 1000 files, you’ll need to loop through the rest until you get all of them.

Then you loop through the list of files. For each file you check if server-side encryption is already turned on by reading the existing metadata of the file. If not, you set the flag for encryption, and then essentially copy the file onto itself. This will save the new metadata, and will turn on server-side encryption.

Encrypting files in AWS S3 using Java API

If you use AWS S3 Java API, and would like to see how you can encrypt files on S3, this post is for you.

First of all, there are two ways you can encrypt files in S3. One is to encrypt files on the server side, and one is to encrypt files on the client side. With using the server side option, you don’t have to worry about too much. S3 encrypts the files for you when they are written to disk, and decrypts them when they are read, seamlessly. With the client side option, the client (your application) has to encrypt files before transmitting them to S3, and decrypt them after receiving the file from S3.

In this post I’ll cover server side encryption. We opted to use this one because it’s just simpler, and seamless. You don’t have to worry about encrypting/decrypting files yourself, nor do you have to worry about the key.

I’m assuming that you’re already familiar with the AWS Java API. For most things related to S3, AWS provides a class called AmazonS3Client. Once you have AmazonS3Client instantiated with your configuration, you will need to enable encryption in the matadata for each file you upload.


File fileForUpload = new File(...);
AmazonS3Client amazonS3Client = new AmazonS3Client(...);
ObjectMetadata meta = new ObjectMetadata();
amazonS3Client.putObject(s3Bucket, s3FullDestinationPath, new FileInputStream(fileForUpload), meta);

Let’s examine. First you instantiate the File you want to upload, and AmazonS3Client. Next you set the metadata on the file. This includes setting the content type of the file (important because having the wrong content-type can cause issues down the line), and sets the encryption flag for the file. Then when you upload the file using AmazonS3Client.putObject(…), the file will be encrypted by S3 before it is stored, and automatically decrypted when it is retrieved, all by S3’s servers. And that’s it!

Note that according to AWS Java API documentation, AmazonS3Client uses SSL under the hood so you don’t have to worry about transmitting unencrypted files over the network.

Getting the number of active sessions in Vaadin

Last week I explored how to get the number of active sessions that Tomcat reports: https://adeelscorner.wordpress.com/2016/03/13/getting-the-number-of-active-sessions-in-tomcat/

The problem with that metric is that some of those sessions, or all, may not actually be active users. What if someone is connected to your java web application (Tomcat in this case), but has just been sitting idle, for hours or even days. They’re not really using your application per se. However, Tomcat will report them as active users, especially if your application has a heartbeat, or a long polling push connection established.

So an additional metric that is useful in determining if your java application is actually in use is to track the number of active sessions within your java application. In our case we’re using Vaadin, so that’s what this post is geared towards. However, you’ll see the general idea here and can apply this to any user based application.

Before I continue, I want to point out a useful Vaadin add-on to measure idle time: https://vaadin.com/directory#!addon/idle. This is pretty handy in checking the actual idle time for a user.

The other important thing to explore first is as follows. Each Vaadin Session is based on a cookie, so if a browser has multiple tabs open to the same application server running Vaadin, the Vaadin Session will be shared among those tabs, because the cookie is shared. Hence, when a Vaadin session ends (perhaps when the user logs out, or times out), all user browser tabs open corresponding to that session are disconnected.

Let’s get started. At the core of how this will work is a public static HashMap which we can use to track a browser tab that belongs to a particular Vaadin session, and how long it’s been active or idle. Since this is a static data structure, it will be shared among all sessions at the Tomcat level. So it’s pretty handy in tracking all sessions on that particular Tomcat server. We’ll be able to track not only how many browser tabs a user has open to your application, but how long they’ve been idle in each tab.

I created a class called ApplicationLevelTracker to assist, which is pretty simple really. It’s as follows:

public class ApplicationLevelTracker {
 public static HashMap < String, Pair < Boolean, Long >> browserTabUniqueIDToIdleTimeHashMap = new HashMap < String, Pair < Boolean, Long >> ();

 private String thisBrowserTabUniqueID;

 public ApplicationLevelTracker(int userID, String vaadinSessionID) {
  thisBrowserTabUniqueID = userID + ":" + vaadinSessionID + ":" + UUID.randomUUID().toString();

 public void registerIdle(boolean idle) {
  Date date = new Date();
  browserTabUniqueIDToIdleTimeHashMap.put(thisBrowserTabUniqueID, new Pair < Boolean, Long > (idle, date.getTime()));

 public void deRegisterThisTab() {

 public void deRegisterAllTabsForThisVaadinSession() {
  String userID = thisBrowserTabUniqueID.split(":")[0];
  String vaadinSessionID = thisBrowserTabUniqueID.split(":")[1];
  Set < String > keySet = ApplicationLevelTracker.browserTabUniqueIDToIdleTimeHashMap.keySet();
  List < String > browserTabUniqueIDsToRemove = new ArrayList < String > ();
  for (String key: keySet) {
   String _userID = key.split(":")[0];
   String _vaadinSessionID = key.split(":")[1];
   if (_userID.equals(userID) && _vaadinSessionID.equals(vaadinSessionID))
  for (String browserTabUniqueID: browserTabUniqueIDsToRemove) {

Here’s a breakdown of the class, and the methods: The constructor takes the user’s ID (any identifier you have for your user, in our case an integer), and the Vaadin Session ID. Using these keys, and a random ID, each browser tab that a user opens to your application will get its own unique ID created in the constructor.

When a user goes idle, registerIdle(true) is called. When a user is active, registerIdle(false) is called. This way we can track whether they are active or idle in the corresponding browser tab. These methods will be used by the Vaadin Idle add-on (more on that soon).

When a browser tab is closed, deRegisterThisTab() is called. When the Vaadin session is being ended (for example, you may do that when a user logs out from your application, or when Vaadin determines the user has timed out), deRegisterAllTabsForThisVaadinSession() is meant to be called, to clean them out of the tracking HashMap.

Next, to actually make use of the ApplicationLevelTracker, you’ll have to set it up and use it properly in your Vaadin UI class, where your application is created and each user’s instance is initiated. So in your UI class that creates your Vaadin application (the class usually extends Vaadin’s “UI”), you’ll need to set up the following code for all this to come together:

First, define this variable in your UI class:

private ApplicationLevelTracker applicationLevelTracker = null;

Next, when the views are being created, after the User is logged in (note: you should set it up so this code is called on every new creation of the UI class, and not just the creation of a new Vaadin Session (the same Vaadin session can have multiple UIs attached to it)), you’ll want to set up ApplicationLevelTracker:

applicationLevelTracker = new ApplicationLevelTracker(user.USER_ID, VaadinSession.getCurrent().getSession().getId());
Idle.track(UI.getCurrent(), 120000, new Idle.Listener() {
 public void userInactive() {
 public void userActive() {

Notice that the Vaadin Idle add-on is used here. We create an instance of ApplicationLevelTracker with the user’s ID, and the Vaadin session ID, which registers this browser tab in static (shared) memory. And then when the user goes idle, or when the user goes active, as reported by the Idle add-on, we register that with ApplicationLevelTracker.

Now that things are setup, we need to be sure they are cleaned up when the user goes away.

You can use this neat trick to register whenever that particular browser tab is closed, or when the user navigates away from your application. This is where you’ll want to call applicationLevelTracker.deRegisterThisTab():

JavaScript.getCurrent().addFunction("browserIsLeaving", new JavaScriptFunction() {
 public void call(JsonArray arguments) {
  if (applicationLevelTracker != null)
Page.getCurrent().getJavaScript().execute("window.onbeforeunload = function (e) { var e = e || window.event; browserIsLeaving(); return; };");

You’ll also want to use applicationLevelTracker.deRegisterThisTab() when the Vaadin detach listener is fired. A detach listener corresponds to the UI, and not the whole Vaadin session. So it will be fired when one particular instance of UI belonging to a singular browser tab (which may one of several tabs belonging to the same Vaadin session) is detected to have gone away, this code will be called to clean up that particular browser tab. You can add the following block of code in the init() method in your Vaadin UI class:

addDetachListener(new DetachListener() {
 public void detach(DetachEvent event) {
  if (applicationLevelTracker != null)

Lastly, wherever in your code the user logs out, and you end the Vaadin session, you’ll want to call applicationLevelTracker.deRegisterAllTabsForThisVaadinSession():

if (applicationLevelTracker != null)

And that’s it! Now you have an application wide HashMap which fully tracks all individual browser tabs open to your application, and information on how long the user has been active or idle in each tab. In my next blog post I’ll cover how to actually pull this information and use it in a meaningful way to make a determination of how many users are using your application at any given moment.

Before wrapping up I want to point out one final thing on how to interpret this tracking information after you’ve started collecting it. If a user is connected, and is idle for too long (say 2 hours, or several days), you can reliably know that they are not using the application. They are either connected and left their machine running and browser open, just sitting idle. Or they’re gone (no longer connected) and their session was never cleaned up for whatever reason. If the user has been active for a long time (say, several days), that means something is wrong too, unless they really have been working every minute in your application for the last several days without a break. In this case you can assume the user is not really there, that their browser connection went away and their tracking didn’t get cleaned up for whatever reason (for example, Vaadin didn’t fire the detach listener reliably, which I’ve noticed happens sometimes though it may be fixed in new Vaadin versions). The only case you can be pretty certain that a user is using your application at the present moment is if they’ve just been idle for a short period (say within a 30 minute window), or if they’ve been active within a short period (say 30 minutes again).

Getting the number of active sessions in Tomcat

This is really useful when you’re trying to determine if your Java application running on a Tomcat server is currently serving any clients. For example, if your server is under use, you can avoid taking it offline for maintenance or upgrade. Or, you can use it to determine the load on a server.

Essentially Tomcat has some internal statistics that can be accessed using JMX (Java Management Extension). More information and details for that are available here: https://tomcat.apache.org/tomcat-8.0-doc/monitoring.html

The Tomcat JMX stats can be accessed in several ways. This article will cover how to access them from within the tomcat serer, which was our requirement. Since the application deployed to the tomcat server will be running locally on the JVM running tomcat, JMX can be accessed locally from the Java code, and no remote setup is required.

The code is as simple as this:

MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer();
ObjectName objectName = new ObjectName("Catalina:type=Manager,context=/,host=localhost");
int activeSessions = (Integer) mBeanServer.getAttribute(objectName, "activeSessions");

And that’s it! You’ll get the number of active sessions tomcat is reporting. And since this is running within the application deployed to tomcat, no remote setup or custom JVM arguments for tomcat are required.

Vaadin upgrade from 7.4.5 to 7.6.1

I was recently tasked with upgrading our Vaadin applications at work from version 7.4.5 to version 7.6.1. I came across an issue that took me a long time to figure out, so I thought I’d share in case anyone else is running into the same.

The process went pretty smoothly at first. We use Ivy for our projects, and simply updating the Vaadin version in ivy.xml allowed all the new library files to be downloaded. Then I re-compiled the widgetset, and took one of our applications out for a spin.

Lo and behold I got an error stating: “Failed to load the bootstrap javascript: ./VAADIN/vaadinBootstrap.js?v=7.6.1”. I did some investigating, and noticed that the vaadinBootstrap.js served to the browser looked like a binary file, whereas the browser was trying to interpret it as a JavaScript file.

I eventually realized that the vaadinBootstrap.js is compressed. I did everything I could to verify that the compression filters were correct, but the browser for some reason seemed like it was not uncompressing vaadinBootstrap.js before trying to interpret it.

Turns out that in prior versions of Vaadin, with the Vaadin demo applications, Vaadin had stuck in a CompressionFilter using net.sf.ehcache.constructs.web.filter.GzipFilter to web.xml. It compressed all *.js, *.html, *.css files (which only works if the browser/client announces it supports it in the HTTP headers) before serving it. HOWEVER, in the new version of Vaadin (7.6.1) it seems that there are some new compression filters already built in! Hence, the content (in this case vaadinBootstrap.js) was compressed twice! So the browser would uncompress it as it should, but that would yield a second compressed file! Sheesh. Had almost pulled my hair out before I figured this out.

The solution was to get rid of all the custom compression filters using net.sf.ehcache.constructs.web.filter.GzipFilter from our web.xml. And then everything started working smoothly.

Vaadin: Executing custom JavaScript from a thread, or loading custom JavaScript functions into global scope

As you may know already, in Vaadin you can use com.vaadin.ui.JavaScript.getCurrent().execute(…) function to execute some custom JavaScript on the client browser.

  1. Executing custom JavaScript from a thread:
    The above works well as long as the JavaScript execute() method is being called on the main UI thread. However, if JavaScript.getCurrent().execute() is called from a background thread, the JavaScript won’t get executed until there is a periodic refresh of the UI, or there’s a UI event (triggered by the user, such as a mouse click somewhere). This can seem to cause erratic behavior, with the JavaScript executing at unpredictable times. (Side note: any Vaadin UI access/manipulation from a background thread needs to be done inside com.vaadin.UI.getCurrent().access(new Runnable() { … });, and also note that you want to do your time-consuming heavy lifting first (such as retrieving data from the back end) and then go into UI.getCurrent().access(…) to manipulate the UI).To get around this problem, simply use Vaadin Push. You’ll need to enable push if not already (see Vaadin documentation on how). Then depending on the push mode you’ve used (manual or automatic) you’ll either need to call com.vaadin.ui.UI.getCurrent().push() or not (for manual mode you’ll need to call the push() method, for automatic mode it will be called after the runnable you send to the UI.access(…) method finishes executing). So call get JavaScript.execute(…), and then UI.push() last. Example:

    new Thread() {
        public void run() {
            //some long running background work
                 UI.getCurrent.access(new Runnable() {
                public void run() {
                    JavaScript.getCurrent().execute("alert('Background Task Complete!');");
  2. Loading custom JavaScript functions into global scope:
    This is extremely useful so you can define a JavaScript function which you can use later from JavaScript.getCurrent().execute(…), such as inside an event (a button click, for example). However, the JavaScript function will need to be in the global scope by injecting it into thetag for the HTML page served by Vaadin. To do this, use the following code while your Vaadin view is being created, or is being enter()’ed.

    StringBuilder script = new StringBuilder(); script .append("var head = document.getElementsByTagName('head')[0];") .append("var script = document.createElement('script');") .append("script.content='function sayHello() { alert(\"Hello!\");}';") .append("head.appendChild(script);"); JavaScript.getCurrent().execute(script.toString());

    Note: as I mentioned, a JavaScript function can only be loaded this way when the view is being created, or in the view’s enter() method. To create a function this way AFTER the page is already loaded (such as through some event), you’ll need to use Vaadin Push, and call UI.getCurrent().push() after the JavaScript.getCurrent().execute() even though you’re not on a background thread.

  3. You can define a function in the script tag being created above, which can then be called later on through JavaScript.getCurrent().execute(“sayHello();”); perhaps inside a Vaadin button click listener.


Making a Vaadin window transparent/translucent

We had a need to do this recently, and it wasn’t obvious, so I thought I’d share.

One thing to note is that if the window is modal, this trick won’t work. Since for modal windows, Vaadin adds some more styling around the window to the back the background “dimmed”, thus giving a stronger visual focus on the modal window. For regular (non-modal) windows, this trick works great, especially if you just want to overlay some information on content behind the window.

In your java code, add the following style to your Vaadin Window:


In your theme stylesheet for your application (most likely a “.scss” file), add the following styles:

  background-color: rgba(255,255,255,0.6) !important;
div.v-window-translucent .v-window-contents {
  background: rgba(255,255,255,0.6);

Then compile your theme, and refresh your page. Viola! Your window will now be translucent.

To tweak the translucency, modify the “a” (alpha) value in the rgba inside the theme style. 0.6 worked well for us, making it a bit translucent. To make it less translucent, try a value greater than 0.6. And to make it completely transparent, try 0.