Threads. Objectives You must be able to answer the following questions –What code does a thread execute? –What states can a thread be in? –How does a

Threads

Objectives

• You must be able to answer the following questions– What code does a thread execute?– What states can a thread be in?– How does a thread change its state?– How does synchronization work?

Java ThreadsA Quick Definition:

A Thread is a lightweight process (chunk of code) that can act independently of another any other code in an application. They allow us to give the illusion that more than one process is running at the same time.

How-To in Java:

Subclass java.lang.Thread, or implement the interface Runnable and provide a method:

public void run(){

// threaded code goes here}

The method run() is started via a call to “start()”. (One can also override start(), but this is not always necessary.)

Thread Objects

• As is everything else, threads in Java are represented as objects.

• The code that a thread executes is contained in its run() method.– There is nothing special about run, anyone can

call it.

• To make a thread eligible for running you call its start() method

Example

public class CounterThread extends Thread { public void run() { for ( int i=0; i<10; i++) System.out.println(“Count: “ + i); }

public static void main(String args[]) { CounterThread ct = new CounterThread(); ct.start(); }}

Interface Runnable

• Classes that implement Runnable can also be run as separate threads

• Runnable classes have a run() method

• In this case you create a thread specifying the Runnable object as the constructor argument

Examplepublic class DownCounter implements Runnable { public void run() { for (int i=10; i>0; i--) System.out.println(“Down: “+ i); }

public static void main(String args[]) { DownCounter ct = new DownCounter(); Thread t = new Thread(ct);

t.start(); }}

Thread Creation Diagram

Thread t = new BThread();

t.start();

doMoreStuff();

Object A

BThread() {}

void start() {// create thread

}

void run() {doSomething();

}

Object BThread (extends Thread)



t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}




t.start();

doMoreStuff();

Object A

BThread() {}


}


}


Thread Lifecycle

Born

BlockedRunnable

Dead

stop()

start()

stop()

Active

block on I/O

I/O available

JVM

sleep(500)

wake up

suspend()

resume()

wait

notify

Blocking Threads

• When reading from a stream, if input is not available, the thread will block

• Thread is suspended (“blocked”) until I/O is available

• Allows other threads to automatically activate• When I/O available, thread wakes back up again

– Becomes “runnable”

– Not to be confused with the Runnable interface

Thread Scheduling

• In general, the runnable thread with the highest priority is active (running)

• Java is priority-preemptive– If a high-priority thread wakes up, and a low-priority

thread is running– Then the high-priority thread gets to run

immediately

• Allows on-demand processing– Efficient use of CPU

Thread Starvation

• If a high priority thread never blocks

• Then all other threads will starve

• Must be clever about thread priority

Thread Priorities: General Strategies

• Threads that have more to do should get lower priority

• High priority for short tasks

• Give your I/O-bound threads high priority– Wake up, immediately process data, go back to

waiting for I/O

Race Conditions

• Two threads are simultaneously modifying a single object

• Both threads “race” to store their value

• In the end, the last one there “wins the race”

• (Actually, both lose)

Race Condition Example

class Account {

int balance;

public void deposit(int val)

{

int newBal;

newBal = balance + val;

balance = newBal;

}

}

Thread Synchronization

• Language keyword: synchronized

• Takes out a monitor lock on an object– Exclusive lock for that thread

• If lock is currently unavailable, thread will block

Thread Synchronization

• Protects access to code, not to data– Make data members private

– Synchronize accessor methods

• Puts a “force field” around the locked object so no other threads can enter

• Actually, it only blocks access to other synchronizing threads

Thread Deadlock

• If two threads are competing for more than one lock

• Example: bank transfer between two accounts– Thread A has a lock on account 1 and wants to

lock account 2

– Thread B has a lock on account 2 and wants to lock account 1

Avoiding Deadlock

• No universal solution

• Ordered lock acquisition

• Check and back off

• Timeout

• Minimize or remove synchronization

Wait and Notify

• Allows two threads to cooperate

• Based on a single shared lock object

Wait and Notify: Code

• Consumer:

synchronized (lock) {

while (!resourceAvailable()) {

lock.wait();

}

consumeResource();

}

Wait and Notify: Code

• Producer:

produceResource();

synchronized (lock) {

lock.notifyAll();

}

Wait/Notify Sequence

Lock Object

ConsumerThread

ProducerThread

1. synchronized(lock){

2. lock.wait();

3. produceResource()4. synchronized(lock) {5. lock.notify();6.}

7. Reacquire lock8. Return from wait()

9. consumeResource();10. }


Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();





Lock Object

ConsumerThread

ProducerThread


2. lock.wait();




Wait/Notify: Details

• Must loop on wait(), in case another thread grabs the resource...– After you are notified– Before you acquire the lock and return from

wait()

• Use lock.notifyAll() if there may be more than one waiting thread

Wait/Notify Example: Blocking Queue

class BlockingQueue extends Queue {

public synchronized Object remove() {

while (isEmpty()) {

wait(); // really this.wait()

}

return super.remove();

}

public synchronized void add(Object o) {

super.add(o);

notifyAll(); // this.notifyAll()

}

}

Documents

Threads. Objectives You must be able to answer the following questions –What code does a thread execute? –What states can a thread be in? –How does a