Introduction to Software Engineering (2+1 SWS) Winter Term 2009 / 2010 Dr. Michael Eichberg Vertretungsprofessur Software Engineering Department of

Transcription

1 Introduction to Software Engineering (2+1 SWS) Winter Term 2009 / 2010 Dr. Michael Eichberg Vertretungsprofessur Software Engineering Department of Computer Science Technische Universität Darmstadt

2 Dr. Michael Eichberg Fachgebiet Software Engineering Department of Computer Science Technische Universität Darmstadt Introduction to Software Engineering The Proxy Design Pattern For details see Gamma et al. in Design Patterns

3 Motivation 3 Imagine a document editor in which you do not want to handle all elements in a straightforward manner: You only want to display an outline of an image if the image is large and requires a huge amount of memory Document paragraph paragraph paragraph You want to show some content only after the user has been authenticated... But, any special treatment should be transparent for the editor s implementation. Image

4 Example - Lazy Image 4 Document paragraph paragraph paragraph Document DocElement draw() bounds() store() load() Image Paragraph draw() bounds() store() load() Image draw() bounds() store() load()

5 Example - Lazy Image 5 Support objects that may take long to create. Opening a document should be fast. Load the image if it becomes visible. The user just wants to look up some text (head of the document), the image is not shown.

6 Example - Lazy Image 6 How can I hide the fact that the image is created on demand? Support objects that may take long to create. Load the image if it becomes visible. Opening a document should be fast. I don't want to complicate the editor's implementation. The optimization shouldn't impact the rendering and formatting code. The user just wants to look up some text (head of the document), the image is not shown.

7 Example - Lazy Image - Solution 7 16Kb 2Mb imageproxy image :Document image :ImageProxy :Image filename:string in memory on demand on disc We use another object, an image proxy, that acts as a stand-in for the real image.

8 Example - Lazy Image - Solution 8 16Kb imageproxy The Image Proxy... implements the same interface as the real object Client code is unaware that it doesn't use the real object. instantiates the real object when required, e.g., when the editor asks the proxy to display itself by invoking its draw() operation Keeps a reference to the image after creating it to forward subsequent requests to the image.

9 Example - Lazy Image - Solution 9 16Kb imageproxy Document Document DocEleme DocElement Paragraph Graph Paragraph Image Image old new

10 Example - Lazy Image - Solution 10 16Kb imageproxy Document Document DocElement DocElement Paragraph Graphic Paragraph Image Image ImageProxy old new

11 Example - Lazy Image - Solution 11 16Kb imageproxy Document DocElement Graphic Image ImageProxy image : image draw() bounds()... «method» { if (image ==null) { loadimage(); } image.draw(); }

12 Structure 12 Client Subject request() RealSubject request() realsubject Proxy request() «method» {... realsubject.request()... }

13 Intent 13 Provide a surrogate or placeholder for another object to control access to it (From the client s point of view, the proxy behaves just like the actual object.) Also known as Surrogate (surrogate =dt. Stellvertreter)

14 Variations 14 Virtual Proxies Placeholders (as in image example). Smart References Additional functionality. Remote Proxies Make distribution transparent. Protection Proxies Rights management.

15 Variations - Virtual Proxy 15 Idea Create expensive objects only on demand. Objects associated with a large amount of data in a disk file or persistent objects may only be loaded into memory if the operation on the proxy demands that it be loaded. Implementation Some subset of operations may be able to be carried out without bothering to load the entire object; e.g., return the extent of image.

16 Variations - Smart Reference 16 Idea Replace bare pointer and provide additional actions when accessed. Examples Locking / unlocking references to objects used from multiple threads Reference counting, e.g., for resource management (garbage collection, observer activities)

17 Variations - Remote Proxy 17 Idea Provide a local interface for communicating with objects in a different address space. Operations on the proxies are delegated to a remote object and return values are passed through the proxy back to the client. Issues From the client s view, the proxy responds just like if the object were local, even though it is actually sending requests over a network. Network failures may be impossible to hide

18 Variations - Protection Proxy 18 Idea Verify that the caller has sufficient access to perform the operation. Issues Different clients may have different access levels for operating on an object Read-only objects may be protected from unauthorized modifications this way Exceptions are thrown in such violation cases

19 Summary 19 The Proxy Pattern describes how to replace an object with a surrogate object,...without making clients aware of that fact, achieving a benefit of some kind: lazy creation, resource and/or rights management, or distribution transparency.

20 Java s Dynamic Proxy Classes The GoF Design Patterns 20 A dynamic proxy class is a class that implements a list of interfaces specified at runtime such that a method invocation through one of the interfaces on an instance of the class will be encoded and dispatched to another object through a uniform interface.

21 Java s Dynamic Proxy Classes The GoF Design Patterns 21 A dynamic proxy class is a class that implements a list of interfaces specified at runtime when the class is created. A proxy interface is such an interface that is implemented by a proxy class. A proxy instance is an instance of a proxy class. Proxy classes, as well as instances of them, are created using the static methods of the class java.lang.reflect.proxy.

22 Java s Dynamic Proxy Classes The GoF Design Patterns 22 public interface Foo { Object bar(object obj) throws BazException; } public class FooImpl implements Foo { Object bar(object obj) throws BazException { //... } }

23 Java s Dynamic Proxy Classes public class DebugProxy implements java.lang.reflect.invocationhandler { The GoF Design Patterns 23 private Object obj; public static Object newinstance(object obj) { return java.lang.reflect.proxy.newproxyinstance( obj.getclass().getclassloader(), obj.getclass().getinterfaces(), new DebugProxy(obj)); } private DebugProxy(Object obj) { this.obj = obj; } } public Object invoke(object proxy, Method m, Object[] args) throws Throwable { Object result; try { System.out.println("before method " + m.getname()); result = m.invoke(obj, args); }... return result; } Foo foo = (Foo) DebugProxy.newInstance(new FooImpl()); foo.bar(null);