同步方法中的等待与不等待（java）

Question

Hi I'm pretty new to Java and now I'm getting into java concurrency. And I have a little doubt about Synchronized methods: i have seen that I can get the same results using an If else inside a Synchronized method, checking every time If the condition to do an action is fullfilled, as using a wait / notify approach.

Since i get the same result I'm wondering If the If else approach has any advantages or disadvantages over t'he wait and notify approach? I supose that efficiency will be a disadvantage, since If is always checking the condition, Who le wait Just stops and waits for notify. But are any other advantages or disadvantages?

Thx!

Answer 1

You are mixing two concepts. If-Else vs Wait-Notify are totally different. You want two threads to communicate with each-other that is where Wait-Notify would be used while if-else is general conditional statement.

You cannot have two threads communicate with each other simply using if-else condition. You can write your code that makes it look like it does however you are simply not allowing threads to interact with each other.

Moreover it can lead to undesirable consequences/computational states. Sooner or later you would have hotchpotch code.

Answer 2

synchronized block makes the code thread safe. You would want to use wait() and notify() or notifyAll() if you want to be more efficient.

For example if your shared resource is a list, multiple threads share. If you put it in synchronized block of a monitor then threads will constantly jump in and run the code, during context switches. Even if the list is enpty!!

The wait() is hence used on the monitor (the object inside the synchronized(..) ) as a mechanism to 'tell' all threads to chill out and stop using CPU cycles until further notice or notifyAll() .

synchronized(monitor) {

    while( list.isEmpty() ) 
       monitor.wait();
    doSomething(...)
}

In the above example, doSomething() will be executed only when the list is not empty, after another thread executed notify() or notifyAll() somewhere else in the code.

• read more why use while surrounding wait()

BUT with the following code:

synchronized(monitor) {

    if(!list.isEmpty())
        doSomething(...)
}

When a thread comes in to the synchronized block, there are 3 possible scenarios:

1. The list is empty: doSomething() will not be executed.
2. The list is NOT empty: doSomething() may be executed properly, or...
3. If there was a context switch right after the if and before doSomething , and the other thread got all list's items out, after another context-switch out thread will execute doSomethig() on an empty list.

So, just to sum everything up, if you use wait/notify, you guarantee more efficient code! thread will not work when they don't need to.