What's the best way to get the current date/time in Java?
There is no "best" way.
It depends on what form of date / time you want:
If you want the date / time as a single numeric value, then
System.currentTimeMillis()gives you that, expressed as the number of milliseconds after the UNIX epoch (as a Javalong). This value is a delta from a UTC time-point, and is independent of the local time-zone1.If you want the date / time in a form that allows you to access the components (year, month, etc) numerically, you could use one of the following:
new Date()gives you aDateobject initialized with the current date / time. The problem is that theDateAPI methods are mostly flawed ... and deprecated.Calendar.getInstance()gives you aCalendarobject initialized with the current date / time, using the defaultLocaleandTimeZone. Other overloads allow you to use a specificLocaleand/orTimeZone. Calendar works ... but the APIs are still cumbersome.new org.joda.time.DateTime()gives you a Joda-time object initialized with the current date / time, using the default time zone and chronology. There are lots of other Joda alternatives ... too many to describe here. (But note that some people report that Joda time has performance issues.; e.g. https://stackoverflow.com/questions/6280829.)in Java 8, calling
java.time.LocalDateTime.now()andjava.time.ZonedDateTime.now()will give you representations2 for the current date / time.
Prior to Java 8, most people who know about these things recommended Joda-time as having (by far) the best Java APIs for doing things involving time point and duration calculations.
With Java 8 and later, the standard java.time package is recommended. Joda time is now considered "obsolete", and the Joda maintainers are recommending that people migrate3.
Note: the Calendar, org.joda.time and java.time solutions can use either the platform's default timezone or an explicit timezone provided via constructor arguments. Generally, using an explicit timezone rather than the default zone will make your application's behavior more predictable / less susceptible to problems if (for example) you redeploy to a data center in a different timezone.
But no matter what you do, you (and maybe your application) should be aware that the timezone of the user, your service and the data center can all be different. The concept of the "current date/time" is complicated.
1 - System.currentTimeMillis() gives the "system" time. While it is normal practice for the system clock to be set to (nominal) UTC, there will be a difference (a delta) between the local UTC clock and true UTC. The size of the delta depends on how well (and how often) the system's clock is synced with UTC.
2 - Note that LocalDateTime doesn't include a time zone. As the javadoc says: "It cannot represent an instant on the time-line without additional information such as an offset or time-zone."
3 - Note: your Java 8 code won't break if you don't migrate, but the Joda codebase may eventually stop getting bug fixes and other patches. As of 2020-02, an official "end of life" for Joda has not been announced, and the Joda APIs have not been marked as Deprecated.
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What's the best way to get the current date/time in Java?
There is no "best" way.
It depends on what form of date / time you want:
If you want the date / time as a single numeric value, then
System.currentTimeMillis()gives you that, expressed as the number of milliseconds after the UNIX epoch (as a Javalong). This value is a delta from a UTC time-point, and is independent of the local time-zone1.If you want the date / time in a form that allows you to access the components (year, month, etc) numerically, you could use one of the following:
new Date()gives you aDateobject initialized with the current date / time. The problem is that theDateAPI methods are mostly flawed ... and deprecated.Calendar.getInstance()gives you aCalendarobject initialized with the current date / time, using the defaultLocaleandTimeZone. Other overloads allow you to use a specificLocaleand/orTimeZone. Calendar works ... but the APIs are still cumbersome.new org.joda.time.DateTime()gives you a Joda-time object initialized with the current date / time, using the default time zone and chronology. There are lots of other Joda alternatives ... too many to describe here. (But note that some people report that Joda time has performance issues.; e.g. https://stackoverflow.com/questions/6280829.)in Java 8, calling
java.time.LocalDateTime.now()andjava.time.ZonedDateTime.now()will give you representations2 for the current date / time.
Prior to Java 8, most people who know about these things recommended Joda-time as having (by far) the best Java APIs for doing things involving time point and duration calculations.
With Java 8 and later, the standard java.time package is recommended. Joda time is now considered "obsolete", and the Joda maintainers are recommending that people migrate3.
Note: the Calendar, org.joda.time and java.time solutions can use either the platform's default timezone or an explicit timezone provided via constructor arguments. Generally, using an explicit timezone rather than the default zone will make your application's behavior more predictable / less susceptible to problems if (for example) you redeploy to a data center in a different timezone.
But no matter what you do, you (and maybe your application) should be aware that the timezone of the user, your service and the data center can all be different. The concept of the "current date/time" is complicated.
1 - System.currentTimeMillis() gives the "system" time. While it is normal practice for the system clock to be set to (nominal) UTC, there will be a difference (a delta) between the local UTC clock and true UTC. The size of the delta depends on how well (and how often) the system's clock is synced with UTC.
2 - Note that LocalDateTime doesn't include a time zone. As the javadoc says: "It cannot represent an instant on the time-line without additional information such as an offset or time-zone."
3 - Note: your Java 8 code won't break if you don't migrate, but the Joda codebase may eventually stop getting bug fixes and other patches. As of 2020-02, an official "end of life" for Joda has not been announced, and the Joda APIs have not been marked as Deprecated.
(Attention: only for use with Java versions <8. For Java 8+ check other replies.)
If you just need to output a time stamp in format YYYY.MM.DD-HH.MM.SS (very frequent case) then here's the way to do it:
String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(Calendar.getInstance().getTime());
There is always the old-fashioned way:
long startTime = System.nanoTime();
methodToTime();
long endTime = System.nanoTime();
long duration = (endTime - startTime); //divide by 1000000 to get milliseconds.
I go with the simple answer. Works for me.
long startTime = System.currentTimeMillis();
doReallyLongThing();
long endTime = System.currentTimeMillis();
System.out.println("That took " + (endTime - startTime) + " milliseconds");
It works quite well. The resolution is obviously only to the millisecond, you can do better with System.nanoTime(). There are some limitations to both (operating system schedule slices, etc.) but this works pretty well.
Average across a couple of runs (the more the better) and you'll get a decent idea.
Unfortunately, none of the ten answers posted so far are quite right.
If you are measuring elapsed time, and you want it to be correct, you must use System.nanoTime(). You cannot use System.currentTimeMillis(), unless you don't mind your result being wrong.
The purpose of nanoTime is to measure elapsed time, and the purpose of currentTimeMillis is to measure wall-clock time. You can't use the one for the other purpose. The reason is that no computer's clock is perfect; it always drifts and occasionally needs to be corrected. This correction might either happen manually, or in the case of most machines, there's a process that runs and continually issues small corrections to the system clock ("wall clock"). These tend to happen often. Another such correction happens whenever there is a leap second.
Since nanoTime's purpose is to measure elapsed time, it is unaffected by any of these small corrections. It is what you want to use. Any timings currently underway with currentTimeMillis will be off -- possibly even negative.
You may say, "this doesn't sound like it would ever really matter that much," to which I say, maybe not, but overall, isn't correct code just better than incorrect code? Besides, nanoTime is shorter to type anyway.
Previously posted disclaimers about nanoTime usually having only microsecond precision are valid. Also it can take more than a whole microsecond to invoke, depending on circumstances (as can the other one), so don't expect to time very very small intervals correctly.
Which types to use in order to accomplish this in Java?
The short answer is a long. Now, more on how to measure...
System.currentTimeMillis()
The "traditional" way to do this is indeed to use System.currentTimeMillis():
long startTime = System.currentTimeMillis();
// ... do something ...
long estimatedTime = System.currentTimeMillis() - startTime;
o.a.c.l.t.StopWatch
Note that Commons Lang has a StopWatch class that can be used to measure execution time in milliseconds. It has methods methods like split(), suspend(), resume(), etc that allow to take measure at different points of the execution and that you may find convenient. Have a look at it.
System.nanoTime()
You may prefer to use System.nanoTime() if you are looking for extremely precise measurements of elapsed time. From its javadoc:
long startTime = System.nanoTime();
// ... the code being measured ...
long estimatedTime = System.nanoTime() - startTime;
Jamon
Another option would be to use JAMon, a tool that gathers statistics (execution time, number of hit, average execution time, min, max, etc) for any code that comes between start() and stop() methods. Below, a very simple example:
import com.jamonapi.*;
...
Monitor mon=MonitorFactory.start("myFirstMonitor");
...Code Being Timed...
mon.stop();
Check out this article on www.javaperformancetunning.com for a nice introduction.
Using AOP
Finally, if you don't want to clutter your code with these measurement (or if you can't change existing code), then AOP would be a perfect weapon. I'm not going to discuss this very deeply but I wanted at least to mention it.
Below, a very simple aspect using AspectJ and JAMon (here, the short name of the pointcut will be used for the JAMon monitor, hence the call to thisJoinPoint.toShortString()):
public aspect MonitorAspect {
pointcut monitor() : execution(* *.ClassToMonitor.methodToMonitor(..));
Object arround() : monitor() {
Monitor monitor = MonitorFactory.start(thisJoinPoint.toShortString());
Object returnedObject = proceed();
monitor.stop();
return returnedObject;
}
}
The pointcut definition could be easily adapted to monitor any method based on the class name, the package name, the method name, or any combination of these. Measurement is really a perfect use case for AOP.