Wiley & Sons, Inc

Y L F M A E T Team-Fly® JavaJava™ EnterpriseEnterprise Design Patterns Patterns in JavaTM, Volume 3 MARK GRAND MARK GRAND John Wiley & Sons, Inc. New York ● ChichesterJohn● Weinheim Wiley &● Sons,Brisbane Inc. ● Singapore ● Toronto New York ● Chichester ● Weinheim ● Brisbane ● Singapore ● Toronto Publisher: Robert Ipsen Editor: Theresa Hudson Developmental Editor: Kathryn A. Malm Managing Editor: Angela Smith New Media Editor: Brian Snapp Text Design & Composition: Designations used by companies to distinguish their products are often claimed as trademarks. In all instances where John Wiley & Sons, Inc., is aware of a claim, the product names appear in initial capital or ALL CAPITAL LETTERS. Readers, how- ever, should contact the appropriate companies for more complete information regarding trademarks and registration. This book is printed on acid-free paper. ●∞ Copyright © 2002 by Mark Grand. All rights reserved. Published by John Wiley & Sons, Inc., New York Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4744. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012, (212) 850-6011, fax (212) 850-6008, E-Mail: PERMREQ @ WILEY.COM. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold with the understanding that the publisher is not engaged in professional services. If professional advice or other expert assistance is required, the services of a competent professional person should be sought. Library of Congress Cataloging-in-Publication Data: Grand, Mark. Java Enterprise design patterns / Mark Grand. p. cm. ISBN 0-471-33315-8 (pbk.: alk. paper) 1. Java (Computer program language) 2. Web servers. 3. Object- oriented programming (Computer science) I. Title. QA76.73.J38 G72 2001 005. 13'3—dc21 2001045611 Printed in the United States of America. 10987654321 CONTENTS Acknowledgments vii About the Author viii Chapter 1 Introduction to Software Patterns 1 Description of Patterns 2 Pattern Name 3 Synopsis 3 Context 3 Forces 3 Solution 4 Consequences 4 Implementation 4 Known Uses 4 Code Example 4 Related Patterns 4 A Very Brief History of Patterns 5 Organization of This Book 5 iv ■ CONTENTS Chapter 2 Overview of UML 7 Class Diagram 8 Collaboration Diagram 18 Statechart Diagram 26 Deployment Diagram 28 Chapter 3 The Software Life Cycle 29 Chapter 4 Transaction Patterns 33 Acid Transaction 37 Composite Transaction 55 Two Phase Commit 65 Audit Trail 75 Chapter 5 Distributed Architecture Patterns 81 Shared Object 83 Object Request Broker 89 Object Replication 99 Redundant Independent Objects 109 Prompt Repair 115 Mobile Agent 119 Demilitarized Zone 129 Process Pairs 133 Chapter 6 Distributed Computing Patterns 137 Object Identifier 139 Registry 149 Protection Proxy 157 Publish-Subscribe 175 Retransmission 187 Mailbox 195 Heavyweight/Lightweight 203 Heartbeat 209 Connection Multiplexing 229 Chapter 7 Concurrency Patterns 275 Session Object 277 Lock File 285 Static Locking Order 291 Optimistic Concurrency 297 Thread Pool 303 Ephemeral Cache Item 325 Transaction State Stack 337 Contents ■ v Chapter 8 Temporal Patterns 347 Time Server 349 Versioned Object 355 Temporal Property 373 Chapter 9 Database Patterns 387 Persistence Layer 389 CRUD 407 Stale Object 413 Type Conversion 423 IsDirty 431 Lazy Retrieval 439 Appendix A Persistence Framework 445 Bibliography 475 Index 477 ACKNOWLEDGMENTS This book would not have been possible without the inspiration, encouragement and assistance of others. I want to thank Brad Appleton for his diligent reviews and concern with form. Wolfgang W. Keller provided extensive feedback on the transaction patterns chapter. Frank Sauer provided excellent feedback on the database patterns chapter. I also want to thank the members of the pattern discussion group at University of Illinois, Champaign-Urbana, for their invaluable comments on my manuscript: Joe Yoder, Brian Foote, Hiroaki Nakamura, Roger Whitney, Ralph Johnson, Brian Marick, Wanghong Yuan, Paul Rubel, Frederico Balaguer, Alejandra Garrido, Don Roberts, Zhijiang “John” Han, Weerasak Witthawuskul, Peter Hatch, Dragos Malolescu, and Les Tyrrell. Last, but not least, I would like to thank my wife Nicole for her sup- port and encouragement through the most difficult of times. This book could not have been finished without her understanding and patience. ABOUT THE AUTHOR Y L F M Mark Grand is a consultant specializingA in distributed systems, object- oriented design, and Java.E He is currently working on an open source framework for gluing components and programs into an application. Mark Grand is theT author of a series of books titled Patterns in Java. He is a consultant who specializes in Distributed Systems, Object Oriented Design, and Java. He was the architect of the first commercial B2B e-commerce product for the Internet. Team-Fly® CHAPTER 1 Introduction to Software Patterns Software patterns are reusable solutions to recurring problems that occur during software development. For purposes in this book, we refer to software patterns simply as patterns. What makes a bright, experienced programmer so much more produc- tive than a bright but inexperienced programmer? Experience. Experience gives programmers wisdom. As programmers gain experience, they recognize the similarity between new problems and those problems that they have solved before. With even more experience, they recognize that the solutions for similar problems follow recurring patterns. Experienced programmers recognize the situations where these patterns apply and quickly draw on existing solutions without having to stop, analyze the problems, and then pose possible strategies. When a programmer discovers a pattern, it’s just an insight. In most cases, to go from a nonverbalized insight to a well-thought-out idea that the programmer can clearly articulate is surprisingly difficult. It’s also an extremely valuable step. When we understand a pattern well enough to put it into words, we are able to intelligently combine it with other patterns. More important, once put into words, a pattern can be used in discussions among programmers who know the pattern. That allows programmers to more effectively collaborate and combine their wisdom. It can also help to 1 2 ■ CHAPTER ONE avoid the situation where programmers argue over various solutions to a problem only to find out later that they were really thinking of the same solution but expressing it in different ways. Putting a pattern into words has an additional benefit for less experienced programmers who have not yet discovered the pattern. Once a pattern has been put into words, more experienced programmers can teach it to programmers who are new to the pattern. This book provides experienced programmers with a common vocab- ulary to discuss patterns. It also allows programmers who have not yet discovered a pattern to learn about the pattern. Though this book includes a substantial breadth of patterns, additional patterns did not make it into this book. You, dear reader, may discover some of these patterns for yourself. Some patterns you discover may be highly specialized and of interest to only a small number of people. Other patterns may be of very broad interest and worthy of inclusion in a future volume of this book. If you wish to communicate such a pattern to me, my e-mail is [email protected]. The patterns cataloged in this book convey constructive ways of orga- nizing parts of the software development cycle. Other patterns that recur in programs are not constructive. These types of patterns are called AntiPatterns. Because AntiPatterns can cancel out the benefits of patterns, this book does not attempt to catalog them. Description of Patterns Patterns are usually described using a format that includes the following information: • A description of the problem that includes a concrete example and a solution specific to the concrete problem • A summary of the forces that lead to the formulation of a general solution • A general solution • The consequences, good and bad, of using the given solution to solve a problem • A list of related patterns Pattern books differ in how they document patterns. The format used in this book varies depending on which phase of the software life cycle the pattern addresses. The patterns in this book are related to a few different phases of the software life cycle. The pattern descriptions are organized into the section headings described here. Because the nature of the patterns varies, not every heading is used in every pattern. Introduction to Software Patterns ■ 3 Pattern Name The Pattern Name section consists of the name of the pattern and a bibliography reference that indicates where the pattern originated. Most patterns don’t have any additional text under this heading. For those that do, this section contains information about the derivation or general nature of the pattern. The bibliography reference indicates where the ideas in the pattern were first written in the form of a pattern. Because patterns are based on established practices, in many cases there are other sources of the ideas in the pattern other than the bibliography reference. Usually, the author of a pattern is not the first person to discover the ideas that underlie the pattern. In particular, I do not claim to be the first person to discover the ideas presented in this book. Those patterns with a bibliographic reference to this book merely indicate that I know of no other place where that particular set of ideas has been documented as a pattern.

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