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Chapter 7 Threads Part 1
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• Overview
• Why using Thread?
• What is Thread?
• Thread and Process
• How to use Thread in Java?
• How to control Thread in Java? (covered by part
  2)

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• Why using Thread?
• There are times when a program needs to do
  multiple tasks concurrently. For example, The Web
  browser is multithreaded, allowing this
  flexibility.
• Performing more than one job is similar to having
  more than one computer.(Virtual CPU)
• A simplistic view of a computer is that it has a
  CPU that performs computations, read-only memory
  (ROM) which contains the program that the CPU
  executes, and random-access memory (RAM) which
  holds the data on which the program operates.
• Multithreading is useful even in programs running
  on a single-processor computer. In this scenario,
  thread two runs while thread one is blocked
  waiting for I/O. The I/O can take place in
  parallel with the CPU running the other thread.
  In a single-threaded case, thread two would have
  to wait until thread one finished.

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• What is Thread?
• A thread is is composed of three main parts
• A virtual CPU (In Java programming, the virtual
  CPU is encapsulated in an instance of the Thread
  class. When a thread is constructed, the code and
  the data that define its context are specified by
  the object passed to its constructor.)
• The code the CPU is executing (Code can be shared
  by multiple threads, independent of data.)
• The data on which the code works (data may or may
  not be shared by multiple threads, independent of
  code. )
• At any moment, at most one object is executing
  per CPU, while others might be waiting for
  resources, or waiting for a chance to execute, or
  sleeping, or dead.

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• Thread and Process
• A process gives a program a place to live,
  allowing access to memory and resources. A
  process provides a level of isolation that keeps
  different applications from inadvertently
  interacting with each other. The goal of this
  isolation is to increase the overall stability of
  the system.
• Every process contains one or more threads. You
  can think of a thread as the moving part of the
  process. Without a thread interacting with
  elements within a process, nothing interesting
  will happen.
• Threads are paths of execution. The threads
  perform the operations while the process provides
  the isolation. A single-threaded application has
  only one thread of execution.

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• Application Loading
• How an application is loaded into a process
• When an executable is launched, the OS creates a
  process for the executable to run in.
• The OS then loads the executable into the
  processs memory and looks for an entry point, a
  specially marked place to start carrying out the
  instructions contained within the executable.
• Once the entry point is identified, a thread is
  created and associated with the process. The
  thread is started, executing the code located at
  the entry point. From that point on the thread
  follows the series of instructions. This first
  thread is referred to as the main thread of the
  process.
• Example Windows Task Manager

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• Some MultiThreading Terms
• Only one thread can be executing at a given point
  in time and all other threads are waiting for
  their turn to execute. When more than one
  application is executing, there must be some
  means of determining whose turn it is to execute.
  This is generally referred to as scheduling.
• A single processor executes one thread at a time.
  When the processor decides that a different
  thread should be executed, it saves the
  information the current thread requires to
  continue and switches to a different thread. This
  is called a context switch.

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• Thread in Java - Overview
• Javas thread support resides in three places
• The java.lang.Thread class
• The java.lang.Object class
• Java language and JVM
• Most support resides in the Thread class.
• Thread support methods in java.lan.Object class
  are wait(), notify() and notifyAll(), these are
  used to control Thread, and will be covered in
  part 2.

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• Thread in Java - Thread Class
• Some important methods
• start() Causes this thread to begin execution
  the Java Virtual Machine calls the run method of
  this thread.
• run() If this thread was constructed using a
  separate Runnable run object, then that Runnable
  object's run method is called otherwise, this
  method does nothing and returns.
• join() Waits for this thread to die.
• Constructors and other methods see API Document

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• What a Thread Executes
• To make a thread execute, you call its start( )
  method.
• This registers the thread with thread scheduler.
• The scheduler can be either JVM or the host OS.
• Note calling start( ) doesnt immediately cause
  the thread to run it just makes it eligible to
  run.
• When a thread get a change to executed by CPU, it
  is the method run( ) being executed.
• The thread can execute its own run( ) method
• The thread can execute the run( ) method of some
  other object.

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• Threads Own run() Method
• To execute its own run( ) method, you need to
  subclass the Thread class and Override run( )
  method.
• Example
• public class CounterThread extends Thread
• public void run()
• for(int I0Ilt10I)
• System.out.println(Counting I)
• To invoke this thread, construct an instance of
  CounterThread class and then invoke its start( )
  method
• CounterThread ct new CounterThread()
• ct.start()
• It is legal to call run(), but nothing would
  happen, and youre still performing the task in
  current thread. So let the scheduler call run()
  method.

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• Executing Other Objects
• You have to call the Thread constructor to
  specify which object owns the run( ) method that
  you want.
• Use Runnable interface in Thread constructor.
• public Thread(Runnable target)
• Example
• public class DownCounter implements Runnable
• public void run()
• for(int I10Igt1I--)
• System.out.println(Counting Down I)
• To invoke this thread
• DownCounter dc new downCounter()
• Thread t new Thread(dc)
• t.start()

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• Runnable Interface
• The Runnable interface should be implemented by
  any class whose instances are intended to be
  executed by a thread. The class must define a
  method of no arguments called run.
• public void run( ) // You can only override
  this.
• In addition, Runnable provides the means for a
  class to be active while not subclassing Thread.
  In most cases, the Runnable interface should be
  used if you are only planning to override the
  run() method and no other Thread methods.

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• Comparison of Two Methods
• Use interface implements Runnable allows the
  object which you want to thread can have access
  of all private member data.
• Use class extends Thread will restrict you to
  inherit from another class. (single inheritance
  rule).
• Use extends Thread is like to say This class is
  a thread
• Use implements Runnable, which says this class
  is associated with a thread. From OO design
  point of view, make it a better design.

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• When Executions Ends
• When the run( ) method returns, the thread has
  finished its task and is considered dead.
• Once a thread is dead, it may not be started
  again
• if you want the threads task to be performed
  again, you have to construct and start a new
  thread instance.
• The dead thread continues to exist you can still
  access its data and call its methods (think about
  it as a normal object). You just cant make it
  run again.
• Dead Thread property
• You can NOT restart a dead thread.
• You CAN call the methods of a dead thread.

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• Kill a Thread
• You can kill the thread by forcing it into the
  dead state.
• Before JDK 1.2, stop() method is used. But
  because is can cause data corruption or deadlock,
  this method is deprecated, you should call
  interrupt() instead.

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• Performance Consideration
• It is a expensive operation to create, use and
  discard threads
• It is better to create a pool of reusable worker
  threads that can be assigned chores as needed.

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• Thread States
• Thread states are
• Running The state that all threads aspire to
• Various waiting states Waiting, Sleeping,
  Suspended(deprecated), Blocked
• Ready Not waiting for anything except the CPU
• Dead all done
• Only thread scheduler can move a ready thread
  into the CPU.

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• Thread Priority
• Every thread has a priority
• The priority is an integer from 1 to 10 (with 10
  being highest)
• threads with higher priority get preference over
  threads with lower priority.
• If there is more than one waiting thread with the
  same priority, the scheduler chooses one of them
  with no guarantee that the thread chosen will be
  the one that has been waiting the longest.

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• Thread Priority(cont.)
• Thread class provides some constant related to
  priority
• MAX_PRIORITY (10)
• NORM_PRIORITY (5)
• MIN_PRIORITY (1)
• The default priority is 5.
• How thread priority affect the scheduler are
  platform dependent.
• Example to set/get thread priority
• int oldPriority theThread.getPriority()
• int new newPriority Math.min(oldPriority 1,
  Trhead. MAX_PRIORITY)
• theThread.setPriority(newPriority )

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