Threads in Java

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Threads in Java

• Programming parallel processes

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Single Threaded Programs

• A computer program is a set of instructions that
  the machine follows
• A program that is in its execution is known as
  process
• The order in which the instructions in a program
  are executed is known as flow of control.

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Single Threaded Program

• Usually, a process has only one flow of control
  in a program
• At times it is necessary to have several flows of
  control in a program that work in parallel
• For example
• A program may be taking input from file, writing
  to network, displaying animation on screen and
  playing music on the sound card at the same time

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Threads

• The flow of control in a program is called the
  thread
• A program with multiple flows of control is known
  as multi-threaded
• Most of the program we have written in java so
  far are single threaded

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An example
Single Thread
Input 1
Calculate
Output
Input 2
Calculate
Output
Input 3
Calculate
Output
Multiple Threads
Input 1
Calculate
Output
Input 2
Calculate
Output
Input 3
Calculate
Output
Time
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Threads on a single processor

• When using threads with machines having one
  processor, actually only one thread executes at
  one time.
• Threads take turns, one by one to execute their
  code.
• It happens so quickly that it appears to the user
  that threads are executing in parallel

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Thread

• Threads are useful on single processor machines
  as well as multiple processor machines
• As seen in example, one thread may be waiting for
  I/O, one thread calculating the result and so
  on. can improve the efficiency of the program.

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Process vs Threads

• Every running program is known as a process
• Operating system handles the processes running on
  a machine
• Threads are actually sub-processes within a
  process that execute in parallel
• It still depends on operating system, how threads
  within a process are managed and executed

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Threads

• A nice example of a multi-threaded program is
  MS-Word
• While you type a document in Word, a thread keeps
  checking the spelling mistakes in your document

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How to create threads in Java

• java.lang.Thread is the basic thread class
  provided by JDK
• Creating Thread is a 3 step process in java
• Extend your class by java.lang.Thread
• Define a function public void run() in it
• Start the thread

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Creating ThreadExtending Thread

• public class ThreadTest extends Thread
• This makes our ThreadTest class capable of
  running a separate thread.

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Define a function public void run()

• The code you want to run in parallel should be
  written as a body of run method of the thread
• public class ThreadTest extends Thread
• public void run()
• for(int i0 ilt20 i)
• System.out.println("Running thread step
  "i" of 20000")

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Starting the thread

• public static void main(String args)
• ThreadTest t1 new ThreadTest()
• ThreadTest t2 new ThreadTest()
• t1.start() // first thread started
• t2.start() // second thread started
• // without waiting for t1

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Tentative OutputOutput will vary from machine
to machine

• Running thread step 0 of 20000
• Running thread step 1 of 20000
• Running thread step 2 of 20000
• Running thread step 3 of 20000
• Running thread step 4 of 20000
• Running thread step 5 of 20000
• Running thread step 6 of 20000
• Running thread step 7 of 20000
• Running thread step 8 of 20000
• Running thread step 0 of 20000
• Running thread step 1 of 20000
• Running thread step 2 of 20000
• Running thread step 3 of 20000
• Running thread step 4 of 20000
• Running thread step 9 of 20000
• Running thread step 10 of 20000
• Running thread step 11 of 20000
• Running thread step 12 of 20000
• Running thread step 13 of 20000

Thread 1
Thread 2
Thread 1
Thread 2
Thread 1
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How does thread stop ?

• Thread automatically stop when run() method
  finishes it execution.

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Thread Priorities

• All threads have by default the same priority in
  the JVM
• You can set one thread to have higher or lower
  priority
• Thread.setPriority( )and Thread.getPriority(
  )are the methods that can change or check the
  priority of a thread

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Priorities in java.lang.Thread

• static int MAX_PRIORITY The maximum priority
  that a thread can have.
• static int MIN_PRIORITY  The minimum priority
  that a thread can have.
• static int NORM_PRIORITY The default priority
  that is assigned to a thread.

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Trym out