Java Multi-Threading

Question 1

Data Race

Answer 1

Two or more threads in a single process access the same memory location concurrently and, at least one of the accesses is for writing, and the threads are not using any exclusive locks to control their accesses to that memory

Question 2

Deadlock

Answer 2

Two threads hold locks on different resources, each waiting indefinitely for the other to release its lock

Question 3

Synchronized method

Answer 3

A method that can only be executed by a single thread at a time

Question 4

Method ‘lock’

Answer 4

enclosed in a try/finally because if the ‘try’ code throws an exception, the lock must be unlocked or it will block other threads forever. Only one thread can lock/unlock at a time (for each lock)

Question 5

Semaphore

Answer 5

Controls access to shared resources through the use of a counter. If counter > 0, access is allowed, otherwise denied. If counter is < 0, thread will be blocked until a different thread releases their permit.

Question 6

Thread vs Process

Answer 6

Thread: smallest units of the particular process. Has the ability to execute different parts of the program at the same time.
Process: refers to a program that is in execution

Question 7

User Thread vs Daemon Thread

Answer 7

User thread: JVM waits for user threads to finish their tasks before termination. Created by user to execute tasks concurrently. High-priority, required to run in foreground
Daemon thread: JVM does not wait for daemon threads to finish their tasks before termination. Created by JVM. Low-priority used for garbage collection, releasing memory of unused objects, etc.

Question 8

Wait() vs Sleep() methods

Answer 8

Wait - causes current thread to wait and sleep. Releases the lock for inter-thread communication.
Sleep - pauses or stops the execution of the current thread. Does not release lock.

Question 9

Start() vs Run()

Answer 9

Start - starts the execution of a newly created thread. this newly created thread executes the task that is kept in the run method
Run - starts the execution of the same thread. No new thread is created

Question 10

Join()

Answer 10

pauses the execution of a thread until thread 1 in thread.join(thread1) is dead or completed.

Question 11

Garbage Collection

Answer 11

automatic memory management. Three phases - mark, delete, compact/copy. A garbage collector finds objects that are no longer required by the program and deletes them to free up memory space

Question 12

Volatile variable

Answer 12

Allows a field to become visible to all readers (threads) after a write operation completes on it. Without volatile, some threads could see some non-updated value.

Question 13

Synchronized Block

Answer 13

Locks part of the program between parentheses after the synchronized keyword. Preferred to synchronized method because it doesn’t lock up the whole method.

Question 14

Livelock

Answer 14

state of threads changes between one another without making any progress. Threads are not blocked but their execution is stopped due to the unavailability of resources

Question 15

BlockingQueue

Answer 15

blocks a thread from trying to insert into a full queue until another thread takes from it, and vice versa

Question 16

CyclicBarrier

Answer 16

enables a set of threads to wait for each other until they reach a common execution point, then lets them all continue

Question 17

CountDownLatch

Answer 17

enables main threads to wait until mandatory operations are performed and completed by other threads.

Question 18

What is multi-threading?

Answer 18

a process of executing multiple threads simultaneously. Consumes less memory. Threads share the same address space, thread is lightweight, cost of communication between processes is low

Question 19

What is a thread?

Answer 19

a subprocess that shares the resources of a process.

Question 20

5 states of a Thread

Answer 20

New, runnable, running, waiting/blocked, dead/terminated

Question 21

Functions used to perform inter-thread communication?

Answer 21

notify, wait, notifyAll

Question 22

Context Switching

Answer 22

a feature through which the current state of a thread is saved for it to be restored and executed later. This allows multiple processes to share the same CPU

Question 23

How to avoid deadlock?

Answer 23

avoid nested thread locks and provide locks to only one thread at a time. Use thread.join(thread2)

Question 24

Lock-free data structure

Answer 24

allows individual threads to starve but guarantees at least one thread always making progress. All wait-free algorithms are lock-free

Question 25

Four phases of lock-free algorithms

Answer 25

complete one’s own operation, assist an obstruction operation, abort an obstructing operation, wait.

Question 26

Atomic variable and example

Answer 26

a = 28 (a an int) is an atomic operation but a++ is not because it requires a read of the value ‘a’, an incrementation, and then a write to ‘a’. Atomic operation incrementAndGet(i) solves this dilemma

Question 27

Avoid data-race without java.util.concurrent

Answer 27

synchronized method/statement. Each object in Java is associated with a monitor which a thread can lock or unlock. Only one thread at a time can hold a lock on a monitor.