This presentation introduces the concept of synchronization beatween threads, as implemented in the Java platform. It is the first part of a series of slides dedicated to thread synchronization. This slides introduces the following concepts: - Thread safety - Types of race conditions - Locking (reentrant locks, intrinsic locks, synchronized blocks) - Locking pitfalls The presentation is took from the Java course I run in the bachelor-level informatics curriculum at the University of Padova.

1. CONCURRENT PROGRAMMING
SYNCHRONIZATION (PART 1)
PROGRAMMAZIONE CONCORRENTE E DISTR.
Università degli Studi di Padova
Dipartimento di Matematica
Corso di Laurea in Informatica, A.A. 2015 – 2016
rcardin@math.unipd.it

2. Programmazione concorrente e distribuita
SUMMARY
 Introduction
 Thread-safety
 Race conditions
 Locking
 Locking pitfalls
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3. Programmazione concorrente e distribuita
INTRODUCTION
 Threads can reduce development cost and
improve the performance application
 Exploiting multiple processors
 Improved throughput by utilizing available processors
 Simplicity of modeling
 Use every thread to do a specific task is simplier than using
one thread to do them all
 Simplified handling of asynchronous events
 Prevention of server’s stall while it is fulfilling a request
 More reponsive user interfaces
 Use of different threads for GUI and event management
 Event dispatch thread (EDT)
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4. Programmazione concorrente e distribuita
INTRODUCTION
 Risks of threads
 Safety hazards
 Without sufficient synchronization, the ordering of
operations in multiple threads is unpredictable
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public class UnsafeSequence {
private int value;
/** Returns a unique value. */
public int getNext() {
return value++; // Three operations: read, add and store
}
}
With unlucky timing, two
threads could call
getNext and receive the
same value

5. Programmazione concorrente e distribuita
INTRODUCTION
 Risks of threads
 Liveness hazards
 An activity gets into a state such that it is permanently unable
to make forward progress
 Dining philosophers problem
 Performance hazards
 Poor service time, responsiveness, throughput,
resource consumption, or scalability
 Context switch is not cost free
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If thread A is waiting for a resource that thread B holds
exclusively, and B never releases it, A will wait forever.

6. Programmazione concorrente e distribuita
INTRODUCTION
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7. Programmazione concorrente e distribuita
THREAD SAFETY
 Writing thread-safe code is about managing
access to shared and mutable state
 Object state is represented by its data
 By Shared, we mean that a variable could be
accessed by multiple threads
 By mutable, that its value could change
 It is far easier to design a class to be thread-safe than
to retrofit it later
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Whenever more than a thread accesses a given state variable, and one
of them might write to it, they all must coordinate their access to it
using synchronization
-- Brian Goetz

8. Programmazione concorrente e distribuita
THREAD SAFETY
 There are three ways to fix a mutable shared
variable
 Don’t share the state variable across threads
 Make the state variable immutable
 Use synchronization whenever accessing it
 Object oriented techniques favor thread safety
 Encapsulation
 Immutability
 Clear specification of invariants
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Stateless objects are always thread-safe.
-- Brian Goetz

9. Programmazione concorrente e distribuita
THREAD SAFETY
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10. Programmazione concorrente e distribuita
RACE CONDITIONS
 Race conditions
 The correctness of a computation depends on
relative timing of multiple threads at runtime
 Atomicity
 A set of statements is not atomic if they are not executed in a
single, indivisible operation
 Read-modifiy-write
 Check-then-act
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A class is thread-safe if it behaves correctly when accessed from
multiple threads, regardless of the scheduling or interleaving of the
execution of those threads by the runtime environment.
-- Brian Goetz

11. Programmazione concorrente e distribuita
RACE CONDITIONS
 Read-modify-write race condition
 The value of value is read, then modified adding 1
and finally stored into value variable
 Among the execution of every statement, control flow could
be preempted by another thread
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public class UnsafeSequence {
private int value;
/** Returns a unique value. */
public int getNext() {
value = value + 1;
return value;
}
}
read value value + 1 store value
Possible preemption

12. Programmazione concorrente e distribuita
RACE CONDITIONS
 Check-then-act race condition
 Lazy initialization
 The boolean expression depends on a value that is
modified according to it
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public class LazyInitRace {
private ExpensiveObject instance = null;
public ExpensiveObject getInstance() {
// Check-then-act
if (instance == null)
instance = new ExpensiveObject();
return instance;
}
}
check instance create object store instance
Possible preemption

13. Programmazione concorrente e distribuita
RACE CONDITIONS
 Compound actions
 Sequences of operations that must be executed
atomically in order to remain thread-safe.
 Read-modify-write and Check-then-act must always be
atomic to be thread-safe
 Atomicity is relative to operation that are executed
on shared state
 The java.util.concurrent.atomic package contains
atomic variable classes for effecting atomic state transitions
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public class SafeSequence {
private AtomicInteger value;
public int getNext() {
return value.incrementAndGet(); // Atomic read-modify-write
}
}

14. Programmazione concorrente e distribuita
RACE CONDITIONS
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15. Programmazione concorrente e distribuita
SHARING STATE
 Writing correct concurrent programs is primarily
about managing access to shared, mutable state
 It’s all about memory visibility
 We want to ensure that when a thread modifies the state of
an object, other threads can actually see those changes
 If shared state is represented by more than one variable,
atomic classes are not useful
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public class UnsafeCachedSequence {
private AtomicInteger lastValue;
private AtomicInteger value;
public int getNext() {
// Invariant of the class is not satisfied anymore
lastValue.set(value.get());
return value.incrementAndGet();
}
}

16. Programmazione concorrente e distribuita
LOCKING
 To preserve state consistency, update related
state variable in a single atomic operation
 Java has a built-in locking mechanism for enforcing
atomicity: synchronized block
 But, it is easier to understand the synchronized
keyword after having seen locks in isolation...
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synchronized (lock) {
// Access or modify shared state guarded by lock
}
Object that will serve
as lock
Block code to be guarded by
the lock

17. Programmazione concorrente e distribuita
LOCKING
 Reentrant locking
 Use a Lock to protect a code block
 The construct guarantees that only one thread at time
can enter the critical section
 Always release the lock in a finally block to prevents deadlocks
 The class ReentranctLock implements basic functionalities of
a lock
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myLock.lock(); // a ReentrantLock object
try {
// Operation in this block are executed atomically
} finally {
// make sure the lock is unlocked even if an
// exceptions thrown
myLock.unlock();
}

18. Programmazione concorrente e distribuita
LOCKING
 Reentrant locking
 The lock acts as mutual exclusion locks
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public class SafeCachedSequence {
private Lock lock = new ReentrantLock();
private int lastValue;
private int value;
public int getNext() {
// Invariant of the class is now satisfied
lock.lock();
try {
lastValue = value;
value = value + 1;
int result = value;
} finally {
lock.unlock();
}
return result;
}
}
Now the invariant of
the class is
guaranteed by the
lock: value and
lastValue will always
be updated in a
consistent way

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LOCKING
 Reentrant locking
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Thread 1 Thread 2
getNext getNext
Unsynchronized
Thread 1 Thread 2
getNext
getNext
Synchronized

20. Programmazione concorrente e distribuita
LOCKING
 Reentrant locking
 Different threads have to synchronize using the same
instance of the lock
 It is called reetrant because a thread can repeatedly
acquire a lock that it already owns
 The lock has a hold count that keeps track of the nested calls
to the lock method
 Prevents deadlocks wrt the subclass mechanism
 Every object in Java (since 1.0) has an intrinsic lock
 The synchronized keyword on a method protects the
access to that method using this reference as lock object
 All method’s code is guarded
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21. Programmazione concorrente e distribuita
LOCKING
 Intrinsic locking
 To call the method, a thread must acquire the
intrinsic object lock
 Static synchronized methods use the Class object as lock.
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public synchronized void method() {
// method body
}
// ...is equivalent to
public void method() {
this.intrinsicLock.lock();
try {
// method body
} finally {
this.intrinsicLock.unlock();
}
}

22. Programmazione concorrente e distribuita
LOCKING
 Intrinsic locking suffers of performance issues
 Use synchronization by Lock object
 Or synchronized block
 It uses the reference to an object that will serve as lock
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public class SafeCachedSequence {
private Object lock = new Object();
private int lastValue;
private int value;
public int getNext() {
synchronized (lock) {
lastValue = value;
value = value + 1;
int result = value;
}
return result;
}
}

23. Programmazione concorrente e distribuita
LOCKING
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24. Programmazione concorrente e distribuita
LOCKING PITFALLS
 All the accesses to a mutable shared variable
must be performed with the same lock held
 Not only compound actions
 Not all data needs to be guarded by a lock
 Only mutable data
 All the variables involved in the same invariant
must be guarded by the same lock
 It is not sufficient to use intrinsic lock on every
method
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// Not thread-safe
if (!vector.contains(element))
vector.add(element);

25. Programmazione concorrente e distribuita
LOCKING PITFALLS
 Poor concurrency
 Limits by the availability of processing resources, not
by the structure of application itself
 CPU intensive and I/O operations must be outside
synchronized blocks
 Acquiring and releasing a lock has some overhead
 Not break down synchronized blocks too far
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There is frequently a tension between simplicity and performance.
When implementing a synchronization policy, resist the temptation to
prematurely sacrifice simplicity (potentially compromising safety) for
the sake of performance.
-- Brian Goetz

26. Programmazione concorrente e distribuita
EXAMPLES
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https://github.com/rcardin/pcd-snippets

27. Programmazione concorrente e distribuita
REFERENCES
 Chap. 1 «Introduction», Java Concurrency in Practice, Brian Goetz,
2006, Addison-Wesley Professional
 Chap. 2 «Thread Safety», Java Concurrency in Practice, Brian Goetz,
2006, Addison-Wesley Professional
 Chap. 3 «Sharing Objects», Java Concurrency in Practice, Brian
Goetz, 2006, Addison-Wesley Professional
 Dining philosophers problem
https://en.wikipedia.org/wiki/Dining_philosophers_problem
 Chap. 14 «Multithreading», Core Java Volume I - Fundamentals, Cay
Horstmann, Gary Cornell, 2012, Prentice Hall
 Intrinsic Locks and Synchronization
https://docs.oracle.com/javase/tutorial/essential/concurrency/lock
sync.html
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