FULL PRODUCT VERSION :
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b17, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux wunderkund 3.2.0-61-generic #92-Ubuntu SMP Mon Mar 31 23:47:59 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
The following error was generated using JRE 8u65, activation.jar 1.1, and javax.mail 1.5.2.

java.lang.NoClassDefFoundError: sun/nio/cs/ext/MS1256$Decoder
    at sun.nio.cs.ext.MS1256.newDecoder(MS1256.java:43) ~[na:na]
    at sun.nio.cs.StreamDecoder.<init>(StreamDecoder.java:232) ~[na:1.8.0_66]
    at sun.nio.cs.StreamDecoder.forInputStreamReader(StreamDecoder.java:69) ~[na:1.8.0_66]
    at java.io.InputStreamReader.<init>(InputStreamReader.java:100) ~[na:1.8.0_66]
    at com.sun.mail.handlers.text_plain.getContent(text_plain.java:107) ~[javax.mail-1.5.2.jar:1.5.2]
    at javax.activation.DataSourceDataContentHandler.getContent(DataHandler.java:795) ~[na:1.8.0_66]
    at javax.activation.DataHandler.getContent(DataHandler.java:542) ~[na:1.8.0_66]
    at javax.mail.internet.MimeBodyPart.getContent(MimeBodyPart.java:657) ~[javax.mail-1.5.2.jar:1.5.2]

It appears that Java is looking for the classes MS1256$Decoder and JIS_X_0201$Decoder. I do see JIS_X_0201.class and MS1256.class inside the charsets.jar, by they don't appear to have a Decoder inner class.

jre64/jre/lib/sun/nio/cs/ext# ls *JIS_X_0201*
JIS_X_0201.class

jre64/jre/lib/sun/nio/cs/ext# ls *MS1256*
MS1256.class

Note: There may be others charsets that observe the same behaviour. These are the ones we have come across.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
 create an InputStreamReader using JIS_X_0201 or MS1256

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
We do not expect to see a java.lang.NoClassDefFoundError. This is a critical error and in some cases can cause servers to due completely.
ACTUAL -

java.lang.NoClassDefFoundError: sun/nio/cs/ext/MS1256$Decoder
java.lang.NoClassDefFoundError: sun/nio/cs/ext/JIS_X_0201$Decoder

ERROR MESSAGES/STACK TRACES THAT OCCUR :

java.lang.NoClassDefFoundError: sun/nio/cs/ext/MS1256$Decoder
    at sun.nio.cs.ext.MS1256.newDecoder(MS1256.java:43) ~[na:na]
    at sun.nio.cs.StreamDecoder.<init>(StreamDecoder.java:232) ~[na:1.8.0_66]
    at sun.nio.cs.StreamDecoder.forInputStreamReader(StreamDecoder.java:69) ~[na:1.8.0_66]
    at java.io.InputStreamReader.<init>(InputStreamReader.java:100) ~[na:1.8.0_66]
    at com.sun.mail.handlers.text_plain.getContent(text_plain.java:107) ~[javax.mail-1.5.2.jar:1.5.2]
    at javax.activation.DataSourceDataContentHandler.getContent(DataHandler.java:795) ~[na:1.8.0_66]
    at javax.activation.DataHandler.getContent(DataHandler.java:542) ~[na:1.8.0_66]
    at javax.mail.internet.MimeBodyPart.getContent(MimeBodyPart.java:657) ~[javax.mail-1.5.2.jar:1.5.2]

REPRODUCIBILITY :
This bug can be reproduced always.