Java 13 have introduced three new methods to java.nio.file.FileSystems to treat the content of a file as a file system easily.
newFileSystem(Path, Map<String, ?>)
newFileSystem(Path, Map<String, ?>, ClassLoader)
Following are other major changes added to language.
java.time – Japanese era name added
javax.crypto – MS Cryptography Next Generation (CNG)support
javax.security – jdk.sasl.disabledMechanisms property added disabling SASL mechanisms
javax.xml.crypto – String constants introduced to represent Canonical XML 1.1 URIs
javax.xml.parsers – Methods added to instantiate DOM and SAX factories for namespaces support
Unicode support is upgraded to version 12.1
Kerberos principal name canonicalization support added, cross-realm referrals are supported.
SocketImpl implementations prior to JDK 1.4
javadoc tool old features
Other details are available at following link.APIs proposed for removal.