Thread safe creating and deleting file in Java

Question

Are methods Files.createFile() and Files.delete() thread safe? I have read in documentation that createFile() always an atomic operation but delete() is not. Should I somehow synchronize these blocks in my Java application and how? What atomic operation means for multihreading task?

Answer 1

a. What atomic operation means for multihreading task?

In context of multi-threading atomicity is the ability of a thread to execute a task in such a manner so that other threads have apparently no side-effect over the state varibles of that task when it was being executed by this thread.

File.createNewFile() :- For this method the state is existence or non existence of the file, when the thread was about to execute this method. Lets say that when this method was being executed by the thread the file did not exist. Now lets say that this method takes 5 ms of time to execute and create the file. So according to the concept of Atomicity no other thread should be able to create the same file(which was not existing before) during these 5ms otherwise the very first assumption of this thread about the state of the file will change and hence the output.

So in this case the executing thread does-this by obtaining a write lock over the directory where file is to be created.

Files.delete():- The Java doc for this method says

this method may not be atomic with respect to other file system operations. If the file is a symbolic link, then the symbolic link itself, not the final target of the link, is deleted.

the above statement says that this operation is also atomic but in case if this method is invoked on a symbolic link, the link is deleted and not the file. Which implies that the original file exists and file system operations on that file are feasible by other threads.

to determine if a file is a symlink see the reference:- determine symlink

b. Should I somehow synchronize these blocks in my Java application and how?

You need not handle any multi-threading scenarios in both the cases. However you can use the method mentioned in the link above to determine symlinks and handle that separately as you would wish. But no synchronization is required from your end for sure.

Answer 2

Do you mean File.createNewFile()?

Javadoc says: The check for the existence of the file and the creation of the file if it does not exist are a single operation that is atomic with respect to all other filesystem activities that might affect the file.

With other words, between the check if the file exist and the creation of the file will be no other file operation, changing the existence of the file.

If two threads want to create the same non existing file, only one will create the file and return true. The other thread will return false.

Usually you dont need to synchronize these operations but do a proper exception handling. Maybe other programs operate on your files too.