Java Threads Concurrent...

Fachbereich Informatik und Elektrotechnik

Programming in Java, Helmut Dispert

Java Threads

Concurrent Programming

Concurrency versus Parallelism

Task 1

Task 2

Time Time

Task 1 Task 2

Concurrency Parallelism

Requires a

multiple-CPU systemSingle-CPU system

Java Thread Definition

Synonyms:

• Lightweight Process

• Execution Context

Definition:

• A thread is a single sequential flow of control within a

program.

Java Threads

Thread

A Program

Java Threads

Threads

A Program

Threads and Parallelism

Threads enable parallelism within a program process.

Each thread may take a different path of code execution

through a program.

• Single processor machines simulate simultaneous

execution by running each thread for a very short time,

then performing a context switch to another thread.

• Threads make use of multiple processor machines (with

two or more CPUs on a board). Each thread has it own

resource overhead of local variables and program

counters.

http://www.wilsonmar.com

Operating System

Java Threads

Process 1

Process 2

Process 3(Virtual Java Machine)

Storage

Local Variables 1

Local Variables 2

Local Variables 3

Thread 1

Thread 2

Thread 3

Multithreaded Processes in a multithreaded environment

Java Threads

Program Counter

Stack Segment

Program Counter

Stack Segment

Thread 1

Thread 2

Code Segment

Data Segment Files

Task - Process

Implicit Threads

Creating implicit threads with AWT, Swing:

Implicit Threads

//AwtThread.java - doing two things at once

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

class AwtThread {

public static void main(String[] args)

throws InterruptedException

createGUI();

int count = 0;

while (true) {

count++;

// go to sleep for 1 second = 1000 milliseconds

Thread.currentThread().sleep(1000);

System.out.println("count is now " + count);

System.out.flush(); // force output to print now

continued

thread executing

main()

Implicit Threads

static void createGUI() {

JFrame frame = new JFrame("AwtThread");

Container pane = frame.getContentPane();

JButton quit = new JButton("Quit");

quit.addActionListener(new GoodBye());

pane.add(quit, BorderLayout.NORTH);

JButton counter = new JButton("Click to count");

counter.addActionListener(new ClickCounter());

pane.add(counter, BorderLayout.SOUTH);

frame.pack();

frame.show();

continued

thread reading

GUI events

Implicit Threads

//ClickCounter.java - count button clicks

class ClickCounter implements ActionListener {

public void actionPerformed(ActionEvent e) {

count++;

System.out.println("Total clicks is " + count);

int count = 0;

class GoodBye implements ActionListener {

public void actionPerformed(ActionEvent e) {

System.out.println("Goodbye!");

System.exit(0);

Ira Pohl, Charlie McDowell,

Java by Dissection

Implicit Threads - AWT

thread executing

main()

thread reading

GUI events

Java Thread Definition

Thread Attributes:

• Thread Body: method run ()

• Thread State: state diagram

(running, sleeping, dead)

• Thread Priority: information for Java scheduler

• Daemon Threads: service for other threads

• Thread Group: group of related threads

Daemon Threads:Daemon threads are threads that work in the background to

support the runtime environment, they are service providers for

other threads running in the same process.

Any Java thread can be a daemon thread.

Java Thread Implementation

Thread Implementation:

• Way 1: Extend class Thread:

class ClassName extends Thread

Extended class must implement method run()

public void run()

• Way 2: Implementation of interface Runnable

public class ClassNameextends SuperClassimplements Runnable

Rule of Thumb:If a class must subclass some other class (the most common example

being Applet), Runnable has to be used as described in option 2.

Erweitern der Klasse Thread oder Implementieren von

Runnable?

Kann sich nur mit

Umwegen selbst starten.

Allgemein: Thread –

Methoden können nur

über Umwege genutzt

werden.

Die Klasse kann von

einer anderen,

problemspezifischen

Klasse erben

Implementieren von

Runnable

(implements Runnable)

Da es in Java keine

Mehrfachvererbung gibt,

kann die Klasse nur

Thread erweitern

Programmcode in

run() kann die

Methoden der Klasse

Thread nutzen

Ableiten von Thread

(extends Thread)

NachteilVorteil

Methods of class Thread that can be called:

void start()

void stop()

void sleep (long wait_time)

void suspend()

void resume()

void join(long wait_time)

boolean isAlive()

Remove a thread from the waiting state and place it

in the ready-to-run state. void notify

moves an object into a waiting state where it waits

for a lock to be released. void wait

Suspend a thread for a specified amount of time (in

milliseconds). void sleep

Start the thread by calling its run method. void start

This method is the entry point for threads, like the

main method for applications. void run

Determine if the thread is currently running. boolean isAlive

Retrieve the thread instance's priority. int getPriority

Retrieve the name of the thread object or instance. String getName

Returns an object reference to the thread in which it

is invoked. Thread currentThread

Description Return Type Method

Java.lang.Thread Class Methods (native Java v1.3+)

The wait(), notify(), and notifyAll() methods for synchronized thread control

in Java 1.2+ are part of the Object class. Ref.:

Java Thread Methods

� void start()

Causes this thread to begin execution; the Java Virtual Machine calls the

run method of this thread.

� void stop()

Deprecated. This method is inherently unsafe.

� void suspend()

Deprecated. This method has been deprecated, as it is inherently

deadlock-prone.

� void resume()

Deprecated. This method exists solely for use with suspend(), which

has been deprecated.

continued

Java Thread Methods

� void join()

Waits for this thread to die.

� void join(long millis)

Waits at most millis milliseconds for this thread to die.

� void join(long millis, int nanos)

Waits at most millis milliseconds plus nanos nanoseconds for this

thread to die.

� boolean isAlive()

Tests if this thread is alive.

� getName()

Returns this thread's name.

� getPriority()

Returns this thread's priority.

Java Threads: Constructor Summary

Thread(String name)

Thread(Runnable target, String name)

Thread(Runnable target)

Thread()

Allocates a new Thread object.

Java Thread Life Cycle (State Diagram)

New Thread Runnable Not Runnable

new Thread()

start()

stop() stop()

stop() or

run() exits

yield()

Thread States according to Java 1.2

Thread States according to Java 1.2New

When a Thread is created it is into the New state. A New State Thread is not

running.

Running (Runnable)

A call to the Thread method start() is required to move the Thread from the New

State to the Runnable State. A Thread in the Runnable State is eligible to execute

on the CPU. A Runnable State Thread will move on and off the CPU due to

scheduling assignments.

A Thread moves from Runnable State to Dead State when the run() method exits

or the Thread stop() method is called. Note that a Thread that has stopped is

dead, not suspended.

Blocked (Not Runnable)

The last Thread state is Blocked State. The Blocked State contains the Threads

that can not execute due to a wait(), suspend(), sleep(), or I/O wait. When the

reason for the block is removed, the Thread moves back to the Runnable State.

Thread methods suspend( ), resume( ), and stop( ) were deprecated by Java

v1.3. Ref.:

another thread closes socket

Ready-to-run Sleeping

(finished)

Running

(executing)

Waiting

Blocked on

I/O or Sync

Object.notify();

Object.notifyAll()

start()

scheduler swap or

Thread.yield()

data/sync

received

Thread.sleep()

chosen by

scheduler

Object.wait()

Thread States according to Java 1.3

Thread States according to Java 1.3• Ready-to-run

waiting for its turn to be picked for execution by the thread scheduler based on thread priorities.

• Running (IsAlive)The thread's code is being actively executed by a processor and will run until it is swapped out, becomes blocked, or voluntarily give up its turn with a yield method which generally, because of context switching overhead, should not be used more than about five times per second

• WaitingA Thread is in a blocked state while it waits for some external processing (such as file I/O) to finish.

• SleepingThreads are forcibly put to sleep (suspended) by a thread.sleep() java call.

• Blocked on I/OWill move to Ready-to-Run after I/O condition changes (reading a byte of data).

• Blocked on SyncWill move to Ready-to-Run when a lock is acquired (passes synchronized statement).

• Dead (Terminated)The thread has finished working and cannot be resumed.

Java Thread Example

public void run()

// Code that will be executed in it's own thread

continued

Implement method run()

Java Thread Example

class CntThread extends Thread

String ThreadName="";

CntThread(String Name)

this.ThreadName=Name;

public void run()

for (int I=0; I<=5; I++)

System.out.println ("Thread " + ThreadName + " :"+I);

continued

Example Program: ThreadCaller

Thread Code

Java Thread Example

Start Threads

Create Thread Objects

class ThreadCaller

public static void main(String args[])

CntThread Cnt1 = new CntThread("U.S.A. ");

CntThread Cnt2 = new CntThread("Germany");

Cnt1.start();

Cnt2.start();

Java Thread Example

Thread U.S.A. :0

Thread Germany :0

Thread U.S.A. :1

Thread Germany :1

Thread Germany :2

Thread U.S.A. :2

Thread Germany :3

Thread Germany :4

Thread Germany :5

Thread U.S.A. :3

Thread U.S.A. :4

Thread U.S.A. :5

Thread Germany :0

Thread U.S.A. :0

Thread U.S.A. :1

Thread U.S.A. :2

Thread Germany :1

Thread U.S.A. :3

Thread U.S.A. :4

Thread U.S.A. :5

Thread Germany :2

Thread Germany :3

Thread Germany :4

Thread Germany :5

1st Test 2nd Test

Expectation:

Java Thread Example

Thread U.S.A. :0

Thread Germany :0

Thread U.S.A. :1

Thread Germany :1

Thread U.S.A. :2

Thread Germany :2

Thread U.S.A. :3

Thread Germany :3

Thread U.S.A. :4

Thread Germany :4

Thread U.S.A. :5

Thread Germany :5

Thread U.S.A. :0

Thread Germany :0

Thread U.S.A. :1

Thread Germany :1

Thread U.S.A. :2

Thread Germany :2

Thread U.S.A. :3

Thread Germany :3

Thread U.S.A. :4

Thread Germany :4

Thread U.S.A. :5

Thread Germany :5

1st Test 2nd Test

Results (I = 5):

ordered

sequence

Java Thread Example

Thread U.S.A. :985

Thread Germany :997

Thread U.S.A. :986

Thread Germany :998

Thread U.S.A. :987

Thread Germany :999

Thread U.S.A. :988

Thread Germany :1000

Thread U.S.A. :989

Thread U.S.A. :990

Thread U.S.A. :991

Thread U.S.A. :992

Thread U.S.A. :993

Thread U.S.A. :994

Thread U.S.A. :995

Thread U.S.A. :996

Thread U.S.A. :997

Thread U.S.A. :998

Thread U.S.A. :999

Thread U.S.A. :1000

Thread U.S.A. :997

Thread Germany :985

Thread U.S.A. :998

Thread Germany :986

Thread U.S.A. :999

Thread Germany :987

Thread Germany :988

Thread Germany :989

Thread Germany :990

Thread U.S.A. :1000

Thread Germany :991

Thread Germany :992

Thread Germany :993

Thread Germany :994

Thread Germany :995

Thread Germany :996

Thread Germany :997

Thread Germany :998

Thread Germany :999

Thread Germany :1000

1st Test 2nd Test

Results (I = 1000):

Java Thread Methods

public static void sleep(long millis)

Causes the currently executing thread to sleep (temporarily cease

execution) for the specified number of milliseconds. The thread does not

lose ownership of any monitors.

Parameters:

millis - the length of time to sleep in milliseconds.

public static void sleep(long millis, int nanos)

Causes the currently executing thread to sleep (cease execution) for the

specified number of milliseconds plus the specified number of

nanoseconds. The thread does not lose ownership of any monitors.

Parameters:

millis - the length of time to sleep in milliseconds.

nanos - 0-999999 additional nanoseconds to sleep.continued

Java Thread Methods

public static void yield()

Causes the currently executing thread object to temporarily pause and

allow other threads to execute.

Sleep()

actively suspends the thread for the specified

yield()

will give another waiting thread a chance.

class CntThread2 extends Thread

String ThreadName="";

CntThread2(String Name)

this.ThreadName=Name;

public void run()

for (int I=0; I<=10; I++)

System.out.println("Thread " + ThreadName + " :"+I);

sleep((long) (Math.random() * 1000));

catch(InterruptedException e) {}

Java Thread Example

Example Program: ThreadCaller with sleep()

sleep()

JavaThreadCaller2

Java Thread Example

Thread U.S.A. :0

Thread Germany :0

Thread U.S.A. :1

Thread Germany :1

Thread U.S.A. :2

Thread Germany :2

Thread Germany :3

Thread U.S.A. :3

Thread U.S.A. :4

Thread Germany :4

Thread U.S.A. :5

Thread Germany :5

Thread Germany :0

Thread U.S.A. :0

Thread Germany :1

Thread U.S.A. :1

Thread Germany :2

Thread Germany :3

Thread U.S.A. :2

Thread U.S.A. :3

Thread Germany :4

Thread U.S.A. :4

Thread Germany :5

Thread U.S.A. :5

1st Test 2nd Test

Results with sleep() (I = 5):

Java Thread Example

Run Threads

class ThreadCallerR

public static void main(String args[])

CntThread Cnt1 = new CntThread("U.S.A. ");

CntThread Cnt2 = new CntThread("Germany");

Cnt1.run();

Cnt2.run();

Alternative Solution:

Simple Method CallJava ThreadCallerR

Java Thread Example

Thread U.S.A. :0

Thread U.S.A. :1

Thread U.S.A. :2

Thread U.S.A. :3

Thread U.S.A. :4

Thread U.S.A. :5

Thread U.S.A. :6

Thread U.S.A. :7

Thread U.S.A. :8

Thread U.S.A. :9

Thread U.S.A. :10

Thread Germany :0

Thread Germany :1

Thread Germany :2

Thread Germany :3

Thread Germany :4

Thread Germany :5

Thread Germany :6

Thread Germany :7

Thread Germany :8

Thread Germany :9

Thread Germany :10

Thread U.S.A. :0

Thread U.S.A. :1

Thread U.S.A. :2

Thread U.S.A. :3

Thread U.S.A. :4

Thread U.S.A. :5

Thread U.S.A. :6

Thread U.S.A. :7

Thread U.S.A. :8

Thread U.S.A. :9

Thread U.S.A. :10

Thread Germany :0

Thread Germany :1

Thread Germany :2

Thread Germany :3

Thread Germany :4

Thread Germany :5

Thread Germany :6

Thread Germany :7

Thread Germany :8

Thread Germany :9

Thread Germany :10

1st Test 2nd Test

Results for the alternative solution (I = 20):

Java Thread Example

Thread U.S.A. :0

Thread U.S.A. :1

Thread U.S.A. :2

Thread U.S.A. :3

Thread U.S.A. :4

Thread U.S.A. :5

Thread U.S.A. :6

Thread U.S.A. :7

Thread U.S.A. :8

Thread U.S.A. :9

Thread U.S.A. :10

Thread Germany :0

Thread Germany :1

Thread Germany :2

Thread Germany :3

Thread Germany :4

Thread Germany :5

Thread Germany :6

Thread Germany :7

Thread Germany :8

Thread Germany :9

Thread Germany :10

Thread U.S.A. :0

Thread U.S.A. :1

Thread U.S.A. :2

Thread U.S.A. :3

Thread U.S.A. :4

Thread U.S.A. :5

Thread U.S.A. :6

Thread U.S.A. :7

Thread U.S.A. :8

Thread U.S.A. :9

Thread U.S.A. :10

Thread Germany :0

Thread Germany :1

Thread Germany :2

Thread Germany :3

Thread Germany :4

Thread Germany :5

Thread Germany :6

Thread Germany :7

Thread Germany :8

Thread Germany :9

Thread Germany :10

1st Test 2nd Test

Results for the alternative solution with sleep() (I = 20):

Synchronization

Synchronization among threads

Problem: producer/consumer relationship � threads often need to share a common resource (file, etc.) with one

thread reading from it while another thread is writing to it.

Race conditions:� race conditions occur when multiple, asynchronously executing

threads access the same object returning unexpected (wrong)

results;

� race conditions can be avoided by synchronizing the methods

which access the shared resource.

Example: Producer/Consumer Problem

Ref.: Sun Microsystems

Basic Idea:

� The producer generates an integer (0 to 9) and stores it in an object (Storage);

� The consumer consumes the integers from the Storage

object;

� Producer and consumer share data through acommon Storage object.

class Storage {

private int contents;

private boolean available = false;

public int get() {

available = false;

return contents;

public void put(int value) {

contents = value;

available = true;

continued

No Synchronization:

class Producer extends Thread {

private Storage stored;

private int number;

public Producer(Storage c, int number) {

stored = c;

this.number = number;

public void run() {

for (int i = 0; i < 10; i++) {

stored.put(i);

System.out.println("Producer #" + this.number

+ " put: " + i);

sleep((int)(Math.random() * 100));

} catch (InterruptedException e) { }

} continued

class Consumer extends Thread {

private Storage stored;

private int number;

public Consumer(Storage c, int number) {

stored = c;

this.number = number;

public void run() {

int value = 0;

for (int i = 0; i < 10; i++) {

value = stored.get();

System.out.println("Consumer #" + this.number

+ " got: " + value);

continued

public class PCTest {

public static void main(String[] args) {

Storage c = new Storage();

Producer p1 = new Producer(c, 1);

Consumer c1 = new Consumer(c, 1);

p1.start();

c1.start();

Start Threads

Create Thread Objects

Producer #1 put: 0

Consumer #1 got: 0

Producer #1 put: 1

Consumer #1 got: 1

Producer #1 put: 2

Producer #1 put: 3

Producer #1 put: 4

Producer #1 put: 5

Producer #1 put: 6

Producer #1 put: 7

Producer #1 put: 8

Producer #1 put: 9

Producer #1 put: 0

Consumer #1 got: 0

Producer #1 put: 1

Producer #1 put: 2

Producer #1 put: 3

Producer #1 put: 4

Producer #1 put: 5

Producer #1 put: 6

Producer #1 put: 7

Producer #1 put: 8

Producer #1 put: 9

1st Test 2nd Test

Results:

Synchronization

Synchronizing two threads

Two types of thread synchronization can be distinguished:

� Mutual exclusion synchronization:

used to protect certain critical sections of code from being executed

simultaneously by two threads.

� Signal-wait synchronization:

used when one thread has to wait until another thread has

completed its action.

Synchronization

Mutual exclusion synchronization:

storeload

storeload storeload

storethread 1

thread 2

Context switch Context switch

Synchronization

Java program:

� Statements are compiled as a sequence of bytecodes,

� code is executed using the VM stack (temp values, results),

� each Java thread has its own stack used to execute bytecodes.

Context switch:

� When VM switches from one running thread to another, it also

switches to another stack.

Synchronization

Java platform dependence:

� Atomicity in Java:difficult to write truly platform-independent multithreaded Java

programs, because the implementation of atomic operations

depend on the OS-level concept.

Examples:

� assignment to variables of most types is atomic,� assignment to long or double is not atomic.

� shortcut operators do not support atomicity,e.g. "x = ++y" or "x += y" could be preempted after the

increment operation but before the assignment.

Monitors

Monitors:

The Java language and runtime system support thread

synchronization through the use of monitors

C. A. R. Hoare:

An Operating System Structuring Concept

(Communications of the ACM, 17(10), 549-557, 1974).

��Reader

Tony Hoare

"I conclude that there are two ways of constructing a software design:

One way is to make it so simple that there are obviously no deficiencies and

the other way is to make it so complicated that there are no obvious

deficiencies."

Charles Antony Richard Hoare

(Tony Hoare)

One of the fathers of modern

computer science, having made

fundamental contributions to the

definition and design of

programming languages.

http://research.microsoft.com/users/thoare/

Tony Hoare

Tony Hoare:

� 1934: born in Colombo (Sri Lanka)

� 1956: BA in Literae Humaniores (Classics)

� 1968: appointed as Professor of Computing Science at the

Queen's University, Belfast

� 1977:

Professor of Computing at Oxford University

� developed Quicksort

� developed the formal language:

Communicating Sequential Processes (CSP)

(interactions of concurrent processes)

� 1980: Turing Award

� since 1999:

Emeritus Professor of Computing, Oxford University Computing

Laboratory,

Senior researcher with Microsoft Research in Cambridge

Thread Synchronization - Monitors

A monitor is associated with a specific data item (a

condition variable) and functions as a lock on that data.

When a thread holds the monitor for some data item,

other threads are locked out and cannot inspect or

modify the data.

The code segments within a program that access the

same data from within separate, concurrent threads are

known as critical sections.

In the Java language, critical sections in a program are marked with the synchronized keyword.

To avoid corruption of data during the execution of

threads Java provides two levels of thread

synchronization:

a) Protection of shared resources

Locking an Object;

b) Signalling between threads to indicate changes in

conditions.

Synchronized Methods

Protection of shared resources,

Locking an Object:

� The code segments within a program that access the same object

from separate, concurrent threads are called critical sections.

In the Java language, a critical section can be a block or a method and are identified with the synchronized keyword.

The Java platform associates a lock with every object that has

synchronized code.

Ref.: Sun Microsystems

Signalling between threads

to indicate changes in conditions.

� Several methods exist to indicate to a thread when a

condition that it is waiting for has been met:

- wait()

- notify()

- notifyAll()

Synchronization Methods

� public final void wait()

Causes current thread to wait until another thread invokes the notify()

method or the notifyAll() method for this object.

The current thread must own this object's monitor. The thread releases

ownership of this monitor and waits until another thread notifies threads

waiting on this object's monitor to wake up either through a call to the

notify method or the notifyAll method.

� public final void notify()

Wakes up a single thread that is waiting on this object's monitor. If any

threads are waiting on this object, one of them is chosen to be

awakened. The choice is arbitrary and occurs at the discretion of the

implementation.

� public final void notifyAll()

Wakes up all threads that are waiting on this object's monitor.

class Storage {

private int contents;

private boolean available = false;

public synchronized int get() {

while (available == false) {

wait();

available = false;

notifyAll();

return contents;

continued

Example with Synchronization:

public synchronized void put(int value) {

while (available == true) {

wait();

contents = value;

available = true;

notifyAll();

continued

With Synchronization:

Java PCTestSync.java

Consumer #1 got: 0

Producer #1 put: 0

Producer #1 put: 1

Consumer #1 got: 1

Producer #1 put: 2

Consumer #1 got: 2

Consumer #1 got: 3

Producer #1 put: 3

Consumer #1 got: 4

Producer #1 put: 4

Consumer #1 got: 5

Producer #1 put: 5

Producer #1 put: 6

Consumer #1 got: 6

Producer #1 put: 7

Consumer #1 got: 7

Producer #1 put: 8

Consumer #1 got: 8

Consumer #1 got: 9

Producer #1 put: 9

Producer #1 put: 0

Consumer #1 got: 0

Producer #1 put: 1

Consumer #1 got: 1

Producer #1 put: 2

Consumer #1 got: 2

Consumer #1 got: 3

Producer #1 put: 3

Producer #1 put: 4

Consumer #1 got: 4

Producer #1 put: 5

Consumer #1 got: 5

Producer #1 put: 6

Consumer #1 got: 6

Consumer #1 got: 7

Producer #1 put: 7

Producer #1 put: 8

Consumer #1 got: 8

Producer #1 put: 9

Consumer #1 got: 9

1st Test 2nd Test

Results:

Ending a Thread

A thread normally ends when it's execution completes,

most other methods of stopping a thread are deprecated

� public void interrupt()

Interrupts this thread

� public boolean isInterrupted()

Tests whether this thread has been interrupted. The interrupted status

of the thread is unaffected by this method.

� public final void join()

Waits for this thread to die:

one thread can wait for another to complete,

invoking join() guarantees that the method will not return until the threads

run() method has completed.

� public static void yield()

Causes the currently executing thread object to temporarily pause and

allow other threads to execute (lower priority threads are ignored).

Ending a Thread

� public final void setDaemon(boolean on)

Marks this thread as either a daemon thread or a user thread. The Java

Virtual Machine exits when the only threads running are all daemon

threads.

Parameters:

on - if true, marks this thread as a daemon thread.

Daemon Threads:

By default a new thread is not a daemon thread. However a

thread can be turned into a daemon thread using the method

setDaemon(true).

Ending a Thread

Deprecated methods to stop a thread:

� public final void stop()

Forces the thread to stop executing.

stop() is a deprecated method and should not be used.

� public final void suspend()

Suspends this thread.

� public final void resume()

Resumes a suspended thread.

Ending a Thread

Ending a Thread using "interrupt()"

• interrupt()

setzt eine interne Variable auf true, die in der

run() – Methode abgefragt werden kann.

• isInterrupted()

wird in der run() – Methode aufgerufen um zu

überprüfen, ob der Thread beendet werden soll.

Diese Methode sollte regelmäßig in der

run() – Methode aufgerufen werden.

Example: Ending a Thread

class ThreadInterrupt extends Thread

public void run()

System.out.println( "Der Anfang" );

while ( ! isInterrupted() )

System.out.println( "Und er laeuft und er laeuft

und er laeuft" );

Thread.sleep( 500 );

catch ( InterruptedException e )

interrupt();

System.out.println( "Unterbrechung in sleep()" );

System.out.println( "Das Ende" );

} continued

Example: Ending a Thread

public static void main( String args[] )

ThreadInterrupt t = new ThreadInterrupt();

t.start();

catch ( InterruptedException e ) { }

t.interrupt();

Ending a Thread using join()

Using "join()" to wait for the end of a

thread

• Threads können beim Beenden keine Ergebnisse

nach außen geben, dafür verwendet man Klassen -

oder Objekt –Variablen.

• Andere Threads wissen aber nicht, wann ein

bestimmter Thread fertig ist.

• Daher warten diese Threads mit Hilfe der Methode

join() auf deren Beendigung.

Example: Ending a Thread using join()

class JoinTheThread

static class JoinerThread extends Thread

public int result;

public void run()

result = 1;

public static void main( String args[] ) throws Exception

JoinerThread t = new JoinerThread();

t.start();

t.join(); // Ohne diesen Aufruf ist das Resultat 0 – mit ist es 1

// Kann wahlweise auch mit einem Timeout versehen werden

System.out.println( t.result );

} Java

Waiting for the slowest thread

Zwei Threads A und B arbeiten an einem Problem. Die

Methode go() startet A und B und wartet dann, bis beide

fertig sind. Dann kann ein weiterer Thread die Ergebnisse

verwenden.

void go() throws Exception

Thread a = new A();

Thread b = new B();

a.start();

b.start();

a. join() ;

b. join() ;

Putting a thread to sleep

Um einen Thread schlafen zu legen, verwendet man die

Methode sleep() :

public class SleepInInnerClass {

public static void main( String args[] ) {

new Thread() {

{ start(); } {

public void run() {

sleep(2000);

System.out.println("Zeit ist um.");

} catch ( InterruptedException e ) { }

Stopping and resuming a thread

• Mit den folgenden Methoden kann man einen Thread

vorübergehend deaktivieren:

– void suspend()

Versetzt den Thread in den Zustand „nicht laufend“,

bis resume() aufgerufen wird.

– void resume()

Versetzt ihn wieder in den Zustand „laufend“.

• Beide Methoden sind veraltet ("deprecated") und sollten

nicht mehr verwendet werden.

Priorities

• Jeder Thread wird auf einer Skala von 1 bis 10 mit der

Standardpriorität 5 gestartet.

• Die Priorität wirkt sich auf die Häufigkeit, mit der der

Thread vom Scheduler ausgeführt wird, aus und kann

entweder im Konstruktor oder mit der Methode

setPriority() gesetzt werden.

• Hohe Prioritäten sollten zugunsten von niedrigeren

Prioritäten vermieden werden.

Example

continued

Example

Applets and Threads

Clock-Applet

import java.awt.*;

import java.awt.Graphics;

import java.util.*;

import java.text.DateFormat;

import java.applet.Applet;

public class Clock extends Applet implements Runnable {

private Thread clockThread = null;

public void start() {

if (clockThread == null) {

clockThread = new Thread(this, "Clock");

clockThread.start();

public void run() {

Thread myThread = Thread.currentThread();

while (clockThread == myThread) {

repaint();

Thread.sleep(1000);

} catch (InterruptedException e){ }

Clock implements the Runnable interface (and

therefore implements the run method defined in it)

continued

Clock-Applet

public void paint(Graphics g) {

// get the time and convert it to a date

Calendar cal = Calendar.getInstance();

Date date = cal.getTime();

// format it and display it

DateFormat dateFormatter = DateFormat.getTimeInstance();

Font arial200 = new Font ("Arial", Font.PLAIN, 200);

g.setFont(arial200);

g.drawString(dateFormatter.format(date), 75, 250);

// overrides Applet's stop method, not Thread's

public void stop() {

clockThread = null;

<HTML> <HEAD> <TITLE>Clock</TITLE> </HEAD>

<BODY>

</APPLET>

</BODY>

</HTML> Demo

Clock-Applet

Programming Errors

Programming errors

Programming Errors

Common Programming Errors

in multithreaded programs:

1. Race condition:Two threads with references to a shared object interfere with

each other.

2. Deadlock:A thread in a group of threads is holding one lock and

waiting for another lock that is held by another thread in the

group.Consequence: not simply possible to add synchronized to

all methods.

Deadlock

Programming Errors

Deadlock:

Wait for lock for objectOne

Enter methodOne() for objectOne

Call ObjectTwo.methodTwo()

Wait for lock for objectTwo

Enter methodOne() for objectTwo

Call ObjectOne.methodTwo()

Wait for lock for objectOne

Context

switch

waiting

Thread 1 Thread 2

Deadlock Example

class Deadlock {

static Object a = new Object(), b = new Object();

static class T1 extends Thread {

public void run() {

synchronized( a ) {

System.out.println( "T1: Get lock on a" );

waitDelay();

synchronized( b ) {

System.out.println( "T1: Get lock on b" );

private void waitDelay() {

} catch ( InterruptedException e ) { }

Deadlock Example

static class T2 extends Thread {

public void run() {

synchronized( b ) {

System.out.println( "T2: Get lock on b" );

synchronized( a ) {

System.out.println( "T2: Get lock on a" );

public static void main( String args[] ) {

new T1().start();

new T2().start();

T1: Get lock on a

T2: Get lock on b Java

Threadgroups

Thread-1

Thread-2

Thread-3

Thread-c

Thread-a

Thread-b

Thread-d

Thread-A

Thread-αααα

Thread-ββββ

"main"

subgroup1 subgroup2

subgroup3

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