Can Multiple Threads Interleave Output When Using System.out.println()?

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

In Java, the System.out class provides a println method for printing messages to the console. However, there is some ambiguity regarding whether output from multiple threads calling this method can be interleaved.

API Documentation

The Java API documentation for System.out does not explicitly state whether it is thread-safe or not. As such, it cannot be assumed that the output from multiple threads will be consistent.

Potential Interleaving

In theory, it is possible for the output from multiple threads to be interleaved. This can occur if the underlying implementation of the Java Virtual Machine (JVM) does not ensure atomic write operations for System.out.println. In such cases, the output could become garbled, with characters from different messages appearing interspersed.

Example Case

For example, consider the following code:

Thread thread1 = new Thread(() -> System.out.println("ABC"));
Thread thread2 = new Thread(() -> System.out.println("ABC"));

thread1.start();
thread2.start();

In this example, it is possible (but not guaranteed) that the output could appear interleaved, as follows:

AABC
BC

JVM Implementation

In practice, the actual behavior may depend on the specific JVM implementation. Some JVMs may optimize the System.out class to ensure atomic write operations, preventing interleaving. However, this is not guaranteed across all platforms and implementations.

Enforcing Mutual Exclusion

To ensure that the output from multiple threads is not interleaved, you can manually enforce mutual exclusion using the synchronized keyword. For instance:

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

By protecting the System.out object with a lock, this method guarantees that only one thread can access it at any given time. As a result, the output will be consistent and non-interleaved.

Remember that it is crucial to use this method consistently throughout your code. If any thread calls System.out.println directly, the output may still become interleaved.

The above is the detailed content of Can Multiple Threads Interleave Output When Using System.out.println()?. For more information, please follow other related articles on the PHP Chinese website!