Understanding Proxies in Smalltalk
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This document discusses proxy objects in Smalltalk. It begins by defining a proxy as a surrogate or placeholder that controls access to another target object. It then describes different approaches to implementing proxies and their limitations, such as issues with traditional approaches that use #doesNotUnderstand:. The document proposes an alternative "ghost model" using #cannotInterpret: to intercept all messages in a uniform way while keeping proxies and handlers separate. It explains how this approach works and advantages it has over traditional proxies. The document also discusses extending this approach to proxy classes and methods.
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Understanding Proxies in Smalltalk
- 1. RMod 1 Mariano Martinez Peck marianopeck@gmail.com http://marianopeck.wordpress.com/ Monday, August 22, 2011
- 2. What is a proxy? 2 A proxy object is a surrogate or placeholder that controls access to another target object. Monday, August 22, 2011
- 3. Glossary Target: object to proxify. Client: user of the proxy. Interceptor: object that intercepts message sending. Handler: object that performs a desired action as a consequence of an interception. 3 Monday, August 22, 2011
- 4. Forwarder and Logging example 4 Monday, August 22, 2011
- 5. With or without Object replacement? In proxies with object replacement (#become:), the target object is replaced by a proxy. Proxies without object replacement are a kind of factory. 5 Monday, August 22, 2011
- 6. With or without Object replacement? In proxies with object replacement (#become:), the target object is replaced by a proxy. Proxies without object replacement are a kind of factory. 5 Monday, August 22, 2011
- 7. Traditional proxy implementations 6 Usage of a minimal object together with an implementation of a custom #doesNotUnderstand doesNotUnderstand: executeBeforeHandling executeAfterHandling target Proxy identityHash pointersTo nextObject ... ProtoObject Monday, August 22, 2011
- 8. 7 We are going to play a little game... Monday, August 22, 2011
- 9. 8 Are both prints in Transcript the same or not? Monday, August 22, 2011
- 10. 8 Are both prints in Transcript the same or not? Conclusion: methods understood are NOT intercepted. Is that bad? Monday, August 22, 2011
- 11. 8 Are both prints in Transcript the same or not? Conclusion: methods understood are NOT intercepted. Is that bad? Different execution paths Errors difficult to find Monday, August 22, 2011
- 12. 9 Do we want the regular #doesNotUnderstand or to intercept the message? Monday, August 22, 2011
- 13. 10 Do we want the regular #doesNotUnderstand or to intercept the message? Monday, August 22, 2011
- 14. 11 I wanted the normal #doesNotUnderstand!!! Monday, August 22, 2011
- 15. 12 I wanted the normal #doesNotUnderstand!!! Monday, August 22, 2011
- 16. Problems #doesNotUnderstand: cannot be trapped like a regular message. Mix of handling procedure and proxy interception. Only methods that are not understood are intercepted. No separation between proxies and handlers 13 This approach is not stratified Monday, August 22, 2011
- 17. 14 Subclassing from nil does not solve the problem. Monday, August 22, 2011
- 18. 15 VM CRASH This solution is not uniform Monday, August 22, 2011
- 19. 16 A Uniform, Light-weight and Stratified Proxy Model and Implementation. Monday, August 22, 2011
- 20. Used hooks Object replacement (#become:) Change an object’s class (#adoptInstance:) Objects as methods (#run:with:in:) Classes with no method dictionary (#cannotInterpret:) 17 Monday, August 22, 2011
- 21. Object replacement 18 A B C D A B C D c become: d Monday, August 22, 2011
- 22. Objects as methods 19 The VM sends #run: aSelector with: anArray in: aReceiver Monday, August 22, 2011
- 23. Objects as methods 19 The VM sends #run: aSelector with: anArray in: aReceiver So.....We can implement in Proxy: Monday, August 22, 2011
- 24. CLasses with no method dictionary 20 aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass Monday, August 22, 2011
- 25. Ghost model 21 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object handleInterception: anInterception ProxyHandler handleInterception: anInterception SimpleForwarderHandler message proxy proxyState Interception Monday, August 22, 2011
- 26. How it works? 22 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object Monday, August 22, 2011
- 27. How it works? 22 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object Proxy class >> Monday, August 22, 2011
- 28. How it works? 22 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object aProxy ‘Mariano’aHandler instanceOf Proxy class >> Monday, August 22, 2011
- 29. How it works? 22 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object aProxy ‘Mariano’aHandler instanceOf Proxy class >> ProxyTrap class >> Monday, August 22, 2011
- 30. How it works? 22 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object aProxy ‘Mariano’aHandler instanceOf Proxy class >> ProxyTrap class >> Monday, August 22, 2011
- 31. How it works? 22 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object aProxy ‘Mariano’aHandler instanceOf Proxy class >> Monday, August 22, 2011
- 32. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Monday, August 22, 2011
- 33. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Monday, August 22, 2011
- 34. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Monday, August 22, 2011
- 35. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Monday, August 22, 2011
- 36. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Monday, August 22, 2011
- 37. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Proxy >> Monday, August 22, 2011
- 38. aProxy aProxy username 1: #username send methodDict := nil ProxyTrap cannotInterpret: aMessage Proxy Object 2: #aProxy lookup 3: Since the method dictionary was nil, the VM sends #cannotInterpret to the receiver but starting the lookup in the superclass 4: #cannotInterpret: lookup References instance of message send lookup subclass How it works? 23 Proxy >> SimpleForwarderHandler >> Monday, August 22, 2011
- 39. 24 Traditional Ghost #doesNotUnderstand: cannot be trapped like a regular message. #cannotInterpret: is trapped like a regular message. Mix of handling procedure and proxy interception. No mix of handling procedure and proxy interception. Only methods that are not understood are intercepted. “All” methods are intercepted. No separation between proxies and handlers. Clear separation between proxies and handlers. Monday, August 22, 2011
- 40. 25 is stratified Monday, August 22, 2011
- 41. Methods not intercepted 26 1) Optimizations done by the Compiler 2) Special shortcut bytecodes between Compiler and VM 2.1) Methods NEVER sent: #== and #class 2.2) Methods that may or may not be executed depending on the receiver and arguments: e.g. in ‘1+1’ #+ is not executed. But with ‘1+$C’ #+ is executed. 2.3)Always executed, they are just little optimizations. Examples #new, #next, #nextPut:, #size, etc. Monday, August 22, 2011
- 42. Methods not intercepted 26 1) Optimizations done by the Compiler 2) Special shortcut bytecodes between Compiler and VM 2.1) Methods NEVER sent: #== and #class 2.2) Methods that may or may not be executed depending on the receiver and arguments: e.g. in ‘1+1’ #+ is not executed. But with ‘1+$C’ #+ is executed. 2.3)Always executed, they are just little optimizations. Examples #new, #next, #nextPut:, #size, etc. Monday, August 22, 2011
- 43. Methods not intercepted 26 1) Optimizations done by the Compiler 2) Special shortcut bytecodes between Compiler and VM 2.1) Methods NEVER sent: #== and #class 2.2) Methods that may or may not be executed depending on the receiver and arguments: e.g. in ‘1+1’ #+ is not executed. But with ‘1+$C’ #+ is executed. 2.3)Always executed, they are just little optimizations. Examples #new, #next, #nextPut:, #size, etc. Monday, August 22, 2011
- 48. Proxy for classes 27 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object cannotInterpret: proxyFor: superclass methodDict format handler target ClassProxy initialize nilMethodDict ClassProxyTrap aUser aClassProxy methodDict = nil superclass = ClassProxy logIn: validate: username age User instance of Monday, August 22, 2011
- 49. Proxy for classes 27 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object cannotInterpret: proxyFor: superclass methodDict format handler target ClassProxy initialize nilMethodDict ClassProxyTrap aUser aClassProxy methodDict = nil superclass = ClassProxy logIn: validate: username age User instance of User name Monday, August 22, 2011
- 50. Proxy for classes 27 cannotInterpret: proxyFor: handler target Proxy initialize nilMethodDict ProxyTrap Object cannotInterpret: proxyFor: superclass methodDict format handler target ClassProxy initialize nilMethodDict ClassProxyTrap aUser aClassProxy methodDict = nil superclass = ClassProxy logIn: validate: username age User instance of aUser username User name Monday, August 22, 2011
- 51. Proxy for methods 28 Monday, August 22, 2011
- 52. Proxy for methods 28 Regular message Monday, August 22, 2011
- 53. Proxy for methods 28 Regular message Method execution Monday, August 22, 2011
- 54. Proxy for methods 28 Just handling #run:with:in correctly is enough to also intercept method execution. Regular message Method execution Monday, August 22, 2011
- 55. is 29 Uniform Monday, August 22, 2011
- 56. is 29 moreUniform Monday, August 22, 2011
- 57. More features Low memory footprint. Compact classes. Store the minimal needed state. Easy debugging. Custom list of messages. 30 Monday, August 22, 2011
- 58. Conclusion 31 With a little bit of special support from the VM (#cannotInterpret hook), we can have an image-side proxy solution much better than the classic #doesNotUnderstand: Monday, August 22, 2011
- 59. Future work Experiment with immediate proxies (memory address tag) in VM side. Think how to correctly intercept non- executed methods. 32 Monday, August 22, 2011
- 61. Mariano Martinez Peck marianopeck@gmail.com http://marianopeck.wordpress.com/ RMod A Uniform, Light-weight and Stratified Proxy Model and Implementation Monday, August 22, 2011