Can Multi-threaded `System.out.println` Output Be Interleaved?

Can Multi-Threaded System.out.println Output Be Interleaved?

The Java API for System.out.println(String) lacks explicit synchronization guarantees, raising questions about the interleaving of output from multiple threads.

Can Output Get Interleaved?

Without synchronization, it is possible for output from multiple threads to become interleaved. This means that characters from different threads could be mixed within a single line of output.

For instance, consider multiple threads executing the code:

System.out.println("ABC");

The expected output should be:

ABC
ABC

However, interleaving could result in:

AABC
BC

Does Buffering Prevent Interleaving?

While buffering and the VM memory model can potentially prevent interleaving in some cases, this cannot be relied upon. The Java API specification does not guarantee atomic line writes for System.out.println.

Ensuring Non-Interleaved Output

To prevent interleaving and ensure that output is synchronized, manual synchronization must be enforced. This can be achieved by using synchronized blocks or by utilizing thread-safe printing methods provided by the Java library.

For example, the following code adds a synchronized block to System.out.println:

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

By using this method consistently throughout the code, output interleaving can be eliminated, ensuring that lines from different threads remain separate and orderly.

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