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Multithreading

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Construct a thread object with the threadname. public Thread() Construct a thread object whose name = 'Thread-' number. void start ... – PowerPoint PPT presentation

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Title: Multithreading

1
Multithreading
2
Outline

Introduction
Class thread
Thread States
Scheduling and Priority
Thread Synchronization
Daemon Thread

3
Introduction

One of the characteristics that makes Java a
powerful programming language is its support of
multithreaded programming as an integrated part
of the language.
Multithreading is the ability of a program or an
operating system process to manage its use by
more than one user at a time and to even manage
multiple requests by the same user without having
to have multiple copies of the programming
running in the computer. It involves multiple
threads of control within a single program.

4
Introduction

A thread is a single sequence of execution
within a program. Threads allow you to do many
things at the same time.
One of the example of Multithreading is Javas
automatic garbage collection. The garbage
collector thread automatically reclaims
dynamically allocated memory that is no longer
needed.
Furthermore writing multithreaded programming can
be tricky. Why use threads?

5
Class Thread

public Thread(String threadname)
Construct a thread object with the threadname
public Thread()
Construct a thread object whose name
Thread- number
void start()
Cause the thread to begin execution. Call run
method
void run()
Method to place the work of the thread.

6
Class Thread

void sleep(long milisec)
Causes the currently executing thread to sleep
for the specified number of milliseconds.
boolean isIinterrupted()
Tests whether the thread has been interrupted.
void interrupt()
Interrupts the thread.
boolean isAlive()
Test whether start method has been called.

7
Class Thread

void join()
Waits for the Thread to which the message is
sent to die before the calling Thread can
proceed.
public class classname extends Thread
To add thread functionality to a class, simply
derive the class from Thread and override the run
method.
public class classname implements Runnable
The Runnable interface provides the overhead
for adding thread functionality to a class.
Classes that implement the Runnable interface
provide a run method that is executed by an
associated thread object that is created
separately.

8
Thread States
9
Thread States

When a thread is first created, it does not
exist as an independently executing set of
instructions. Instead, it is a template from
which an executing thread will be created. This
state is refer to born state.
When the thread start method is called, the
thread is now enter the ready state.
When the system assigns a processor to the
thread , the thread is in the running state.
A thread enters the dead state when its run
method completes or terminates.

10
Thread States

The running thread will enter the block state
when it issues an input or output request.
The running thread will enter sleeping state
when sleeping method is called. The sleeping
state will expire after the sleep time expires.
The running thread will enter waiting state when
wait method is called. One thread in the waiting
state will become ready when notify method is
called. Every thread in the waiting state will
become ready when notifyAll method is called.

11
Creating Executing Threads
12
(No Transcript)
13
Output
14
Scheduling and Priority

Thread scheduling is the mechanism used to
determine how runnable threads are allocated CPU
time.
A thread-scheduling mechanism is either
preemptive
nonpreemptive.

15
Preemptive Scheduling

The thread scheduler preempts a running thread
to allow different threads to execute.
A preemptive scheduler can be either time-sliced
or nontime-sliced. With time-sliced scheduling,
the scheduler allocates a period of time
(quantum) that each thread can use the CPU. A
nontime-sliced scheduling uses other criteria
such as priority or I/O status to preempt a
thread.
A yield method() is appropriate for a non
timesliced systems in which a thread would
complete before another thread (same priority)
would have opportunity to run.

16
Non-Preemptive Scheduling

The non-preemptive scheduler never interrupts a
running thread, but relies on the running thread
to yield control of the CPU so that other threads
may execute.
Java scheduler ensures that several equally
high-priority threads execute for a quantum in
round-robin fashion.
Sometimes, the new high priority threads could
postpone possibly indefinitely by the execution
of the lower priority threads, which refer to
Starvation.

17
Scheduling and Priority

Every thread has a priority. When a thread is
created, it inherits the priority of the thread
that created it. A thread has a priority in the
range Thread.MIN_PRIORITY (1) and
Thread.MAX_PRIORITY (10). By default, each thread
is given priority Thread.NORM_PRIORITY (5). The
highest the numbers, the highest the priority.
void setPriority(int newPriority)
To adjust the threads priority.
int getPriority()
Returns this thread's priority.

18
Thread Synchronization

Every object with synchronized methods is a
monitor.
Java uses monitors to perform synchronization.
It allows one thread at a time to execute a
synchronized method on the object.
When the synchronized method is invoked, the
object is locked. All other threads attempting to
invoke synchronized methods must wait. All these
threads will call wait method.
When a synchronized method finishes executing,
the lock on the object is released. The highest
priority ready thread that invoke a synchronized
method will proceed.

19
Thread Synchronization

The thread can notify a waiting thread to become
ready again so that it can attempt to obtain the
lock on the monitor object again and then
execute.
Monitor objects maintain a list of all threads
waiting to enter the monitor object to execute
synchronized methods.
Threads that explicitly invoked wait can only
proceed when notified via a call by another
thread to notify or notifyAll.
One of the example of thread synchronization is
thread coordination.

20
Thread Synchronization

One of the problem of thread coordination is
bounded buffer. This problem involves using a
fixed-size memory buffer to communicate between
two processes or threads.
In order to solve this problem, the following
requirement must be fulfill
When the writer thread attempts to write to a
full buffer, the writer is suspended until some
items are removed from the buffer.
When the reader thread removes items from the
full buffer, the writer thread is notified of the
buffer's changed condition and may continue
writing.

21
Thread Synchronization

When the reader thread attempts to read from an
empty buffer, the reader is suspended until some
items are added to the buffer.
When the writer adds items to the empty buffer,
the reader thread is notified of the buffer's
changed condition and may continue reading.
Shared data can get corrupted if we do not
synchronize access among multiple threads.
By using a circular buffer, we can perform
synchronize access more optimally.

22
Daemon Threads

A Daemon thread is a thread that providing
service to other threads and it runs in the
background.
Unlike conventional user threads, daemon threads
do not prevent a program from terminating.
The garbage collection thread is one of the
example of daemon thread
When only daemon threads remain alive, the Java
virtual machine process exits.
void setDaemon(boolean on)
Marks this thread as either a daemon thread
(true) or a user thread (false).

23
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