How does Groovy's metaClass enable dynamic method addition?

Every object in Groovy has an associated metaClass, which is the object responsible for actually resolving method calls and property access at runtime - it's the layer Groovy's dynamic dispatch goes through instead of directly hitting a fixed compiled vtable the way Java does.

Because the metaClass is itself modifiable at runtime, you can add a brand-new method to an existing class, even one you don't own, like String, by assigning a closure to that class's metaClass, and every instance of that class gains the new method afterward.

This is a powerful but double-edged capability: it enables things like custom DSLs and testing utilities that patch behavior temporarily, but overusing it in production code can make behavior harder to reason about, since a method's implementation isn't fixed purely by its declared class anymore.

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