11. Threads• A way to support several parallel activities in a

program• Execute asynchronously and ”simultaneously” e.g.

timeslicing• If two threads are dependent on each other

synchronization may be necessary• If they depend on a shared resource then deadlock,

livelock or race conditions can occur.• One solution to synchronization is the semaphore

11.1 Example: Web browser

Problem: fetching images, files, sound etc

takes time. Don’t want to lock up screen while

waiting, e.g. can’t scroll down screen or abort

fetch.

Solution: create separate thread for page fetch

and start this on each new fetch.

11.2 Threads in Java

• Interface Runnable with method run()• classes Thread, Write run() yourself

• To start a thread start()

• start() calls run() immediately

• To stop a thread interrupt()• Try to avoid stop(), suspend(), resume()

11.3 Thread Safety/Management

• Problem: threads running methods on the same object may corrupt that object. E.g. internal bank transfer.

acc[i] = acc[i]+x acc[i] = acc[i]+yacc[j] = acc[j]-x acc[j] = acc[j]-y

Problem: keep the bank total constant!

• Solution: synchronization on objects

• Aka Monitors

• Give each object a lock.

• Thread gets a lock by calling a synchronized object method.

• When one thread has the lock on an object no other thread may get the lock.

• Lock is on the object, not its methods

• Unsynchronized methods can still be called.

• Lock a method with synchronized

public synchronized void transfer (int from, int to){

acc[from] -= amount;acc[to] += amount;num_transactions++;

}

Looks fine, but what if an account mustn’t gonegative (no credit facility)??

• Problem: what happens if we get the lock and can’t proceed?

• Another thread which had the lock could allow us to proceed!

• Thread calls wait() method

• Gives up the lock temporarily,

• Enters queue of waiting threads on the object.

• notify() reawakens one thread in queue

• notifyAll() reawakens all threads in queue

public synchronized void transfer (int from, int to)

{

while ( acc[from] < amount )

{ wait(); } // inactive wait!

acc[to] += amount;

num_transactions++;

notifyAll();

}

Try to avoid deadlocks, livelocks and races!!

11.4 Rules (for the WC!)

1. If 2 or more threads modify an object, declare them as synchronized. Read-only methods which are affected must also be synchronized. (Decency Rule 1)

2. If a thread must wait for a change, wait inside the object by entering a synchronized method and doing a wait.(Decency Rule 2)

3. Don’t spend a long time in a synchronized method. If you can’t complete, perform a wait(). (Fairness Rule 1)

4. Whenever a method changes the state of the object, execute a notifyAll(). (Fairness Rule 2)

5. wait() and notifyAll()/notify() are methods of the Object class, not the Thread class. make sure waits are matched by notifies on the same object. (Fairness Rule 3)

11.5 Real-Time Issues• Real-time systems are not necessarily fast• Programmer gives guarantees for execution

time, etc• Operating system must handle time, parallel

activities, scheduling and priority of processes, as well as interrupts

• Program uses synchronization (semaphores), conditional execution (event variables) e.g. using monitors (ADT)

11.6 Timers

• Class Timer

• Constructor Timer(int delay, ActionListener l)

• void start() : starts the timer• void stop(): stops the timer sending events

• actionPerformed() of l is called whenever a timer interval has elapsed.

11.7 Real-Time Example

When we burn a CD the hardware must not

pause but write continuously. The data buffer

may therefore not become empty. In

Windows or Unix there is no guarantee for

this under heavy loading. In a real-time OS

such loading is dealt with by lowered priorities

(burn has highest).

12. Data Structures

• What kind of problem is to be solved?– Queue (FIFO)– Stack (LIFO)

– Random Access (sorted or unsorted)• Table with lookup using keys• Set , 1 copy of each• List many copies

12.1 Collections

• Collection Interfaces– JList – JTree– Vector, LinkedList– Set (1 copy of each object)– HashSet, TreeSet– Map (lookup key + value in a table)– AbstractMap, HashMap, TreeMap

12.2 Pattern Iterator

• For each data structure we need a method to traverse it, a so called iterator

• Example: Trees, breadth-first, depth-first

• Method will affect run-time performance, but end-user shouldn’t see this, only see iterator interface.

Iterator (Cont.)

• Data Structure Vector• Start of traversal i = 0;• Increment traversal i++;• Get i-th element

myVector.get(i);• Finished traversal?

i < myVector.size();

12.3 Trees

• Need a node called root

• A node can have several child nodes

• A childless node is called a leaf

• Example: DefaultMutableTreeNode

• parent.add(child); adds a child to a node

• See also DirTree.java in Lab ex. 4.

TreeModel model = … ;

Jtree tree = new Jtree(model);

• Make a class that implements TreeModel

interface. DefaultTreeModel already does this.

TreeNode root = …;

DefaultTreeModel model = new DefaultTreeModel(root);

• TreeNode is another interface.

• DefaultMutableTreeNode already implements MutableTreeNode a subinterface of TreeNode.

DefaultMutableTreeNode node = newDefaultMutableTreeNode(“Stockholm”);ornode.setUserObject(“Göteborg”)

Now to construct a tree!

DefaultMutableTreeNode root = new

DefaultMutableTreeNode(“World”);

DefaultMutableTreeNode country = new

DefaultMutableTreeNode(“Sverige”);

DefaultMutableTreeNode town = new

DefaultMutableTreeNode(“Stockholm”);

root.add(country);

country.add(town);

DefaultTreeModel model = new DefaultTreeModel(root);

JTree tree = new JTree(model);

Container contentPane = getContentPane();

contentPane.add(new JScrollPane(tree));

• What about the iterators?

enumeration e = root.breadthFirstEnumeration();

while (e.hasMoreElements()) {DefaultMutableTree node =(DefaultMutableTree)e.nextElement();/* do something with node … e.g. */if (node.getUserObject().equals(obj) )

return node;}

13. Java and Operating Systems

• Java is OS independent

• Class Runtime gives an interface to the OS

• Applets execute in a sandbox where a security manager controls access to file system, network connections, devices, etc.

• Applications can be given a security manager.

13.1 Execution of Java

• Class file is not binary and isn’t executed directly in hardware.

• Java byte code executes on a virtual java machine jvm … slow??

• Applets execute in an appletviewer or a web-browser with a jvm

• Beans execute in a BeanBox

13.2 Execution of Java (Cont.)

• Can connect Java code to other code (C, C++, Fortran etc.) vis JNI java native interface

• 3rd party suppliers have compilers to binary code. Increases speed but loses platform independence.

• Write-once, debug everywhere!! ?? (NM!)

14. BNF Grammars

• Standard format to define new context-free grammars, Backus-Naur-Form BNF.

• Example: Proposition ::= Subject PredicateSubject ::= “I” | “you”Predicate ::= “know” | “think”

Gives strings I know, I think, you know, you think

14.1 BNF Example

Statement ::= Proposition |

Proposition Conjunction Statement

Conjunction ::= “and” | “that”

GivesI know that you think that I think and you know

14.2 Parse Trees

I know that you think that I think

sub predpredpred subsub conjconj

prop propstatement

statement

statement

14.3 Ambiguity

This language is unambiguous.

i.e. every string in the language has a

unique parse tree.

Tip: use recursive descent in lab ex. 4 and a

method for each symbol in the grammar.

I know that you think that I think

sub predpredpred subsub conjconj

prop statementprop/statement

statement

statement

14.4 Language Analysis• Skaldic (old Icelandic) poetry – kennings

man ::= “man” | “tree of the” battle |

“thrower of the” sword | “giver of the” gold

gold ::= “gold” | “fire of the” war

war ::= “war” | “storm of the” spear

spear ::= “spears” | “witch of the” shield

shield ::= “shield” | “moon of the ship”

Giver of the fire of the storm of the witch of the moon of the ship

Construct a parse tree for this kenning to

understand its structure. Are kennings simply

metaphors? Only a great poet was allowed to

introduce a new kenning rule! Add some rules!

M. I. Steblin-Kamenskii, Icelandic Culture, 1967.

“As a rule, any kenning for a warrior was no richer in content than the pronoun “he” ”

14.5 XML

• Not related to UML!!!

• XML = eXtensible Markup Language

• An internet standard for data interchange,

e.g. between databases.

• Defines a data interface.

• Originates from SGML and is related to HTML

14.6 XML Methodology

• Tagged data

• e.g. <flower>rose</flower>

• <color>rose</color>

• Data marked up with its identity e.g.

<?xml version=”1.0” encoding=”UTF-8”?><!DOCTYPE configuration …><configuration>

<title><font>

<name>Helvetica</name><size unit=”pt”>36</size>

</font><author>

Karl Meinke<phone>6337</phone>

</author></title>

</configuration>

• Optional (recommended) header<?xml version=”1.0”?>

• Optional document type declaration (DTD)<!DOCTYPE configuration SYSTEM

”http://myserver.com/config.dtd”>• Finally the root element

<configuration> … <configuration>

• An element can contain child elements, text or both (but both makes a messy DTD!)

• Use attributes to modify data interpretation, and not to specify values

Parsing an XML document

• Document Object Model (DOM) parser reads an XML document into a tree structure

• Simple API for XML (SAX) pareser generates events as it reads an XML document.

• Sun provides its own DOM parser• Be careful with whitespace around tags!

import javax.xml.parsers.*;

import javax.swing.tree.*;

…

DocumentBuilderFactory factory =

DocumentBuilderFactory.newInstance();

DocumentBuilder builder =

factory.newDocumentBuilder();

File f = …

Document doc = builder.parse(f); /*tree structure of the document*/

Element root = doc.getDocumentElement();

NodeList children = root.getChildNodes();

Node firstchild = children.item(1);

14.7 DTD

• Usually an external file (small DTDs can be internal)

• Specifies the format of tagged data.

• An XML document is valid (wrt a DTD) if it satisfies the format of the DTD

• Can build XML validators – prevent data corruption, more efficient parse, etc.

• factory.setValidating(true);

• e.g. A font element consists of a name element followed by a size element

<!ELEMENT font (name,size)>• e.g. A kenning

<!ELEMENT man ( manText | treeText, battle |throwerText, sword | giverText, gold )>

<!ELEMENT manText #PCDATA > • DTDs define regular expressions• XML assumes grammar is unambiguous!!

Rule Meaning

E* 0 or more occurrences of E

E+ 1 or more occurrences of E

E? 0 or 1 occurrences of E

E1 | … | En E1 or E2 …. or En

E1 , … , En E1 followed by E2 followed

by … followed by En

#PCDATA Text

Any Any children allowed

Empty No children allowed

• Can also specify attributes of elements

<!ATTLIST font style (plain|bold|italic) plain>• Style attribute can be plain, bold or italic, but

default is plain.

<!ATTLIST size unit CDATA #IMPLIED>• Unit attribute of size is a character sequence

and optional

<!ATTLIST size unit CDATA #REQUIRED>• Now unit is not optional!

<!DOCTYPE Date [

<!ELEMENT Date (Name*, Moon?)>

<!ATTLIST Date year CDATA #REQUIRED>

<!ATTLIST Date month #REQUIRED>

<!ATTLIST Date day #REQUIRED>

<!ATTLIST Date weekday #IMPLIED>

<!ATTLIST Date CDATA #REQUIRED>

<!ELEMENT Name(#PCDATA)>

<!ELEMENT Moon EMPTY>

<!ATTLIST Moon phase (new|between|full)

“between”>

]>