Threads and Singleton

Threads  The JVM allows multiple “threads of execution”  Essentially separate programs running concurrently in one memory space  No inherent guarantees about the order in which different threads execute  Threads take turns executing and one may be stopped to let another run at any time  The JVM allows multiple “threads of execution”  Essentially separate programs running concurrently in one memory space  No inherent guarantees about the order in which different threads execute  Threads take turns executing and one may be stopped to let another run at any time

Dining Philosophers  Five philosophers around a table with five forks spread between them  All philosophers do is think and eat  To eat, a philosopher must hold both of the forks next to it.  Can you cause deadlock?  Which rules for the philosophers cause deadlocks and which ones prevent it?  Five philosophers around a table with five forks spread between them  All philosophers do is think and eat  To eat, a philosopher must hold both of the forks next to it.  Can you cause deadlock?  Which rules for the philosophers cause deadlocks and which ones prevent it?

Look at Code  Each philosopher is a separate thread  The state of the forks is shared (static)  Will it run to completion?  Each philosopher is a separate thread  The state of the forks is shared (static)  Will it run to completion?

Preventing deadlock  Pick up lower numbered fork  Pick up higher numbered fork  Eat  Put down higher numbered fork  Put down lower numbered fork  Think  Forever  Pick up lower numbered fork  Pick up higher numbered fork  Eat  Put down higher numbered fork  Put down lower numbered fork  Think  Forever

Singleton  Used when you only want one instance of a class  Make constructor private and, instead, give everyone a reference to the same instance  Used when you only want one instance of a class  Make constructor private and, instead, give everyone a reference to the same instance

Example  Check out the code in Singleton

public class Professor { private boolean isTeaching; // other variables private static Professor uniqueProfessor; private Professor() { isTeaching = false; //other assignments } public static Professor getProfessor() { if (uniqueProfessor == null) { uniqueProfessor = new Professor(); return uniqueProfessor; } return uniqueProfessor; } //other methods }

Singleton Pattern Definition  The Singleton Pattern ensures a class has only one instance, and provides a global point of access to that instance.

Singleton Pattern UML Singleton static uniqueInstance private Singleton() static getInstance()

Why Did I Show You Threads?  What happens if we have multiple threads getting singletons?  Is there a place a thread could be interrupted that would cause problems?  What happens if we have multiple threads getting singletons?  Is there a place a thread could be interrupted that would cause problems?

Synchronized  Adding the keyword “synchronized” to the definition of a method means that only one thread can be executing that method at any point in time  Adding Synchonized to the getProfessor would certainly fix our problem  Has a non-trivial performance penalty  Adding the keyword “synchronized” to the definition of a method means that only one thread can be executing that method at any point in time  Adding Synchonized to the getProfessor would certainly fix our problem  Has a non-trivial performance penalty

public class SyncProfessor { private boolean isTeaching; // other variables private static SyncProfessor uniqueProfessor; private SyncProfessor() { isTeaching = false; //other assignments } private static synchronized SyncProfessor getProfessor() { if (uniqueProfessor == null) { uniqueProfessor = new SyncProfessor(); } return uniqueProfessor; } //other methods }

Eager Creation vs. Lazy Creation  Eager: initialize at variable declaration (created when the class is loaded)  WATCH OUT! If construction depends on information not available at load time (constructor parameters), can’t use eager construction.  Eager: initialize at variable declaration (created when the class is loaded)  WATCH OUT! If construction depends on information not available at load time (constructor parameters), can’t use eager construction.

Double Checked Locking  Look at DoubleCheckedProfessor  “Volatile” marks a member variable not to be used in optimization, during compilation  “synchronized” obtains and releases locks on monitors  “volatile” only available with Java 1.5 and later!  Look at DoubleCheckedProfessor  “Volatile” marks a member variable not to be used in optimization, during compilation  “synchronized” obtains and releases locks on monitors  “volatile” only available with Java 1.5 and later!

public class DoubleCheckedProfessor { private boolean isTeaching; private volatile static DoubleCheckedProfessor uniqueProfessor; private DoubleCheckedProfessor() { isTeaching = false; } private static DoubleCheckedProfessor getProfessor() { if (uniqueProfessor == null) { synchronized (DoubleCheckedProfessor.class) { if (uniqueProfessor == null) { uniqueProfessor = new DoubleCheckedProfessor(); } return uniqueProfessor; } //other methods }