Multiple Threads

Sometimes you need more than multiple objects; you need multiple subtasks. The Java Virtual Machine supports multi-threading – independent subtasks in the program’s address space.

The slides and notes in this presentation are adapted either from Thinking in Java or Groovy in Action (see Recommended Reading).

An index to the source code for all the examples in this lecture is available.

The Thread

Each thread acts like its own program.
Underlying mechanism divides up CPU time between multiple threads.
Typically used for more responsive user interfaces, animation, networking.

Basic threads

Simplest way to create a thread is to inherit from Thread
Override run()
Body of run() method is executed “simultaneously” with other threads in program

Class SimpleThread

Example 1: simpleThread.groovy creates five threads, each with a unique id. run() method counts down from 5 to 0.

1|Example 1: SimpleThread (at-m42/Examples/lecture09/simpleThread.groovy)

extern> http://www.cpjobling.org.uk/~eechris/at-m42/Examples/lecture09/simpleThread.groovy

Class SimpleThread (at run time)

Typical Run

#1: 5		
#1: 4		
#1: 3		
#1: 2		
#1: 1		
#2: 5		
#2: 4		
#2: 3		
 ...  
#5: 2
#5: 1

Yielding control

Thread's yield() method gives hint to JVM that task is complete and another thread can be scheduled.
Hint may be ignored … particularly if run() takes too long.
- Pre-emptive scheduler may interrupt thread before yield() is reached.
- E.g. for I/O

A Yielding Thread

public class YieldingThread extends Thread {
  // ...
  void run() {
    while(true) {
      println this
      if (--countDown == 0) {
          return
      }
      yield()
    }
  }
  // ...
}

Example 2: yieldingThread.groovy.

Typical Run

A Sleeping Thread

public class SleepingThread extends Thread {
  // ...
  void run() {
    while(true) {
       println this
      if (--countDown == 0) {
      	return
      }
      try {
        sleep(100) // 100 ms      
      } catch (InterruptedException e) {
        throw new RuntimeException(e)
      }
    }
  }
  // ...
}

Example 3: sleepingThread.groovy.

Method sleep(100) schedules a time delay of 100 ms. The Thread is suspended until this time is up. Call to sleep() must be put in a try block because it could be interrupted before it times out. You should use wait() rather than sleep() if you expect the interrupt!

Priority

Tells the scheduler how important this thread is.
The order that threads will be scheduled is indeterminate.
Scheduler will schedule a higher priority before others.
All threads will run eventually, lower priority threads just run less often.
- Use Threads setPriority(int).
- Only portable settings Thread.MIN_PRIORITY, Thread.MAX_PRIORITY and Thread.NORM_PRIORITY.

Daemon Threads

A daemon is a process that is typically started as a background task and runs as long as the programme runs.
An example of such a process is a a server process running in a client server system. E.g. a web server, a servlet or a database connection.
“Daemonhood” of a thread can be tested with isDaemon()
Thread can be made into a daemon using setDaemon().

Joining a Thread

One thread may call join() on another thread
Thread waits for first thread to complete before completing
Example:
- thread a calls b.join() on thread b.
- thread a is suspended until thread b finishes (when b.isAlive() is false)
- join() can be called with a timeout argument so that if the joined thread does not complete before the timeout, join() will return early

Coding Variations

One way to make a thread is to inherit from Thread class and override run().
Can't do this if your class already inherits from another class (single inheritance property).
Alternative is to implement the Runnable interface (also implemented by Thread)
Runnable interface defines only the run() method.
Implement Runnable, define a run() method, pass object of class to new Thread(Runnable) constructor.

Runnable in Groovy

Groovy Closure implements runnable.

def t = new Thread () {/* closure body */}
t.start()

Even simpler:

Thread.start{/* closure body */}

More Threads with closures

A daemon thread:

Thread.startDaemon() {/* closure body */}

Deferred start with a time:

new Timer().runAfter(1000) {/* closure body */}

Producer consumer

Example 4: using threads with synchronization for the producer/consumer problem.

The producer pushes integers on a stack, consumer pops items when available. The push/pop actions are reported. Actual sequence is not predictable. We use closures to run something (producing and consuming) and sleep to slow down the consumer. We introduce a Storage class that holds the stack and synchronizes access to it. If we try to pop from an empty stack, we will wait until the producer has caught up.

1|Example 4: using threads with synchronization for the producer/consumer problem (at-m42/Examples/lecture09/producerConsumer.groovy)

extern> http://www.cpjobling.org.uk/~eechris/at-m42/Examples/lecture09/producerConsumer.groovy

An unresponsive user interface

Example 5: a responsive user interface.

1|Example 5: a responsive user interface (at-m42/Examples/lecture09/responsiveUI.groovy)

extern> http://www.cpjobling.org.uk/~eechris/at-m42/Examples/lecture09/responsiveUI.groovy

Explanation: this program simulates a problem often encountered with distributed programs, and programs that have a graphical user interface. In the first class, the while loop simulates some long running process that the user interface may have launched. Unfortunately, it never terminates, so the input statement at line 10 is never executes: in other words, the user interface has become unresponsive.

In the second example, the class is made into a daemon and the long running process is coded in the run method. This time the user interface is in a separate thread (and after 300 ms) input can be accepted from the user (which in this case allows the current version of d to be printed).

In interactive programs, the placing of long-running computation (e.g. processing of some distributed process across a network) into a separate thread, is a commonly used technique.

Sharing Limited Resources

Having multiple threads causes problems when they new to share resources.
Presentation given here are from “Head First Java”, by Kathy Sierra and Bert Bates, O'Reilly 2003.

Sharing an Object

Imagine that Ryan and Monica share a bank account.
They both have odd behaviour when making a withdrawal.
- Check balance
- Sleep
- Make the withdrawal

The Bank Account

l|A bank account (at-m42/Examples/lecture09/BankAccount.groovy)

extern> http://www.cpjobling.org.uk/~eechris/at-m42/Examples/lecture09/BankAccount.groovy

The Demo

Example 6

Makes one instance of Ryan and Monica job
Makes two threads with the same Runnable
Name and starts the threads
Watches the threads check the balance and attempt withdrawal

if (account.balance >= amount) {
  try {
       Thread.sleep(500);  
  }  catch (InterruptedException ex)  {  
     ex.printStackTrace();
  }

l| Shared resource problems (at-m42/Examples/lecture09/ryanAndMonicaJob.groovy)

extern> http://www.cpjobling.org.uk/~eechris/at-m42/Examples/lecture09/ryanAndMonicaJob.groovy

Results

Ryan checks the balance, sees there's enough money, goes to sleep
Monica comes in and checks the balance. See's there enough money (Ryan hasn't withdrawn it yet) but doesn't realise that Ryan is going to withdraw it.
Monica falls asleep.
Ryan wakes up and completes the withdrawal.
Monica wakes up and completes the withdrawal … but Ryan has already taken the money, and Monica goes overdrawn.

Monica's check was invalid because Ryan was already making a withdrawal. We need a way to prevent Monica starting a withdrawal if Ryan is already making a withdrawal.

Need a Lock!

<html> <table> <tr> <td> <ul> <li class=“level1”>Associate a lock with withdraw method.</li> <li class=“level1”>If Ryan or Monica wants to make a withdrawal, he/she must acquire the lock, and keep the key.</li> <li class=“level1”>If Ryan has the key, only he can access the bank account.</li> <li class=“level1”>Lock prevents Monica accessing the bank account.</li> </ul> </td> <td> <ul> <li class=“level1”>When Ryan has finished the transaction, he unlocks the object and the key becomes available.</li> <li class=“level1”>Monica can now take the key and lock the bank account to prevent Ryan making a withdrawal.</li> <li class=“level1”>Lock ensures that only one Thread has access to shared object at a time.</li> </ul> </td> </tr> </table> </html>

Creating a Lock in Java

synchronized method runs as an atomic transaction and while running cannot be entered by another thread.
Associated object holds the lock.
Thread holds the key.

Example 7

l| Example 7: Using a lock to control access to a shared resource (at-m42/Examples/lecture09/ryanAndMonicaJob2.groovy)

extern> http://www.cpjobling.org.uk/~eechris/at-m42/Examples/lecture09/ryanAndMonicaJob2.groovy

Object Locks

One lock per object for object's data members
One lock per class for class' static data members
Typically data is private, only accessed through methods
If a method is synchronized, entering that method acquires the lock
No other thread may call that method until the lock is released

How JVM Shares Resources

Only one synchronized method can be called at any time for a particular object

synchronized f() { /* ... */ }	
synchronized g(){ /* ... */ }

synchronized efficiency
- Each object has a lock
- Typical 6x method call overhead, theoretical minimum 4x

The "Lost Update" problem

Occurs when a two-step (or more-step) process is accessed by another thread before it completes.
E.g.

def increment(){ 
  i = balance;
  balance = i + 1
}

Increment balance by adding 1 to value AT TIME IT WAS READ rather than CURRENT value.

Thread A runs for a while

Thread A enters increment method:
1. Put the value of balance into variable i.
2. balance is 0, so i is now 0.
3. Set the balance to the result of i + 1.
4. Now balance is 1.
Thread A re-enters increment
1. Put the value of balance into variable i.
2. balance is 1, so i is now 1.
3. Set the balance to the result of i + 1.
4. Now balance is 2.

Thread B runs for a while

Thread B enters increment method:
1. Put the value of balance into variable i.
2. balance is 2, so i is now 2.
3. Set the balance to the result of i + 1.
4. Now balance is 3.
Thread B re-enters increment
1. Put the value of balance into variable i.
2. balance is 3, so i is now 3.

—-

Scheduler now makes Thread B runnable (ready to run) rather than running. Note that this is before balance has been incremented.

Thread A Runs again

Picking up where it left off:

Thread A re-enters increment
1. Put the value of balance into variable i.
2. balance is 3, so i is now 3.
3. Set the balance to the result of i + 1.
4. Now balance is 4.
Thread A re-enters increment
1. Put the value of balance into variable i.
2. balance is 4, so i is now 4.
3. Set the balance to the result of i + 1.
4. Now balance is 5.

Thread B runs again

And picks up exactly where it left off:

Set the balance to the result of i + 1.
But balance is now 4 !!!!.

—-

i had value 3 when Thread B was stopped. Value of 3 + 1 = 4 passed to balance. Update done by A is completely overridden. Hence “Missing Update”. A common problem in distributed systems e.g. database transactions.

Make increment atomic

synchronized void increment() {
  i = balance
  balance = i + 1
}

The assignment and increment code will run to end.

Thread A Runs for a While

Thread A attempts to enter increment method.
1. synchronized code, so get the key for this object.
2. put the value of balance into variable i.
3. balance is 0, so i is now 0.
4. Set the balance to the result of i + 1.
5. Now balance is 1.
6. Return the key.
Thread A re-enters increment method. Gets the key.
1. Put the value of balance into variable i.
2. balance is 1, so i is now 1.

Thread B is selected to run

Thread B attempts to enter increment method.
1. The method is synchronized so we need to get the key.
2. The key is not available.
3. Thread B is “blocked” (waiting for key to become available).

Thread A Runs again

Picking up where it left off
1. Set the balance to the result of i + 1.
2. Now balance is 2.
3. Return the key

Thread B is selected to run

Thread B attempts to enter increment method.
1. The method is synchronized so we need to get the key.
2. The key is available, get the key
3. Put the value of balance into variable i.
4. Balance is 2, so i is now 2.
5. Set the balance to the result of i + 1.
6. Now balance is 3.
7. Return the key

Synchronizing on other resources

Synchronizing on something other than the memory inside an object
Using non-synchronized objects

Best to wrap everything inside an object and guard it with the object's own synchronized methods, but you can also:

synchronized(syncObject) {
    // This code can only be accessed
    // by one thread at a time
}

Blocking

Four states of a thread:

New: the thread object has been created but it hasn’t been started yet so it cannot run
Runnable: thread can be run, when the time-slicing mechanism has CPU cycles available
Dead: normally after thread returns from its run method
Blocked: the thread could be run but there’s something that prevent it. While a thread is in the blocked state the scheduler will simply skip over it and not give it any CPU time

Becoming Blocked

Thread is sleeping: sleep(milliseconds).
Thread is suspended: suspend( ). It will not become runnable again until the thread gets the resume( ) message.
Thread is waiting with wait( ). It will not become runnable again until the thread gets the notify( ) or notifyAll( ) message.
The thread is waiting for some I/O to complete.
The thread is trying to call a synchronized method on another object and that object's lock is not available.

Deadlock

A chain of threads waiting on each other, looping back to the beginning
No language support to prevent it
Tough to debug
To create a deadlock only need two threads and two objects

Thread deadlock

Thread deadlock scenario

Thread A enters synchronized method of object foo and gets the key.
Thread A goes to sleep

Thread deadlock scenario

Thread B enters synchronized method of object bar and gets the key.
Thread B tries to enter a synchronized method of object foo, but can't get that key (A has it).
B “blocks” waiting for foo's key to become available.
B keeps the bar key.

Thread deadlock scenario

Thread A wakes up and tries to enter a synchronized method on object bar, but can't get that key because B has it.
A “blocks” waiting for bar's key to become available.
Thread A can't run until it gets bar key that B is holding.
Thread B can't run until it gets the foo key that A is holding.
Deadlock!

Summary

Single-threaded programming: live by yourself, own everything, no contention for resources
Multi-threading: suddenly you can have collisions and destroy information, get locked up over the use of resources
Multi-threading makes programming on the Java Platform complicated.
Groovy multithreading easier because of the use of the closure.
Multi-threading is hard!

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Table of Contents

Multiple Threads

The Thread

Basic threads

Class SimpleThread

Class SimpleThread (at run time)

Typical Run

Yielding control

A Yielding Thread

Typical Run

A Sleeping Thread

Priority

Daemon Threads

Joining a Thread

Coding Variations

Runnable in Groovy

More Threads with closures

Producer consumer

An unresponsive user interface

Sharing Limited Resources

Sharing an Object

The Bank Account

The Demo

Results

Need a Lock!

Creating a Lock in Java

Object Locks

How JVM Shares Resources

The "Lost Update" problem

Thread A runs for a while

Thread B runs for a while

Thread A Runs again

Thread B runs again

Make increment atomic

Thread A Runs for a While

Thread B is selected to run

Thread A Runs again

Thread B is selected to run

Synchronizing on other resources

Blocking

Becoming Blocked

Deadlock

Thread deadlock

Thread deadlock scenario

Thread deadlock scenario

Thread deadlock scenario

Summary