volatile关键字与同步Java块的重要性是什么？

Question

请问谁能告诉我Java中的volatile关键字是什么，它的主要功能，如何在同步块中使用它，如果将其从块中完全删除，会发生什么？

Answer 1

Let's start with what's a volatile variable in java:

Volatile keyword in Java is used as an indicator to Java compiler and Thread that do not cache value of this variable and always read it from main memory

so what is the benefit of reading the variable value from the memory, consider the following sample code:

public class Singleton{
private static volatile Singleton _instance; //volatile variable 

public static Singleton getInstance(){

if(_instance == null){
        synchronized(Singleton.class){
          if(_instance == null)
          _instance = new Singleton();
        }

}
return _instance;

}

in the previous sample:

1) We are only creating instance one time

2) We are creating instance lazily at the time of first request comes.

If we do not make _instance variable volatile then Thread which is creating instance of Singleton is not able to communicate other thread, that instance has been created until it comes out of the Singleton block, so if Thread A is creating Singleton instance and just after creation lost the CPU, all other thread will not be able to see value of _instance as not null and they will believe its still null.

Conclusion:

volatile keyword will be more useful. When multiple threads using the same variable, each thread will have its own copy of the local cache for that variable. So, when it's updating the value, it is actually updated in the local cache not in the main variable memory. The other thread which is using the same variable doesn't know anything about the values changed by the another thread

Answer 2

Volatile keyword in Java is used as an indicator to Java compiler and Thread that do not cache value of this variable and always read it from main memory.

What will happen if we remove it from a synchronized block?