Java Synchronization

Question 1

Why we need Java synchronization ?

Answer 1

When there is two or more threads, they might use same resource ,for example, if multiple threads try to write within the same file , they may corrupt data , one of threads can override data or while one thread try to open a file at the same time another thread may close it, so it produce unforeseen results.
So , there is need to synchronize action of multiple threads and make sure only one thread can access the resource at a given point in time.This is implemented using a concept called monitors

Question 2

What is monitor concept ?

Answer 2

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 .

Question 3

synchronized

Answer 3

used to synchronize java thread tasks.

Question 4

synchronized Syntax

Answer 4

synchronized(objectRef){

}

=> objectRef is a reference to an object associated with a monitor (which only one thread can hold a lock at a time)

Question 5

Example of synchronized use

Answer 5

We write two threads for a counter program
=> Without synchronized() the count is not in sequence but when we write print counter method inside synchronized(PrintCounterRef){
PrintCounterMethod()} the count is in sequence.

Question 6

Static Synchronization

Answer 6

when we use this type of synchronization , the thread on synchronization region have access and all other threads are blocked until the thread allowed ends

Question 7

Syntax of Static Synchronization

Answer 7

static synchronized returnType nameOfMethod( Type parameters) {
// code
}

Question 8

inter-thread communication

Answer 8

happens when threads exchange information

Question 9

How to achieve inter-thread communication ?

Answer 9

we use this methods:
*wait(): a thread wait until another thread invoke notify().
*notify():Wakes up a single thread waiting on this object’s monitor
*notifyAll(): Wakes up all threads that called wait() on the same object.

Question 10

Reentrant Monitor(ReentrantLock)

Answer 10

is mechanism for synchronization that allows same thread acquire same lock multiple times without causing deadLock.
=> It is one of deadLock solutions

Question 11

How Reentrant Monitor works ?

Answer 11

*Reentrancy: a monitor is re-entrant when its thread can re-enter same synchronized block or method it already holds lock on
*Hold count : when a thread acquire a reentrant lock it increments hold count , same thing happens when it acquires the same lock again and opposite happen when it release the lock.Hold count is set to 0 when a lock is fully released

Question 12

Reentrant Monitor Benefits

Answer 12

*Avoids DeadLock
*More flexible than synchronized