Multithreading

Chapter 23

2

Introduction

• Consider ability of human body to ___________– Breathing, heartbeat, chew gum, walk …

• In many situations we need a computer to multitask

• Concurrency normally available in __________

• Java provides built-in multithreading– Multithreading improves the ___________ of some

programs

3

Thread States: Life Cycle of a Thread

• __________________ state– New thread begins its life cycle in the new state– Remains in this state until program starts the

thread, placing it in the runnable state

• runnable state– A thread in this state is ___________ its task

• waiting state– A thread ___________ to this state to wait for

another thread to perform a task

4

Thread States: Life Cycle of a Thread

• __________________ state– A thread enters this state to wait for another

thread or for an amount of time to elapse– A thread in this state returns to the ___________

state when it is signaled by another thread or when the timed interval expires

• terminated state– A runnable thread enters this state when it

_____________ its task

5

Operating System View Of runnable State

• ready state– ____________ waiting for another thread– Waiting for the ______________ to assign the

thread a processor

6

Operating System View Of runnable State

• running state– Currently has a _________________ and is

executing– Often executes for a small amount of processor

time called a _______________________ before transitioning back to the ready state

7

Thread Priorities and Thread Scheduling

• Java thread priority– Priority in range ______________

• Timeslicing– Each thread assigned time on the processor (called

a quantum)– Keeps ______________ threads running

8

Priorities and

Scheduling

Thread.MAX_PRIORITY

9

Creating and Executing Threads•Runnable interface

– Preferred means of creating a multithreaded application

– Declares method _______________– Executed by an object that implements the Executor interface

•Executor interface– Declares method ___________________– Creates and manages a group of threads called

a thread pool

10

Creating and Executing Threads

•ExecutorService interface– ______________________ of Executor that

declares other methods for managing the life cycle of an Executor

– Can be created using _______________ methods of class Executors

– Method shutdown _______________ when tasks are completed

11

Creating and Executing Threads

•Executors class– Method newFixedThreadPool creates a pool

consisting of a __________________________ of threads

– Method newCachedThreadPool creates a pool that creates new threads _____________________________

12

Creating and Executing Threads

• PrintTask class Figure 23.4

• RunnableTester, Figure 23.5

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

milliseconds– When awakened, it displays name, etc.

13

Producers and Consumers

• Producer– Generating

_______________

• Consumer– Receives and

_________________the output

14

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 ______________ gets ahead of the producer

• There are no objects to be processed (_____________ buffer)

• The consumer must wait for the producer

15

Thread Synchronization• Thread synchronization

– Provided to the programmer with _____________________• Exclusive access to a shared object

– Implemented in Java using _____________

•Lock interface– lock method obtains the lock, enforcing mutual

exclusion– unlock method ________________ the lock– Class ReentrantLock implements the Lock

interface

16

Thread Synchronization

• Condition variables– If a thread holding the lock cannot continue with its

task until a condition is satisfied, the thread can wait on a ____________________

– Create by calling Lock method newCondition– Represented by an object that implements the ___________________ interface

17

Thread Synchronization

• Condition interface– Declares methods await, to make a thread wait, – ____________________, to wake up a waiting

thread, and – signalAll, to wake up all waiting threads

18

Producer/Consumer Relationship without Synchronization

• Buffer– ____________________ memory region

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

buffer– Writes to empty buffer and calls ____________ for

consumer

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

• Synchronize threads to avoid corrupted data

19

Producer/Consumer Relationship without Synchronization

• View source code which establishes– Buffer, Figure 23.6

• An interface which specifies get and set methods

– Producer, Figure 23.7• ___________________ of Thread• Uses a shared Buffer object• Method run is _________________ from Thread class• Uses Buffer.set() method

– Consumer, Figure 23.8• Also a subclass of Thread, also uses shared Buffer• Uses the Buffer.get() method

20

Producer/Consumer Relationship without Synchronization

• View Figure 23.9 which implements the Buffer interface– Implements the _________________ methods

• This UnsynchronizedBuffer object is used in Figure 23.10 program– Buffer object declared, instantiated– Also Producer and Consumer objects– Both threads call method start()

21

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 _______________ a value– Consumer reads same value multiple times

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

22

Producer/Consumer Relationship with Synchronization

• Solution is to _________________ the producer and consumer objects

• Figure 23.11 implements a buffer and synchronizes – Consumer consumes only ______ produces a value– Producer produces a value only after consumer

consumes ____________ value produced– Condition variable occupiedBufferCount

determines whose turn it is

• Program which uses this, Figure 23.12

23

Using Thread Methods

• Create Wait class

• Has _______________ methods

• Provide capability to have another application ______________ for a certain amount of time

• Note Example to act as a simple timer.

24

Circular Buffer• Features

– Multiple memory cells– Produce item if one or more empty cells– Consume item if one or more filled cells

• Caveats– Producer and consumers must be relatively

______________ speed• Otherwise buffer fills up or stays empty

– Synchronization still necessary– Seek to optimize buffer size

• ______________________ thread-wait time

25

Circular Buffer

• Circular Buffer class Figure 23.13– ______________ for mutual exclusion– Condition variables to control writing and reading– Circular buffer; provides three spaces for data– Obtain the lock ____________ writing data to the

circular buffer– Wait until a buffer space is ___________– Impose circularity of the buffer– ___________ waiting thread when to read data

from buffer– Release lock

26

Circular Buffer

• Circular Buffer test, Figure 23.14– Create instance of circular buffer– Execute producer, consumer in separate threads

27

Daemon Threads

• Run for benefit of _____________________– Do not prevent program from terminating– __________________ is a daemon thread

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

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

28

Runnable Interface

• May be necessary to __________ a class that already extends a class other than Thread

• Java does not allow a class to extend more than one class at a time– Implement Runnable for ____________ support

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

class

29

Runnable Interface

• Illustration of using a Runnable interface– Figure 23.18

• Note methods start, stop, run