Multithreaded Java

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COMP1681 / SE15Introductionto Programming
Fast Track Session 3

• Multithreaded Java

2
Todays Learning Objectives

• For you to see how Java programs can be made
  multithreaded through the use of Thread objects
  the Runnable interface
• For you to appreciate the problems that can occur
  when threads interfere with one another
• and for you to understand how to fix those
  problems

3
Session Outline

• What are threads?
• Thread class vs. Runnable interface
• Network servers revisited
• Dealing with race conditions
• Scheduled tasks

4
What Are Threads?

• A multitasking OS time-slices between processes
  to give the illusion of concurrency
• Threads are a finer-grained concept allowing
  concurrent tasks within a single process
• Much easier to share data between threads,
  because they operate in the same address space

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Threads in Java

• Basic features in java.lang
• Thread class
• Runnable interface
• Additional features in java.util
• Timer TimerTask (since J2SE 1.3)
• Advanced features (J2SE 5.0)
• Thread pools
• Thread-aware collections
• Locks, semaphores, etc

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Thread vs. Runnable

• Extend Thread class
• Override run method with code that needs to be
  executed in a separate thread
• Cant do this if the code is in a class that
  already has a superclass (and it may not make
  sense, anyway)
• Implement Runnable interface
• Forces us to write a run method, containing code
  that needs to be executed in a separate thread
• Need to create separate Thread objects to operate
  on our Runnable objects

7
Writing Servers Revisited

• Socket alone isnt sufficient to write a server
• We need something to sit by the phone, waiting
  for incoming calls
• Java provides ServerSocket class to
• Listen on a particular port
• Negotiate connection with client
• Open a Socket connection between hosts
• Spawn thread to handle communication with client

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Race Conditions

• What happens if two threads have access to the
  same object and each tries to modify object
  state?
• Outcome depends on how the instructions executed
  by the two threads are interleaved
• Example bank transfers

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accountsfrom - amountaccountsto amount
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Synchronization

• Methods that should not be interrupted can be
  tagged with synchronized modifier
• When a synchronized method is called by a thread,
  a mutex lock on the object is acquired
• The lock prevents other threads from calling any
  synchronized method on that object
• until the first thread exits its synchronized
  method

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wait notify

• wait method allows a thread to wait within a
  synchronized method and relinquish the object
  lock
• Waiting thread goes onto wait list and will be
  ignored by thread scheduler
• For a thread to be removed from wait list,
  another thread must invoke notify or notifyAll on
  same object
• notifyAll method unblocks all waiting threads,
  freeing them to compete for the object lock after
  current thread has exited the synchronized method

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Scheduled Tasks

• Implement task to be scheduled as a class that
  extends abstract class TimerTask
• Override run method with code be executed
• Timer class has methods to execute TimerTask code
  at a later time, once only or repeatedly
• Scheduled tasks are placed in an ordered queue
  and are executed sequentially by a single thread
• Tasks should be short-lived

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Summary

• We have
• Seen how Java provides the Thread class and
  Runnable interface to support multithreaded code
• Considered how a simple network server can be
  made multithreaded
• Examined how race conditions can arise with
  threads, and how synchronization can help resolve
  problems
• Looked at how tasks can be scheduled to run at
  regular intervals in Java programs