Context::sessionAttribute(String) can return null value if attribute is not set but is typed as 'T' instead of 'T?'

Actual behavior (the bug) Getting an attribute with Context::sessionAttribute(String) (which returns null if the attribute isn’t set) returns a T instead of T?.

This forces Kotlin users to add @Suppress("SENSELESS_COMPARISON") when checking if an attribute is null (or have an unexpected runtime NullPointerException).

Expected behavior sessionAttribute(String) should return T? instead of T.

Additional context Although an easy fix, this will “break” the API for Kotlin users that currently don’t have a null check when getting an attribute.

[FredDeschenes]

Yeah I’ve been looking at all possible paths the setAttribute calls can take in Jetty’s source and turns out they don’t all have the same side effects when sending a null value: all of them will remove the attribute, except UpgradeHttpServletRequest which will just add the attribute with a null value -_-

I’ll go ahead and revert this back to what was already present except for the nullable stuff.

[tipsy]

I see that other attribute functions have the same problem:

fun <T> attribute(attribute: String): T
fun <T> attributeMap(): Map<String, T>
fun <T> sessionAttribute(attribute: String): T
fun <T> sessionAttributeMap(): Map<String, T>

If you’d like to create a PR, please include all of them.