Design Patterns & Refactoring Proxy Oliver Haase HTWG Konstanz - - PowerPoint PPT Presentation

Design Patterns & Refactoring

Proxy Oliver Haase

HTWG Konstanz

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Description I

Classification: Object-based structural pattern Also known as: Surrogate Purpose: Control access to a subject through indirection, i.e. a placeholder object (proxy) that provides the same interface and that delegates operation calls to the actual subject. Applicability: Common reasons to hide a subject behind a proxy are:

local placeholder for remote communication; basic idea behind Remote Procecure Call (RPC), Remote Method Invocation (RMI) → Remote Proxy create expensive objects only on demand → Virtual Proxy Example: high resolution graphic in text document, virtual proxy might only know size (bounding box) and position. control access to original proxy → Protection Proxy access control can only mean to filter out interfaces that shall not be usable, or a full-fledged authentication and authorization mechanism.

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Description II

need for an “inteligent” reference that e.g. counts number of existing references to subject (needed for automatic garbage collection) → Smart Reference

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Structure

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Participants

Proxy:

has reference to real subject (might see real subject only as an instance

• f type Subject)

implements type Subject controls access to real subject; might be responsible for creating real subject on demand further functionality depends on proxy kind

Subject: Common interface for RealSubject and Proxy → allows a proxy to substitute for a real subject. RealSubject: implements the real thing.

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Implementation

Example: A service interface that contains methods for usage, for administration, and methods to get a usage or administration proxy:

public i n t e r f a c e ServiceUsage { void use ( ) ; } public i n t e r f a c e S e r v i c e A d m i n i s t r a t i o n { void a d m i n i s t r a t e ( i n t s t a t e ) ; } public i n t e r f a c e S e r v i c e extends S e r v i c e A d m i n i s t r a t i o n , ServiceUsage { S e r v i c e A d m i n i s t r a t i o n getAdministrationProxy ( ) ; ServiceUsage getUsageProxy ( ) ; }

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Implementation

Simple Service implementation:

public c l a s s C o n c r e t e S e r v i c e implements S e r v i c e { private i n t s t a t e = 0; public void a d m i n i s t r a t e ( i n t s t a t e ) { t h i s . s t a t e = s t a t e ; } public void use () { System . out . p r i n t l n ( ” s t a t e : ” + s t a t e ) ; } public S e r v i c e A d m i n i s t r a t i o n getAdministrationProxy () { return new AdministrationProxy ( t h i s ) ; } public ServiceUsage getUsageProxy () { return new UsageProxy ( t h i s ) ; } }

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Implementation

Proxy classes:

p u b l i c c l a s s A d m i n i s t r a t i o n P r o x y implements S e r v i c e A d m i n i s t r a t i o n { p r i v a t e S e r v i c e A d m i n i s t r a t i o n d e l e g a t e ; p u b l i c A d m i n i s t r a t i o n P r o x y ( S e r v i c e A d m i n i s t r a t i o n d e l e g a t e ) { t h i s . d e l e g a t e = d e l e g a t e ; } p u b l i c void a d m i n i s t r a t e ( i n t v a l u e ) { d e l e g a t e . a d m i n i s t r a t e ( v a l u e ) ; } } p u b l i c c l a s s UsageProxy implements ServiceUsage { p r i v a t e ServiceUsage d e l e g a t e ; p u b l i c UsageProxy ( ServiceUsage d e l e g a t e ) { t h i s . d e l e g a t e = d e l e g a t e ; } p u b l i c void use () { d e l e g a t e . use ( ) ; } }

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Implementation

Sample application that creates a ConcreteService, then feeds an administration thread with an AdministrationProxy and a usage thread with a UsageProxy.

p u b l i c c l a s s A p p l i c a t i o n { p u b l i c s t a t i c void main ( S t r i n g [ ] args ) { C o n c r e t e S e r v i c e s e r v i c e = new C o n c r e t e S e r v i c e ( ) ; S e r v i c e A d m i n i s t r a t i o n sa = s e r v i c e . g e t A d m i n i s t r a t i o n P r o x y ( ) ; ServiceUsage su = s e r v i c e . getUsageProxy ( ) ; new Thread (new AdminThread ( sa ) ) . s t a r t ( ) ; new Thread (new UsageThread ( su ) ) . s t a r t ( ) ; }

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Implementation

p r i v a t e s t a t i c c l a s s AdminThread implements Runnable { p r i v a t e S e r v i c e A d m i n i s t r a t i o n sa ; p u b l i c AdminThread ( S e r v i c e A d m i n i s t r a t i o n sa ) { t h i s . sa = sa ; } p u b l i c void run () { i n t v a l u e = 0; while ( true ) { sa . a d m i n i s t r a t e(++v a l u e ) ; t r y { Thread . s l e e p ( 5 0 0 0 ) ; } catch ( Exception e ) {} } } }

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Implementation

p r i v a t e s t a t i c c l a s s UsageThread implements Runnable { p r i v a t e ServiceUsage su ; p u b l i c UsageThread ( ServiceUsage su ) { t h i s . su = su ; } p u b l i c void run () { while ( true ) { su . use ( ) ; t r y { Thread . s l e e p ( 2 0 0 0 ) ; } catch ( Exception e ) {} } } } }

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Implementation

Variant with dynamic proxies: Java allows to create dynamic proxies that implement a set of interfaces specified at run-time.

p u b l i c c l a s s C o n c r e t e S e r v i c e implements S e r v i c e { [ . . . ] p u b l i c S e r v i c e A d m i n i s t r a t i o n g e t A d m i n i s t r a t i o n P r o x y () { I n v o c a t i o n H a n d l e r h a n d l e r = new DelegationHandler ( t h i s ) ; return ( S e r v i c e A d m i n i s t r a t i o n ) Proxy . newProxyInstance ( S e r v i c e A d m i n i s t r a t i o n . c l a s s . g e t C l a s s L o a d e r ( ) , new C l a s s [ ] { S e r v i c e A d m i n i s t r a t i o n . c l a s s } , h a n d l e r ) ; } p u b l i c ServiceUsage getUsageProxy () { I n v o c a t i o n H a n d l e r h a n d l e r = new DelegationHandler ( t h i s ) ; return ( ServiceUsage ) Proxy . newProxyInstance ( ServiceUsage . c l a s s . g e t C l a s s L o a d e r ( ) , new C l a s s [ ] { ServiceUsage . c l a s s } , h a n d l e r ) ; }

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Implementation

Class DelegationHandler forwards all incoming calls to delegate object:

p u b l i c c l a s s DelegationHandler implements I n v o c a t i o n H a n d l e r { p r i v a t e Object d e l e g a t e ; p u b l i c DelegationHandler ( Object d e l e g a t e ) { t h i s . d e l e g a t e = d e l e g a t e ; } p u b l i c Object invoke ( Object arg0 , Method arg1 , Object [ ] arg2 ) throws Throwable { return arg1 . invoke ( delegate , arg2 ) ; } }

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Implementation

Java 5 and later uses dynamic proxies for Java RMI: When an RMI server object is exported, the runtime system (Java virtual machine) creates a dynamic proxy (RMI stub) that implements the respective remote interface, and forwards all methods calls to the remote server object. This obsoletes the necessity to create stub classes (rmic) beforehand.

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