Multithreading

Chapter 23

2

Introduction

• Consider ability of _____________ to multitask– Breathing, heartbeat, chew gum, walk …

• In many situations we need a computer to multitask

• Concurrency normally available in ________________________

• Java provides built-in _______________– Multithreading improves the performance of some

programs

3

Thread States: Life Cycle of a Thread

• _________ state– Thread was just created

• Ready state– Thread’s __________ method invoked– Thread can now execute

• Running state– Thread is _______ a processor and running

• _______________ state– Thread has completed or exited– Eventually disposed of by system

View diagram, Figure 16.1

4

Thread Priorities and Thread Scheduling

• Java thread priority– Priority in range ___________

• Timeslicing– Each thread assigned time on the processor

(called a _________________)– Keeps highest priority threads running

5

Priorities and

Scheduling

Priority 9

Priority 8

Priority 7

Priority 10

Priority 6

Priority 5

Priority 4

Priority 3

Priority 2

Priority 1

A B

D

C

E F

G

H I

J K

Ready threads

Thread.MIN_PRIORITY

Thread.MAX_PRIORITY

Thread.NORM_PRIORITY

6

Creating and Executing Threads

• Figure 16.3

• Demonstrates– Constructing ____________ objects– Using Thread methods start and __________– Creates 3 equal priority threads– Each is put to sleep for random number of

milliseconds– When __________, it displays name, etc.

7

Thread Synchronization

• Java uses monitors for thread synchronization

• The sychronized keyword– Every synchronized method of an object has a

_________________– _____________ inside a synchronized method

at a time– All other threads ___________ until method finishes– Next highest priority thread runs when method

finishes

8

Producers and Consumers

• Producer– Generating

____________

• Consumer– ___________ and

processesthe output

9

Synchronization

• Problem– Sometimes the producer gets

too far ___________ of the consumer • The objects produced fill up the holding area (________)• The producer must wait for space to place objects

– Sometimes the consumer gets ahead of the producer

• There are no objects to be processed (______________)• The consumer must wait for the producer

10

Producer/Consumer Relationship without Synchronization

• Buffer– __________________ memory region

• Producer thread– Generates data to add to buffer– Calls _____________ if consumer has not read previous

message in buffer– Writes to _______ buffer and calls notify for consumer

• ________________ thread– Reads data from buffer– Calls wait if buffer empty

• _______________ threads to avoid corrupted data

11

Producer/Consumer Relationship without Synchronization

• View source code which establishes– Buffer, Figure 16.4

• An interface which specifies ______________ methods

– Producer, Figure 16.5• __________________ of Thread• Uses a shared Buffer object• Method run is overridden from Thread class• Uses Buffer.set() method

– Consumer, Figure 16.6• Also a subclass of Thread, also uses shared Buffer• Uses the _________________ method

12

Producer/Consumer Relationship without Synchronization

• View Figure 16.7 which implements the Buffer interface– Implements the get and set methods

• This ___________________________ object is used in Figure 16.8 program– Buffer object declared, instantiated– Also Producer and Consumer objects– _________________ call method start()

13

Producer/Consumer Relationship without Synchronization

• Example randomly called producer and consumer

• You should note that in some cases the data was _____________________ !!– Consumer reads values _________ producer

generates– Consumer misses a value– Consumer reads _____________ multiple times

• We need to deal with problem so data is not corrupted

14

Producer/Consumer Relationship with Synchronization

• Solution is to synchronize the producer and consumer objects

• Figure 16.9 implements a buffer and synchronizes – Consumer consumes only _______ produces a

value– ________________ produces a value only after

consumer consumes previous value produced– Condition variable occupiedBufferCount

determines __________________________

• Program which uses this, Figure 16.10

15

Circular Buffer• Features

– Multiple memory cells– Produce item if one or more _________ cells– ____________ item if one or more filled cells

• Caveats– Producer and consumers must be relatively

_________________________• Otherwise buffer fills up or stays empty

– Synchronization still necessary– Seek to __________________ buffer size

• minimizes thread-wait time

16

Circular Buffer

• Code in figures noted below set up circular buffer, producer, consumer– Figure 16.11

• Implements RunnableOutput class• Overrides run method

– Figure 16.12• Loads values in buffer• Instantiates a RunnableOutput object

– Figure 16.13• Instantiates RunnableOutput object

17

Circular Buffer

• Figure 16.14 CircularBuffer– Instantiates an ___________, the shared area– Methods get() and set() implemented

• Figure 16.15– Program to demonstrate use of circular buffer– __________________ Producer and Consumer

objects– Note that it is a windowed application

• Sends output to _____________________

18

Daemon Threads

• Run for benefit of other threads– Do not prevent program from ________________– Garbage collector is a daemon thread

• Set daemon thread with method setDaemon– Must be done at ____________________

• Do not assign ___________________ to daemon thread– Will be terminated without warning– May prevent those tasks from completing properly

19

Runnable Interface

• May be necessary to extend a class that already extends a class __________________

• Java does not allow a class to extend more than one class at a time– __________________________ for multithreading

support

• Program that uses a Runnable object to control a thread– Creates a __________________ object– Associates the Runnable object with that Thread

class

20

Runnable Interface

• Illustration of using a Runnable interface– Figure 16.16

• Note methods start, stop, run