How does System.out.println work at the threading level?

I was investigating what system calls are made on a simple HelloWorld java program. With a simple strace I noticed that there is no write call, which I found suspicious:

...
mprotect(0x7f0bcd852000, 4096, PROT_READ) = 0
mprotect(0x7f0bce915000, 790528, PROT_READ) = 0
getpid()                                = 27931
munmap(0x7f0bcf6ac000, 174284)          = 0
getpid()                                = 27931
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f0bcf5d6000
clone(child_stack=0x7f0bcf6d5fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7f0bcf6d69d0, tls=0x7f0bcf6d6700, child_tidptr=0x7f0bcf6d69d0) = 27932
futex(0x7f0bcf6d69d0, FUTEX_WAIT, 27932, NULLHello, World
) = 0
munmap(0x7f0bbfa6c000, 140063364)       = 0
close(3)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

So, as you can see above, with a normal strace call only a futex call is made, which is waiting on the string address.

So, I ran strace with the -f parameter to see all the threads:

[pid 28249] pread64(3, "\312\376\272\276\0\0\0009\0\330\n\0\2\0\3\7\0\4\f\0\5\0\6\1\0\23java/n"..., 6104, 4355323) = 6104
[pid 28249] pread64(3, "\312\376\272\276\0\0\0009\0\306\n\0\2\0\3\7\0\4\f\0\5\0\6\1\0\20java/l"..., 4455, 3263638) = 4455
[pid 28249] write(1, "Hello, World\n", 13Hello, World
) = 13
[pid 28249] pread64(3, "\312\376\272\276\0\0\0009\0(\n\0\2\0\3\7\0\4\f\0\5\0\6\1\0\25java/l"..., 999, 4425942) = 999
[pid 28249] mmap(0x7f9d35ae2000, 16384, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7f9d35ae2000
[pid 28249] mprotect(0x7f9d2c2e7000, 8192, PROT_READ|PROT_WRITE) = 0

Like this, I can see the write call as well as much more futex calls.

My question is, what exactly happens at the threading level when System.out.println is called? Does Java create a dedicated thread for printing, that is then synchronized with the main thread? Also, what do they use for synchronization that in the end makes a futex call.

Solution

System.out.println has nothing to do with threads (except that PrintStream methods are synchronized, but that doesn't matter in an uncontended case).

Java launcher creates a JVM in a new thread, that's why any Java code (not just println) is executed in a non-primordial thread.

Running Java code in a primordial thread used to cause many problems, see JDK-6316197 for details.