Explain the execution flow of a BeanShell method closure capturing its enclosing scope?

When a scripted method is defined inside a script, BeanShell records a reference to the NameSpace that was current at the point of definition, associating that reference with the method - this is the moment the closure's capture actually happens, not later when the method is eventually called.

When that closure is later invoked, potentially from a completely different point in the script's execution or even from a different calling context, BeanShell creates a new, local execution scope for the method call itself, but sets that new scope's parent reference to point back to the captured enclosing NameSpace rather than to whatever context the call happened to originate from.

As the method body executes and references variables, unqualified lookups first check this new local scope, then walk outward through the captured parent reference into the originally-captured enclosing namespace, meaning the closure genuinely reaches back to where it was defined, not to wherever it happens to be called from - this is what distinguishes lexical, definition-site, scoping from a dynamic, call-site, scoping model.

Because the captured reference points to the live NameSpace object rather than a copied snapshot of its contents, any changes made to variables in that enclosing scope after the closure was defined, whether by the same script or by another closure sharing that same enclosing scope, are visible to this closure the next time it's invoked, since it's always reading through to the current, live state of that captured namespace.

This mechanism is what lets multiple closures defined within the same enclosing scope effectively share and coordinate through common state in that scope, functioning collectively similar to private fields shared among the methods of a single object instance, even though none of them were formally declared as a Java class.

flowchart LR
  A[Method defined: NameSpace at that point captured] --> B[Closure holds reference to captured NameSpace]
  B --> C[Closure invoked later, possibly from elsewhere]
  C --> D[New local scope created for this call]
  D --> E[Local scope's parent set to captured NameSpace, not call-site context]
  E --> F[Unqualified lookups: local scope first, then captured parent chain]

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