Threads in Java

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• Threads in Java
• Mutual exclusion in Java

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Threads Concept

• A Thread is a flow of execution, with a beginning
  and an end, of a task in a program.
• 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.

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Threads Concept
Multiple threads on multiple CPUs
Multiple threads sharing a single CPU
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Thread States
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Thinking about threads

• A thread propagates through your program
  executing statements.
• At a given point of time, every thread (that you
  made) is somewhere in your program.
• When you have many threads, different parts of
  your program are executed simultaneously
• In reality, the different threads statements are
  interleaved.

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Threads in Java

• A thread is a strand of execution.
• The main thread
• Executes the main() method
• The thread ends when the main() method returns.
• User-made threads
• Thread subclass
• Implementation of the Runnable interface
• These execute the run() method
• The threads end when the run() method returns.
• The event handler thread
• Started by the API when a GUI is created
• The garbage collection thread
• Started by the JVM

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Creating a thread

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

public class Courier extends Thread
public void run () for(int i 0 i lt
nofVisits i) bank.depositMoney(100)

public static void main(String args)
Courier courier new Courier()
courier.start() for(int i 0 i lt 15 i)
System.out.println(Blah)

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

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• Equivalent method

public class Courier implements Runnable
public void run () for(int i 0 i lt
nofVisits i) bank.depositMoney(100)

public static void main(String args)
Courier courier new Courier() Thread t new
Thread(courier) t.start() for(int i 0 i
lt 15 i) System.out.println(Blah)
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What is mutual exclusion?

• 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.

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When is code critical?

• 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 inconsistensies.
• 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
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• The three statements in the addMoney() method are
  critical code. Example of serial execution

Statement executed by thread 1 Statement executed by thread 2 Value of Variable (temp),balance after this statement
addMoney(50) tempbalance temp t balance temp 1 (0),1000 1(1000),1000 1(1050),1000 1(1050),1050
addMoney(50) tempbalance temp t balance temp 2(0),1050 2(1050),1050 2(1100), 1050 2(1100), 1100
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• The three statements in the addMoney() method are
  critical code. Example of what could go wrong if
  the code isnt treated as an atomic unit

Statement executed by thread 1 Statement executed by thread 2 Value of Variable (temp),balance after this statement
addMoney(50) 1(0), 1000
temp balance temp t 1(1000),1000 1(1050),1000
addMoney(50) 2(0), 1000
temp balance temp t balance temp 2(1000),1000 2(1050),1000 2(1050),1050
balance temp 1(1050),1050
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• We see from the example that the variables
  balance gets wrong value (they should sum to
  1100) because two threads are modifying them
  simultaneously.
• 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.

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Avoiding critical 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
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Enforcing mutual exclusion

• 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
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The synchronized keyword
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The synchronized keyword

• 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.