Presentation of Practical Exercise 1 - PowerPoint PPT Presentation

Presentation of Practical Exercise 1

Threads in Java

Mutual exclusion in Java

TDT4186, H2008

• A thread is an independent path of execution within a program, or a specific task.
• Multithreading refers to two or more tasks executing concurrently within a single program.
• E.g. word processing (type characters and check spelling at the same time)
• Multiprocessor systems: multiple threads can be executed simultaneously.
• Single-processor systems:
• Threads share CPU time.
• The operating system is responsible for scheduling and allocating resources to them.

Multiple threads on multiple CPUs

Multiple threads sharing a single CPU

• Threads are lightweight compared to processes.
• Allow different tasks to be performed concurrently.
• In reality, the different threads’ statements are interleaved.
• Share the process’s resources, e.g. code, data and open files.
• Context switching between threads is usually less expensive than between processes.
• Intercommunication among threads is potentially problematic.

• In Java, you start with just one thread, called the main thread.
• The main thread
• Able to create additional threads
• Executed by invoking a main() method
• Terminated when the main() method returns
• Types of threads
• A user-made thread
• Two ways of creating: implement Runnable interface, or extend Thread class
• The main thread invokes Thread.start() in order to start a new thread
• Executed by the code in a run() method
• Terminated when the run() method returns
• An event handler thread
• Started by the API when a GUI is created
• A garbage collection thread
• Started by the JVM

• When you start a program you start the main thread.
• To start more threads:

public static void main(String[] args) {

Courier courier = new Courier();

courier.start();

for(int i = 0; i < 15; i++) {

System.out.println(”Blah”);

}

}

public class Courier extends Thread {

[…]

public void run () {

for(int i = 0; i < nofVisits; i++) {

bank.depositMoney(100);

}

}

}

• At the courier.start() statement the executing thread (the main thread) branches into two threads, indicated by the blue (main) and red (courier) arrow.

public static void main(String[] args) {

Courier courier = new Courier();

Thread t = new Thread(courier);

t.start();

for(int i = 0; i < 15; i++) {

System.out.println(”Blah”);

}

}

public class Courier implements Runnable

{

[…]

public void run () {

for(int i = 0; i < nofVisits; i++) {

bank.depositMoney(100);

}

}

}

• Mutual exclusion is about guaranteeing that a resource is only accessed by one user at a time.
• In our case, the resource is a code segment, and the users are threads.
• Code that must be mutually exclusive is called critical code.

• When multiple threads may execute a code segment, and
• the code segment performs some operation that consists of multiple statements but must be treated as an atomic operation.
• Similar to a transaction in database systems.
• Violation of the mutual exclusion requirement can

lead to data inconsistencies.

• Example (critical code marked with red):

public class Money {

private int balance = 1000;

public void addMoney(int t) {

int temp = balance;

temp += t;

balance = temp;

}

}

• Solutions:
• Write ”smarter” code.
• Protect critical sections of the code with mutual exclusion.
• Code where these solutions have been applied is called threadsafe because it is safe to have multiple threads executing the code.

• In some cases it is possible to perform operations in such a way that interleaving of statements is not a problem.
• Example:

public class Money {

private int balance = 1000;

public void addMoney(int t) {

balance += t;

}

}

• If critical code is unavoidable, mutual exclusion must be enforced with locks.
• In Java, this is done using the synchronized keyword.

public class Money {

private int balance = 1000;

public synchronized void addMoney(int t) {

int temp = balance;

temp += t;

balance = temp;

}

}

• Used to indicate that a thread must acquire a lock before proceeding.
• When the thread leaves the section of the code marked as synchronized, it releases the lock, allowing waiting threads to enter.
• Only one thread can have the lock at any time, so only one thread can be inside a synchronized block at any time.
• Different synchronized blocks may use the same lock.
• More on the synchronized keyword in exercise 2.

The synchronized keyword

• Your task is to inspect a small, multithreading program.
• Couriers deposit money in a bank.
• Each courier is a thread.
• Somehow, the total amount of money deposited differs from the amount of money actually in the bank.
• This is because the program isn’t threadsafe.

• Analyse the code, find out why it doesn’t work. Correct the error(s).
• Compare the runtime of the correct solution to the runtime of the original code. Explain any differences.
• Show examples of situations where the incorrect code leads to inconsistent money accounts.

• Take advantage of examples and information given in this presentation.
• You should run the incorrect program to get an impression of what the program does, and to see for yourself that inconsistencies sometimes occur.
• You can run the handed out code by running the P1.bat batch file.