Java concurrency

Java Concurrency
Towards a better parallel balanced life …
● By
● - Srinivasan.raghavan@oracle.com
- Vaibhav.x.choudhary@oracle.com

Java Releases
JDK 1.5:
- Generics
-Concurrency
-Auto-boxing
J2SE 6:
- Collection Framework
enhancement
-Drag and Drop
-Improve IO support
J2SE 7:
- Concurrency Utilities
- Swing, -Fork/Join Pool ,
LWC&HWC
- G1
J2SE 8:
- Lambda expressions
- Stream Package
-More concurrency
Updates

Agenda
● Basics of concurrency
● Java Concurrent Package
● Reentanrant Lock / Performance
● Concurrent Collections
● Insight of Fork/Join
● Concurrency in Java 8
● Concurrent Collections java 8 updates

Basics of concurrency
● Amdhal's law
– Speedup <= 1/ (F + (1-F)/N)
● Age of multi-core processors and requirement
of parallel programming in mainstream.
● Writing concurrent programing is hard and
writing correct concurrent program is even
harder.
● Research in the field has been lead by Java
designers and the concurrency interest group

General Design Principles

Its the mutable state stupid

Design for immutability

Make field final unless they need to be mutable

Encapsulate to manage complexity

Guard all variables in an invariant with same
lock

Document the synchronization policy

Java Concurrency Constructs
● In Java 5 a set of synchronisers was built giving
more power than intrinsic locking
● Atomic operations using Compare and swap
● Non-Blocking Collections

Intrinsic locks vs Reentarant Locks
● Intrinsic locks are very low level constructs
● Reentarant Locks offers more features
● Fair queuing
● TryLock
● Explicit Condition Queue

Dead Locks avoidance with try locks

Dead Lock can be avoided using the try locks
●
Try locks can used for Probabilistic lock acquire
of the lock and exponential back-off and spin lock
●
Classic Transfer money Deadlock Problem
Typical traffic problem.

Condition Queues
●
Intrinsic locks can also work as condition queues
with wait and notify
●
Problem occurs when Intrinsic condition queue
may be used with more than one condition
predicate
●
Reentarant locks provides explicit condition
queues which offers more features
●
Fairness in access of the lock which reduce
contention
●
Help us create queues per condition predicate

Condition Queues
● Where a Lock replaces the use of synchronized
methods and statements, a Condition replaces
the use of the Object monitor methods.
● Consider a case of bounded buffer
– final Condition notFull = lock.newCondition();
– final Condition notEmpty = lock.newCondition();
● ArrayBlockingQueue provides the functionality
● A typical consumer/producer problem with
BlockingQueue.

Performance
Can Reentarant lock replace synchronized?
– No , There are big issues when used carelessly
– Use only when Fair queuing, probabilistic locking
and explicit condition queues are required

Java Synchronizers
● Executors
– Execution tasks in thread pools.
– Cyclic barrier ,Future task ,Semaphores
,Countdown latch
● Non-blocking algorithms
– AtomicInteger
– Compare and Swap (CAS)
– Writing a correct non-blocking algorithm are tough.
● Find out the code.

Concurrent Collection
● Copy on write array List
● Concurrent hash-map
● Blocking queues and producer – consumer
pattern

Fork/Join Framework
● New parallel mechanism for compute intensive
tasks, the fork-join framework.
● Runs on work-stealing algorithm.
● Approach
– Partition into subproblems.
– Create subtaks.
– Fork subtasks.
– Join subtaks.
– Compose solution.

Parallelism Built In
● Parallelism is the feature built in Java 8
● U can iterate,filter, sort ,map reduce an array in
parallel
● The streams api makes heavy use of the Fork-
Join framework
● The entire set of features offered by the streams
api can be made to work in parallel by just
asking for the parallel stream
● The inherit complexity of Fork-Join framework is
handled under the hood

Java 8 Concurrency Changes
● Addition of fork-join common pool
● ConcurrentHashmapv8 which supports bulk
operations in parallel
● Completable future
● Changes to the Future Task implementation

Concurrent Hashmap
● Started as a replacement to Hashtable.
● Implements Lock stripping
● Doesn’t throw concurrent modification
exception
● Java 8 concurrenthashmap does a lot
● Built in support for parallelism
● More in the code .

Java concurrency

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