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Java multi threading | PPT
Java Threads




               1
Multitasking and Multithreading
• Multitasking:
  – refers to a computer's ability to perform multiple jobs
    concurrently
  – more than one program are running concurrently, e.g.,
    UNIX
• Multithreading:
  – A thread is a single sequence of execution within a
    program
  – refers to multiple threads of control within a single
    program
  – each program can run multiple threads of control within
    it, e.g., Web Browser
                                                              2
Concurrency vs. Parallelism
CPU             CPU1      CPU2




                                 3
Threads and Processes
                        CPU



                                main


                                run

Process 1   Process 2         Process 3   Process 4



                                 GC




                                                      4
What are Threads Good For?

• To maintain responsiveness of an application
  during a long running task
• To enable cancellation of separable tasks
• Some problems are intrinsically parallel
• To monitor status of some resource (e.g., DB)
• Some APIs and systems demand it (e.g., Swing)



                                                  5
Application Thread

•   When we execute an application:
    1. The JVM creates a Thread object whose task
       is defined by the main() method
    2. The JVM starts the thread
    3. The thread executes the statements of the
       program one by one
    4. After executing all the statements, the method
       returns and the thread dies
                                                        6
Multiple Threads in an Application

• Each thread has its private run-time stack
• If two threads execute the same method, each will
  have its own copy of the local variables the
  methods uses
• However, all threads see the same dynamic
  memory, i.e., heap (are there variables on the
  heap?)
• Two different threads can act on the same object
  and same static fields concurrently
                                                      7
Creating Threads

•   There are two ways to create our own
    Thread object
    1. Subclassing the Thread class and instantiating
       a new object of that class
    2. Implementing the Runnable interface
•   In both cases the run() method should be
    implemented
                                                        8
Extending Thread
public class ThreadExample extends Thread {
    public void run () {
        for (int i = 1; i <= 100; i++) {
            System.out.println(“---”);
        }
    }
}




                                              9
Thread Methods

void start()
  – Creates a new thread and makes it runnable
  – This method can be called only once
void run()
  – The new thread begins its life inside this method
void stop() (deprecated)
  – The thread is being terminated

                                                        10
Thread Methods
void yield()
  – Causes the currently executing thread object to
    temporarily pause and allow other threads to
    execute
  – Allow only threads of the same priority to run
void sleep(int m) or sleep(int m, int n)  
  – The thread sleeps for m milliseconds, plus n
    nanoseconds

                                                      11
Implementing Runnable
public class RunnableExample implements Runnable {
    public void run () {
        for (int i = 1; i <= 100; i++) {
                   System.out.println (“***”);
        }
    }
}




                                                     12
A Runnable Object

• When running the Runnable object, a
 Thread object is created from the Runnable
 object
• The Thread object’s run() method calls the
 Runnable object’s run() method
• Allows threads to run inside any object,
 regardless of inheritance     Example – an applet
                               that is also a thread 13
Starting the Threads
public class ThreadsStartExample {
      public static void main (String argv[]) {
          new ThreadExample ().start ();
          new Thread(new RunnableExample ()).start ();
      }
}




           What will we see when running
             ThreadsStartExample?


                                                         14
15
Scheduling Threads
               start()
                                                Ready queue

    Newly created
    threads


 Currently executed
 thread
                   I/O operation completes

                     •Waiting for I/O operation to be   completed
What happens when    •Waiting to be notified
a program with a     •Sleeping
ServerSocket calls   •Waiting to enter a synchronized   section
accept()?                                                         16
Thread State Diagram

                       Alive


                               Running
new ThreadExample();           while (…) { … }

New Thread                     Runnable                     Dead Thread
             thread.start();
                                                       run() method returns


                               Blocked
                                             Object.wait()
                                             Thread.sleep()
                                             blocking IO call
                                             waiting on a monitor         17
Example
public class PrintThread1 extends Thread {
    String name;
    public PrintThread1(String name) {
        this.name = name;
    }
    public void run() {
        for (int i=1; i<100 ; i++) {
            try {
                sleep((long)(Math.random() * 100));
            } catch (InterruptedException ie) { }
            System.out.print(name);
        }
}
                                                      18
Example (cont)
    public static void main(String args[]) {
         PrintThread1 a = new PrintThread1("*");
         PrintThread1 b = new PrintThread1("-");


         a.start();
         b.start();
    }
}




                                                   19
20
Scheduling

• Thread scheduling is the mechanism used
 to determine how runnable threads are
 allocated CPU time
• A thread-scheduling mechanism is either
 preemptive or nonpreemptive



                                            21
Preemptive Scheduling

• Preemptive scheduling – the thread scheduler
  preempts (pauses) a running thread to allow
  different threads to execute
• Nonpreemptive scheduling – the scheduler never
  interrupts a running thread
• The nonpreemptive scheduler relies on the running
  thread to yield control of the CPU so that other
  threads may execute
                                                     22
Thread Priority

• Every thread has a priority
• When a thread is created, it inherits the
 priority of the thread that created it
• The priority values range from 1 to 10,
 in increasing priority


                                              23
Thread Priority (cont.)

• The priority can be adjusted subsequently using
  the setPriority() method
• The priority of a thread may be obtained using
  getPriority()

• Priority constants are defined:
  – MIN_PRIORITY=1
  – MAX_PRIORITY=10             The main thread is
                                created with priority
  – NORM_PRIORITY=5
                                NORM_PRIORITY
                                                        24
Daemon Threads
• Daemon threads are “background” threads, that
  provide services to other threads, e.g., the garbage
  collection thread
• The Java VM will not exit if non-Daemon threads
  are executing
• The Java VM will exit if only Daemon threads are
  executing
• Daemon threads die when the Java VM exits
• Q: Is the main thread a daemon thread?
                                                     25
Thread and the Garbage Collector

• Can a Thread object be collected by the
 garbage collector while running?
  – If not, why?
  – If yes, what happens to the execution thread?

• When can a Thread object be collected?



                                                    26
ThreadGroup

• The ThreadGroup class is used to create
 groups of similar threads. Why is this
 needed?

   “Thread groups are best viewed as an
   unsuccessful experiment, and you may simply
   ignore their existence.”
               Joshua Bloch, software architect at Sun

                                                         27

Java multi threading

  • 1.
  • 2.
    Multitasking and Multithreading •Multitasking: – refers to a computer's ability to perform multiple jobs concurrently – more than one program are running concurrently, e.g., UNIX • Multithreading: – A thread is a single sequence of execution within a program – refers to multiple threads of control within a single program – each program can run multiple threads of control within it, e.g., Web Browser 2
  • 3.
  • 4.
    Threads and Processes CPU main run Process 1 Process 2 Process 3 Process 4 GC 4
  • 5.
    What are ThreadsGood For? • To maintain responsiveness of an application during a long running task • To enable cancellation of separable tasks • Some problems are intrinsically parallel • To monitor status of some resource (e.g., DB) • Some APIs and systems demand it (e.g., Swing) 5
  • 6.
    Application Thread • When we execute an application: 1. The JVM creates a Thread object whose task is defined by the main() method 2. The JVM starts the thread 3. The thread executes the statements of the program one by one 4. After executing all the statements, the method returns and the thread dies 6
  • 7.
    Multiple Threads inan Application • Each thread has its private run-time stack • If two threads execute the same method, each will have its own copy of the local variables the methods uses • However, all threads see the same dynamic memory, i.e., heap (are there variables on the heap?) • Two different threads can act on the same object and same static fields concurrently 7
  • 8.
    Creating Threads • There are two ways to create our own Thread object 1. Subclassing the Thread class and instantiating a new object of that class 2. Implementing the Runnable interface • In both cases the run() method should be implemented 8
  • 9.
    Extending Thread public classThreadExample extends Thread { public void run () { for (int i = 1; i <= 100; i++) { System.out.println(“---”); } } } 9
  • 10.
    Thread Methods void start() – Creates a new thread and makes it runnable – This method can be called only once void run() – The new thread begins its life inside this method void stop() (deprecated) – The thread is being terminated 10
  • 11.
    Thread Methods void yield() – Causes the currently executing thread object to temporarily pause and allow other threads to execute – Allow only threads of the same priority to run void sleep(int m) or sleep(int m, int n)   – The thread sleeps for m milliseconds, plus n nanoseconds 11
  • 12.
    Implementing Runnable public classRunnableExample implements Runnable { public void run () { for (int i = 1; i <= 100; i++) { System.out.println (“***”); } } } 12
  • 13.
    A Runnable Object •When running the Runnable object, a Thread object is created from the Runnable object • The Thread object’s run() method calls the Runnable object’s run() method • Allows threads to run inside any object, regardless of inheritance Example – an applet that is also a thread 13
  • 14.
    Starting the Threads publicclass ThreadsStartExample { public static void main (String argv[]) { new ThreadExample ().start (); new Thread(new RunnableExample ()).start (); } } What will we see when running ThreadsStartExample? 14
  • 15.
  • 16.
    Scheduling Threads start() Ready queue Newly created threads Currently executed thread I/O operation completes •Waiting for I/O operation to be completed What happens when •Waiting to be notified a program with a •Sleeping ServerSocket calls •Waiting to enter a synchronized section accept()? 16
  • 17.
    Thread State Diagram Alive Running new ThreadExample(); while (…) { … } New Thread Runnable Dead Thread thread.start(); run() method returns Blocked Object.wait() Thread.sleep() blocking IO call waiting on a monitor 17
  • 18.
    Example public class PrintThread1extends Thread { String name; public PrintThread1(String name) { this.name = name; } public void run() { for (int i=1; i<100 ; i++) { try { sleep((long)(Math.random() * 100)); } catch (InterruptedException ie) { } System.out.print(name); } } 18
  • 19.
    Example (cont) public static void main(String args[]) { PrintThread1 a = new PrintThread1("*"); PrintThread1 b = new PrintThread1("-"); a.start(); b.start(); } } 19
  • 20.
  • 21.
    Scheduling • Thread schedulingis the mechanism used to determine how runnable threads are allocated CPU time • A thread-scheduling mechanism is either preemptive or nonpreemptive 21
  • 22.
    Preemptive Scheduling • Preemptivescheduling – the thread scheduler preempts (pauses) a running thread to allow different threads to execute • Nonpreemptive scheduling – the scheduler never interrupts a running thread • The nonpreemptive scheduler relies on the running thread to yield control of the CPU so that other threads may execute 22
  • 23.
    Thread Priority • Everythread has a priority • When a thread is created, it inherits the priority of the thread that created it • The priority values range from 1 to 10, in increasing priority 23
  • 24.
    Thread Priority (cont.) •The priority can be adjusted subsequently using the setPriority() method • The priority of a thread may be obtained using getPriority() • Priority constants are defined: – MIN_PRIORITY=1 – MAX_PRIORITY=10 The main thread is created with priority – NORM_PRIORITY=5 NORM_PRIORITY 24
  • 25.
    Daemon Threads • Daemonthreads are “background” threads, that provide services to other threads, e.g., the garbage collection thread • The Java VM will not exit if non-Daemon threads are executing • The Java VM will exit if only Daemon threads are executing • Daemon threads die when the Java VM exits • Q: Is the main thread a daemon thread? 25
  • 26.
    Thread and theGarbage Collector • Can a Thread object be collected by the garbage collector while running? – If not, why? – If yes, what happens to the execution thread? • When can a Thread object be collected? 26
  • 27.
    ThreadGroup • The ThreadGroupclass is used to create groups of similar threads. Why is this needed? “Thread groups are best viewed as an unsuccessful experiment, and you may simply ignore their existence.” Joshua Bloch, software architect at Sun 27