This presentation introduces the concept of synchronization beatween threads, as implemented in the Java platform. It is the second part of a series of slides dedicated to thread synchronization. This slides introduces the following concepts: - Conditional locking - Volatile variables - Thread confinement - Immutability 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 2)
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
 Conditions
 Volatile variables
 Atomics
 Thread confinement
 Immutability
2Riccardo Cardin

3. Programmazione concorrente e distribuita
CONDITIONS
 Condition variables
 Often a thread enters a critical section only to
discover that it can’t proceed
A condition is not fulfilled
 We can try to use a lock
3Riccardo Cardin
if (bank.getBalance(from) >= amount) {
// Thread might be deactivated at this point
bank.transfer(from, to, amount);
}
public void transfer(int from, int to, int amount) {
bankLock.lock();
try {
while (accounts[from] < amount) {
// wait
}
// transfer funds
}
No thread can
withdraw money, due
to the acquired lock:
DEADLOCK!!

4. Programmazione concorrente e distribuita
CONDITIONS
 To avoid unpleasant deadlock, use conditions
 A condition variable is built from a lock
 A thread owning the lock, calls await on the condition
 The lock is released by the thread
 Thread is not made runnable when the lock i available. It stays
deactivated until the condition will be fulfilled
 Wait set for the condition
4Riccardo Cardin
class Bank {
private Condition sufficientFunds;
public Bank() {
// Getting a condition with an evocative name
sufficientFunds = bankLock.newCondition();
}
}
sufficientFunds.await();

5. Programmazione concorrente e distribuita
CONDITIONS
 When another thread fulfills the condition, it
should notify other awaiting threads
 One of the awaiting thread will be eligible to acquire
the lock and to continue where it left off
 Lock must be available
 The condition may be fulfilled
 Retry to check that condition are met over and over again
 An awaiting thread cannot reactive itself: be carefull!
5Riccardo Cardin
sufficientFunds.signalAll();
while (!(/* ok to proceed */)) {
condition.await();
}

6. Programmazione concorrente e distribuita
CONDITIONS
 It’s important that some thread calls the
signalAll method eventually
 If no other thread bother to reactivate a waiting thread,
it will neve run again
 DEADLOCK!
 Call signalAll whenever the state of an object changes
6Riccardo Cardin
public void transfer(int from, int to, int amount) {
bankLock.lock();
try {
while (accounts[from] < amount)
sufficientFunds.await();
// transfer funds
sufficientFunds.signalAll();
} finally {
bankLock.unlock();
} }

7. Programmazione concorrente e distribuita
CONDITIONS
7Riccardo Cardin

8. Programmazione concorrente e distribuita
CONDITIONS
 Intrinsic locks have a single associated condition
 The wait method adds a thread to the wait set
 The notifyAll method unblocks waiting threads
 Having a single condition per intrinsic lock can be
inefficient
 Which condition has been safisfied? All threads waiting have
to be resumed
8Riccardo Cardin
public synchronized void transfer(int from, int to, int amount)
throws InterruptedException {
while (accounts[from] < amount)
wait(); // wait on intrinsic object lock
// transfer funds
notifyAll(); // notify all threads waiting
}

9. Programmazione concorrente e distribuita
CONDITIONS PITFALLS
 What should you use in your code, Locks or
synchronized methods
 Neither. In many situation it can be used one of the
mechanisms of the java.util.concurrent package
 i.e. – Blocking queues
 If you have to choose, use synchronized blocks
 Use Lock / Condition if you really need the
additional power that gives to you
 You have to define a custom protocol of synchronization
9Riccardo Cardin
Do not underestimate the powers of the dark side of concurrency
-- Riccardo Cardin

10. Programmazione concorrente e distribuita
VOLATILE VARIABLES
 Cached values and operations reodering are evil!
 A volatile variable is not cached by threads
 Share the visibility feature of synchronized
 Threads will automatically see the most up-to-date value
 ...but non of the atomicity features
 Possible race-conditions on multiple operations
10Riccardo Cardin
If you write a variable which may next be read by another thread, or
you read a variable which may have last been written by another
thread, you must use synchronization.
-- Brian Goetz
private volatile boolean done;
public boolean isDone() { return done; }
public void setDone() { done = true; }

11. Programmazione concorrente e distribuita
VOLATILE VARIABLES
 When to use volatile vars instead of locks
 Writes do not depend on its current value
 DO NOT use for implementing counters!
 The variable does not partecipate in invariants with
other variables
 Slightly better performances
11Riccardo Cardin
// Not atomic, you need synchronization
public void flipDone() { done = !done; }
volatile boolean shutdownRequested;
public void shutdown() { shutdownRequested = true; }
public void doWork() {
while (!shutdownRequested) {
// do stuff
}
}
Pattern of use:
status flag

12. Programmazione concorrente e distribuita
VOLATILE VARIABLES
12Riccardo Cardin

13. Programmazione concorrente e distribuita
ATOMICS
 There are operations other than setter and
getter provided by volatile variables
 java.util.concurrent.atomic provides classes
that guarantee atomicity of other operations
 AtomicInteger, AtomicBoolean, AtomicLong, ...
13Riccardo Cardin
class AtomicCounter {
private AtomicInteger c = new AtomicInteger(0);
public void increment() {
c.incrementAndGet();
}
public void decrement() {
c.decrementAndGet();
}
public int value() {
return c.get();
}
}
Uses low level CPU
operations, that don’t need
synchronization (CAS,
compare-and-swap)

14. Programmazione concorrente e distribuita
THREAD CONFINEMENT
 The best solution to concurrency problems is to
not share any mutable state
 Use ThreadLocal helper class to give each thread an
instance of a class
 When thread terminates, value is garbage collected
 Do not use as a replacement for global variables
 Many JDK classes are not thread-safe
 SimpleDateFormat, Random, ...
14Riccardo Cardin
public static final ThreadLocal<SimpleDateFormat> dateFormat =
new ThreadLocal<SimpleDateFormat>() {
protected SimpleDateFormat initialValue() {
return new SimpleDateFormat("yyyy-MM-dd");
}};
String dateStamp = dateFormat.get().format(new Date());

15. Programmazione concorrente e distribuita
IMMUTABILITY
 All the problems described so far have to do
with accessing shared mutable state
 If object state cannot be modified, the risks go away
 Immutable object are simple
 There are not different states for complex objects
 Immutable object are safer
 No untrusted code can modify directly object’s state or retain
a reference to modify it later
 Java does not formally defined immutability
 It is not sufficient declaring all fields as final
15Riccardo Cardin
Immutable objects are always thread-safe
-- Brian Goetz

16. Programmazione concorrente e distribuita
IMMUTABILITY
 An object is immutable if:
 Its state cannot be modified after construction
 So a immutable class has reference only to (effectively)
immutable classes
 All its fields are final
 It is properly constructed
 The this reference does not escape during construction, i.e.
calling code outside the class, and passing this
 Can use mutable state for internal representation
 Are this kind of object useful?
 There is a big difference between an object been immutbale
and the reference to it being immutable
16Riccardo Cardin

17. Programmazione concorrente e distribuita
IMMUTABILITY
17Riccardo Cardin

18. Programmazione concorrente e distribuita
IMMUTABILITY
 The final keyword on fields makes possibile
the guarantee on initialization safety
 A more limited version of the const in C++
 No reorder will be done by the compiler
 So, final fields can be accessed without additional
synchronization
 Better maintainability
 It’s time to have a look to an immutable class!
18Riccardo Cardin
Immutable objects can be used safely by any thread without additional
synchronization.
-- Brian Goetz

19. Programmazione concorrente e distribuita
IMMUTABILITY
19Riccardo Cardin
class OneValueCache {
private final BigInteger lastNumber;
private final BigInteger[] lastFactors;
// Do not use directly a mutable object to construct
// an immutable object
public OneValueCache(BigInteger i, BigInteger[] factors) {
lastNumber = i;
lastFactors = Arrays.copyOf(factors, factors.length);
}
// Do not late ‘escape’ an internal value of the immutable
// object. In this way no other code can maliciously modify
// that state
public BigInteger[] getFactors(BigInteger i) {
if (lastNumber == null || !lastNumber.equals(i))
return null;
else
return Arrays.copyOf(lastFactors, lastFactors.length);
}
}
}

20. Programmazione concorrente e distribuita
EXAMPLES
20Riccardo Cardin
https://github.com/rcardin/pcd-snippets

21. Programmazione concorrente e distribuita
REFERENCES
 Chap. 14 «Multithreading», Core Java Volume I - Fundamentals, Cay
Horstmann, Gary Cornell, 2012, Prentice Hall
 Chap. 3 «Sharing Objects», Java Concurrency in Practice, Brian
Goetz, 2006, Addison-Wesley Professional
 Atomic Access
https://docs.oracle.com/javase/tutorial/essential/concurrency/ato
mic.html
 Java theory and practice: Managing volatility
http://www.ibm.com/developerworks/library/j-jtp06197/
 Java theory and practice: Going atomic
http://www.ibm.com/developerworks/library/j-jtp11234/
 What is the difference of Atomic / Volatile / synchronize?
http://stackoverflow.com/questions/9749746/what-is-the-
difference-of-atomic-volatile-synchronize
21Riccardo Cardin