Attachment

Attachment is the wrapper returned by Context#getAttachment(), removeAttachment(), and findAttachment(int).

Methods

Class<?> getType()
<T> Optional<T> to(Class<T> type)

Behavior

• a context stores at most one attachment value; setting null clears the attachment
• setAttachment(value) replaces the current value
• getAttachment() only reads the current context
• findAttachment(level) reads the current or ancestor context
• removeAttachment() removes the current context's value and returns it as an Attachment

If no attachment is present on the target context (never set, or cleared via setAttachment(null) or removeAttachment()), the Context methods return Optional.empty(). findAttachment(level) still throws if level is negative or the ancestor does not exist.

Attachment#to(...):

• returns Optional.empty() only when the underlying value is absent
• otherwise uses Class#cast, so an incompatible type causes ClassCastException

Example

context.setAttachment(new TestAttachment("value"));

String value = context.getAttachment()
        .flatMap(a -> a.to(TestAttachment.class))
        .map(TestAttachment::value)
        .orElseThrow();

Ancestor lookup

Attachments are not globally shared. A child action can only reach ancestor data by navigating the context hierarchy.

TestAttachment attachment = context.findAttachment(1)
        .flatMap(a -> a.to(TestAttachment.class))
        .orElseThrow();