This issue will track Ruby 2.5 support for JRuby. This will be released in JRuby 9.3.0.0 or JRuby 10.

As with previous releases, we will base our checklist on MRI's NEWS file: https://github.com/ruby/ruby/blob/trunk/NEWS
This bug will track all NEWS items new in Ruby 2.5. This list is based off https://github.com/ruby/ruby/blob/trunk/NEWS.

Only changes relevant to JRuby are listed here. MRI-specific internal changes and features are not included.

NOTE: Pull requests should be done against the ruby-2.5 branch.

Language changes

• Top-level constant look-up is removed. Feature #11547
• rescue/else/ensure are allowed inside do/end blocks. Feature #12906
• refinements take place in string interpolations. Feature #13812

Core classes updates (outstanding ones only)

Array

• Array#append Feature #12746
• Array#prepend Feature #12746

Data

• Now deprecated Feature #3072

Dir

• Dir.glob provides new optional keyword argument, :base. Feature #13056
• Dir.children Feature #11302
• Dir.each_child Feature #11302

File

• :newline option to File.open implies text mode now. Bug #13350
• File#path now raises an IOError for files opened with File::Constants::TMPFILE option. Feature #13568
• Add File.lutime Feature #4052

Hash

• Hash#transform_keys Feature #13583
• Hash#transform_keys! Feature #13583
• Hash#slice Feature #8499

IO

• IO#pread Feature #4532
• IO#pwrite Feature #4532
• IO#copy_stream tries copy offload with copy_file_range(2) Feature #13867
• IO#write accepts multiple arguments Feature #9323

IOError

• exception message "stream closed" is changed Bug #13405

Integer

• Integer.sqrt Feature #13219
• Integer#step does no longer rescue exceptions when given a step value which cannot be compared with #> to 0. Feature #7688
• Integer#{round,floor,ceil,truncate} now always return an Integer. Bug #13420
• Integer#pow added

Kernel

• Kernel#yield_self Feature #6721
• Kernel#pp Feature #14123

Module

• Module#attr, attr_accessor, attr_reader, attr_writer are now public [Feature #14132]
• Module#define_method, alias_method, undef_method, remove_method are now public [Feature #14133]

Numeric

• Numerical comparison operators (< ,<=, >=, >) no longer rescue exceptions of #coerce. Return nil in #coerce if the coercion is impossible. Feature #7688

Process

• Precision of Process.times is improved if getrusage(2) exists. Feature #11952

Range

• Range#initialize no longer rescue exceptions when comparing begin and
  end with #<=> and raise a "bad value for range" ArgumentError
  but instead let the exception from the #<=> call go through.
  Feature #7688

Regexp

• Update to Onigmo 6.1.1.
• Support absent operator Implement Absent Operator k-takata/Onigmo#82

String

• String#-@ deduplicates unfrozen strings. Already-frozen strings remain unchanged for compatibility. Feature #13077
• -"literal" (String#-@) optimized to return the same object (same as "literal".freeze in Ruby 2.1+) Feature #13295
• String#{casecmp,casecmp?} now return nil for non-string arguments instead of raising a TypeError. Bug #13312
• String#delete_prefix is added to remove prefix Feature #12694
• String#delete_prefix! is added to remove prefix destructively Feature #12694
• String#delete_suffix is added to remove suffix Feature #13665
• String#delete_suffix! is added to remove suffix destructively Feature #13665
• String#each_grapheme_cluster and String#grapheme_clusters is added to enumerate grapheme clusters Feature #13780
• String#start_with? supports regexp Feature #13712

Regexp/String:

• Update Unicode version from 9.0.0 to 10.0.0 Feature #13685

Thread

• Thread#fetch Feature #13009 (Implement fetch for Thread.current #5089)
• Description set by Thread#name= is now visible on Windows 10.

Time

• Time#at receives 3rd argument which specifies the unit of 2nd argument. Feature #13919

KeyError

• KeyError#receiver Feature #12063
• KeyError#key Feature #12063

Stdlib updates (outstanding ones only)

Bundler

• Add Bundler to Standard Library. Feature #12733

Pathname

• New method: Pathname#glob Feature #7360 (Started in Pathname glob #5095)

Psych

• Update to Psych 3.0.0.
• Add :symbolize_names option to Psych.load, Psych.safe_load like JSON.parse Add :symbolize_names option to Psych.load like JSON.parse ruby/psych#333, Add :symbolize_names option to .safe_load too ruby/psych#337
• Add Psych::Handler#event_location Add Psych::Handler#event_location ruby/psych#326
• Make frozen string literal = true Make frozen string literal = true ruby/psych#320
• Preserve time zone offset when deserializing times Preserve time zone offset when deserializing times ruby/psych#316
• Removed deprecated method aliases for syck gem Remove deprecated methods ruby/psych#312

RbConfig

• New constants: RbConfig::LIMITS is added to provide the limits of C types. This is available when rbconfig/sizeof is required. (Partial support implemented, additional types can be added as needed)

Ripper

• New method: Ripper#state is added to tell the state of scanner. Feature #13686
• New constants: Ripper::EXPR_BEG and so on for Ripper#state.

SecureRandom

• New methods: SecureRandom.alphanumeric

Set

• Add Set#to_ as alias to #inspect Feature #13676
• Add Set#=== as alias to #include? Feature #13801
• Add Set#reset Feature #6589

StringIO

• StringIO#write accepts multiple arguments

StringScanner

• Add StringScanner#size Feature #836
• Add StringScanner#captures Feature #836
• Add StringScanner#values_at Feature #836

Compatibility issues (excluding feature bug fixes)

BasicSocket

• BasicSocket#read_nonblock and BasicSocket#write_nonblock no longer sets the O_NONBLOCK file description flag as side effect (on Linux only) Feature #13362

Random

• Random.raw_seed renamed to become Random.urandom. It is now applicable to non-seeding purposes due to Bug #9569.

Socket::Ifaddr

• Socket::Ifaddr#vhid is added Feature #13803

ConditionVariable, Queue and SizedQueue reimplemented for speed.

• They no longer subclass Struct. Feature #13552

Stdlib compatibility issues (excluding feature bug fixes)

mathn.rb

• Removed from stdlib. Feature #10169

Rubygems

• Removed "ubygems.rb" file from stdlib. It's needless since Ruby 1.9.

C API updates

(Probably not JRuby-relevant.)

Supported platform changes

Implementation improvements

(Probably not JRuby-relevant.)

• This might not be a "user visible feature change" but) Hash class's
  hash function is now SipHash13. Feature #13017
• SecureRandom now prefers OS-provided sources than OpenSSL. Bug #9569
• Mutex rewritten to be smaller and faster Feature #13517
• Performance of block passing using block parameters is improved by lazy Proc allocation Feature #14045
• Dynamic instrumentation for TracePoint hooks instead of using "trace" instruction to avoid overhead Feature #14104

Miscellaneous changes

• Print backtrace and error message in reverse order if STDERR is unchanged and a tty. Feature #8661 [experimental] (Started in Reverse the order in which we print out the backtrace. #4876 #5094)
• Print error message in bold/underlined text if STDERR is unchanged and a tty. Feature #14140 [experimental] (Started in Reverse the order in which we print out the backtrace. #4876 #5094)