I have some old C++ code that uses stdio for input and output. The code also spawns new processes via forking. It remaps stdio to each new process so each session gets its respective data.
I am looking at using threads in Java to create child processes. However, I am stuck when it comes to finding out how to remap System.in , System.out and System.err to the child threads on creation.
Could anyone please point me in the right direction if this is possible?
The simple answer is not not write your code as directly accessing System.out/in/err. Instead have an InputStream and two OutputStreams passed in to your object's constructor. The object then works directly with those objects and does not depend on what they are actually mapped to. To get access to print() and println(), you will pass the OutputStream in to the constructor of a PrintStream.
Then based on what you actually want to do, you can call the constructor with System.out or some a FileOutputStream.
spawning a thread is not the same as spawning a process. when you spawn a thread in java (and c++) it shares the same memory space as the spawner (i.e. they share the same sdio streams). if you wanted to spawn a new process in java, you would use Runtime.exec() and then you would have to manually pipe the io into the new process, java does not support sharing io streams across process boundaries.
Java doesn't have fork(), but it has ProcessBuilder and Runtime.exec() for starting new processes (objects of the Process class). You can think of it as of fork()/exec() pair, but without the capability to perform something in between like dup2(). This means you can't redirect the child process' stdio, but you can explicitly write something into its stdin and read from its stdout and stderr, using the corresponding Process methods, or, to be precise, the corresponding methods of input/output streams returned by the getInputStream()/getOutputStream()/getErrorStream() methods of the Process class. This can be valid workaround if you want to have processes instead of threads.
If you want to use threads, then they all share the same stdio. You can redirect it, but it would be pointless since the redirection will affect all the threads. You can use some sort of imitation of IPC with threads using custom InputStream/OutputStream implementations, or you may wish to have a look at the PipedInputStream/PipedOutputStream pair. These can actually be used to set up something like an IPC pipe, probably with conjunction with BufferedInputStream/BufferedOutputStream to avoid excessive blocking.
I would recommend using separate processes, or else explicitly assigning a PrintStream to each thread.
However, it is possible to forward writes to System.out to different places for each thread, even though each thread sees the same object for System.out. In your startup code, you would call System.setOut(PrintStream) with a custom PrintStream. This PrintStream would override all the print and write methods. In these methods, it would look up the thread's PrintStream with an InheritableThreadLocal and forward the method call to it.
