Multi tool use

Use sychronized or not below code generating same hashcode for all threads. How?

"we make the global access method synchronized so that only one thread can execute getInstance method at a time" without using synchronized keyword below code acting like threadsafe already.

Car.java

public class Car {

private static Car car;

private Car() {
// TODO Auto-generated constructor stub
}

public static Car getInstance(){
if(car==null){
car=new Car();
}
return car;
}
}


Test.java

public class Test {
public static void main(String args) {
Thread t1=new Thread(new Runnable() {

@Override
public void run() {
System.out.println(Thread.currentThread().getName()+" Running");
System.out.println(Car.getInstance().hashCode());
System.out.println(Thread.currentThread().getName()+" Finishing");
}
});
t1.start();

Thread t2=new Thread(new Runnable() {

@Override
public void run() {
System.out.println(Thread.currentThread().getName()+" Running");
System.out.println(Car.getInstance().hashCode());
System.out.println(Thread.currentThread().getName()+" Finishing");
}
});

t2.start();

Thread t3=new Thread(new Runnable() {

@Override
public void run() {
System.out.println(Thread.currentThread().getName()+" Running");
System.out.println(Car.getInstance().hashCode());
System.out.println(Thread.currentThread().getName()+" Finishing");
}
});
t3.start();
}
}


Output

Thread-0 Running
1828451009
Thread-0 Finishing
Thread-1 Running
1828451009
Thread-1 Finishing
Thread-2 Running
1828451009
Thread-2 Finishing


Without using synchronized method hashcode are same in all thread case, how?

t1.start(); t2.start(); t3.start();

2 Answers
2

If i start all threads at a time

t1.start();
t2.start();
t3.start();


output is varying sometime same hashcode sometimes its different

Thread-1 Running
Thread-2 Running
Thread-0 Running
183297081
Thread-2 Finishing
183297081
Thread-1 Finishing
183297081
Thread-0 Finishing


Thread-0 Running
Thread-2 Running
Thread-1 Running
678111939
1828451009
1828451009
Thread-2 Finishing
Thread-1 Finishing
Thread-0 Finishing


Thread-0 Running
Thread-2 Running
Thread-1 Running
678111939
183297081
558529301
Thread-1 Finishing
Thread-2 Finishing
Thread-0 Finishing


Unable to understand this behaviour

Thread scheduling is part of OS, if you execute the code several times you find differences. If you want your code thread safe in all cases then you have to use synchroized block with double locking.

public class Car {

private static volatile Car car;

private Car() {
// TODO Auto-generated constructor stub
}

public static Car getInstance(){
if(car==null){
synchronized(Car.class) {
if(car==null)
car=new Car();
}
}
return car;
}
}


Note : Defining the whole method synchronized having performance
issue.

https://javarevisited.blogspot.com/2014/05/double-checked-locking-on-singleton-in-java.html

Car

volatile

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.