synchronized keyword not making the function synchronized: Java [duplicate]

Question

I was practicing multithreading in java and wrote the below code

class Printer { synchronized void printHi(String x) { System.out.println(x); } } class MyThread extends Thread { Printer objm; MyThread(Printer a) { objm = a; } @Override public void run() { for (int i = 0; i < 10; i++) { objm.printHi("MyThread" + i); } } } class YourThread extends Thread { Printer objy; YourThread(Printer a) { objy = a; } @Override public void run() { for (int i = 0; i < 10; i++) { objy.printHi("YourThread" + i); } } } public class test { public static void main(String[] args) { Printer ob = new Printer(); MyThread mt = new MyThread(ob); YourThread yt = new YourThread(ob); mt.start(); yt.start(); } }

Sometimes I gets the output as:

MyThread0 YourThread0 MyThread1 YourThread1 MyThread2 YourThread2 MyThread3 YourThread3 YourThread4 MyThread4 MyThread5 MyThread6 YourThread5 MyThread7 YourThread6 MyThread8 MyThread9 YourThread7 YourThread8 YourThread9

Which is asynchronous. Why is it so even after making the function printHi() as synchronized?

sequential is opposite to concurrent. You want first but using second. — rkosegi
– rkosegi, Commented Aug 26, 2021 at 18:38

Gabe Sechan · Accepted Answer · 2021-08-26 18:28:43Z

Synchronized means only one thread can be running a block of code marked synchronized on the same lock object (instance of the object for the case of a synchronized method). It does not mean threads will run in order. It is perfectly correct for the two threads to enter those synchronized blocks in any order, and any number of times before the next thread does. What you want is much more difficult and beyond what a simple synchronized block can do.

If you want it to always go Thread A, Thread B, Thread A, Thread B- first I'd question that those two things should actually be separate threads. Wanting things to run sequentially like that is the number one sign you aren't asynchronous and shouldn't be multiple threads. But if they do, you're probably best off with two threads with message handlers, sending messages to each other about when they're allowed to run. Or using semaphores to signal each other. It's hard to give exact advice because the problem here is obviously a trivialized version of something harder, and without the details the right implementation is hard to guess.

Silvio Mayolo · Accepted Answer · 2021-08-26 18:27:20Z

You're synchronizing the printHi function call. That means two instances of printHi on the same object won't run at the same time. That's great. It means you'll never get output like

MyThYourThreadread77

(Side note: Java's printing primitives are already somewhat synchronized if I recall correctly, so that shouldn't happen anyway. But for demonstrative purposes, it'll do)

However, your loop is not synchronized. If you want the whole loop to only happen in one thread at a time, remove synchronized from print and write your loop as

synchronized (objm) { for (int i = 0; i < 10; i++) { objm.printHi("MyThread" + i); } }

(and the same for objy in YourThread)

There's still no guarantee on which one will happen first, but the two loops won't interrupt each other in this case.

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