Locks CSCI 201L Jeffrey Miller, Ph.D. HTTP :// WWW - SCF. USC. EDU /~ CSCI 201 USC CSCI 201L

Outline USC CSCI 201L2/9 ▪L▪Locks ›L›Lock Conditions

synchronized Keyword ▪The synchronized keyword puts a restriction on a method that only one thread can be inside at a time ›This is accomplished using a monitor ›No other thread will be able to enter that method if another thread is currently executing inside of it, regardless of whether it is in the CPU currently or not USC CSCI 201L3/9 Locks

Explicitly Acquiring Locks ▪A synchronized method or block of code implicitly acquires a lock on the instance before executing ▪Java gives us the ability to explicitly acquire locks using the java.util.concurrent.locks.Lock interface and java.util.concurrent.locks.ReentrantLock class USC CSCI 201L4/9 Locks

synchronized Keyword Example 1 import java.util.concurrent.ExecutorService; 2 import java.util.concurrent.Executors; 3 4 public class AddAPenny implements Runnable { 5 private static PiggyBank piggy = new PiggyBank(); 6 7 public void run() { 8 piggy.deposit(1); 9 } public static void main(String [] args) { 12 ExecutorService executor = Executors.newCachedThreadPool(); 13 for (int i=0; i < 100; i++) { 14 executor.execute(new AddAPenny()); 15 } 16 executor.shutdown(); 17 // wait until all tasks are finished 18 while(!executor.isTerminated()) { } System.out.println("Balance = " + piggy.getBalance()); 21 } 22 } class PiggyBank { 25 private int balance = 0; 26 public int getBalance() { 27 return balance; 28 } 29 public synchronized void deposit(int amount) { 30 int newBalance = balance + amount; 31 Thread.yield(); 32 balance = newBalance; 33 } 34 } USC CSCI 201L5/9 Locks 4 Executions

Explicitly Acquiring Locks Example 1 import java.util.concurrent.ExecutorService; 2 import java.util.concurrent.Executors; 3 import java.util.concurrent.locks.Lock; 4 import java.util.concurrent.locks.ReentrantLock; 5 6 public class AddAPenny implements Runnable { 7 private static PiggyBank piggy = new PiggyBank(); 8 9 public void run() { 10 piggy.deposit(1); 11 } public static void main(String [] args) { 14 ExecutorService executor = Executors.newCachedThreadPool(); 15 for (int i=0; i < 100; i++) { 16 executor.execute(new AddAPenny()); 17 } 18 executor.shutdown(); 19 // wait until all tasks are finished 20 while(!executor.isTerminated()) { } System.out.println("Balance = " + piggy.getBalance()); 23 } 24 } class PiggyBank { 27 private int balance = 0; 28 private Lock lock = new ReentrantLock(); 29 public int getBalance() { 30 return balance; 31 } 32 public void deposit(int amount) { 33 lock.lock(); 34 try { 35 int newBalance = balance + amount; 36 Thread.yield(); 37 balance = newBalance; 38 } finally { 39 lock.unlock(); 40 } 41 } 42 } USC CSCI 201L6/9 Locks 4 Executions

Thread Conditions ▪Threads are able to communicate with each other based on specific conditions ▪Threads can perform the following actions using the java.util.concurrent.Condition interface ›await() Wait for the condition to be signaled ›signal() Wake up one thread waiting on the condition ›signalAll() Wake up all of the threads waiting on the condition ▪A Condition is created from a Lock object by calling the newCondition() method USC CSCI 201L7/9 Locks – Lock Conditions

Thread Conditions Example 1 import java.util.concurrent.ExecutorService; 2 import java.util.concurrent.Executors; 3 import java.util.concurrent.locks.Condition; 4 import java.util.concurrent.locks.Lock; 5 import java.util.concurrent.locks.ReentrantLock; 6 7 public class AddAndRemoveAPenny implements Runnable { 8 private static PiggyBank piggy = new PiggyBank(); 9 private boolean isWithdrawal; public void run() { 12 if (isWithdrawal) { 13 piggy.deposit((int)(Math.random() * 9 + 1)); 14 } 15 else { 16 piggy.withdraw((int)(Math.random() * 9 + 1)); 17 } 18 } public static void main(String [] args) { 21 ExecutorService executor = Executors.newCachedThreadPool(); 22 for (int i=0; i < 100; i++) { 23 AddAndRemoveAPenny penny = new AddAndRemoveAPenny(); 24 if (i < 50) { // exactly 50 threads will withdraw 25 penny.isWithdrawal = false; 26 } 27 else { // and exactly 50 threads will deposit 28 penny.isWithdrawal = true; 29 } 30 executor.execute(penny); 31 } 32 executor.shutdown(); 33 // wait until all tasks are finished 34 while(!executor.isTerminated()) { } System.out.println("Balance = " + piggy.getBalance()); 37 } 38 } USC CSCI 201L8/9 Locks – Lock Conditions 39 class PiggyBank { 40 private int balance = 0; 41 private Lock lock = new ReentrantLock(); 42 private Condition depositMade = lock.newCondition(); 43 public int getBalance() { 44 return balance; 45 } 46 public void withdraw(int amount) { 47 lock.lock(); 48 try { 49 while (balance < amount) { 50 System.out.print("Waiting for deposit...”); 51 System.out.print(“ to withdraw $" + amount); 52 System.out.println(" from balance of $" + balance); 53 depositMade.await(); 54 } 55 balance -= amount; 56 System.out.print("$" + amount + " withdrawn,”); 57 System.out.println(“ leaving balance of $" + balance); 58 } catch (InterruptedException ie) { 59 System.out.println("IE: " + ie.getMessage()); 60 } finally { 61 lock.unlock(); 62 } 63 } 64 public void deposit(int amount) { 65 lock.lock(); // acquires lock 66 try { 67 balance += amount; 68 System.out.print("$" + amount + " deposited,”); 69 System.out.println(“ making balance of $" + balance); 70 depositMade.signalAll(); 71 } finally { 72 lock.unlock(); // release the lock 73 } 74 } 75 }

Thread Conditions Issues ▪Once a thread invokes await() on a condition, the thread will move to the waiting state until it is signaled ›If signal() or signalAll() is never called on the condition, the thread will wait forever ›In the previous example, some executions of the program never terminate if a withdrawal was not able to be made based on the balance after all the deposits were made ▪You must first obtain the lock on the object before you are able to invoke a method on its Condition ›When a thread calls await(), it will release its lock It will not be able to start executing again, even after it is signaled, if it is not able to obtain the lock again USC CSCI 201L9/9 Locks – Lock Conditions