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

Concurrency Tutorial

Synchronized Methods

Intrinsic Locks

ConcurrentMap

compute

Code To Investigate

YUCK

class:	CheckThenActCourse.java	DEMO:
methods:	isSpaceRemaining add drop
package:	atomicity.course.studio
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

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

method: public boolean addIfSpace(Student student) (sequential implementation only)

method: public boolean drop(Student student) (sequential implementation only)

Code To Fix

get then put is not atomic

call compute
call compute
use ConcurrentHashMap
use ConcurrentHashMap
or say shoot!
or say shoot!

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

private final ConcurrentMap<String, Integer> map;

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;
}

Testing Your Solution

Correctness

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