Coming soon: serial lint warning enabled in the jdk build

[Joe Darcy]

Hello,

In my JDK 9 development forest, I have a collection of changes that will 
resolve all the serial warnings in the jdk repo. [1] Over the next few 
weeks, these changes will go through review and integration. Once all 
the warnings are resolved, the serial lint warning will be enabled in 
the build. [2]

Once that warning is enabled, all serializable classes must either 
declare a serialVersionUID or must have a @SuppressWarnings("serial") 
annotation.

If a class has the usual long-term serialization contract, then it 
should declare a serialVersionUID. This is true  both for classes that 
are part of Java SE (generally the java.* and javax.* namespaces) and 
classes that are marked as jdk.Exported(true) in the com.sun.* and jdk.* 
packages. If the class existed in JDK 8 or earlier, the serialVersionUID 
should match the result of the serialver computation in the earlier 
release. If the class is new in JDK 9, any value for the 
serialVersionUID can be used at the discretion of the maintainer of the 
class.

If a class is serializable but does *not* have a long-term serialization 
contract, such as many swing classes, then the class should have a 
@SuppressWarnings("serial") annotation with a comment explaining why the 
warning should be suppressed. In the patches I've worked on explanation 
have included:

* "Same-version serialization only"  -- swing serial contract
* "Superclass is not serializable across versions" -- subclass of a 
swing class
* "JDK implementation class" -- e.g. a sun.* class or a 
jdk.Exported(false) class
* "Anonymous class" -- anonymous classes and long-term serialization 
don't interact well

In particular, since all exception types are serializable (thanks RMI!), 
new exception types introduced in JDK 9 must either have a 
serialVersionUID or a @SuppressWarnings("serial") annotation.

Cheers,

-Joe

[1] JDK-8032976 Fix serial lint warnings in jdk libraries,
https://bugs.openjdk.java.net/browse/JDK-8032976

[2] JDK-8032977, Add serial lint warning to build of jdk repository,
https://bugs.openjdk.java.net/browse/JDK-8032977