JDK 9 Early Access builds might may also be provided by third parties. Oracle publishes regular JDK 9 builds at http://jdk9.java.net, for example.
Some of the changes targeted for JDK 9 might may affect code that relies on deprecated, removed, unsupported, internal or unspecified functionality. The list below has been compiled from the list of JEPs targeted for JDK 9 and other resources. It is not an exhaustive list of changes - there may be other changes you run into that have not been listed here.
Unified JVM Logging
This JEP might may change log message formats and possibly the meaning of some JVM options.
Remove GC Combinations Deprecated in JDK 8
This JEP might may affect code that relies on the removed GC combinations.
Modular Run-Time Images
This JEP might may affect code that relies on the removed endorsed-standards override mechanism, the removed extension mechanism, and the existence of rt.jar and tools.jar files as well as the old directory layout of files in the JDK & JRE installation image.
New Version-String Scheme
This JEP might may affect code that relies on the old version string scheme to distinguish major, minor, and security-update releases.
Remove Launch-Time JRE Version Selection
This JEP might may affect code that relies on the removed ability to request, at JRE launch time, a version of the JRE that is not the JRE being launched.
Remove the JVM TI hprof Agent
This JEP might may affect code that relies on the removed
hprof agent library as part of the JDK.
Remove the jhat Tool
This JEP might may affect code that relies on the removed experimental, unsupported, and out-of-date jhat tool.
Milling Project Coin
This JEP might may affect source code that uses underscore ("_") as an identifier. Its use generated a warning as of Java SE 8, and has been turned into an error in JDK 9.
Simplified Doclet API
JEP 221 may affect code that uses the Doclet API. It replaces the Doclet API with a simpler design that leverages newer alternative APIs with improved functionality, and updates the standard doclet to use it.
New JDK 9 features
REPL (aka Project Kulla)