• [BREAKING] Generics handling is more robust now, the former optimization for smaller size (but increased serialization time) is now optional and disabled by default.
  Important: This change breaks the serialization format of the FieldSerializer for generic fields, therefore generic classes serialized with Kryo 3 and FieldSerializer cannot be deserialized with Kryo 4 by default. To deserialize such Kryo 3 serialized generic classes you have to set kryo.getFieldSerializerConfig().setOptimizedGenerics(true);!
  For details see #433: Disable the optimization of generics serialization and provide an API for enabling/disabling it (9923d05).
  This improves/fixes #377 "Kryo does not correctly support parameterized type hierarchy", #384 "ClassCastException When Writing a Map After Previously Reading a Map with Different Generic Type"
• #424 Fixed (Compatible)FieldSerializer inheritance issue (5a7b7c5)
• Update reflectasm to latest version, reduction of synchronization (132456e, d77c752)
• #352 Add forward compatibility to TaggedFieldSerializer (93bff2d)
• #418 Add checks to setting output buffer and throw IllegalArgumentException when setting a buffer size greater than the max buffer size (d57e00c)
• #417 Make FieldSerializer.serializeTransient usable (8aae38d)
• [BREAKING] #415 Make Closure public, moving it to ClosureSerializer (bf80397, 6c402d9) Fixes #299 "Can't register ClosureSerializer"
• #414 Introduce FieldSerializerConfig to encapsulate config in Kryo (4a47981)
• Fix #412: Add default serializer for java.net.URL (c10fd76)
• #404 Use system property to detect Android (c238f97)
• #396 Adds Charset serializer, also set as default serializer (d7e924b) Resolves #364 "Few of the java Charsets classes from rt.jar can not be deserialized"
• #395 Adds support for java 8 java.time.* classes (aff0689)
• [BREAKING] #392 Move Generics{,Resolver} to serializers, reduce public api (e22ffb8)
• #400 Modified utility hash maps to gracefully handle very large number of entries. (2b8e6fd) Resolves #382 "Kryo breaks down while serializing genomic data"
• Adds serializers for java8 Optional{Int,Long,Double} (fb65ee9)
• #362 Add java8 support and serializer for java.util.Optional (12229ca)
• #393 Fix #389 revert ByteOrder on overflow in require during writeVar_() during writeVar_() (7d840a0)
• #394 Fix positioning and order updates in ByteBufferInput and ByteBufferOutput (88f5875)
• #388 Resolve #386: Automatically test serialization compatibility (c904c7e)
• #375 Fix #370 copyTransient is not global (8414444, 481ddf4, 1ada47d)
• #368) Try to load classes with fallback that uses current ClassLoader. (8dee484)
• Update objenesis to 2.2 (ba67801)
• #359 Use binary search to find field in CompatibleFieldSerializer for object with lots of fields (8678e50)
• Fixed #346 Issue resizing UnsafeMemoryOutput on OSX 10.10.5 (50d1a6f)
• Fixed bug #340 KryoException: Encountered unregistered class ID: <negative_number> (2fdf64e)
• #344 Propagate flush to underlying stream (b3d6bda)
• #342 Prevent creation of cyclic Generics stack when serializing specific generics object trees
• #337/#338 ByteBufferInputStream violates InputStream contract (cf06241)
• #297 Warn unregistered classes option (fd7d1b2)

Here's the list of all commits since 3.0.3

And most importantly; many thanks to all contributors!

Compatibility

• Serialization compatible
  • Standard IO: No (format for generic fields written by FieldSerializer changed, compatibility can be achieved with kryo.getFieldSerializerConfig().setOptimizedGenerics(true);)
  • Unsafe-based IO: No (format for generic fields written by FieldSerializer changed, compatibility can be achieved with kryo.getFieldSerializerConfig().setOptimizedGenerics(true);)
• Binary compatible - No (Details)
• Source compatible - No (Details)