Download embjava.pa2

Java and
embedded systems
About me
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Peter Kriens
Work as a consultant
(mainly for for ERICSSON)
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Finnasandsvagen 22
43933 Onsala, Sweden
+46 705950899
Peter.Kriens@aQute.se
aQute Copyright © 2002 All rights reserved
The language
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Simple key word based language with lots of
curly braces
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public class A {
public void main( String args[] ) {
System.out.println( “Hello world” );
}
}
Close resemblance to C/C++ basic syntax
Formally defined
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The Language: Names
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Class name derived from directory and file name
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Class names are globally unique
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watch case sensitiveness on PC’s
redundant
com.ericsson.bcm.BCM
packages/classes can be imported for
convenience
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import com.ericsson.bcm.*;
full name is real name
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The language: Access controls
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Access control build into language
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public
private
protected
default: package private
deprecated
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The Language: Interfaces
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Interface new concept
Instead of multiple inheritance
Verified promise to the type system to implement
a method
Decouples sender from receiver
Slight (very slight) overhead in current
implementations
Versioning problems
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Interfaces
public interface Log {
public void log(String s);
}
client
uses
public class SimpleLog {
public void log(String s) {
System.out.println( s );
}
}
Simple
Log
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interface
Log
implements
IBM
Log
Motorola
Log
Interfaces and message dispatch
log(“yes”)
invokeinterface
IBM
Log
an object
resolve name
Log
public void log(String s) {
System.out.println( s );
}
lookup method
log(String)
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The Language: Nested classes
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Used for callbacks
Expensive
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> 500 bytes overhead per class
More linking
Requires quirks like final variables
Ugly syntax
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Anonymous classes
void foo( final int offset ) {
window.addActionListener(
new Action() {
public void performAction() {
_count+=offset;
}}});
com/ibm/log/IBMLog.class
IBMLog
com/ibm/log/IBMLog$1.class
IBMLog$1
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The Language: Object Oriented
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Java is mainly OO
int, float, char, byte, long are not objects
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problematic with for example reflection
A class is an object
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Threads
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Easy to create a new thread
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Threadgroups
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Thread thread = new Thread() {
public void run() { …. }
};
Treat a group of threads as one
Monitor life of threads
Expensive resource!
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Stack
Scheduling
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Threads
Thread
Group
Thread
Thread
Data area
the heap
Stack area
Stack area
Thread
Stack area
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Threads: Monitors
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Synchronized keyword
Each object has a monitor
Difficult to understand for many people
But powerful
Wait gives up lock
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Monitors
void push(Object o) {
synchronized( _vector ) {
_vector.addElement( o );
if ( _vector.size() == 1 )
notifyAll();
} }
Queue
waiting
Object pop() {
synchronized( _vector ) {
while ( _vector.size() == 0 )
wait();
Object o = _vector.elementAt(0);
_vector.removeElementAt(0);
} }
in
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synchronized
monitor
out
aThread
aThread
Threads: killing them
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Threads cannot be killed due to locks!
Use variable and close() to get rid of threads
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class DNS implements Runnable {
boolean
_continue = true;
ServerSocket
_socket;
public void run() {
try {
_server = new ServerSocket(53);
while ( _continue ) {
Socket socket = server.accept();
process(socket);
}
} catch( IOException e ) { Log.report(e); }
}
public void quit() throws IOException {
_continue = false;
server.close();
}}
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Garbage collection
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Never delete an object!
Java will clean up after you.
When no more references exist, an object is
finalized
Do not get too sloppy, careful programming
always pays in the end
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Finalization
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Careful with static variables
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A static variable can keep a class alive
finalize
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Gets called just before an object is removed
No guarantee in what context
 Threads!
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Not as important as C++ destructor
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Exceptions
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Extra flow of control
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call/return and call/exception
Checked exceptions for errors that cannot be
prevented (environment): IO errors, Not found
Unchecked (programmer errors): Null pointers
Errors (integrity): Link errors
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Exception hierarchy
Object
Throwable
checked!
do not catch
Error
Exception
unchecked
Verify
Error
...
Error
IO
Exception
...
Exception
Runtime
Exception
NullPointer
Exception
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...
Exception
Exceptions
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Exceptions very useful for life cycle management
for reliable functions
Interfaces often forget to throw no Exceptions
while they should
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complicates implementation
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public interface Printer {
void print( String s ) /* throws IOException */;
}
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Exceptions: problems
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Checked exceptions create tight coupling
between layers
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Force implementors to catch exceptions
No standard logging mechanisms
Absolutely fatal:
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public void foo() {
try {
process();
}
catch( Exception e ) {}
}
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Reflection
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Access an object untyped
Methods, Fields, Constructors, inheritance and
interfaces
No type safety
Can significantly reduce code size
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Method m= String.class.getDeclaredMethod( “size”, new Class[]
{} );
Integer i = (Integer) m.invoke( “abc”, new Object[] {} );
aQute Copyright © 2002 All rights reserved
Dynamic linking
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References are resolved in run time by name
and signature
Pretty lenient
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Static initialization when first referenced
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Addition of new variables/methods/signatures
Removal of unused methods
static {
doSomething();
}
Size/Performance hit
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Dynamic linking
Def.class
Constants
"foo"
"bar"
Abc.class
Constants
"bar"
"kim"
Methods
1: invoke 2
Methods
1: ...
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Class path
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Hardest thing to get right
ClassNotFoundException is dreaded
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Exception could be on class A while class B
referenced by A could not be found!
Use A, link in
aClient
refers to
also link in
A
B
extends
reports A!!
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Classpath
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Rules:
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Names are case sensitive even if file system is not
Current directory is not default included in class path
Use a make file to maintain class path
Do not hard code paths to 3pp products everywhere
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Class loaders
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Java abstracts where code comes from
aClient
links in class
aClass
Loader
retrieves byte codes
aFile
refers to
is loaded by
anObject
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belongs to class
aClass
db, network, etc
Class Loaders
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Code can from anywhere
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network, database, file system
calculation on the fly (new RMI, Voyager)
Class loader defines security scope
Very simple to implement
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Class identity crisis
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Two identical class loaded via two different class
loaders are different classes!
aObject
These objects are NOT of the same class
bObject
bound to
AClass
aClient
AClass
is loaded by
is loaded by
A
Loader
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Aclass
file
B
Loader
Type safety
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Java is type safe by design
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However, an object can be cast to another class.
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String s = (String) new Integer()
Does not compile, does not get past verifier
Allows optimizations
Verified in run time
Expensive
Type safety verified by byte code verifier
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Byte codes
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A byte code is an instruction to a virtual machine.
Compare with an op code for a real processor
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RETURN = 0xB1
SALOAD = 0x35
Byte codes generated by compiler or assembler
The VM can directly interpret the byte codes
A JIT is a Just In Time Compiler that translates
the byte codes to native op codes
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Byte codes
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Disassemble code with javap
javap -c -classpath /src ericsson.net.ipv4.IP
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Local variables for method int dotted(java.lang.String)
java.lang.String s pc=0, length=54, slot=0
java.util.StringTokenizer st pc=11, length=43, slot=1
int[] n pc=15, length=39, slot=2
int i pc=17, length=21, slot=3
Method int dotted(int, int, int, int)
0 iload_0
1 bipush 24
3 ishl
4 iload_1
5 bipush 16
7 ishl
8 iadd
9 iload_2
10 bipush 8
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Class files
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A class file contains all the byte codes and linking
information for one class
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format version
constant pool
interfaces
super class
fields
methods
debug info
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Class files
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Contains always only 1 class
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Nested classes are <name>$<n>
No optimization for performance and size
Long class names cause your class files to grow
exponentially!
+/- 500 bytes overhead per class
Class name = file path is confusing
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Classpath problems are a serious problem in Java
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Jar Files
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Packs a number of class files in a compressed
ZIP file
Faster downloading in HTTP 1.0 servers made a
connection for each class file
Contains classes + resources
 images
 web pages
 translations
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Easier to ship
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Jar Files
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No optimization or pre linking
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Plain zip file
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Java support for parsing/extracting JAR files
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Example content
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jar tvf ericsson*.jar
31 Mon Nov 22 12:21:42
1572 Mon Nov 22 12:21:44
2759 Mon Nov 22 12:21:44
1441 Mon Nov 22 12:21:44
32 Mon Nov 22 12:21:44
3486 Mon Nov 22 12:21:44
11401 Mon Nov 22 12:21:44
2481 Mon Nov 22 12:21:44
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CET
CET
CET
CET
CET
CET
CET
CET
1999
1999
1999
1999
1999
1999
1999
1999
ericsson/net/ipv4/resources.txt
ericsson/net/ipv4/UDP.class
ericsson/net/ipv4/TCP.class
ericsson/net/ipv4/ICMP.class
ericsson/rcur/btest/resources.txt
ericsson/rcur/btest/Lme.class
ericsson/net/ipv4/IP.class
ericsson/net/ipv4/Network.class
Jar Files: Manifest
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Manifest
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Signing of files for security
Options: main class, package versions
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Manifest-Version: 1.0
Created-By: Signtool (signtool 1.1)
Comments: PLEASE DO NOT EDIT THIS FILE. YOU WILL BREAK IT.
Name: java/awt/Adjustable.class
SHA1-Digest: 181v4ECne8mD6ZqcHP3JVD6l17k=
Name: java/awt/AWTError.class
SHA1-Digest: /ekvoK3hUnQ+amWPopPc2iujHMU=
Name: java/awt/AWTEvent.class
SHA1-Digest: Jm/yZUSuRs7yZX2IGGVIG4ULD/M=
aQute Copyright © 2002 All rights reserved
Performance
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Class loading overhead.
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Native code is mapped to memory and paged in.
Class linking overhead. Linking is symbolic
Two VM’s do not share byte codes in memory
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Modern OS'es share executable memory images
VM-1
VM-2
Class files
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Performance
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Interpretation or poor optimization when JIT is
used
No dirty tricks:
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C:
Java:
char c[100]; int x = (int) *c;
byte c[] = new byte[100]; int x =((0xFF&c[0])<<24) +
((0xFF&c[1])<<16) + ((0xFF&c[2])<<8) + (0xFF&c[3]);
No pre-processor
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distinction between develop/release difficult
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Security
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Classes are authorized by their “codebase”.
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The class loader defines the security scope
Privilege is minimum privilege of all callers on the
stack
Significant change from Java 1.1
SecurityManager to Java 2 AccessController
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Security
implies(FilePermission)
anA
A
Policy
Protection
Domain
implies(FilePermission)
foo()
aB
B
Code
Source
Permission
Collection
Permissions
Permission
File
Permissions
File
Permission
Check permission
bar()
stack
aC
C
open()
Open
file
Security
Manager
checkRead(File)
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A
B
B
B
C
C
check(FilePermission)
implies(FilePermission)
get stack trace
use minimal permission
Access
Controller
Security Java 2
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Each class loader has a protection domain
A protection domain holds a collection of
Permission object.
Permission objects have a target and actions
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FilePermission
 /tmp/- + read,write, execute, delete
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SocketPermission
 people.ericsson.se:80 + accept, connect, listen, resolve
aQute Copyright © 2002 All rights reserved
Java Profiles
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Java 2 Enterprise Edition
Java 2 Standard Edition
Java 2 Micro Edition
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CDC = Standard VM
CLDC = KVM
Migration
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Personal Java
Java Card
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User Interfaces: AWT
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AWT, original UI library
Poor event handling
Uses peer objects
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Native look & feel (when you are lucky)
Impossible to get right on all platforms
Client
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Text
Component
PeerText
Field
User Interfaces: Swing
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UI library fully implemented in Java
Big ….. and slow
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Uses many, many classes
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> 16 Button related classes
Pluggable UI
>700 classes loaded for "Hello world"
Surprisingly easy to use and good looking
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User Interfaces: IFC (Netscape)
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Same concept as Swing
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same designers!
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No more maintenance by Netscape
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no bug fixes (wonderful stability)
 source code available
Has UI builder called Constructor
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Small, lean
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Whole library < 400K jar file
embedded in Netscape
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Java versus C++
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No more stray pointer related core dumps
Useful exceptions
No more memory leaks
Cleaner, simpler syntax
Less performance
Better productivity
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Open Service Gateway initiative
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ERICSSON, SUN, IBM, Telia, Nokia, Toshiba,
Nortel, Siemens, EDF, …
Standardize the Java API for applications
residing on the residential gateway
Service
Service
provider
provider
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Aggregator
OSGi
OSGi
e-box
e-box
OSGi
Clients
PC, e-box
video,...
OSGi
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Framework
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Life cycle management (install,start,stop, update,
uninstall)
Registry
Http server
Logging
Client access
Remote Admin
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Why did Java get so big?
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Politics, APIs included that should not have been
in the base
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CORBA
Swing
Design method
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Analysis models confused with design models
Peanut sized classes: Swing ‘Hello world’ loads 700
classes ...
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Why did Java get so big?
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Requirement for perfection
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API’s must cover all cases perfectly (though they
rarely do)
Lack of ‘dirty’ optimization tricks
Design by committee
Lack of time?
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Optimizing
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Threads are expensive
Design with sense
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just enough classes
Use reflection
Minimize short lived objects
Use tools to build JAR file
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JAX (see alphaworks)
Deliver (ericsson Utility)
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Conclusion
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Java clearly improves productivity over C++
Performance and size are issues
Libraries available for any thinkable subject
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Not always well designed
The de-facto language of today
It is not Smalltalk, but it is usually fun to work
with …
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References
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Java: www.javasoft.com
Java Developers Connection:
http://developer.java.sun.com
OSGi: www.osgi.org
JPadPro (simple IDE):
http://www.modelworks.com/products.html
aQute Copyright © 2002 All rights reserved
References
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JProbe:
http://www.klgroup.com/jprobe/profiler/index.html
IBM source code: alphaworks.ibm.com
Voyager: www.objectspace.com
PSE Pro: www.odi.com
Links to java related:
http://www.taxon.demon.nl/JW
aQute Copyright © 2002 All rights reserved
Java Books
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Java in a Nutshell
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Java Secrets
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David Flanagan. ISBN 1-56592-183-6
Elliote Rusty Harold.ISBN 0-7645-8007-8
Java 2 Performance and idiom guide
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Craig Larman, Rhett Guthrie. ISBN 0-13-014260-3
aQute Copyright © 2002 All rights reserved
Java Books
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The Java Virtual Machine Specification
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Java Security
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Scott Oaks. ISBN 1-56592-403-7
Java Developers Almanac
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Tim Lindholm, Frank Yellin, ISBN 0-201-63452-X
Patrick Chan. ISBN 0-201-37967-8
Late night IFC

Jason Beaver, Jamie Costa, Jason Wehling. ISBN 156276-540-X
aQute Copyright © 2002 All rights reserved