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

Design Patterns & Refactoring Proxy Oliver Haase HTWG Konstanz Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 1 / 14

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. Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 2 / 14

Description II need for an “inteligent” reference that e.g. counts number of existing references to subject (needed for automatic garbage collection) → Smart Reference Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 3 / 14

Structure Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 4 / 14

Participants Proxy : has reference to real subject (might see real subject only as an instance of 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. Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 5 / 14

Implementation Example : A service interface that contains methods for usage, for administration, and methods to get a usage or administration proxy: ServiceUsage { public i n t e r f a c e use ( ) ; void } 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 ( ) ; } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 6 / 14

Implementation Simple Service implementation: C o n c r e t e S e r v i c e S e r v i c e { public c l a s s implements s t a t e = 0; private i n t a d m i n i s t r a t e ( i n t s t a t e ) { public void 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 ) ; } } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 7 / 14

Implementation Proxy classes: A d m i n i s t r a t i o n P r o x y p u b l i c c l a s s implements S e r v i c e A d m i n i s t r a t i o n { 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 r i v a t e A d m i n i s t r a t i o n P r o x y ( 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 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 { ServiceUsage d e l e g a t e ; p r i v a t e UsageProxy ( ServiceUsage d e l e g a t e ) { p u b l i c 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 ( ) ; } } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 8 / 14

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 ( ) ; } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 9 / 14

Implementation AdminThread Runnable { p r i v a t e s t a t i c c l a s s implements 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 ; AdminThread ( 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 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 ) {} } } } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 10 / 14

Implementation UsageThread Runnable { p r i v a t e s t a t i c c l a s s implements p r i v a t e ServiceUsage su ; UsageThread ( ServiceUsage su ) { p u b l i c 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 ) ; } ( Exception e ) {} catch } } } } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 11 / 14

Implementation Variant with dynamic proxies : Java allows to create dynamic proxies that implement a set of interfaces specified at run-time. C o n c r e t e S e r v i c e S e r v i c e { p u b l i c c l a s s implements [ . . . ] 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 () { p u b l i c 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 ) ; ( ServiceUsage ) Proxy . newProxyInstance ( return ServiceUsage . c l a s s . g e t C l a s s L o a d e r ( ) , C l a s s [ ] { ServiceUsage . c l a s s } , new h a n d l e r ) ; } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 12 / 14

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 ) Throwable { throws return arg1 . invoke ( delegate , arg2 ) ; } } Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 13 / 14

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. Oliver Haase (HTWG Konstanz) Design Patterns & Refactoring 14 / 14