Since the date patterns, are likely to stay the same on each page. (Because 
usually the date is dynamic, and the date patterns static). Would it be an idea 
to have a pool of SimpleDateFormatters each with its own pattern + locale, so 
we could reuse the same patterns to avoid unnecessary calls to 
Calendar.getInstance?

e.g. in org.apache.taglibs.standard.tag.common.fmt.FormatDateSupport

Something like:
    static Map sdfCache=new HashMap();

    public synchronized SimpleDateFormat getSimpleDateFormat(String pattern, 
Locale locale) {
    	System.out.println("getSimpleDateFormat "+pattern+":"+locale);
    	String key=pattern+locale;
    	SimpleDateFormat sdf;
    	if (sdfCache.containsKey(key)) {
    		sdf=(SimpleDateFormat) sdfCache.get(key);
    	}
    	else {
        	sdf=new SimpleDateFormat(pattern, locale);
        	sdfCache.put(key,sdf);	    		
    	}
    	return sdf;
    }

And later:
Instead of
	    DateFormat formatter = createFormatter(locale);
            // Apply pattern, if present
	    if (pattern != null) {
		try {
		    ((SimpleDateFormat) formatter).applyPattern(pattern);
		} catch (ClassCastException cce) {
		    formatter = new SimpleDateFormat(pattern, locale);
		}
	    }

Something like
	    DateFormat formatter = getSimpleDateFormat (pattern, locale);	
	

After making a prototype of the code mentioned earlier, I saw a tremendous 
increase in the performance; and no calendar.getInstance appeared in the 
stacktrace...
However... Now we are experiencing locking issues with this method:
org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager.getEvaluator
ByName(ExpressionEvaluatorManager.java:144)
org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager.evaluate
(ExpressionEvaluatorManager.java:128)

Fortunatly I don't seem to be alone anymore with this problem:
http://www.mail-archive.com/taglibs-user@jakarta.apache.org/msg04179.html

Is there any progress on this? (After all the above message was posted over 1 
year ago)

Ah the above locking seems to be resolved in 1.0.6:
http://jakarta.apache.org/taglibs/doc/standard-1.0-doc/ReleaseNotes.html

The suggestion below of a static cache seems bad as SimpleDateFormat is not
thread safe. 

However we only access the formatter object in a very small location, so we
could change:

            if (tz != null) {
                formatter.setTimeZone(tz);
            }
            formatted = formatter.format(value);

to

         synchronized(formatter) {
            if (tz != null) {
                formatter.setTimeZone(tz);
            }
            formatted = formatter.format(value);
         }

The current code does also access formatter when creating, but that's because
it's doing daft things in creation. The code below gets rid of that.

So this is fixable and needs a demonstrative test to be created. 

As with 17700, the question of what to do with the cache over time exists.

In this case it seems unlikely that we would have an enormous number of patterns
being created; so I wouldn't expect the map to grow hugely.

One solution might be to implement a very basic pool - but that seems very
likely to lead to bugs while we get it right.

SPI style plugin cache added for this (see attachment to issue
https://issues.apache.org/bugzilla/show_bug.cgi?id=31789 ).

Created attachment 20900 [details]
Fix+Test patch

Attaching a fix and a test. The test fails because Cactus does not seem to set
the locale on the request. The default action then is to print date.toString().
Switching that to use the Locale.getDefault() shows that the code is working.

Probably have to just ditch the test and just go with the fix, along with a
sample app showing that things are okay.

Created attachment 20987 [details]
Fix+More Tests

Adding a few more tests, though it is more of a proof and not a test.

svn ci -m "Applying the fix and the tests from #32311 - LRUMap used as a caching
system to improve on performance"

Sending        src/org/apache/taglibs/standard/tag/common/fmt/FormatDateSupport.java
Adding         test/org/apache/taglibs/standard/tag/el/fmt
Adding         test/org/apache/taglibs/standard/tag/el/fmt/TestDateTag.java
Adding         test/web/org/apache/taglibs/standard/tag/el/fmt
Adding         test/web/org/apache/taglibs/standard/tag/el/fmt/TestDateTag.jsp
Transmitting file data ...
Committed revision 585044.