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JEP 213 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.

Module System

JEP 261 may affect source code that is not compiled with the javac compiler using the -source, -target or -release options to select the use of legacy mode. 

The stack traces generated for exceptions at run time have been extended to include, when present, the names and version strings of relevant modules. The detail strings of exceptions such as ClassCastException,IllegalAccessException, and IllegalAccessError have also been updated to include module information. Any code analysing, or filtering, based on the stack trace element's detail strings should be updated appropriately, to handle this.

In addition, this feature may affect code that assumes that the application class loader or the extension class loader is an instance of URLClassLoader, rather than of an internal class.

In addition, this feature may affect code that attempts to set or read the bootclasspath using the -Xbootclasspath  or the -Xbootclasspath/p options, or the removed  JDK-specific system property sun.boot.class.path.

Removed support for 1.5 and earlier source and target options

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