问题
If I try to evaluate the following code in my emacs cider-repl, nil is returned, as expected, but none of the printing takes place in the repl buffer or console. How can I make this print out as intended?
(dotimes [i 5]
(.start
(Thread.
(fn []
(Thread/sleep (rand 500))
(println (format "Finished %d on %s" i (Thread/currentThread)))))))
;=> nil
This works fine, however:
(println (format "Finished 1 on %s" (Thread/currentThread)))
;=> Finished 1 on Thread[nREPL-worker-18,5,main]
----------- mini-buffer -----------------
nil
回答1:
The behavior of println is to use a dynamically bound var called *out* as its output stream. emacs dynamically binds *out* to go to the repl buffer for code evaluated in the repl buffer, but if you create a thread, that thread's *out* gets the root binding of *out*, which in the case of cider will not be the repl buffer.
If you started the repl using cider-jack-in, when you look at you buffer list there should be a buffer with a name like *nrepl-server* which contains the output of the root *out* binding. Here is the contents of mine after running your code:
nREPL server started on port 52034 on host 127.0.0.1 - nrepl://127.0.0.1:52034
Finished 1 on Thread[Thread-9,5,main]
Finished 0 on Thread[Thread-8,5,main]
Finished 2 on Thread[Thread-10,5,main]
Finished 3 on Thread[Thread-11,5,main]
Finished 4 on Thread[Thread-12,5,main]
If you did not use cider-jack-in, the output will print to the terminal where you started the nrepl process.
回答2:
*out* is the dynamic variable determining where output from println and similar functions goes. It is thread-bound to someplace that causes stuff to be sent back to emacs for display by cider; if you start a new thread, that binding is not present, and the output goes elsewhere (probably to the stdout of the nrepl server emacs/leiningen started in the background).
You can address this in a few ways. You could capture the value of *out* from the parent thread, and then pass it along to the child thread in a closure, and rebind *out* to it:
(let [out *out*]
(.start (Thread. (fn []
(binding [*out* out]
(println "test"))))))
Or you can use a future instead of starting the thread yourself: Clojure automatically conveys relevant thread-local bindings to new threads started for a future.
回答3:
Execute the following expression in the repl, then all output will end up in the repl:
(alter-var-root #'*out* (constantly *out*))
original answer:
https://groups.google.com/d/msg/cider-emacs/bIVBvRnGO-U/nDszDbGoVzgJ
回答4:
If you are using Figwheel, then doing prn/println in ring handlers (which are actually similar to the Threads example shown above) can also be swallowed by Fighweel itself. Check your project's project.clj (look for the key :server-logfile inside the :figwheel map), where you can control if out should go to the repl or to a logfile. Please note, this only applies if you are using figwheel, otherwise the printing to the REPL of course works fine.
See my answer on this question for more details: Output compojure server print statements into figwheel terminal?
来源：https://stackoverflow.com/questions/26743958/why-cant-i-print-from-background-threads-in-clojure-cider-repl-in-emacs