Is System.out.println() Thread-Safe in Multithreaded Java?

Multi-Threaded System.out.println() Interleaving: A Thread Safety Dilemma

The question arises: Can output from System.out.println(String) by multiple threads become interleaved if there's no synchronization?

The API remains silent on thread safety, leaving room for doubt. While an interleaved output is theoretically feasible, the reality may be different due to buffering, memory models, or JVM implementation.

For instance, if each thread executes System.out.println("ABC"), one might expect the output to be "ABCnABC", indicating no interleaving. However, "AABCnBC" is also plausible.

Addressing the Conundrum

The API documentation does not guarantee thread safety for System.out, implying that interleaving is possible. Nevertheless, some JVM implementations may utilize thread-safe mechanisms within println (like glibc's printf), ensuring output fidelity.

However, relying on JVM-specific implementations is risky. To ensure complete output integrity, manual mutual exclusion is necessary:

public void safePrintln(String s) {
  synchronized (System.out) {
    System.out.println(s);
  }
}

Note that this only prevents interleaving if all code consistently uses safePrintln and avoids direct calls to System.out.println.

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