HTML5 script async

The HTML5 “async” attribute simplifies page-load performance improvements and dynamic script loading, it can be useful in modern web browsers.

Simply put, this tag allows for the browser to asynchronously load and execute external javascript files in a parallel vs. serial manner. Unfortunately while most modern browser support it, MSIE versions prior to MSIE10+ are problematic.

<script src="example.js" async="async"></script>

This is particularly useful when using third-party javascript libraries and utilities that have no dependeny relationships with your existing website javascript.


Java User-Agent detector and caching

It’s often important for a server side application to understand the client platform. There are two common methods used for this.

  1. On the client itself, “capabilities” can be tested.
  2. Unfortunately, the server cannot easily test these, and as such must usually rely upon the HTTP Header information, notably “User-Agent”.

Example User-agent might typically look like this for a common desktop browser, developers can usually determine the platform without a lot of work.

"Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3)"

Determining robots and mobile platforms, unfortunately is a lot more difficult due to the variations. Libraries as those described below simplify this work as standard Java Objects expose the attributes that are commonly expected.

With Maven, the dependencies are all resolved with the following POM addition:


/* Get an UserAgentStringParser and analyze the requesting client */
final UserAgentStringParser parser = UADetectorServiceFactory.getResourceModuleParser();
final ReadableUserAgent agent = parser.parse(request.getHeader("User-Agent"));

out.append("You're a '");
out.append("' on '");

As indicated on the website documentation, running this query for each request uses valuable server resources, it’s best to cache the responses to minimize the impact!

NOTE: the website caching example is hard to copy-paste, here’s a cleaner copy.

package com.example.cache;

import java.util.concurrent.TimeUnit;
import net.sf.uadetector.ReadableUserAgent;
import net.sf.uadetector.UserAgentStringParser;
import net.sf.uadetector.service.UADetectorServiceFactory;

* Caching User Agent parser
* @see
* @author Scott Fredrickson [skotfred]
* @since 2015jan28
* @version 2015jan28
public final class CachedUserAgentStringParser implements UserAgentStringParser {

private final UserAgentStringParser parser = UADetectorServiceFactory.getCachingAndUpdatingParser();
private static final int CACHE_MAX_SIZE = 100;
private static final int CACHE_MAX_HOURS = 2;
* Limited to 100 elements for 2 hours!
private final Cache<String , ReadableUserAgent> cache = CacheBuilder.newBuilder().maximumSize(CACHE_MAX_SIZE).expireAfterWrite(CACHE_MAX_HOURS, TimeUnit.HOURS).build();

* @return {@code String}
public String getDataVersion() {
return parser.getDataVersion();
* @param userAgentString {@code String}
* @return {@link ReadableUserAgent}
public ReadableUserAgent parse(final String userAgentString) {
ReadableUserAgent result = cache.getIfPresent(userAgentString);
if (result == null) {
result = parser.parse(userAgentString);
cache.put(userAgentString, result);
return result;
public void shutdown() {


Custom HTTP 404 image vs. page on Apache

In most cases, server administrators define error pages in their apache configuration, but they often neglect to appropriately handle images, and other content types for that matter.

Generally, something similar to the following will be found in the Apache httpd.conf file:

ErrorDocument 404 /errors/404.html

You can use the file extension of the request to provide content that is at least “usable” to the browser, you could even provide a “cachable” response unless you expect to fix it quickly.

The advantage of this approach is that the HTTP Response code remains 404 but the user gets something other than a “broken” image on their screen. I generally make it a small 1×1 pixel gif.

An example for images would be…

<FilesMatch ".(jpg|png|gif|ico)$">
ErrorDocument 404 "/path/to/404.gif"

NOTE: you could use a similar mechanism for .css and .js files and provide cachable responses with comments within them to avoid the validation errors caused by 404’s.


Blocking access to files by extension in Apache

Usually, you might have a simple rule to prevent users from accessing sensitive files such as “.htaccess“, that rule might look like:

<FilesMatch "^\.ht">
Order deny,allow
Deny from all
Satisfy all

You can also use this capability to prevent other file extensions. For example, if you wanted to block common image formats extensions, you might add the following:

<FilesMatch "\.(gif|png|jpg|ico)$">
Order allow,deny
Deny from all
Satisfy all

Some other file extensions to consider, *.bak, *.old, *.inc