Java Thread Synchronization With Example

Java Thread Synchronization:

When synchronization  is required?

synchronization  is reqired when two threads are accessing same resource if you want to protect that resource the synchronization is required.

How many ways we can achieve synchronization ?

there is only two ways we can achieve 1.method level2.block level

how to achieve synchronization ?

using synchronization  key word in javasynchronized keyword was only way to provide synchronization of shared object in Java. Any code written by using  synchronized block or enclosed inside synchronized method will be mutually exclusive, and can only be executed by one thread at a time. You can have both static synchronized method and non static synchronized method and synchronized blocks in Java

How synchronization worked  in java?

synchronization  works on based on object lock mechanism.every object in java has a built in lock that only comes to play when that object has synchronization  method codewhen thread enters a synchronized non-static method,

we automatically acquire the lock associated with current instance of the class there is only one lock for object the one thread pick the lock means no other thread can't pick the lock until current thread release the lock.
using blocks with in method you can restrict some code only for synchronization  remaining code in the method can accessible for other thread using this block performance will be goodwhy because only piece code in the method only synchronized.

Difference between static synchronization  and non-static synchronization ?

static synchronization  is class level lockall instance of that particular class have only one lock
example:Student s1=new Student();
Student s2=new Student();both s1 and s2 have only one lock in class level lock in non-synchronization  in case of non-static synchronization  both s1 and s2 have different locksthat mean one thread only need to modify s1 object it not allow any other threadbut two threads can change both s1 and s2 parallel in nonstatic-synchronization  

the static method lock on Class instance while non-static method lock on this instance


*only method or block can be synchronized*each object has only one lock*a class can have both synchronization  and non-synchronization 

Consider one example raju and rani have one join account account balance is 50.and they are accessing same time what will happen?
see this example

package com.multi;
public class ProblemWithTh implements Runnable { 

Account account=new Account();

public static void main(String[] args) { 

ProblemWithTh problemWithTh=new ProblemWithTh();
Thread thread1=new Thread(problemWithTh); 

Thread thread2=new Thread(problemWithTh); thread1.setName("raju");

thread2.setName("rani");

thread1.start();

thread2.start(); 

}

@Override public  void run()

 {  

for(int i=0;i<=5;i++){  

makeWithDraw(10);  

if(account.getBalance()<0){ System.out.println("overdrwannnn"); 

}  

}  

}

private   void makeWithDraw(int i) {  

if(account.getBalance()>=i){  

System.out.println(Thread.currentThread().getName()+"::::is going to with draw current balance is"+account.getBalance()); account.withdrawl(i); 

System.out.println("::after "+Thread.currentThread().getName()+"::with drawl account balance:::::::"+account.getBalance()); 

try {

Thread.sleep(500);  

} catch (InterruptedException e) {

// TODO Auto-generated catch block e.printStackTrace(); } 

}

else{ System.out.println("no nougat money for"+Thread.currentThread().getName()); 

}  

}

}

Output:
raju::::is going to with draw current balance is50

::after raju::with drawl account balance:::::::40

rani::::is going to with draw current balance is50

::after rani::with drawl account balance:::::::30

raju::::is going to with draw current balance is30::

after raju::with drawl account balance:::::::20

rani::::is going to with draw current balance is30

::after rani::with drawl account balance:::::::10

rani::::is going to with draw current balance is10

::after rani::with drawl account balance:::::::0

raju::::is going to with draw current balance is10

::after raju::with drawl account balance:::::::-10

overdrawn

overdrawn

no enough money for rajuoverdrawnno enough money for ranioverdrawnno enough money for rajuoverdrawnno enough money for ranioverdrawnno enough money for ranioverdrawnno enoug money for rajuoverdrawn


raju and rani  using one account if one preforming  withdrawal or any other operation another thread need to wait in above example
i did n't use any synchronization that why in output after two threads are accessing simultaneously.
because of this reason after with drawl of raju for rani it displaying wrong amountrani will shock before withdrawal account  balance is 50 after withdrawal 10 balance is 30 :)

make sure that output you will not be same all time two are different. based on thread scheduler selection threads will get chance that's why out put will not be same all time.

just i added synchronized word on which method i need protect 

by accessing multiple threads.

private   synchronized void makeWithdraw(int i) {

if(account.getBalance()>=i){

System.out.println(Thread.currentThread().getName()+"::::is going to with draw current balance is"+account.getBalance()); account.withdrawl(i);  

System.out.println("::after "+Thread.currentThread().getName()+"::with drawl account balance:::::::"+account.getBalance()); try { Thread.sleep(500);

}

 catch (InterruptedException e) {

// TODO Auto-generated catch block e.printStackTrace(); }

}

else{ System.out.println("no enough money for"+Thread.currentThread().getName()); 

}  

}



output:rani::::is going to with draw current balance is50

::after rani::with drawl account balance:::::::40

rani::::is going to with draw current balance is40

::after rani::with drawl account balance:::::::30

rani::::is going to with draw current balance is30

::after rani::with drawl account balance:::::::20

raju::::is going to with draw current balance is20::

after raju::with drawl account balance:::::::10

raju::::is going to with draw current balance is10::

after raju::with drawl account balance:::::::0

no enough money for rajuno enough money for rani

no enough money for rani

no enough money for rani

no enough money for raju

no enough money for raju

no enough money for raju

problem is solved using synchronized key word it allow only one thread instance at a time
while rani withdrawal raju will wait raju withdraw rani will wait :)