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description= Detailed Java 12 guide: migration, versions; switch expressions, teeing collectors, indenting/transforming strings (and more); default CDS, Shenandoah, G1.;
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Text of the page (random words):
ty before it is set in stone incubator modules for apis preview features for language and jvm changes both mechanisms work in a similar way they allow easy experimentation with new and possibly unstable features they prevent accidental dependencies on them by requiring command line flags during compilation and execution the only incubator module was the reactive http 2 client in java 9 and 10 got finalized in 11 and the only preview feature are switch expressions in java 12 see below which means we can stick to the latter to activate preview features you need to use the flag enable preview this is how to do it in maven plugin artifactid maven compiler plugin artifactid configuration compilerargs enable preview compilerargs configuration plugin plugin artifactid maven surefire plugin artifactid configuration argline enable preview argline configuration plugin and in gradle compilejava options compilerargs enable preview test jvmargs enable preview in intellij set the language level to 12 preview switch expressions for me this only works if i set for the module i e setting it just for the project doesn t cut it language features and api updates you ve decided to move to java 12 updated your tools and dependencies and bumped the compiler s source and target version here s what you get in exchange switch expressions so much has already been written about switch expressions that i don t want to keep you for long the numerous details aside it comes down to switch no longer just being a statement which directs where computation goes like if but an expression which is itself computed to a result like the conditional ternary operator the main use case will be to assign the computed value to a variable https thedailywtf com articles what_is_truth_0x3f_ boolean bool switch ternarybool case true true case false false case file_not_found throw new uncheckedioexception this is ridiculous new filenotfoundexception some of the details i just put aside are greatly anticipated improvements multiple case labels e g case true false no fall through from one case to the next compiler checks exhaustiveness that s why there s no default branch in the example definitive guide to switch expressions in java 12 first contact with switch expressions in java 12 video jep 325 switch expressions remember that this is a preview feature so be aware that it may change in future releases until it s stabilized don t bet too much of your internal code on it and never publish code that uses it to use it in experiments add enable preview to compiler and jvm commands don t publish code that uses switch expressions teeing collectors sometimes you need to collect two pieces of information from a stream pipeline but doing that before java 12 wasn t exactly comfortable see this example where i want to determine a stream s smallest and greatest element so i can use them to create a range range integer range stream of 1 8 2 5 reduce the initial range parameters are min and max in that order so this range is empty range of integer max_value integer min_value combining an existing range with the next number from the stream _range number int newmin math min number _range min int newmax math max number _range max return range of newmin newmax combining two ranges needed at the end of a parallel stream _range1 _range2 int newmin math min _range1 min _range2 min int newmax math max _range1 max _range2 max return range of newmin newmax the annoying thing is that there are collectors minby and maxby which could do most of the work for me if only i could use both of them from java 12 on we can do just that by passing them to the teeing collector static method collectors teeing just like unix tee command it forwards each element the stream passes to it to the two specified collectors once the stream is exhausted it combines the two results into a single instance with the third argument you specify a function with the teeing collector i can solve the problem as follows range string range stream of 1 8 2 5 collect collectors teeing the collectors produce optional integer collectors minby integer compareto collectors maxby integer compareto i wrote a static factory method that creates a range from two optional integer range ofoptional orelsethrow new illegalstateexception non empty stream was empty much better right teeing collector in java 12 jdk 8209685 create collector which merges results of two other collectors more versatile error recovery with completablefuture the api of completablefuture is already immense here s a thorough introduction and in java 12 it gets a little larger the reason why it s so big in the first place is the combinatorial explosion of mostly orthogonal requirements various actions e g thenapply for result bearing methods akin to stream map or completablefuture bearing methods akin to stream flatmap after one one of two or two of two actions complete e g thenapply applytoeither and thencombine in an unspecified thread explicitly as a new task with async suffix or as a new task with a specific executor with async suffix and executor argument java 12 ticks a few boxes on the feature matrix that were previously empty they relate to error recovery and are add ons to the pre existing method exceptionally function throwable t which recovers from a failed computation by turning the exception into a normal result there are five new methods exceptionallycompose function throwable completionstage t is to exceptionally like stream flatmap is to stream map you can pass a function that produces a completionstage supertype of completablefuture new async overloads for exceptionally and exceptionallycompose once with the same arguments once with an additional executor this gives us more tools to recover from all the things that can break out there jdk 8210971 indenting and transforming strings imagine we had raw string literals in java 12 alas we don t wouldn t it be handy to quickly fix their indentation public string createhtml assume four space indentation return html body h1 header h1 body two levels of indentation imply eight spaces to get rid of indent 8 what if after discovering the new line character for new lines you decided to go all in on not abusing spaces and use an indentation character for indentation crazy i know or what if you prefer to specify the target indentation instead of the change of indentation which would be stable under refactoring given a method that does what you want you could of course simply call it with the string public string createhtml assume tab indentation we want no indentation so we call our method string setindentationtodepth string int with the raw string and 0 return html setindentationtodepth body h1 header h1 body 0 ugh i know compared to indent that s clumsy thankfully there s a way out public string createhtml same as before return html body h1 header h1 body but here we call transform which expects a function string t transform s setindentationtodepth s 0 the method transform simply passes the instance that it s called on to the specified function neat now as you may have noticed we didn t get raw string literals in java 12 so why are these methods still in there good question for once i didn t scour the mailing lists but dustin marx did and i highly recommend you read his article on the topic if you want to know more about string transform the brief but complicated history of jdk 12 s string transform method an interesting tidbit is that transform may show up in other places for example on stream or on optional which means you could apply your methods that modify a stream or optional in a fluent call chain inspired by jdk 8140283 given this method public stream t maybeaddfilter stream t s if condition return s filter else return s and this pipeline source stream map collect tolist this is how to call maybeaddfilter maybeaddfilter source stream map collect tolist once again ugh what about transform source stream map transform this maybeaddfilter collect tolist that would be pretty awesome compact number format need to format upvotes subscribers or followers in a social media compliant manner where 5412 becomes 5 4 k the new compactnumberformat is there for you compactnumberformat has a constructor but getting an instance from numberformat getcompactnumberinstance is easier numberformat followers numberformat getcompactnumberinstance new locale en us style short followers setmaximumfractiondigits 1 prints 5 4k followers system out println followers format 5412 followers compact number formatting comes to jdk 12 mismatching files the utility class files got a new utility the method mismatch path path compares the two specified files and returns the index of the first byte where they differ or 1 if they don t long mismatchindex files mismatch path1 path2 boolean match mismatchindex 1 if match system out println files match else system out println files first difference is at index mismatchindex jvm improvements besides the language and apis the jvm has of course also seen improvements default cds archives what does the jvm do when it needs to load a class looks the class up in a jar loads the bytes verifies the bytecode pulls it into an internal data structure for each used class this process is repeated every time the jvm is relaunched even though as long as the class is unchanged it always leads to the same result class data sharing cds removes the redundancy by storing the internal data structure the so called class data archive in a file and then memory mapping it on future launches so the classes don t have to be loaded again there are two kinds of cds regular cds archives jdk classes appcds archives jdk and application classes cds has been in the jdk for a while and java 10 unlocked application class data sharing as a free feature now java 12 ships with an archive for the jdk classes and uses it by default java 12 uses a cds archive for jdk classes you can easily observe the effect by launching an application with java 12 once without additional command line options cds is on by default and once with xshare off turning cds off on my laptop launching a simple hello world takes 50 ms with and 100ms without cds if i launch it as a single source file java helloworld java java xshare off helloworld java it s 120ms and 170ms so apparently the compiler doesn t need a ton of classes there s nothing you need to do to benefit from default cds archives but you may notice a related message when launching a java program from your ide openjdk 64 bit server vm warning sharing is only supported for boot loader classes because bootstrap classpath has been appended the archive is essentially a cache and the jvm needs to invalidate classes that may be different if loaded by a class loader than if taken from the archive if the bootstrap class path is tampered with only archived classes that were originally loaded by the boot class loader are guaranteed to be correct and so the jvm sticks to them that s what the message tells you improve launch times on java 10 with application class data sharing jep 341 default cds archives garbage collection shenandoah red hat is working on a new low pause time garbage collector dubbed shenandoah shenandoah reduces gc pause times by doing evacuation work concurrently with the running java threads pause times with shenandoah are independent of heap size meaning you will have the same consistent pause times whether your heap is 200 mb or 200 gb shenandoah is an appropriate algorithm for applications which value responsiveness and predictable short pauses java 12 ships an experimental version of it shenandoah gc the garbage collector that could talk at javazone by aleksey shipilev jep 189 shenandoah a low pause time garbage collector g1 oracle s garbage first g1 collector also sees continuous improvements most notably that it now promptly returns unused memory to the operating system something shenandoah does as well g1 only returns memory from the java heap at either a full gc or during a concurrent cycle since g1 tries hard to completely avoid full gcs and only triggers a concurrent cycle based on java heap occupancy and allocation activity it will not return java heap memory in many cases unless forced to do so externally from java 12 on g1 uses phases during which the application shows little activity to trigger continue a concurrent cycle thus using the existing functionality to make unused memory available to other processes more quickly this is particularly interesting for applications that run in the cloud because it enables them to be more elastic with their resource requirements thus becoming cheaper to run jep 344 abortable mixed collections for g1 jep 346 promptly return unused committed memory from g1 constants api apparently an improvement that helps bytecode parsers and manipulation libraries compilers and offline transformers like jlink as well as some code in the jdk but i m in over my head here you ve got to read the jep yourself jep 334 jvm constants api security enhancements java 12 ships with a number of security enhancements that oracle s sean mullan discusses on his blog jdk 12 security enhancements removed and deprecated removed finalize on fileinputstream fileoutputstream inflater deflater zipfile overrides of getcause in classnotfoundexception exceptionininitializererror undeclaredthrowableexception privilegedactionexception the only notable new deprecations i found are in sun misc unsafe getobject getobjectvolatile getobjectacquire getobjectopaque putobject putobjectvolatile putobjectopaque putobjectrelease getandsetobject getandsetobjectacquire getandsetobjectrelease compareandsetobject compareandexchangeobject compareandexchangeobjectacquire compareandexchangeobjectrelease weakcompareandsetobject weakcompareandsetobjectacquire weakcompareandsetobjectplain weakcompareandsetobjectrelease all of these methods are marked for removal summary executive summary on migration considerations no known technical challenges when moving to 12 except increased bytecode level free lts for java 12 16 is unlikely commercial lts for 12 16 is sparse moving from 11 to 12 likely implies that you have to make it all the way to 17 new features switch as an expression preview feature use two stream collectors and combine their results with collectors teeing more varied completablefuture error recovery with exceptionallycompose exceptionallycomposeasync and exceptionallyasync manage a multi line string s indentation with string indent call string processing methods in a method chain with string transform format numbers as 5 4k with compactnumberformat that you get from numberformat getcompactnumberinstance compare files byte by byte with files mismatch improved jvm shorter jvm boot times thanks to default class data sharing for jdk classes new garbage collector shenandoah improvements to default garbage collector g1 something something jvm constants have fun when you delve into java 12 i ll sh...
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