Multithreading

Question 1

Thread vs Runnable

Answer 1

you can implement Runnable and extend from another class as well

Question 2

start() vs run()

Answer 2

start a Thread
vs
just execute run() method

Question 3

synchronization of java blocks vs methods

Answer 3

synchronized method acquires a lock on the whole object.
This means no other thread can use any synchronized method
in the whole object while the method is being run by one thread.

vs

synchronized blocks acquires a lock in the object between
parentheses after the synchronized keyword. Meaning no other
thread can acquire a lock on the locked object until the synchronized block exits.

Question 4

wait()/notify() usage

Answer 4

Object.wait() – to suspend a thread
Object.notify() – to wake a thread up

Question 5

sleep() vs wait()

Answer 5

wait() – until call notify(), notifyAll() from object
sleep() – until at least time expire or call interrupt().

Question 6

atomic operations
(internal implementation of atomic)

Answer 6

through CPU instructions, volatile and CAS

Question 7

CAS operation

Answer 7

CAS operation (compare-and-swap):
memory location M
existing expected value: A
new value B

The CAS operation updates atomically the value in M to B,
but only if the existing value in M matches A, otherwise no action is taken.

If so - handle failure manually

Question 8

volatile
should we rely on volatile?

Answer 8

it cannot be cached into the computer’s(microprocessor) cache memory by any thread
If there is a write operation going on a volatile variable, and suddenly a read operation is requested, it is guaranteed that the write operation will be finished prior to the read operation.

race condition is possible if previous value is needed to define next value
several threads could read same value

Question 9

Thread local? What are they needed for?

Answer 9

variables that can only be read and written by the same thread

Question 10

Does child thread see the value
of parent thread local?

Answer 10

If you mean InheritableThreadLocal
(extending ThreadLocal), then yes, each
child thread will have the initial default value
to be the same as the parent thread value.
But any changes by the child thread will be local to the child.

Question 11

Deadlock definition
example

Answer 11

deadlock is a state in which each member of
a group of actions, is waiting for some other
member to release a lock

Question 12

Livelock definition
example

Answer 12

special case of resource starvation where two
processes follow an algorithm for resolving a deadlock
that results in a cycle of different locked states because
each process is attempting the same strategy to avoid the lock.

Question 13

Starvation definition
example

Answer 13

situation where a thread is unable to gain regular
access to shared resources and is unable to make progress.
This happens when shared resources are made unavailable
for long periods by “greedy” threads.

Question 14

Race condition definition
exampple

Answer 14

when two or more threads can access
shared data and they try to change it at the same time

Question 15

Executors
Types

Answer 15

ExecutorService can execute Runnable and Callable tasks

cachedThreadPool creates new threads as needed, but will reuse previously constructed threads when they are available
SingleThreadPool single worker thread operating off an unbounded queue, and uses the provided ThreadFactory to create a new thread when needed.
FixedThreadPool reuses a fixed number of threads

If you want to process all submitted tasks in order of arrival, just use newFixedThreadPool(1)
If you want to optimize performance of big computation of recursive tasks, use ForkJoinPool or newWorkStealingPool
If you want to execute some tasks periodically or at certain time in future, use newScheduledThreadPool

Question 16

Happens-before

Answer 16

When thread A enter sync for operation block and thread B is waiting. Then JVM guarantee A happens before B when thread B will know correct all values inialized by first thread before A operation. Other values JVM can reorder by hardware specific.
When A and B in the diffenent processes then JVM guarantee communication from A to B.
Transitivity of ““happens before””: A -> B and B -> C then A -> C

Question 17

volatile vs atomic

Answer 17

Vollatile - one atomic operation. Atomics - few operations can be atomic.
volatile read vs getAndAdd(7)

Question 18

Garbage definition plus example

Answer 18

unused objects or objects that are out of reach

Question 19

Execution order
instruction-reordering-problems

Answer 19

In theory, the JVM could reorder
the instructions but the outcome of the
method has to remain correct.
it never reorders code around native instructions.

Question 20

Atomicity of long and double assignment
operations

Answer 20

single write to a non-volatile long or double
value is treated as two separate writes:
one to each 32-bit half

volatile grants atomicity

Question 21

Lock-free operations, how to create
lock-free implementation of field reassignment

Answer 21

The Lock-Free property guarantees that at least some thread is doing progress on its work.
atomic

Question 22

What is Fork-Join?

Answer 22

provides tools to help speed up parallel processing
recursively breaking the task into smaller independent subtasks
results of all subtasks are recursively joined into a single result

Question 23

ForkJoinPool

Answer 23

Worker threads can execute only one task at the time,
but the ForkJoinPool doesn’t create a separate thread
for every single subtask. Instead, each thread in the pool
has its own double-ended queue (or deque, pronounced deck) which stores tasks.

Question 24

What is Phaser? (Java 7)

Answer 24

barrier on which the dynamic number of threads need to wait before continuing execution

Question 25

Reentrant lock

Answer 25

As the name says, ReentrantLock allow
threads to enter into lock on a resource
more than once. When the thread first enters
into lock, a hold count is set to one.
Before unlocking the thread can re-enter into
lock again and every time hold count is
incremented by one. For every unlock request,
hold count is decremented by one and when
hold count is 0, the resource is unlocked.

Question 26

work-stealing algorithm

Answer 26

free threads try to “steal” work from deques of busy threads.

Question 27

thread pools

Answer 27

ForkJoinPool
FixedThreadPool
CachedThreadPool
SingleThreadExecutor

Question 28

CountDownLatch

Answer 28

thread which calls CountDownLatch.await() will wait until count reaches zero or it’s interrupted by another thread.
other threads are required to count down by calling CountDownLatch.countDown() once they are completed or ready.

Question 29

CyclicBarier

Answer 29

synchronizer that allows a set of threads to wait for each other
to reach a common execution point, also called a barrier.

Question 30

access barrier

Answer 30

A barrier for a group of threads or processes
in the source code means any thread/process
must stop at this point and cannot proceed
until all other threads/processes reach this barrier.

Question 31

Exchanger

Answer 31

A synchronization point at which threads can pair and swap elements within pairs.

Question 32

Monitor

Answer 32

mechanism to control concurrent access to an object
prevents Threads 1 and 2 accessing the monitored (synchronized) section at the same time.

Question 33

Semaphore

Answer 33

to limit the number of concurrent threads accessing a specific resource.

Question 34

ForkJoinPool vs Executor Framework

Answer 34

1 accepts ForkJoinTask vs FutureTask
2 - ForkJoinPool uses work stealing pattern
executors

Question 35

Mutex

Answer 35

acts similarly to a binary semaphore