Atomicity

Motivation

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

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

get then put is not atomic
call compute
call compute
use ConcurrentHashMap
use ConcurrentHashMap
or say shoot!
or say shoot!

Exercise

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

Code To Fix: transfer

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

For this part of the studio we are presented with broken code. Despite using a thread-safe concurrent map:

private final ConcurrentMap<String, Integer> map;

it suffers from an atomicity race:

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

NOTE: do not be concerned about ConcurrentMap vs ConcurrentHashMap. The supplier's get method will return an instance of a thread-safe map. (It will, in fact, be an instance of ConcurrentHashMap.)

Course

Code To Investigate: CheckThenActCourse

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);
		}
	}
}

Code To Implement: Course

We will build a thread-safe Course class with methods which do not lead clients towards atomicity races.

class:	Course.java
methods:	addIfSpace drop
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.

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: Must be thread safe.

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