We Keep Coding

Java local variable vs global field - Performance

I have a private method which takes date in as String type and returns XMLGregorianCalendar object with specific format. This method is being invoked from various other methods within the same class.
The format defined as SimpleDateFormat which is same for every invocation. Below are the 2 versions of the method.
Version 1:
private XMLGregorianCalendar getXmlGregorianCalendar(final String strDt) throws ParseException, DatatypeConfigurationException{
Date date = null;
// local variable dtFormat
DateFormat dtFormat = new SimpleDateFormat("yyyy-MM-dd-hh.mm.ss");
if (strDt != null) {
date = dtFormat.parse(strDt);
GregorianCalendar cal = new GregorianCalendar();
cal.setTime(date);
XMLGregorianCalendar xmlCal = DatatypeFactory.newInstance().newXMLGregorianCalendar(cal);
return xmlCal;
}else {
return DatatypeFactory.newInstance().newXMLGregorianCalendar(new GregorianCalendar());
}
}
Version 2:
// declare date format once, global variable
private DateFormat dtFormat = new SimpleDateFormat("yyyy-MM-dd-hh.mm.ss");
// getter setter for dtFormat
private XMLGregorianCalendar getXmlGregorianCalendar(final String strDt) throws ParseException, DatatypeConfigurationException{
Date date = null;
if (strDt != null) {
date = this.dtFormat.parse(strDt); // of course, getDtFormat() can be used
GregorianCalendar cal = new GregorianCalendar();
cal.setTime(date);
XMLGregorianCalendar xmlCal = DatatypeFactory.newInstance().newXMLGregorianCalendar(cal);
return xmlCal;
}else {
return DatatypeFactory.newInstance().newXMLGregorianCalendar(new GregorianCalendar());
}
}
Version 1 vs Version 2. Which has better performance?
Note: The enclosed class is annotated as #Component and involved in creating response for my web service.

SimpleDateFormat is not thread-safe, so that would be something to consider. Performance has little to do with this question, unless you really don't have any other performance hotspots (which would be surprising).
You can however use a ThreadLocal SimpleDateFormat, but remember that ThreadLocals have their own issues, and it probably doesn't really matter.

If that SimpleDateFormat is a constant, you can make it static final and it would improve your performance just a little bit. But you should be aware of the concept of static fields in classes. static fields are allocated once in memory for all instances of your #Component class. So if this cannot creating a bug in your code, it's better to define it as a constant using static final keywords.

ThreadLocal and SimpleDateFormat array

Using a pattern very similar to that described in a recent question, for a multithreaded application, I am getting weird date values (e.g., years like 2025 or 2035, when clearly no such value exists in the source data). It seems that a concurrency issue is occuring.
The source code looks something like
// Various Java DateFormat patterns, e.g. "yyyy-MM-dd".
private static final String[] DATE_PATTERNS = new String[] {...};
private static SimpleDateFormat[] getFormats(final String[] patterns)
{
ThreadLocal<SimpleDateFormat[]> LOCAL_FORMATS = new ThreadLocal<SimpleDateFormat[]>()
{
#Override
protected SimpleDateFormat[] initialValue()
{
List<SimpleDateFormat> formatList = new ArrayList<SimpleDateFormat>();
for (String pattern:patterns)
{
formatList.add(new SimpleDateFormat(pattern));
}
return formatList.toArray(new SimpleDateFormat[formatList.size()]);
}
};
return LOCAL_FORMATS.get(); // Create a thread-local copy
}
private static final SimpleDateFormat[] DATE_FORMATS = getFormats(DATE_PATTERNS);
After its static initialization, the DATE_FORMATS array is accessed by numerous classes, which in turn use the SimpleDateFormat objects of the array for parsing or formatting several date strings.
Can there be any concurrency issue in such a usage scenario, especially given the use of ThreadLocal?

Yes, there can be concurrency issues. Your thread local variable doesn't serve any purpose. It's only used when the class is initialized, to temporarily store an array of date formats that is immediately retrieved and stored in a static constant.
All the threads, after, always use the same instances of date formats concurrently, without getting them from any thread local variable.
The code should rather be:
private static final String[] DATE_PATTERNS = new String[] {...};
private static final ThreadLocal<SimpleDateFormat[]> DATE_FORMATS =
new ThreadLocal<SimpleDateFormat[]>() {
#Override
protected SimpleDateFormat[] initialValue() {
List<SimpleDateFormat> formatList = new ArrayList<SimpleDateFormat>();
for (String pattern : DATE_PATTERNS)
{
formatList.add(new SimpleDateFormat(pattern));
}
return formatList.toArray(new SimpleDateFormat[formatList.size()]);
}
};
public static SimpleDateFormat[] getDateFormats() {
return DATE_FORMATS.get();
}
I would also use an unmodifiable List<SimpleDateFormat> rather than an array, to be safer.

// Various Java DateFormat patterns, e.g. "yyyy-mm-dd".
The format 'yyyy-mm-dd' is likely to give you weird results because 'mm' is minutes and not months. From the javadoc:
M Month in year Month July; Jul; 07
...
m Minute in hour Number 30

JSF custom Converter for Date - Is is thread safe?

I have created a custom Converter in JSF 1.2 to convert Date objects. The dates have a very particular format. I have implemented my converter using the core Java SimpleDateFormat class to do the conversion, using the formatter string shown in my code comments below. This all works fine.
My question is about thread safety. The SimpleDateFormat API docs state that it is not thread safe. For that reason I have created a separate instance of the date format object for each instance of my converter object. However, I'm not sure if this is enough. My DateFormat object is stored as a member of the DTGDateConverter.
QUESTION: Will two threads every simultaneously access the same instance of a Converter object in JSF?
If the answer is yes, then my Converter is probably at risk.
/**
* <p>JSF Converter used to convert from java.util.Date to a string.
* The SimpleDateFormat format used is: ddHHmm'Z'MMMyy.</p>
*
* <p>Example: October 31st 2010 at 23:59 formats to 312359ZOCT10</p>
*
* #author JTOUGH
*/
public class DTGDateConverter implements Converter {
private static final Logger logger =
LoggerFactory.getLogger(DTGDateConverter.class);
private static final String EMPTY_STRING = "";
private static final DateFormat DTG_DATE_FORMAT =
MyFormatterUtilities.createDTGInstance();
// The 'format' family of core Java classes are NOT thread-safe.
// Each instance of this class needs its own DateFormat object or
// runs the risk of two request threads accessing it at the same time.
private final DateFormat df = (DateFormat)DTG_DATE_FORMAT.clone();
#Override
public Object getAsObject(
FacesContext context,
UIComponent component,
String stringValue)
throws ConverterException {
Date date = null;
// Prevent ParseException when an empty form field is submitted
// for conversion
if (stringValue == null || stringValue.equals(EMPTY_STRING)) {
date = null;
} else {
try {
date = df.parse(stringValue);
} catch (ParseException e) {
if (logger.isDebugEnabled()) {
logger.debug("Unable to convert string to Date object", e);
}
date = null;
}
}
return date;
}
#Override
public String getAsString(
FacesContext context,
UIComponent component,
Object objectValue)
throws ConverterException {
if (objectValue == null) {
return null;
} else if (!(objectValue instanceof Date)) {
throw new IllegalArgumentException(
"objectValue is not a Date object");
} else {
// Use 'toUpperCase()' to fix mixed case string returned
// from 'MMM' portion of date format
return df.format(objectValue).toUpperCase();
}
}
}

Will two threads every simultaneously access the same instance of a Converter object in JSF?
Depends on how you use the converter. If you use
<h:inputWhatever>
<f:converter converterId="converterId" />
</h:inputWhatever>
then a new instance will be created for every input element in view, which is threadsafe (expect of the very rare edge case that the enduser has two identical views in two browser tabs in the same session and simultaneously issues a postback on the both views).
If you however use
<h:inputWhatever converter="#{applicationBean.converter}" />
then the same instance will be shared across all views of the entire application, which is thus not threadsafe.
You're however cloning a static DataFormat instance everytime you create the converter. That part is already not threadsafe. You may risk that you're cloning an instance while its internal state is been changed because it's been used somewhere else. Also, cloning an existing instance isn't necessarily cheaper than creating a new instance.
I would recommend to just declare it threadlocal (i.e. inside the method block), regardless of how you use the converter. If the expensiveness of creating the DateFormat everytime is a major concern (did you profile it?), then consider replacing it by JodaTime.

Date formats are not synchronized. It
is recommended to create separate
format instances for each thread. If
multiple threads access a format
concurrently, it must be synchronized
externally.
Yes it is not thread safe here.
Put it local to method and create instance per thread

Synchronizing access to SimpleDateFormat

The javadoc for SimpleDateFormat states that SimpleDateFormat is not synchronized.
"Date formats are not synchronized. It
is recommended to create separate
format instances for each thread. If
multiple threads access a format
concurrently, it must be synchronized
externally."
But what is the best approach to using an instance of SimpleDateFormat in a multi threaded environment. Here are a few options I have thought of, I have used options 1 and 2 in the past but I am curious to know if there are any better alternatives or which of these options would offer the best performance and concurrency.
Option 1: Create local instances when required
public String formatDate(Date d) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
return sdf.format(d);
}
Option 2: Create an instance of SimpleDateFormat as a class variable but synchronize access to it.
private SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
public String formatDate(Date d) {
synchronized(sdf) {
return sdf.format(d);
}
}
Option 3: Create a ThreadLocal to store a different instance of SimpleDateFormat for each thread.
private ThreadLocal<SimpleDateFormat> tl = new ThreadLocal<SimpleDateFormat>();
public String formatDate(Date d) {
SimpleDateFormat sdf = tl.get();
if(sdf == null) {
sdf = new SimpleDateFormat("yyyy-MM-hh");
tl.set(sdf);
}
return sdf.format(d);
}

Creating SimpleDateFormat is expensive. Don't use this unless it's done seldom.
OK if you can live with a bit of blocking. Use if formatDate() is not used much.
Fastest option IF you reuse threads (thread pool). Uses more memory than 2. and has higher startup overhead.
For applications both 2. and 3. are viable options. Which is best for your case depends on your use case. Beware of premature optimization. Only do it if you believe this is an issue.
For libraries that would be used by 3rd party I'd use option 3.

The other option is Commons Lang FastDateFormat but you can only use it for date formatting and not parsing.
Unlike Joda, it can function as a drop-in replacement for formatting.
(Update: Since v3.3.2, FastDateFormat can produce a FastDateParser, which is a drop-in thread-safe replacement for SimpleDateFormat)

If you are using Java 8, you may want to use java.time.format.DateTimeFormatter:
This class is immutable and thread-safe.
e.g.:
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
String str = new java.util.Date().toInstant()
.atZone(ZoneId.systemDefault())
.format(formatter);

Commons Lang 3.x now has FastDateParser as well as FastDateFormat. It is thread safe and faster than SimpleDateFormat. It also uses the same format/parse pattern specifications as SimpleDateFormat.

Don't use SimpleDateFormat, use joda-time's DateTimeFormatter instead. It is a bit stricter in the parsing side and so isn't quite a drop in replacement for SimpleDateFormat, but joda-time is much more concurrent friendly in terms of safety and performance.

I would say, create a simple wrapper-class for SimpleDateFormat that synchronizes access to parse() and format() and can be used as a drop-in replacement. More foolproof than your option #2, less cumbersome than your option #3.
Seems like making SimpleDateFormat unsynchronized was a poor design decision on the part of the Java API designers; I doubt anyone expects format() and parse() to need to be synchronized.

Another option is to keep instances in a thread-safe queue:
import java.util.concurrent.ArrayBlockingQueue;
private static final int DATE_FORMAT_QUEUE_LEN = 4;
private static final String DATE_PATTERN = "yyyy-MM-dd HH:mm:ss";
private ArrayBlockingQueue<SimpleDateFormat> dateFormatQueue = new ArrayBlockingQueue<SimpleDateFormat>(DATE_FORMAT_QUEUE_LEN);
// thread-safe date time formatting
public String format(Date date) {
SimpleDateFormat fmt = dateFormatQueue.poll();
if (fmt == null) {
fmt = new SimpleDateFormat(DATE_PATTERN);
}
String text = fmt.format(date);
dateFormatQueue.offer(fmt);
return text;
}
public Date parse(String text) throws ParseException {
SimpleDateFormat fmt = dateFormatQueue.poll();
if (fmt == null) {
fmt = new SimpleDateFormat(DATE_PATTERN);
}
Date date = null;
try {
date = fmt.parse(text);
} finally {
dateFormatQueue.offer(fmt);
}
return date;
}
The size of dateFormatQueue should be something close to the estimated number of threads which can routinely call this function at the same time.
In the worst case where more threads than this number do actually use all the instances concurrently, some SimpleDateFormat instances will be created which cannot be returned to dateFormatQueue because it is full. This will not generate an error, it will just incur the penalty of creating some SimpleDateFormat which are used only once.

I just implemented this with Option 3, but made a few code changes:
ThreadLocal should usually be static
Seems cleaner to override initialValue() rather than test if (get() == null)
You may want to set locale and time zone unless you really want the default settings (defaults are very error prone with Java)
private static final ThreadLocal<SimpleDateFormat> tl = new ThreadLocal<SimpleDateFormat>() {
#Override
protected SimpleDateFormat initialValue() {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-hh", Locale.US);
sdf.setTimeZone(TimeZone.getTimeZone("America/Los_Angeles"));
return sdf;
}
};
public String formatDate(Date d) {
return tl.get().format(d);
}

Imagine your application has one thread. Why would you synchronize access to SimpleDataFormat variable then?

Determine if a String is a valid date before parsing

I have this situation where I am reading about 130K records containing dates stored as String fields. Some records contain blanks (nulls), some contain strings like this: 'dd-MMM-yy' and some contain this 'dd/MM/yyyy'.
I have written a method like this:
public Date parsedate(String date){
if(date !== null){
try{
1. create a SimpleDateFormat object using 'dd-MMM-yy' as the pattern
2. parse the date
3. return the parsed date
}catch(ParseException e){
try{
1. create a SimpleDateFormat object using 'dd/MM/yyy' as the pattern
2. parse the date
3. return parsed date
}catch(ParseException e){
return null
}
}
}else{
return null
}
}
So you may have already spotted the problem. I am using the try .. catch as part of my logic. It would be better is I can determine before hand that the String actually contains a parseable date in some format then attempt to parse it.
So, is there some API or library that can help with this? I do not mind writing several different Parse classes to handle the different formats and then creating a factory to select the correct6 one, but, how do I determine which one?
Thanks.

See Lazy Error Handling in Java for an overview of how to eliminate try/catch blocks using an Option type.
Functional Java is your friend.
In essence, what you want to do is to wrap the date parsing in a function that doesn't throw anything, but indicates in its return type whether parsing was successful or not. For example:
import fj.F; import fj.F2;
import fj.data.Option;
import java.text.SimpleDateFormat;
import java.text.ParseException;
import static fj.Function.curry;
import static fj.Option.some;
import static fj.Option.none;
...
F<String, F<String, Option<Date>>> parseDate =
curry(new F2<String, String, Option<Date>>() {
public Option<Date> f(String pattern, String s) {
try {
return some(new SimpleDateFormat(pattern).parse(s));
}
catch (ParseException e) {
return none();
}
}
});
OK, now you've a reusable date parser that doesn't throw anything, but indicates failure by returning a value of type Option.None. Here's how you use it:
import fj.data.List;
import static fj.data.Stream.stream;
import static fj.data.Option.isSome_;
....
public Option<Date> parseWithPatterns(String s, Stream<String> patterns) {
return stream(s).apply(patterns.map(parseDate)).find(isSome_());
}
That will give you the date parsed with the first pattern that matches, or a value of type Option.None, which is type-safe whereas null isn't.
If you're wondering what Stream is... it's a lazy list. This ensures that you ignore patterns after the first successful one. No need to do too much work.
Call your function like this:
for (Date d: parseWithPatterns(someString, stream("dd/MM/yyyy", "dd-MM-yyyy")) {
// Do something with the date here.
}
Or...
Option<Date> d = parseWithPatterns(someString,
stream("dd/MM/yyyy", "dd-MM-yyyy"));
if (d.isNone()) {
// Handle the case where neither pattern matches.
}
else {
// Do something with d.some()
}

Don't be too hard on yourself about using try-catch in logic: this is one of those situations where Java forces you to so there's not a lot you can do about it.
But in this case you could instead use DateFormat.parse(String, ParsePosition).

You can take advantage of regular expressions to determine which format the string is in, and whether it matches any valid format. Something like this (not tested):
(Oops, I wrote this in C# before checking to see what language you were using.)
Regex test = new Regex(#"^(?:(?<formatA>\d{2}-[a-zA-Z]{3}-\d{2})|(?<formatB>\d{2}/\d{2}/\d{3}))$", RegexOption.Compiled);
Match match = test.Match(yourString);
if (match.Success)
{
if (!string.IsNullOrEmpty(match.Groups["formatA"]))
{
// Use format A.
}
else if (!string.IsNullOrEmpty(match.Groups["formatB"]))
{
// Use format B.
}
...
}

If you formats are exact (June 7th 1999 would be either 07-Jun-99 or 07/06/1999: you are sure that you have leading zeros), then you could just check for the length of the string before trying to parse.
Be careful with the short month name in the first version, because Jun may not be June in another language.
But if your data is coming from one database, then I would just convert all dates to the common format (it is one-off, but then you control the data and its format).

In this limited situation, the best (and fastest method) is certinally to parse out the day, then based on the next char either '/' or '-' try to parse out the rest. and if at any point there is unexpected data, return NULL then.

Assuming the patterns you gave are the only likely choices, I would look at the String passed in to see which format to apply.
public Date parseDate(final String date) {
if (date == null) {
return null;
}
SimpleDateFormat format = (date.charAt(2) == '/') ? new SimpleDateFormat("dd/MMM/yyyy")
: new SimpleDateFormat("dd-MMM-yy");
try {
return format.parse(date);
} catch (ParseException e) {
// Log a complaint and include date in the complaint
}
return null;
}
As others have mentioned, if you can guarantee that you will never access the DateFormats in a multi-threaded manner, you can make class-level or static instances.

Looks like three options if you only have two, known formats:
check for the presence of - or / first and start with that parsing for that format.
check the length since "dd-MMM-yy" and "dd/MM/yyyy" are different
use precompiled regular expressions
The latter seems unnecessary.

Use regular expressions to parse your string. Make sure that you keep both regex's pre-compiled (not create new on every method call, but store them as constants), and compare if it actually is faster then the try-catch you use.
I still find it strange that your method returns null if both versions fail rather then throwing an exception.

you could use split to determine which format to use
String[] parts = date.split("-");
df = (parts.length==3 ? format1 : format2);
That assumes they are all in one or the other format, you could improve the checking if need be

An alternative to creating a SimpleDateFormat (or two) per iteration would be to lazily populate a ThreadLocal container for these formats. This will solve both Thread safety concerns and concerns around object creation performance.

A simple utility class I have written for my project. Hope this helps someone.
Usage examples:
DateUtils.multiParse("1-12-12");
DateUtils.multiParse("2-24-2012");
DateUtils.multiParse("3/5/2012");
DateUtils.multiParse("2/16/12");
public class DateUtils {
private static List<SimpleDateFormat> dateFormats = new ArrayList<SimpleDateFormat>();
private Utils() {
dateFormats.add(new SimpleDateFormat("MM/dd/yy")); // must precede yyyy
dateFormats.add(new SimpleDateFormat("MM/dd/yyyy"));
dateFormats.add(new SimpleDateFormat("MM-dd-yy"));
dateFormats.add(new SimpleDateFormat("MM-dd-yyyy"));
}
private static Date tryToParse(String input, SimpleDateFormat format) {
Date date = null;
try {
date = format.parse(input);
} catch (ParseException e) {
}
return date;
}
public static Date multiParse(String input) {
Date date = null;
for (SimpleDateFormat format : dateFormats) {
date = tryToParse(input, format);
if (date != null) break;
}
return date;
}
}

On one hand I see nothing wrong with your use of try/catch for the purpose, it’s the option I would use. On the other hand there are alternatives:
Take a taste from the string before deciding how to parse it.
Use optional parts of the format pattern string.
For my demonstrations I am using java.time, the modern Java date and time API, because the Date class used in the question was always poorly designed and is now long outdated. For a date without time of day we need a java.time.LocalDate.
try-catch
Using try-catch with java.time looks like this:
DateTimeFormatter ddmmmuuFormatter = DateTimeFormatter.ofPattern("dd-MMM-uu", Locale.ENGLISH);
DateTimeFormatter ddmmuuuuFormatter = DateTimeFormatter.ofPattern("dd/MM/uuuu");
String dateString = "07-Jun-09";
LocalDate result;
try {
result = LocalDate.parse(dateString, ddmmmuuFormatter);
} catch (DateTimeParseException dtpe) {
result = LocalDate.parse(dateString, ddmmuuuuFormatter);
}
System.out.println("Date: " + result);
Output is:
Date: 2009-06-07
Suppose instead we defined the string as:
String dateString = "07/06/2009";
Then output is still the same.
Take a taste
If you prefer to avoid the try-catch construct, it’s easy to make a simple check to decide which of the formats your string conforms to. For example:
if (dateString.contains("-")) {
result = LocalDate.parse(dateString, ddmmmuuFormatter);
} else {
result = LocalDate.parse(dateString, ddmmuuuuFormatter);
}
The result is the same as before.
Use optional parts in the format pattern string
This is the option I like the least, but it’s short and presented for some measure of completeness.
DateTimeFormatter dateFormatter
= DateTimeFormatter.ofPattern("[dd-MMM-uu][dd/MM/uuuu]", Locale.ENGLISH);
LocalDate result = LocalDate.parse(dateString, dateFormatter);
The square brackets denote optional parts of the format. So Java first tries to parse using dd-MMM-uu. No matter if successful or not it then tries to parse the remainder of the string using dd/MM/uuuu. Given your two formats one of the attempts will succeed, and you have parsed the date. The result is still the same as above.
Link
Oracle tutorial: Date Time explaining how to use java.time.