Chapter 9 (Horstmann’s Book)Multithreading

Hwajung Lee

Thread Basics Thread Synchronization

Thread: program unit that is executed independently Multiple Threads: Program units what can be executed in

parallel. Multiple threads run simultaneously.

When executes

▪ Multiprocessor computers: threads actually run in parallel

OR

▪ Illusion of threads running in parallel.

▪ That is, Virtual machine executes each thread for short time slice. Thread scheduler activates, deactivates threads.

How to implement Running Threads

Define class that implements Runnable interface. Runnable has one method.

void run()

Place thread action into run() method

Construct an object of the class which implements the Runnable Interface

Construct an object of Thread class

start() thread

public class MyRunnable implements Runnable{

public void run(){

thread action}

} ... Runnable r = new MyRunnable(); Thread t = new Thread(r); t.start();

Run two threads in parallel Each thread

prints 10 greetings

for (int i = 1; i <= 10; i++){

System.out.println(i + ": " + greeting);Thread.sleep(100);

}

After each printout, sleep for 100 milliseconds All threads should occasionally yield control sleep() throws InterruptedException

Ch9/greeting/GreetingProducer.java Ch9/greeting/ThreadTester.java

Sample Output (Note: output not exactly interleaved.)

1: Hello, World! 1: Goodbye, World! 2: Hello, World! 2: Goodbye, World! 3: Hello, World! 3: Goodbye, World! 4: Hello, World! 4: Goodbye, World! 5: Hello, World! 5: Goodbye, World! 6: Hello, World! 6: Goodbye, World! 7: Hello, World! 7: Goodbye, World! 8: Goodbye, World! 8: Hello, World! 9: Goodbye, World! 9: Hello, World! 10: Goodbye, World! 10: Hello, World!

☞The thread scheduler gives no guarantee about the order in which threads are executed. Moreover, there will always be slight variations in running times, especially when calling operating system services (such as input and output). Thus, you should expect that the order in which each thread gains control appears to be somewhat random.

Each thread has thread state

priority

Thread states: new

▪ before start() called

runnable

blocked

dead

▪ after run() method exits

Reasons for entering the blocked state:

Sleeping

Waiting for I/O

Waiting to acquire lock (later)

Waiting for condition (later)

Unblocks only if the reason for the blocking goes away

Scheduler activates a new (next) thread if a thread has completed its time slice a thread has blocked itself a thread with higher priority has become runnable

Scheduler determines a new thread to run by looking only at all runnable threads, and picking one with the highest priority value. (The priority values are

system-dependant and not adjustable by an application programmer.)OR Picking one at random or using a round-robin scheme to gives each

thread a chance.

Normal way to terminate a thread

Thread terminates when run() exits

However, you sometimes need to terminate running thread (Ex) you may have several threads attempting to find a solution to a

problem. As soon as the first one has succeeded, you can terminate the other ones.

Example

Two producers, one consumer

Each producer thread inserts 100 greetings into the same queue.

Each consumer thread removes all of the greetings in the queue.

In the try{ } blockint i = 1; while (i <= greetingCount) {

if (!queue.isFull()) {

queue.add(i + ": " + greeting); i++;

} Thread.sleep((int)(Math.random() * DELAY));

}

In the try{ } blockint i = 1; while (i <= greetingCount) {

if (!queue.isEmpty()) {

Object greeting = queue.remove(); System.out.println(greeting); i++;

} Thread.sleep((int)(Math.random() * DELAY));

}

1: Hello, World!

1: Goodbye, World! 2: Hello, World! 3: Hello, World! ... 99: Goodbye, World! 100: Goodbye, World!

Sometimes, the program will corrupt the queue and not work correctly. The consumer thread gets stuck and won’t complete. Even though 200

greetings were inserted in the queue, it can’t retrieve them all. It may complete, but print the same greetings repeatedly.

Can you spot the problem? Can see problem better when turning debugging on

queue.setDebug(true); Ch9/queue1/ThreadTester.java Ch9/queue1/Producer.java Ch9/queue1/Consumer.java Ch9/queue1/BoundedQueue.java

Race Condition occurs if the effect of multiple threads on shared data depends on the order in which the threads are scheduled.

Multiple threads, in their race to complete their respective task, rush to win the race.

Race Condition Scenario

First thread calls add and executes

elements[tail] = anObject;

First thread at end of time slice

Second thread calls add and executes

elements[tail] = anObject;

tail++;

Second thread at end of time slice

First thread executestail++;

To fix the race conditions, you need to ensure that only one thread manipulates the queue at any given moment.

Locking Mechanism Thread can temporarily acquire ownership of a lock

When another thread tries to acquire same lock, it is blocked.

When first thread releases the lock, other threads are unblocked and try again

Two kinds of locks Objects of ReentrantLock class or another class implementing

java.util.concurrent.Lock interface type in java.util.concurrent.locks packageLocks

Locks that are built into every Java object

aLock = new ReentrantLock();. . .aLock.lock();try{

protected code}finally{

aLock.unlock();} ☞ The finally clause ensures that the lock is

unlocked even when an exception is thrown in the protected code.

A deadlock occurs if no thread can proceed because each thread is waiting for another to do some work first.

Object = phone booth Thread = person Locked object = closed booth Blocked thread = person waiting for

booth to open