Can Multiple Threads Interleave Output Using Java's System.out.println?

Can System.out.println Output Be Interleaved by Multiple Threads?

Multi-threaded programming often raises questions about thread safety, and Java's System.out.println is no exception. Without explicit synchronization, can multiple threads interleave their outputs written to the console?

The API and Thread Safety

The Java API documentation doesn't specify the thread safety of either the System.out object or the PrintStream#println(String) method. Therefore, it's not safe to assume they are thread-safe.

Implementation-Dependent Behavior

However, it's possible that specific JVM implementations employ thread-safe functions for the println method, ensuring that output always appears as ABCn followed by ABCn. But keep in mind that JVM implementations can vary, and they only adhere to the Java specification.

Ensuring Thread Safety

To guarantee non-interleaved outputs, you must manually enforce mutual exclusion. For instance, you could use a synchronized method:

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

Remember that this example is illustrative only and should not be regarded as the complete solution. It's crucial that all code uses this method and never calls System.out.println(...) directly to achieve full thread safety.

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