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Multitasking Guide

Sun Java™ Wireless Client Software, Version 2.0

Java Platform, Micro Edition

May 2007

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L’ABSENCE DE CONTREFAÇON.

iii

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

iv Multitasking Guide • May 2007

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

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 2-9 Implementing the Native n_cook() Method 17

vi Multitasking Guide • May 2007

vii

Tables

TABLE 3-1 Constant Definitions for the Resource Management Policy 26

viii Multitasking Guide • May 2007

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 various Java

platforms. Java Platform, Enterprise Edition (Java EE) was formerly Java 2 Platform,

Enterprise Edition (J2EE™). Java Platform, Standard Edition (Java SE) was formerly

Java 2 Platform, Standard Edition (J2SE™), and Java Platform, Micro Edition (Java

ME) was formerly Java 2 Platform, Micro Edition (J2ME™).

References in this guide to specific documents, specifications, and products that

were released when the old naming scheme was in use retain their original names.

General references in this guide to Java platforms use the new, simplified naming

scheme.

Before You Read This Guide

Readers using this guide must be familiar with the MIDP 2.1 Specification. The

specification is available from http://www.jcp.org/. It is also useful if the reader

is familiar with the Java Wireless Client software code.

x Multitasking Guide • May 2007

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 multitasking.

Chapter 3 covers strategies and implementations for managing device resources.

Chapter 4 includes a brief overview of the multitasking policies and alternatives in

the Java Wireless Client software.

Related Documentation

The following documentation is included with this release of the Java Wireless

Client software:

Application Title

All Release Notes

Building the Sun Java Wireless Client software Build Guide

Porting the Sun Java Wireless Client software Porting Guide

Porting implementations of optional Java Specification Requests

(JSRs)

Optional Packages

Porting Guide

Creating a new Adaptive User Interface (skin) for the Java

Wireless Client software

Skin Author’s Guide to

Adaptive User Interface

Technology

Configuration and Testing Tools Tools Guide

Preface xi

Typographic Conventions Used in This

Guide

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

Typeface Meaning Examples

Courier

AaBbCc123

The names of commands, files,

and directories; on-screen

computer output

Edit your .login file.

Use ls -a to list all files.

% You have mail.

Bold

AaBbCc123

What you type, when

contrasted with on-screen

computer output

Important parts of a code

sample

Password:

Italic

AaBbCc123

Book titles, new words or terms,

words to be emphasized

Command-line variable; replace

with a real name or value

Read Chapter 6 in the User’s Guide.

These are called class options.

You must be super user to do this.

To delete a file, type rm filename.

Application Title

xii Multitasking Guide • May 2007

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.

CHAPTER

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, meets this

need.

Device manufacturers and service providers also benefit from being able to run

multiple MIDlets concurrently. This functionality enables them to write built-in

(resident) applications, such as address books, in the Java programming language.

Applications written in the Java programming language (Java applications) are

easier to write and maintain, more portable, and easier to customize than those

written in native code. The Java Wireless Client software meets this need too.

In addition, the Java Wireless Client software enables MIDlet writers to take

advantage of multitasking without changing their code. MIDlet suites that run in the

Java Wireless Client software’s multitasking environment are no different from any

other MIDlets: They have no new APIs and they have the same life cycle. The MIDP

2.1 Specification advises all MIDlet writers about managing resources when the

MIDlet is paused and resumed. In a multitasking environment, a MIDlet might

compete with other MIDlets for resources, so it is more important than ever for

MIDlet writers to follow that advice and to use resources carefully.

2 Multitasking Guide • May 2007

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 machine, in a single OS process, to function as multiple virtual machines.

From the standpoint of each Java application, it is running in a separate virtual

machine.

When a MIDlet exits or encounters an error, it always leaves the virtual machine in a

consistent state. This works because each MIDlet is isolated from other MIDlets that

might also be running at the same time. If the MIDlets were not isolated from each

other, an error in one MIDlet might be visible to other MIDlets. This might result in

deadlock or crashes caused by corrupted data structures.

Multitasking, then, runs multiple logical virtual machines within a single OS

process. The CLDC HotSpot Implementation enables this by providing fundamental

mechanisms in the virtual machine. Mechanisms include threads, the isolation of

objects in the Java runtime environment (Java objects) belonging to different

programs, and safe termination. See the CLDC HotSpot Implementation Virtual

Machine White Paper at http://java.sun.com/j2me/docs/pdf/CLDC-

HI_whitepaper-February_2005.pdf for more information.

The OS mechanisms are important and necessary, but they aren’t sufficient to

provide a complete application environment. A complete system needs mechanisms

for managing the application life cycle (starting, stopping, and switching between

applications) and for enabling the user to control the application life cycle. It also

needs mechanisms for managing competition for limited shared resources, such as

memory and network sockets, among multiple applications. The Java Wireless Client

software provides all of these additional mechanisms.

The Java Wireless Client software is a high-performance, feature-rich, deployment-

ready implementation of Java Technology for the Wireless Industry that takes

advantage of the multitasking functionality in the CLDC HotSpot Implementation.

The logical virtual machines in the Java Wireless Client software are called tasks .

Each application has its own task. The Application Management System (AMS) runs

at all times in its own task. This architecture provides improved performance and

robustness.

Chapter 1 Introduction 3

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 separate threads within a single

virtual machine. When applications share a virtual machine, it is impossible to

guarantee that an application can exit cleanly, because objects might be left in an

inconsistent state. In contrast, tasks can and do terminate cleanly.

Note that in MIDP, a Java application is a MIDlet suite. A MIDlet suite is one or

more MIDlets in the same Java Archive (JAR) file. Each MIDlet suite, then, runs in its

own task. The user interacts with one of the MIDlets in the MIDlet suite.

Mechanisms Compared With Policies

Mechanisms are actions that the system carries out, such as starting, pausing, and

stopping a MIDlet. Policies are the system-wide decisions that device manufacturers

and service providers make about when and how to use the available mechanisms.

Policies take generic mechanisms and combine them into a predictable set of system

behaviors.

The Java Wireless Client software provides both the necessary mechanisms for a

complete application environment, such as mechanisms for managing competition

for limited shared resources, and a default set of policies. For example, memory is a

limited shared resource for which the Java Wireless Client software provides

mechanisms and a few default policies. The mechanism is the allocation of heap

memory. One default policy is to allow the applications to compete for available

heap memory.

The default policies are useful, but device manufacturers and service providers are

likely to need to tailor policies to their particular platforms. Devices of different

capabilities might use different policies. Devices with different user interface styles

might use different policies so that their Java applications interact with users in

ways that match the native applications interactions. Because policy decisions are

often intertwined with user interface style and user model decisions, the Java

Wireless Client software enables the development of alternative policies.

4 Multitasking Guide • May 2007

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.

CHAPTER

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 MIDlet is started, a new task is created for it. When a

MIDlet exits, its task is destroyed. The Application Management Software (AMS)

runs in a dedicated task, and it is running the entire time the Java Wireless Client

software is active.

Although tasks cannot interact (they cannot access each other’s objects, for example),

they do share native code, process resources, and external resources. This sharing

leads to some implementation issues, both in native code and in Java programming

language code (Java code). The Java Wireless Client software takes these issues into

account. Those who want to integrate existing libraries, which might not have been

written with multitasking in mind, also need to be aware of the issues so that they

can add the source code correctly.

Code integrated into the Java Wireless Client software must be multitask safe. That is,

it must maintain the independence of one task from another. Because tasks run in a

single operating system process, the OS process can run native code for any task. To

keep tasks from interfering with each other and with each other’s data, the code

must be made aware of the task on whose behalf it is being called and possibly

allocating resources. When the task context is established, the code is considered

multitask safe.

This chapter points out issues that might arise in ensuring native code is multitask

safe. It also provides techniques for making the code multitasking safe in different

situations.

6 Multitasking Guide • May 2007

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 access to native memory, so any data structures in native memory must be

protected from concurrent access. This is typically accomplished through the use of

locks to provide mutual exclusion.

Similarly, multiple Java platform threads can run in the same Java virtual machine.

Each Java platform thread has access to the objects in the Java virtual machine, so

these objects must also be protected from concurrent access. Java code typically

accomplishes this through the use of synchronized code blocks or higher-level

constructs.

In the Java Wireless Client software, each task has one or more threads. Each of these

threads has concurrent access to the objects in that task, and so the same multithread

safety issues occur in Java Wireless Client software as in conventional Java virtual

machines. However, a thread in one task has access only to objects within its task,

and it has no access to objects in any other task. The multitasking nature of Java

Wireless Client software thus has no impact on the multithreaded safety of

applications.

All of the tasks in Java Wireless Client software run in a single OS process, and

therefore they all have access to the same native memory and data structures.

Threads from different tasks are scheduled arbitrarily. Therefore, native methods

that read or update native data structures must be prepared to deal with operations

from different tasks being interleaved in an arbitrary order. Furthermore, because

different tasks are generally running different applications, these different

applications are likely to place quite different demands on the underlying native

system.

Chapter 2 Multitasking Safety 7

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

another application’s files. Code that is aware of this possibility is multitask safe.

While operations from different tasks can occur in an arbitrary order, no possibility

exists of actual concurrency among native methods. One native thread exists in the

Java Wireless Client software. When it is running a native method, no possibility

exists of a context switch that causes another native method to be run at the same

time. Each native method runs to completion and returns before the next native

method can begin. For this reason, every native method is a critical section, and it is

usually not necessary to perform any OS-level locking (such as with Pthreads

mutexes) in native methods. It is only necessary to use OS-level locking if other

native threads are active in the system while the Java Wireless Client software is

running.

Thus, while native code that runs within Java Wireless Client software must be

multitask safe, it need not be multithread safe.

Global and Static Data

In a single-tasking system, it is common for native code to use global or static

variables. This works because only one application is running at a time. Global data

is implicitly associated with the currently running application. In a multi-tasking

system, the multiple applications likely conflict over global data. Therefore, to make

your code multitask safe, you might need to rearrange your native data to be

allocated on a per-task basis instead of globally.

To do this, you must allocate native data dynamically instead of statically. In

addition, you need to associate each piece of native data with a particular task.

To manage native data on a per-task basis, store a pointer to the native data into an

int field in a Java object. Associating the native resource with a single Java object

implicitly provides multitasking safety because each Java object resides in exactly

one task. It is also a good object-oriented approach.

Create a class to represent the native resource. Give the class a private field to hold

the pointer to the resource, for example:

private int nativePtr;

8 Multitasking Guide • May 2007

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 a NULL value before they use the pointer, and throw

an appropriate exception if the field does not have a pointer. An appropriate

exception is a NullPointerException.

Singletons

CLDC HotSpot Implementation 2.0 isolates the logical virtual machines, but there is

one situation to consider if you add or change Java code: Singletons. Singleton

classes are often used when only one instance of a class should exist. Sometimes the

class itself is used as a singleton, and no instance of it is ever created. The way to

handle singleton depends on whether you mean it to be used on a per-application

(that is, per-task) basis or whether you mean it to be used by the entire system.

For example, in Java Wireless Client software, an event queue is a per-task singleton,

because each task has its own event queue. Per-task singletons are not a problem as

they are handled by the VM. Static state is automatically replicated on a per-task

basis. On the other hand, the singleton that holds the foreground display is a

system-global singleton. Only one Display can be in the foreground in the entire

system. All other display instances must be in the background. System-global

singletons require additional work.

To handle a system-global singleton, consider either maintaining the singleton’s state

in a single task (such as the AMS task) and communicate the updates through

messages, or migrating key pieces of information into native memory, with access to

them arbitrated through native methods.

If you maintain the singleton’s state in a single task and update it using events, be

aware that events are asynchronous messages. Organize the updates so that

operations are tolerant of being executed out of order. Although messages are

generally processed in the order received, messages from different tasks might not

be processed in the order they are sent. Also organize the updates so that requestors

can proceed asynchronously. Asynchronous messages do not have any

acknowledgement by default, so the sender cannot know when the message is

processed or whether it is processed successfully.

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