Client-Server Programming Spring Semester, 2005 Threads part 1
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Contents

Concurrent Programming

Threads - Light Weight Processes

Creating Threads

Multiple Processors

Thread Scheduling

Priorities

Time-slicing

Java Types of Threads: use and daemon18

Thread Control

Thread States

Yield

Killing a Thread

Copyright ©, All rights reserved. 2005 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA. OpenContent ( http://www.opencontent.org/opl.shtml) license defines the copyright on this document.

References

The Java Programming Language , 2 ^nd Ed. Arnold & Gosling, Addison-Wesley, 1998

The Java Language Specification , Gosling, Joy, Steele, Addison-Wesley, 1996, Chapter 17 Threads and Locks.

Java 1.4.1 on-line documentation http://java.sun.com/products/jdk/1.2/docs/index.html

Reading

Java Network Programming, 3nd Ed., Harold, Chapter 5. (Java)

Concurrent Programming

The ability to perform concurrent programming is part of the Java programming language. That is different parts of the same program can be executing at the same time, or behave if they are executing at the same time. Java uses threads to achieve concurrency. Writing concurrent programs presents a number of issues that do not occur in writing sequential code.

Safety

Two different threads could write to the same memory location at the same time, leaving the memory location in an improper state.

Liveness

Threads can become deadlocked, each thread waiting forever for the other to perform a task. Threads can become livelocked, waiting forever to get their turn to execute.

Nondeterminism

Thread activities can become intertwined. Different executions of a program with the same input can produce different results. This can make program hard to debug.

Communication

Different threads in the same program execute autonomously from each other. Communication between threads is an issue.

Threads - Light Weight Processes

A thread is an active entity that shares the same name space as the program that created the thread. This means that two threads in a program can access the same data.

Difference from Processes (fork())

Processes (Heavy Weight)

• Child process gets a copy of parent’s variables
• Relatively expensive to start
• Don't have to worry about concurrent access to variables

Thread (Light Weight Process)

• Child process shares parents variables
• Relatively cheap to start
• Concurrent access to variables is an issue

Creating Threads

Java

There are two different methods for creating a thread: extending the Thread class or implementing the Runnable interface. The first method is shown on this slide, the second on the next slide. In the Thread subclass, implement the run() method. The signature of run() must be as it is in this example. run() is the entry point or starting point (or main) of your thread. To start a thread, create an object from your Thread class. Send the "start()" method to the thread object. This will create the new thread, start it as an active entity in your program, and call the run() method in the thread object. Do not call the run() method directly. Calling the run() directly executes the method in the normal sequential manner.

class  ExtendingThreadExample  extends Thread {
   public void run() {
      for  ( int  count = 0;  count < 4; count++)
         System.out.println( "Message " + count +
                          " From: Mom" );
   }
    
   public static void main( String[]  args ) {
      ExtendingThreadExample  parallel  = 
         new ExtendingThreadExample();
      System.out.println( "Create the thread");
      parallel.start();
      System.out.println( "Started the thread" );
      System.out.println( "End" );
   }
}

Output

Create the thread
Message 0 From: Mom
Message 1 From: Mom
Message 2 From: Mom
Message 3 From: Mom
Started the thread
End

Second Method for Creating a Thread

First, have your class implement the Runnable interface, which has one method, run(). This run() plays the same role as the run() in the Thread subclass in the first method. Second, create an instance of the Thread class, passing an instance of your class to the constructor. Finally, send the thread object the start() method.

class  SecondMethod  implements  Runnable {
   public  void  run() {
      for  ( int  count = 0;  count < 4; count++)
         System.out.println( "Message " + count +  " From: Dad");
   }
   
   public static void main( String[]  args ) {
      SecondMethod  notAThread  = new SecondMethod();
      Thread  parallel  =  new Thread( notAThread );
   
      System.out.println( "Create the thread");
      parallel.start();
      System.out.println( "Started the thread" );
      System.out.println( "End" );
   }
}

Output

Create the thread
Message 0 From: Dad
Message 1 From: Dad
Message 2 From: Dad
Message 3 From: Dad
Started the thread
End

We can give each thread a string id, which can be useful.

public class  WithNames  implements  Runnable {
   public  void  run() {
      for  ( int  count = 0;  count < 2; count++)
         System.out.println( "Message " + count +  " From: " + 
                        Thread.currentThread().getName() );
   }
   
   public static void main( String[]  args ) {
      Thread  a  =  new Thread(new WithNames(), "Mom" );
      Thread  b  =  new Thread(new WithNames(), "Dad" );
   
      System.out.println( "Create the thread");
      a.start();
      b.start();
      System.out.println( "End" );
   }
}
   

Output

Create the thread
Message 0 From: Mom
Message 1 From: Mom
Message 0 From: Dad
Message 1 From: Dad
End

SimpleThread for Use in Future Examples

This class will be used in future examples.

public class  SimpleThread  extends Thread
   {
   private  int  maxCount =  32;
   
   public  SimpleThread(  String  name )
      {
      super( name );
      }
   
   public  SimpleThread(  String  name, int repetitions )
      {
      super( name );
      maxCount  =  repetitions;
      }
   
   public  SimpleThread(  int repetitions )
      {
      maxCount  =  repetitions;
      }
   
   public  void run()
      {
      for  ( int  count = 0;  count < maxCount; count++)
         {
         System.out.println( count + " From: " + getName() );
         }
      }
   }

In this example we show some actual parallelism. Note that the output from the different threads is mixed.

public class  RunSimpleThread
   {
   public static void main( String[]  args )
      {
      SimpleThread  first      = new SimpleThread( 5 );
      SimpleThread  second  = new SimpleThread( 5 );
      first.start();
      second.start();
      System.out.println( "End" );
      }
   }

Output- On Rohan

End
0 From: Thread-0
1 From: Thread-0
2 From: Thread-0
0 From: Thread-1
1 From: Thread-1
2 From: Thread-1
3 From: Thread-0
3 From: Thread-1
4 From: Thread-0
4 From: Thread-1

Multiple Processors

Java

Java on a Solaris machine with multiple processors can run threads on different processors

If you run the last example on a single processor machine the results may be completely different.

CS 580 Spring 05
Doc 9, Threads part 1 Slide # 11

Thread Scheduling

• Priorities

• Timeslicing

Priorities

Each thread has a priority

If there are two or more active threads

• If one has higher priority than others
• The higher priority thread is run until it is done or not active

Java Priorities

CS 580 Spring 05

Doc 9, Threads part 1 Slide # 13

Setting Priorities

Continuously running parts of the program should have lower-priority than rare events

User input should have very high priority

A thread that continually updates some data is often set to run at MIN_PRIORITY

Java Examples

public class  PriorityExample
   {
   public static void main( String[]  args )
      {
      SimpleThread  first      = new SimpleThread( 5 );
      SimpleThread  second  = new SimpleThread( 5 );
      second.setPriority( 8 );
      first.start();
      second.start();
      System.out.println( "End" );
      }
   }
   

Output

Threads Run Once

When a Java or Smalltalk thread ends it cannot be restarted

public class RunOnceExample extends Thread {
   public void run() {
      System.out.println(  "I ran" );
   }
   
   public static void main( String args[] ) throws Exception {
      RunOnceExample onceOnly = new RunOnceExample();
      onceOnly.setPriority( 6 );
      onceOnly.start();
   
      System.out.println( "Try restart");
      onceOnly.start();
   
      System.out.println( "The End");
   }
}

Output

I ran
Try restart
The End

Thread Scheduling

Time-slicing

Time-slicing

• A thread is run for a short time slice and suspended,
• It resumes only when it gets its next "turn"

• Threads of the same priority share turns

Non time-sliced threads run until:

• They end
• They are terminated
• They are interrupted

• Higher priority threads interrupts lower priority threads

• They go to sleep
• They block on some call

• Reading a socket
• Waiting for another thread

• They yield

Java

Does not specify if threads are time-sliced or not

Implementations are free to decide

public class  InfinityThread  extends Thread
   {
   public  void run()
      {
      while ( true )
         System.out.println(  "From: " + getName() );
      }
   
   public static void main( String[]  args )
      {
      InfinityThread  first      = new InfinityThread( );
      InfinityThread  second  = new InfinityThread( );
      first.start();
      second.start();
      }
   }
   

Output if Time-sliced

A group of "From: Thread-a" will be followed by a group of "From: Thread-b" etc.

Output if not Time-sliced, Single Processor

"From: Thread-a" will repeat "forever"

Multiple Processor

"From: Thread-a" and "From: Thread-b" will intermix "forever"

Java Types of Threads: use and daemon

We have seen several examples now of a program that continues to execute after its main has finished. So, when does a Java program end? To answer this question we need to know about the different types of threads. There are two types of threads: user and daemon.

Daemon thread

Daemon threads are expendable. When all user threads are done, the program ends all daemon threads are stopped

User thread

User threads are not expendable. They continue to execute until their run method ends or an exception propagates beyond the run method.

When a thread is created, it is the same type of thread as its creator thread. The type a thread can be changed before its start() method is called, but not after its start() method has been called. See example on next slide. The main of your program is started in a user thread.

The Java Virtual Machine continues to execute the program until either of the following occurs:

• The exit method of class Runtime has been called and the security manager has permitted the exit operation to take place.

• All threads that are not daemon threads have died, either by returning from the call to the run method or by throwing an exception that propagates beyond the run method.

Daemon example

The thread "shortLived" has the same priority as the thread running main. Hence on a single processor machine, "shortLived" will not start until main ends or main uses up its time-slice. Main is short enough to finish in one time-slice. However, since "shortLived" is a daemon thread, it does not run after all the user threads are done. Hence, "shortLived" never starts and does not print anything.

public class DaemonExample extends Thread
   {
   public  static  void  main( String  args[] )
      {
      DaemonExample  shortLived      = new DaemonExample( );
      shortLived.setDaemon( true );
      shortLived.start();
      System.out.println( "Bye");
      }
   
   public  void run()
      {
      while (true)
         {
         System.out.println(  "From: " + getName()  );
         System.out.flush();
         }
      }
   }
   

Output

From: Thread-0 (Repeated many times)
Bye
From: Thread-0 (Repeated some more, then the program ends)
CS 580 Spring 05
Doc 9, Threads part 1 Slide # 20

Thread Control

Thread States

• Executing

• Only one thread per processor can be running at a time

• Runnable

• A thread is ready to run but is not currently running

• Not Runnable

• A thread that is suspended or waiting for a resource

Yield

Allow another thread of the same priority to run

public class  YieldThread  extends Thread {
   public  void run() {
      for  ( int  count = 0;  count < 4; count++) {
         System.out.println( count + " From: " + getName() );
         yield();
      }
   }
   
   public static void main( String[]  args )  {
   
      YieldThread  first      = new YieldThread();
      YieldThread  second  = new YieldThread();
        first.setPriority( 1);
        second.setPriority( 1);
      first.start();
      second.start();
      System.out.println( "End" );
   }
}
   

Output (Explain this)

0 From: Thread-0
0 From: Thread-1
1 From: Thread-0
1 From: Thread-1
2 From: Thread-0
2 From: Thread-1
3 From: Thread-0
End
3 From: Thread-1

CS 580 Spring 05

Doc 9, Threads part 1 Slide # 22

Suspend & Resume – Java

The following Thread methods are not thread safe

• suspend
• resume
• stop

These methods can leave your Java program in unstable states

You should not use them

Killing a Thread

Killing a Thread - Java

stop

• This Thread method is unsafe do not use it

destroy

• This method does nothing. It was never implemented

There is no good way to really kill a Java thread

Later lectures will cover some suggestions for doing this

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