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Threads

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Windows 95, Unix and many other operating systems allow multitasking. ... All classes inheriting from Thread must override the method run ... – PowerPoint PPT presentation

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Title: Threads

1
Threads
2
Multitasking

Windows 95, Unix and many other operating systems
allow multitasking. printing while doing word
processing,etc
The operating system gives the user the
impression of concurrent operation.
The user works in the foreground, in idle moments
a background task is executed.

3
Types of Multitasking

Co-operative multitasking
also known as non-pre-emptive
Windows 3.1
program is responsible for giving up processor
Pre-emptive multitasking
Windows 95 (32-bit), NT, Unix
Operating system shares resources
More difficult to implement but better results

4
Processes

Multitasking is achieved by way of processes.
Processes are active programs with their own
memory space.
Operating systems prevent processes from
interfering with each other.
Processes can communicate via an inter-process
communication mechanism.

5
Process vs Thread

A process is an executing program.
Allocated memory by the operating system.
A thread is an execution in the address space of
a process the program counter register points to
the next instruction to be executed.
A process can have more than one thread.
All the threads in a process have their own
program counter and their own stack for local
(also called automatic) variables and return
addresses of invoked procedures.

6
Multithreading

Concurrent strands (threads) of activity within a
program.
Threads run as if each had its own CPU, registers
and memory.
BUT threads share memory with each other.
Less secure than a process but less overhead.
The JDK comes with a Thread class to enable
multithreading.

7
Concurrent Program Execution
8
Concurrent Program Execution

In Java, a thread in the run-time interpreter
calls the main() method of the class on the java
command line.
Each object created can have one or more threads,
all sharing access to the data fields of the
object.

9
Thread class

All classes inheriting from Thread must override
the method run().
This method should not be called directly from
an application or applet. It is invoked by the
JVM when the thread is executed.
The start() method is called to begin execution
of a thread. The method requests the JVM to
schedule the thread.

10
Using Threads

Java provides two ways of associating a Thread
object with a run method
Declare a subclass of Thread that defines a run()
method. When such a class is instantiated and the
object's start() method is called, the thread
invokes this run() method.
Pass a reference to an object that implements the
Runnable interface to a Thread constructor. When
the start() method of such a Thread object is
called, the thread invokes the run() method of
the Runnable object.

11
Thread class methods

void run()
Must be overridden, not called directly
void start()
Called to begin execution of a thread.
static native void sleep (long m)
Makes the currently running thread pause for the
number of milliseconds specified.

12
Extending the Thread class ex. 1

/
This is a thread class that prints the numbers
from 1 to 10. It
is run by the main program, which also prints
numbers from 1 to 10
/
public class PrintNums extends Thread
public void run()
for ( int i 0 i lt10 i)
System.out.println( "\tPrintNums
Thread " i)

For a thread to do anything meaningful, it must
override the Thread class run() method
13
Extending the Thread class ex. 1

/ Main program which is the main thread. This
prints numbers from one to 0 and controls the
thread PrintNums, which also prints numbers
from 1 to 10.
/
public class IntroThread
public static void main ( String args )
// Create a new thread
PrintNums t1 new PrintNums()
// start the run method of the thread
t1.start()
for ( int i1 ilt10 i )
System.out.println ("Main thread " i)
// end IntroThread

PrintNums extends Thread.
14
Implementing the Runnable interface Ex. 2

public class RunEx1 extends Object implements
Runnable
// User must implement the run() in
Runnable
public void run()
for ( int i1 ilt10 i )
System.out.println("Thread 1 " i)
// end RunEx1

The Runnable interface contains only one method
run()
15
Implementing the Runnable interface Ex. 2

// This is a main thread that prints the numbers
from 1 to 10
public class Run1
public static void main ( String args )
RunEx1 a1 new RunEx1() //
create a runnable object
Thread t1 new Thread (a1) // t1
runs a1's run()
// or alternatively Thread t1 new
Thread ( a1, "threadname")
t1.start()
for ( int i1 ilt 10 i)
System.out.println ("gt Main
thread " i)

16
Should I use Runnable or extend Thread?

Rule of Thumb
If your class must subclass some other class (the
most common example being Applet), you should
use Runnable

17
Synchronizing threads

Threads carry out operations at unpredictable
times.
Timing depends on the changing operating
environment.
If two threads are running, the order of their
operations is unpredictable.
If two threads operate on the same data then
output is unpredictable.

18
Thread States

New
runnable
blocked
dead

19
Thread States
20
New threads

A just created thread
Not yet executed.
Requires memory allocation etc before it can
execute.
Invoking the start() method for this thread will
allocate memory to the thread and make it
runnable.

21
Runnable threads

Runnable means may be executed.
Only one runnable thread may be executed at a
time.
On execution code is running.
JVM selects which of the runnable threads may
execute.
Difference in operating system implementations.

22
Blocked threads

A thread is blocked when
the sleep() method is invoked
it calls wait()
it has to wait for input/output
the suspend() method is invoked
When a thread moves into the blocked state, the
JVM can select another one to run.

23
Unblocking