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Multitasking Guide
Sun Java™ Wireless Client Software, Version 2.0
Java Platform, Micro Edition
Sun Microsystems, Inc.
www.sun.com
May 2007
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Copyright © 2007 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, California 95054, U.S.A. All rights reserved. Sun Microsystems, Inc. has intellectual property rights relating to technology embodied in the product that is described in this document. In particular, and without limitation, these intellectual property rights may include one or more of the U.S. patents listed at http://www.sun.com/patents and one or more additional patents or pending patent applications in the U.S. and in
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Contents Preface ix 1. Introduction 1 Multitasking 2 Robustness 3 Mechanisms Compared With Policies 3 2. Multitasking Safety 5 Multitask Safety and Multithread Safety 6 Global and Static Data 7 Singletons 8 Multitasking Safety Example 9 Multithread Safety 11 Multitask Safety 13 Establishing Per-Task Context 15 3. Managing Native Resources 21 Resource Management Mechanisms 22 Reservation 22 Limit 23 Revocation 24 iii
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Default Resource Allocation Policies 24 Customization of Resource Allocation Policies 25 4. Other Multitasking Issues 29 Switching the Foreground MIDlet 29 Default Policy 30 Alternative Policies and Their Implementations 30 Scheduling the CPU 30 Default CPU Scheduling Policy 31 Alternative Policies and Their Implementations 31 Interrupting the User 32 Default User Notification Policies 32 Glossary 33 Index 37 iv Multitasking Guide • May 2007
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Code Samples CODE EXAMPLE 2-1 Native API for a Microwave Oven 9 CODE EXAMPLE 2-2 Typical usage of the microwave 10 CODE EXAMPLE 2-3 Simple Java API for the Microwave Oven 10 CODE EXAMPLE 2-4 Introducing a Locking Mechanism for Thread Safety 11 CODE EXAMPLE 2-5 Using the Locking Mechanism 12 CODE EXAMPLE 2-6 Migrating the Initialization Variable to Native Code (Doesn’t Work) 14 CODE EXAMPLE 2-7 Migrating Initialization to Native Code 14 CODE EXAMPLE 2-8 Keeping State in Java Code 16 CODE EXAMPLE
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vi Multitasking Guide • May 2007
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Tables TABLE 3-1 Constant Definitions for the Resource Management Policy 26 vii
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viii Multitasking Guide • May 2007
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Preface The Multitasking Guide highlights multitasking programming issues in the Sun Java™ Wireless Client software. It describes how to make code safe for the multitasking environment of the Java Wireless Client software. There is a special section about resource management. The Multitasking Guide also describes some multitasking policies implemented in the Java Wireless Client software and discusses possible alternatives. Note – Sun Microsystems has simplified the naming schemes for the variou
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Note – Note - Sun is not responsible for the availability of web sites mentioned in this document. Sun does not endorse and is not responsible or liable for any content, advertising, products, or other materials available through such sites. How This Guide Is Organized This book contains the following chapters: Chapter 1 provides an overview of the implementation and pitfalls of the Java Wireless Client software’s multitasking environment. Chapter 2 describes how source code can be made safe for
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Application Title Multitasking Test Tools Multitasking Quality Test Suite Guide Viewing reference documentation created by the Javadoc™ tool Java API Reference Viewing reference documentation created by the Doxygen tool Native API Reference Typographic Conventions Used in This Guide Typeface Meaning Examples Courier The names of commands, files, Edit your .login file. AaBbCc123 and directories; on-screen Use ls -a to list all files. computer output % You have mail. Bold What you type, when % su
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Accessing Sun Documentation Online The Java Developer Connection™ program web site enables you to access Java platform technical documentation at http://java.sun.com/. Sun Welcomes Your Comments We are interested in improving our documentation and welcome your comments and suggestions. Provide feedback to Sun at http://java.sun.com/docs/forms/sendusmail.html. xii Multitasking Guide • May 2007
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CHAPTER 1 Introduction Customers use mobile phones and other handheld devices for many tasks, such as making phone calls, taking photographs, playing games, organizing contact information, keeping a calendar of events, and accessing web sites. It is natural for them to want to do more than one of these tasks at a time. For example, a customer might not want to shut the address book to receive a flight-delay alert. The Java Wireless Client software, which can concurrently run multiple MIDlets, me
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Multitasking The Connected Limited Device Configuration HotSpot™ Implementation can run multiple Java applications within a single operating system (OS) process. Historically, a CLDC virtual machine (VM) could run one Java application at a time, and each virtual machine typically required its own OS process. Running more than one Java application meant running more than one OS process. This could use too many resources on some small devices. The multitasking feature enables a single virtual mach
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Robustness In addition to providing the multitasking that users want, tasks have the following benefits: ■ Fault containment - If a Java application crashes, then any problems caused by this crash are limited to the task. Applications running in other tasks are unaffected. ■ Clean termination - When a Java application exits, it leaves the virtual machine in a consistent state. In the past, some have tried to run multiple Java applications in a single process by running the applications in separa
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Because policies are so device dependent, this book does not recommend specific policy combinations. As you determine the policies for your device, keep in mind that policies interact with each other and not all combinations of policies make sense. For example, if you have a policy to allow a MIDlet to access a sound device while it is in the background, it does not make sense to also have a policy to suspend a MIDlet’s execution when it is in the background. 4 Multitasking Guide • May 2007
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CHAPTER 2 Multitasking Safety The Java Wireless Client software provides the ability to run multiple MIDlets concurrently in a single OS process. From the standpoint of the OS, there is one process and one Java virtual machine. However, from the standpoint of a Java application, it appears as if it is running in its own, independent virtual machine, isolated from other Java applications. These apparently independent virtual machines are called tasks. Each MIDlet runs in its own task. When a new
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The following list summarizes the multitasking safety issues to consider when you update or add native code for your port: ■ Multitask safety and multithread safety ■ Native global or static data ■ Singletons Multitask Safety and Multithread Safety Many systems today are multithreaded, which requires code that runs in these systems to be thread safe or multithread safe. For example, POSIX Threads (Pthreads) enables multiple native threads to run in the same OS process. Each native thread has acc
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For example, certain native functions (such as file storage) must be maintained on a per-application basis. In a single-tasking system, only one application is running, and so all file access is on behalf of that one application. In a multitasking system, several applications are running, and so the file access code can no longer assume that just one application exists. Instead, it must be aware of the possibility of multiple applications, so that one application doesn't accidentally operate on
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Maintain the following invariants: ■ A value of zero means a NULL pointer ■ A nonzero value means a valid native pointer In native code, when you allocate memory, use KNI field access to store the pointer in the private field. When you free the native memory, use KNI field access to store 0 in the field. Using the K Native Interface (KNI) field access avoids race conditions. Have operations that use the native pointer use KNI field access for consistency. Have those operations check for aNULL va
