Atomicity

Motivation

Many race conditions can be prevented by proper encapsulation avoiding check-then-write and read-then-modify-then-write patterns.

Background

Take a look at the synchronized and lock sections of the reference page

Concurrency Tutorial

Synchronized Methods

Intrinsic Locks

ConcurrentMap

compute

Song to Recall

Frère Jacques, Frère Jacques,		Get then put is not atomic
Dormez-vous? Dormez-vous?		Call compute. Call compute.
Sonnez les matines! Sonnez les matines!		Use ConcurrentHashMap! Use ConcurrentHashMap!
Ding, dang, dong. Ding, dang, dong.		Or say shoot. Or say shoot.

SuspectWordCount

Code To Fix: countWords

Warning:Do NOT use synchronized here. ConcurrentMaps are already thread safe.

fix the code to remove the atomicity race.

class:	SuspectWordCount.java
methods:	countWords
package:	atomicity.wordcount.exercise
source folder:	student/src/main/java

method: public static Map<String, Integer> countWords(Iterable<String> words) (parallel implementation required)

StockPortfolio

note: somewhat out of date video.

Code To Fix

For this part of the exercise we are presented with broken code. HashMap is not thread-safe.

private final Map<String, Integer> map;
public StockPortfolio() {
	this.map = new HashMap<>();
}

Luckily, ConcurrentHashMap ensures that all of its methods are thread-safe.

Warning:Do NOT use synchronized anywhere in this class. Leverage ConcurrentMap's thread safety.

Our work is not done here. ConcurrentHashMap is unable to prevent the read-modify-write atomicity race in transfer. How will you fix the atomicity race?

private int transfer(String listingSymbol, int deltaShareCount) {
	Integer oldValue = map.get(listingSymbol);
	Integer newValue;
	if (oldValue != null) {
		newValue = oldValue + deltaShareCount;
	} else {
		newValue = deltaShareCount;
	}
	map.put(listingSymbol, newValue);
	return newValue;
}

class:	StockPortfolio.java
methods:	transfer
package:	atomicity.stockportfolio.exercise
source folder:	student/src/main/java

method: private int transfer(String listingSymbol, int deltaShareCount) (thread-safe required)

CheckThenActCourse and Course

For this section we will investigate class CheckThenActCourse which, although thread-safe, presents a check-then-act API which unfortunately leads clients to atomicity races. We will implement an improved class Course which avoids this issue through better design.

CheckThenActCourse

Code To Investigate

Warning:Although the class below is technically thread-safe, it offers check-then-act usage which leads clients to atomicity races.

class:	CheckThenActCourse.java	DEMO:
methods:	isSpaceRemaining add drop
package:	atomicity.course.demo
source folder:	src//java

public class CheckThenActCourse {
	private final Collection<Student> students;
	private final int limit;

	public CheckThenActCourse(int limit, Supplier<Collection<Student>> collectionSupplier) {
		this.students = collectionSupplier.get();
		this.limit = limit;
	}

	public int getLimit() {
		return this.limit;
	}

	public boolean isSpaceRemaining() {
		synchronized (this.students) {
			return this.students.size() < this.limit;
		}
	}

	public void add(Student student) {
		synchronized (this.students) {
			this.students.add(student);
		}
	}

	public boolean drop(Student student) {
		synchronized (this.students) {
			return this.students.remove(student);
		}
	}
}

Course

We will build a thread-safe Course class with an addIfSpace(student) method in place of the separate methods isSpaceRemaining() to check and add(student) to act from CheckThenActCourse. This better design does not lead clients towards atomicity races.

Code To Investigate

drop

method: public boolean drop(Student student) (thread-safe required)

Removes the student from the course if the student was enrolled (the remove method returns just the status you need).

NOTE: This method is thread safe.

	public boolean drop(Student student) {
		synchronized (this.students) {
			return this.students.remove(student);
		}
	}

NOTE: This method synchonizes on the students Collection.

Code To Implement

addIfSpace

class:	Course.java
methods:	addIfSpace
package:	atomicity.course.exercise
source folder:	student/src/main/java

method: public boolean addIfSpace(Student student) (thread-safe required)

Adds the student if there is space under the limit specified to the constructor and returns whether or not the add actually occurred.

NOTE: Must be thread safe.

NOTE: This method must synchronize on the same Object as drop(student).

Testing Your Solution

Correctness

class:	_AtomicityTestSuite.java
package:	atomicity
source folder:	testing/src/test/java

class:	_WordCountAtomicityTestSuite.java
package:	atomicity.wordcount.exercise
source folder:	testing/src/test/java

class:	_StockPortfolioAtomicityTestSuite.java
package:	atomicity.stockportfolio.exercise
source folder:	testing/src/test/java

class:	_CourseAtomicityTestSuite.java
package:	atomicity.course.exercise
source folder:	testing/src/test/java