Java Threads - Helmut Dispert

Fachbereich Informatik und Elektrotechnik
Java Threads
Java Threads
Concurrent Programming
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Concurrency versus Parallelism
Concurrency
Parallelism
Time
Time
Task 2
Task 2
Task 1
Task 1
Single-CPU system
Programming in Java,  Helmut Dispert
Requires a
multiple-CPU system
Fachbereich Informatik und Elektrotechnik
Java Thread Definition
Synonyms:
• Lightweight Process
• Execution Context
Definition:
• A thread is a single sequential flow of control within a
program.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Threads
One
Thread
A Program
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Threads
Two
Threads
A Program
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Ref.:
http://www.wilsonmar.com
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Threads
Operating System
Process 1
Process 2
Process 3
(Virtual Java Machine)
Storage
Thread 1
Local Variables 1
Thread 2
Local Variables 2
Thread 3
Local Variables 3
Multithreaded Processes in a multithreaded environment
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Threads
Thread 1
Program Counter
Stack Segment
Thread 2
Program Counter
Stack Segment
4
Data Segment
Code Segment
Files
Task - Process
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Implicit Threads
Creating implicit threads with AWT, Swing:
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Implicit Threads
//AwtThread.java - doing two things at once
import java.awt.*;
thread executing
import java.awt.event.*;
main()
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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();
}
}
thread reading
GUI events
continued
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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);
}
}
Ref.:
Ira Pohl, Charlie McDowell,
Java by Dissection
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Implicit Threads - AWT
thread executing
main()
thread reading
GUI events
Java
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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 ClassName
extends SuperClass
implements 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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Implementation
Erweitern der Klasse Thread oder Implementieren von
Runnable?
Vorteil
Nachteil
Ableiten von Thread
(extends Thread)
Programmcode in
run() kann die
Methoden der Klasse
Thread nutzen
Da es in Java keine
Mehrfachvererbung gibt,
kann die Klasse nur
Thread erweitern
Implementieren von
Runnable
(implements Runnable)
Die Klasse kann von
einer anderen,
problemspezifischen
Klasse erben
Kann sich nur mit
Umwegen selbst starten.
Allgemein: Thread –
Methoden können nur
über Umwege genutzt
werden.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Implementation
Methods of class Thread that can be called:
void start()
void stop()
void sleep (long wait_time)
throws InterruptedException
void suspend()
void resume()
void join(long wait_time)
throws InterruptedException
boolean isAlive()
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Implementation
Java.lang.Thread Class Methods (native Java v1.3+)
Method
Return Type
Description
currentThread
Thread
Returns an object reference to the thread in which it
is invoked.
getName
String
Retrieve the name of the thread object or instance.
getPriority
int
Retrieve the thread instance's priority.
isAlive
boolean
Determine if the thread is currently running.
run
void
This method is the entry point for threads, like the
main method for applications.
start
void
Start the thread by calling its run method.
sleep
void
Suspend a thread for a specified amount of time (in
milliseconds).
wait
void
moves an object into a waiting state where it waits
for a lock to be released.
notify
void
Remove a thread from the waiting state and place it
in the ready-to-run state.
The wait(), notify(), and notifyAll() methods for synchronized thread control
Ref.:
in Java 1.2+ are part of the Object class.
http://www.wilsonmar.com
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Methods
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Methods
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Threads: Constructor Summary
Thread()
Allocates a new Thread object.
Thread()
Thread(Runnable target)
Thread(Runnable target, String name)
Thread(String name)
Docu
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Life Cycle (State Diagram)
Thread States according to Java 1.2
yield()
new Thread()
New Thread
start()
Runnable
Not Runnable
stop() or
run() exits
stop()
stop()
Dead
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Life Cycle (State Diagram)
Thread States according to Java 1.2
New
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.
Dead
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.:
http://www.wilsonmar.com
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Life Cycle (State Diagram)
Thread States according to Java 1.3
Object.notify();
Object.notifyAll()
Ready-to-run
Sleeping
start()
chosen by
scheduler
Waiting
Thread.sleep()
Object.wait()
scheduler swap or
Thread.yield()
done
Dead
(finished)
Running
(executing)
data/sync
received
another thread closes socket
Blocked on
I/O or Sync
Ref.:
http://www.wilsonmar.com
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Life Cycle (State Diagram)
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
• Waiting
A Thread is in a blocked state while it waits for some external processing (such
as file I/O) to finish.
• Sleeping
Threads are forcibly put to sleep (suspended) by a thread.sleep() java call.
• Blocked on I/O
Will move to Ready-to-Run after I/O condition changes (reading a byte of data).
• Blocked on Sync
Will move to Ready-to-Run when a lock is acquired (passes synchronized
statement).
• Dead (Terminated)
The thread has finished working and cannot be resumed.
Ref.:
http://www.wilsonmar.com
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Implement method run()
public void run()
{
// Code that will be executed in it's own thread
}
continued
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Example Program: ThreadCaller
class CntThread extends Thread
{
String ThreadName="";
CntThread(String Name)
{
this.ThreadName=Name;
}
Thread Code
public void run()
{
for (int I=0; I<=5; I++)
{
System.out.println ("Thread " + ThreadName + " :"+I);
}
}
}
continued
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Example
class ThreadCaller
{
public static void main(String args[])
{
CntThread Cnt1 = new CntThread("U.S.A. ");
CntThread Cnt2 = new CntThread("Germany");
Cnt1.start();
Cnt2.start();
}
}
Create Thread Objects
Start Threads
Programming in Java,  Helmut Dispert
Java
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Expectation:
1st Test
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
2nd Test
U.S.A.
Germany
U.S.A.
Germany
Germany
U.S.A.
Germany
Germany
Germany
U.S.A.
U.S.A.
U.S.A.
:0
:0
:1
:1
:2
:2
:3
:4
:5
:3
:4
:5
Programming in Java,  Helmut Dispert
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Germany
U.S.A.
U.S.A.
U.S.A.
Germany
U.S.A.
U.S.A.
U.S.A.
Germany
Germany
Germany
Germany
:0
:0
:1
:2
:1
:3
:4
:5
:2
:3
:4
:5
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Results (I = 5):
1st Test
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
2nd Test
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
:0
:0
:1
:1
:2
:2
:3
:3
:4
:4
:5
:5
Programming in Java,  Helmut Dispert
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
:0
:0
:1
:1
:2
:2
:3
:3
:4
:4
:5
:5
ordered
sequence
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Results (I = 1000):
1st Test
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
2nd Test
:985
:997
:986
:998
:987
:999
:988
:1000
:989
:990
:991
:992
:993
:994
:995
:996
:997
:998
:999
:1000
Programming in Java,  Helmut Dispert
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
Germany
Germany
Germany
U.S.A.
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
:997
:985
:998
:986
:999
:987
:988
:989
:990
:1000
:991
:992
:993
:994
:995
:996
:997
:998
:999
:1000
Fachbereich Informatik und Elektrotechnik
Java Thread Methods
public static void sleep(long millis)
throws InterruptedException
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)
throws InterruptedException
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Methods
public static void yield()
throws InterruptedException
Causes the currently executing thread object to temporarily pause and
allow other threads to execute.
Sleep()
actively suspends the thread for the specified
time.
yield()
will give another waiting thread a chance.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Example Program: ThreadCaller with sleep()
ThreadCaller2
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);
try
{
sleep((long) (Math.random() * 1000));
}
catch(InterruptedException e) {}
}
}}
sleep()
Programming in Java,  Helmut Dispert
Java
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Results with sleep() (I = 5):
1st Test
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
2nd Test
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
Germany
Germany
U.S.A.
U.S.A.
Germany
U.S.A.
Germany
:0
:0
:1
:1
:2
:2
:3
:3
:4
:4
:5
:5
Programming in Java,  Helmut Dispert
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Germany
U.S.A.
Germany
U.S.A.
Germany
Germany
U.S.A.
U.S.A.
Germany
U.S.A.
Germany
U.S.A.
:0
:0
:1
:1
:2
:3
:2
:3
:4
:4
:5
:5
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Alternative Solution:
class ThreadCallerR
{
public static void main(String args[])
{
CntThread Cnt1 = new CntThread("U.S.A. ");
CntThread Cnt2 = new CntThread("Germany");
Cnt1.run();
Cnt2.run();
}
Run Threads
}
Java
ThreadCallerR
Programming in Java,  Helmut Dispert
Simple Method Call
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Results for the alternative solution (I = 20):
1st Test
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
2nd Test
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
Programming in Java,  Helmut Dispert
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
Fachbereich Informatik und Elektrotechnik
Java Thread Example
Results for the alternative solution with sleep() (I = 20):
1st Test
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
2nd Test
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
Programming in Java,  Helmut Dispert
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
Thread
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
U.S.A.
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
Germany
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
:10
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
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 a
common Storage object.
Ref.: Sun Microsystems
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
No Synchronization:
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
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);
try {
sleep((int)(Math.random() * 100));
} catch (InterruptedException e) { }
}
}
}
Programming in Java,  Helmut Dispert
continued
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
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();
}
Create Thread Objects
}
Start Threads
Java
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
Results:
1st Test
Producer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Producer
Consumer
Producer
Producer
Producer
Producer
Producer
Producer
Producer
Producer
2nd Test
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
put:
got:
got:
got:
got:
got:
got:
got:
got:
got:
put:
got:
put:
put:
put:
put:
put:
put:
put:
put:
0
0
0
0
0
0
0
0
0
0
1
1
2
3
4
5
6
7
8
9
Programming in Java,  Helmut Dispert
Producer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Consumer
Producer
Producer
Producer
Producer
Producer
Producer
Producer
Producer
Producer
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
put:
got:
got:
got:
got:
got:
got:
got:
got:
got:
got:
put:
put:
put:
put:
put:
put:
put:
put:
put:
0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Synchronization
Mutual exclusion synchronization:
thread 1
thread 2
load
store
load
Context switch
Programming in Java,  Helmut Dispert
store
load
store
load
store
Context switch
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Tony Hoare
Sir
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/
"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."
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Thread Synchronization - Monitors
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Thread Synchronization - Monitors
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()
Copyright (c) Galileo Press GmbH 2004
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Thread Synchronization - Monitors
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
Example with Synchronization:
class Storage {
private int contents;
private boolean available = false;
public synchronized int get() {
while (available == false) {
try {
wait();
} catch (InterruptedException e) { }
}
available = false;
notifyAll();
return contents;
}
continued
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
With Synchronization:
public synchronized void put(int value) {
while (available == true) {
try {
wait();
} catch (InterruptedException e) { }
}
contents = value;
available = true;
notifyAll();
}
}
Java
PCTestSync.java
continued
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Producer/Consumer Problem
Results:
1st Test
Consumer
Producer
Producer
Consumer
Producer
Consumer
Consumer
Producer
Consumer
Producer
Consumer
Producer
Producer
Consumer
Producer
Consumer
Producer
Consumer
Consumer
Producer
2nd Test
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
got:
put:
put:
got:
put:
got:
got:
put:
got:
put:
got:
put:
put:
got:
put:
got:
put:
got:
got:
put:
0
0
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
Programming in Java,  Helmut Dispert
Producer
Consumer
Producer
Consumer
Producer
Consumer
Consumer
Producer
Producer
Consumer
Producer
Consumer
Producer
Consumer
Consumer
Producer
Producer
Consumer
Producer
Consumer
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
#1
put:
got:
put:
got:
put:
got:
got:
put:
put:
got:
put:
got:
put:
got:
got:
put:
put:
got:
put:
got:
0
0
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
Fachbereich Informatik und Elektrotechnik
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).
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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).
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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" );
try
{
Thread.sleep( 500 );
}
catch ( InterruptedException e )
{
interrupt();
System.out.println( "Unterbrechung in sleep()" );
}
}
System.out.println( "Das Ende" );
continued
}
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example: Ending a Thread
public static void main( String args[] )
{
ThreadInterrupt t = new ThreadInterrupt();
t.start();
try
{
Thread.sleep( 5000 );
}
catch ( InterruptedException e ) { }
t.interrupt();
}
}
Java
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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() ;
}
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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() {
try {
sleep(2000);
System.out.println("Zeit ist um.");
} catch ( InterruptedException e ) { }
}
}
};
}
}
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Priorities
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Example
Example
Applets and Threads
continued
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Clock-Applet
import
import
import
import
import
java.awt.*;
java.awt.Graphics;
java.util.*;
java.text.DateFormat;
java.applet.Applet;
Clock implements the Runnable interface (and
therefore implements the run method defined in it)
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();
try {
Thread.sleep(1000);
} catch (InterruptedException e){ }
}
continued
}
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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 CODE= "Clock.class" WIDTH=900 HEIGHT = 300>
</APPLET>
</BODY>
</HTML>
Programming in Java,  Helmut Dispert
Demo
Fachbereich Informatik und Elektrotechnik
Clock-Applet
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Programming Errors
Programming errors
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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.
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Deadlock
Copyright (c) Galileo Press GmbH 2004
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
Programming Errors
Deadlock:
Thread 1
Wait for lock for objectOne
Enter methodOne() for objectOne
Context
switch
Thread 2
Wait for lock for objectTwo
Enter methodOne() for objectTwo
Call ObjectTwo.methodTwo()
Wait for lock for objectTwo
Call ObjectOne.methodTwo()
Wait for lock for objectOne
Both
waiting
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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() {
try {
Thread.sleep( 1000 );
} catch ( InterruptedException e ) { }
}
}
Programming in Java,  Helmut Dispert
Fachbereich Informatik und Elektrotechnik
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
Programming in Java,  Helmut Dispert
Java
Fachbereich Informatik und Elektrotechnik
Threadgroups
"main"
subgroup1
subgroup2
subgroup3
Thread-β
β
Thread-α
α
Thread-A
Thread-1
Thread-3
Thread-2
Thread-a
Thread-d
Thread-b
Thread-c
Programming in Java,  Helmut Dispert