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Embedded Systems
Java in an Embedded System
C.-Z. Yang
http://syslab.cse.yzu.edu.tw/~czyang
Sept.-Dec. 2001
Java for Embedded Systems
Deepak Mulchandani
Wind River Systems
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Shift based on Internet
• Traditional distinctions between desktop
systems and embedded systems are on the
basis of functionality.
• Now the emergence of the Internet is
shifting the idea of fixed-function embedded
devices toward more open systems offering
some form of network connectivity.
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Embedded Java
• Several reasons
– With Java, developers can target a platform-independent
API and migrate their applications to different devices
without recompiling or re-verifying them.
– The OO nature of Java supports well-structured
development and software reuse.
– The Java Virtual Machine provides a dynamic platform
with a secure execution environment.
• Java is not just a programming language; it
is a complete dynamic platform that
requires extra infrastructure to run on
embedded devices.
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Considerations
• Accordingly, its suitability to an application
depends on the device requirements.
• Developers must consider resource,
integration, and real-time performance
requirements to determine Java’s suitability
to an application.
– For example, the bias of the language and platform toward
32-bit processors would not be appropriate for devices
with 4-, 8-, or 16-bit CPUs.
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Considerations
• Issues related to memory management and
the interpreted Java VM would hinder
applications that demand strict real-time
performance.
• Another consideration is the size of the
complete Java platform.
– Many people believe that support of a Java VM is all you
need to run Java applications.
– However, to compute the total size of the Java platform
correctly, you must add the size of the Java VM, Java API
package, Java application, and associated native-code
libraries.
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Embedded Java Platform Architecture
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Overview
• Two sample devices
– The RTOS supports multithreading (scheduling), memory
management, networking, and peripheral management for
the Java VM.
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Architecture of the Java Platform
• The architecture
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Java VM
• The Java VM depends on an RTOS to
provide hardware-specific functionality for
running Java applications.
• This functionality includes threading,
memory management, and execution of
native code.
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Java VM
• The device supports four Java API
packages: java.awt, java.net, java.io, and
java.math.
• The packages are layered directly on top of
native-code libraries
– This technique supports the abstraction of Java
applications from platform-specific functionality and
allows the Java API package to remain portable across
platforms.
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The Java APIs
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Evolution of Java
• As JDK 1.0.2 evolved to JDK 1.1, the Java
API package was enhanced at a penalty of
size.
– As a result, many embedded manufacturers recognized the
benefits of Java but hesitated to pay the added resource
requirements it places on their devices.
• To address this weakness, JavaSoft
developed four APIs that targeted different
market segments: JDK, PersonalJava,
EmbeddedJava, and Java Card.
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Resource Requirements
• Four APIs
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The Proper API
• JDK 1.1.4
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Scaling Java Class Packages
• Static method
– The static method analyzes the application code and
compiles a list of the referenced Java API classes and
packages.
– Using the compiled list, you can go through the Java class
package and remove the unnecessary packages and classes.
– The static method is quick, but prone to some error in
missing classes that are not directly referenced through the
source code.
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Scaling Java Class Packages
• Dynamic method
– The dynamic method requires you to run the application
code and generate a log of Java classes your application
references.
– To perform this analysis, you can run your Java application
on a Java VM with the -verbose flag enabled.
– This technique will generate a fairly complete list of all the
Java classes referenced in an application.
– The dynamic method is a lot more accurate than static
analysis since it compensates for classes loaded from other
sources such as the network.
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Breaking up the JDK
• Two separate archives
– rt.jar
– i18n.jar
• The impact of a 1.3 Mbtye class package is
a lot easier to absorb on an embedded
device.
• Note that the 1.3 Mbyte figure is for the JDK
class package.
• The PersonalJava and EmbeddedJava class
package will be much smaller.
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Memory Management And
Garbage Collection
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Memory Management
• Java applications do not have any explicit
mechanisms for memory management.
• The allocation and deletion of objects is
performed with cooperation between the
Java VM and the garbage collector.
– On the one hand, this isolates the developer from the
process of keeping track of memory use in an application.
– On the other hand, the garbage collection process is
nondeterministic and cannot be scheduled.
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The Problem
• As a result, you cannot be sure exactly
when the Java VM will clean up the object
heap, since
– it runs the garbage collector as a low-priority thread and
– does not include mechanisms to directly control the
behavior of the garbage collector.
• The Java API package does provide a
method, System.gc(), to call the garbage
collector, but this method does not
guarantee anything; it merely sends the
Java VM a suggestion to clean up the object
heap.
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Object Heap States in the Java VM
• The information is based on ratios and
divided into three categories: green, yellow,
or red state.
– When there is no shortage of memory, the Java VM
operates in the green state.
– When memory drops below a specified threshold, the Java
VM enters the yellow state, which means it should
schedule a garbage collection soon.
– If the application keeps executing, the Java VM enters the
red state, which means the object heap is almost saturated
and garbage collection needs to be done immediately.
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The Java VM Threading Model
• An example
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The Problem of the Garbage Collector
• The garbage collector poses a problem in
that it tends to lock down all the Java
threads while it cleans out unused objects
from the system.
• The lockdown ensures that the state of the
heap remains stable, but it adversely affects
real-time performance of Java applications.
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The Problem of the Garbage Collector
• A simple workaround to this problem is to
continue to allow the RTOS to manage
operations of the device—interrupts, device
management, and task scheduling—with
those tasks running at a higher priority.
– A Java application should not be written to rely on any
RTOS or system-specific functionality, nor should it make
any assumptions about scheduling or timing.
– Any specific portions of the Java application that are
performance-critical can be implemented in native code.
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Java Performance
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Performance Issue
• The Java class file format and the Java VM
– the cross-platform promise
• On the target device, each Java bytecode is
executed using the interpreter loop
embedded in the Java VM.
• The Java VM interpreter loop is responsible
for translating each Java bytecode into
equivalent native-code functions while the
application executes.
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Performance Issue
• Java bytecode applications tend to run
slower than native applications since
– optimizations cannot be performed at runtime and
– the translation is a fairly serial process.
• Based on early benchmarks, Java
applications tend to run 20 to 30 percent
slower than applications written in C.
• This is not a significant problem for small
applications, but it can be a serious issue
when it comes to running any sizable
applications.
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Average Execution-time Allocation
• The table shows the time spent in native
code execution at 1 percent of overall
execution time.
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Optimization Technology
• A specific optimization technology to
improve Java application performance can
only be applied to speed the interpretation
of bytecodes.
• The techniques to choose from include the
ROMizing technology presented earlier.
• Other solutions focus on three main areas:
Java native compilers, JITs, and Java Chips.
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Java Compilers
• Using Java Compilers allows developers to
write inter-operable Java, C, and C++ code.
• Pros
– The benefits are the increased performance by having your
system rely more on the mature native compilers rather
than bytecode-based Java compilers.
• Cons
– On the downside, you lose the benefits of the dynamic Java
platform as soon as portions are converted to native code.
– Also, for device updates you will have to rely on native
code patches rather than using the Java VM to selectively
update certain aspects of your application.
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Just-In-Time Compilers
• Just-in-Time Compilers are a popular
approach to improving Java performance.
• JIT compilers convert Java bytecodes to
native code while an application is running.
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Just-In-Time Compilers
• The JIT approach does not work as well:
– If the application performs operations such as graphics or
floating-point operations, then JIT compilers do not give a
large performance boost.
– If the application constantly creates objects, the garbage
collector would interfere with the operation of the JIT.
– If performance is mostly a function of optimizations, JIT
compilers are limited by their status as a runtime
component.
• JIT compilers can also take up large
amounts of RAM.
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Java Chips
• Java chips have been offered as a hardware
option to improve Java performance.
• You still need a Java VM (without interpreter
loop) and RTOS to put together a complete
Java platform, so the Java chips are
primarily performance enhancers.
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Embedded Internet Systems
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Comparisons
• Desktop v.s. Embedded Internet Systems
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Two Java Tech Paths
• Java for “soft” embedded systems with
fewer resource and response-time
constraints.
– The “soft” Java approach ports a subset of the desktop Java
VM directly into the RTOS environment.
– Although it is optimized for small memory footprints and
diverse visual displays, this results in a relatively large (by
embedded standards) and high-cost end product.
• Real-time Java for deeply embedded “hard
real-time” systems.
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Hp’s Native Java Compilers Approach
• HP is also developing a third approach—
native Java compilers—which translate
Java programs directly into CPU machine
code.
• Although native compilers break Java’s
original mantra of platform neutrality, they
run much faster than Java bytecodes that
are interpreted by the Java VM for execution
on the CPU.
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Embedded Java in a Web-Based
Teleradiology System
Andrea Abrardo and A.L. Casini
University of Florence
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Medinet
•
This paper present a pilot teleradiology application based on
embedded Java in the Web. It investigates how the use of Java can
solve the issues involved in the implementation of a teleradiology
system over the web.The proposed solution is being tested in real
hospital environments and can be used as the backbone for the
development of complex multiplatform client-server applications in
the field of teleradiology.
•
Medinet is a Web-based solution for teleradiology currently
undergoing clinical evaluation as part of the European Union
advanced communications technologies and services program. The
system uses embedded Java applets to permit remote access to
radiologic data and diagnostic services.
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Main Network Requirement of
Today’s Telemedicine Application
• Enable the sharing of data across
heterogeneous network environments.
• Enable the implementation of strong
security measures.
– Secure identification
– Access control strategies
– Encryption methods - privacy of patient data
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Overall Architecture of Medinet
• The overall architecture
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Access to Remote Medical Image
Databases on the Web
• Two models
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Collaborative Diagnosis through
Event Broadcasting
• The event broadcasting method works
particularly well with web-based
collaborative application.
– It fits the Java applet.
– Conference agent must distribute only small amounts of
data to the participants.
• The socket interface provides backbone
communication and allows the bidirectional
transmission of data among the applets and
the Java sever.
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Mednet’s Teleradiology Functions
• Access to Medical
Diagnostic Image
database
– proposed solution
• three tiered
• Indirect Access Method
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Mednet’s Teleradiology Functions
• Access to telediagnosis services
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Results
• Users and Performance
– users  -> threads in Java VM  -> performance 
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Results
• Compression or not and Performance
• Latecomers
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Conclusions
• Managing high-volume images and their
processing requires efficient VM.
• The security manager, along with digitally
signed applets must be used to give the
necessary privileges to the retrieved applets.
• Security of the system must be more
sophisticated when dealing with real patient.
• The option to integrate intranet and extranet
services with different privilege access.
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ACM Code of Ethics
• As an ACM member I will ....
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Contribute to society and human well-being.
Avoid harm to others.
Be honest and trustworthy.
Be fair and take action not to discriminate.
Honor property rights including copyrights and patent.
Give proper credit for intellectual property.
Respect the privacy of others.
Honor confidentiality.
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