Volatile is just another modifier like private , protected etc. It modifies the visibility of the variables and is generally more relevant in thread context.
Volatile when used on a variable ensures that every thread that is using it has the latest copy. It is important in multi threaded programming that shared variables which are frequently read and write by the threads are marked as volatile.
JVM is free to do anything with the variables and every thread can, actually does keep its own copy of variable meaning if a thread writes a new value to a variable, the other thread won't get that latest value.
private int x = 5;
Thread 1 uses it and makes it 8, but thread 2 will have its own copy as 5 and won't see new value 8.
This can of course leads to a lot of problems and I don't think there is any need to explain that.
Primary three reasons to use volatile when needed:
1. Every thread keeps it own copy of a variable, so if you want a shared variable with publish sort of functionality i.e. that every where its latest value is used, make it volatile.
2. CPU caches : if the variable is cached it isn't necessary that that value will be updated which could lead some odd values with some threads. Hence volatile.
3. Volatile ensures that all the operations happening on that variable are atomic. For eg changing a Long value is actually two write operations than one. If that variable is not volatile it might happen that when one thread was in middle of process of modifying that variable , some other thread accessed it and got some arbitrary value since write operation was not complete.
The volatile keyword ensures that before any other thread reads that value, the write operations are completed.
Synchronized is used to achieve locks on variables which are supposed to be accessed by the different threads. Where volatile ensures every thread has the latest value, it fails to maintain the write operation or perhaps the sequence of write operations done on a variable. This is where synchronized comes into play.
Synchronized should be implemented properly. Instead of making whole class synchronized , the actual data structures should be identified which needs to be synchronized and should be synchronized. However in some cases when that data structure is in use everywhere avoiding synchronizing every class methods is the only option.
Volatile when used on a variable ensures that every thread that is using it has the latest copy. It is important in multi threaded programming that shared variables which are frequently read and write by the threads are marked as volatile.
JVM is free to do anything with the variables and every thread can, actually does keep its own copy of variable meaning if a thread writes a new value to a variable, the other thread won't get that latest value.
private int x = 5;
Thread 1 uses it and makes it 8, but thread 2 will have its own copy as 5 and won't see new value 8.
This can of course leads to a lot of problems and I don't think there is any need to explain that.
Primary three reasons to use volatile when needed:
1. Every thread keeps it own copy of a variable, so if you want a shared variable with publish sort of functionality i.e. that every where its latest value is used, make it volatile.
2. CPU caches : if the variable is cached it isn't necessary that that value will be updated which could lead some odd values with some threads. Hence volatile.
3. Volatile ensures that all the operations happening on that variable are atomic. For eg changing a Long value is actually two write operations than one. If that variable is not volatile it might happen that when one thread was in middle of process of modifying that variable , some other thread accessed it and got some arbitrary value since write operation was not complete.
The volatile keyword ensures that before any other thread reads that value, the write operations are completed.
Synchronized is used to achieve locks on variables which are supposed to be accessed by the different threads. Where volatile ensures every thread has the latest value, it fails to maintain the write operation or perhaps the sequence of write operations done on a variable. This is where synchronized comes into play.
Synchronized should be implemented properly. Instead of making whole class synchronized , the actual data structures should be identified which needs to be synchronized and should be synchronized. However in some cases when that data structure is in use everywhere avoiding synchronizing every class methods is the only option.
This article gives a useful explanation of how volatile and synchronized address different problems in Java multithreaded programming. The discussion of thread-local copies and visibility makes it easier to understand why a shared variable may need special handling when multiple threads read and modify it.
ReplyDeleteThe distinction between making a value visible across threads and controlling access to shared data is particularly important. For developers building Java applications as part of a Java Full Stack Course, understanding these concepts can help when working with concurrent backend components and application logic.
The article also makes a good point about using synchronization carefully rather than automatically synchronizing an entire class. Identifying the shared data structures that actually require controlled access can lead to more focused and maintainable multithreaded code. These concepts are especially relevant to Backend Training, where thread safety and concurrent processing can affect the reliability of server-side applications.
ReplyDelete