What is the Proxy design pattern?

The Proxy pattern provides a surrogate or placeholder for another object to control access to it. The proxy implements the same interface as the real object, so clients cannot tell the difference. Common types include: Virtual Proxy (lazy initialization — creates the expensive object only when first needed), Protection Proxy (access control — checks permissions before forwarding), and Remote Proxy (communicates with an object in a different address space over a network). Java's RMI stubs and Spring's @Transactional AOP proxies are well-known examples.