How to know if a specific Java class is thread-safe?

Question

1. Is it true that all classes not under java.util.concurrent are unsafe in a multi-threaded environment?
2. If number 1 is wrong, how do you know if a specific class is thread-safe?

Answer 1

• It is not accurate to claim that all classes not under java.util.concurrent are unsafe in a multi-threaded environment in Java. Whether a class is thread-safe or not depends on its design and how it handles concurrent access by multiple threads. Java's java.util.concurrent package contains a set of classes that are explicitly designed and tested to work correctly in multi-threaded environments. However, there are many other classes in Java, both in the standard library and in custom code, that can be used safely in multi-threaded environments if they are used correctly and with proper synchronization mechanisms. These classes may not be explicitly labeled as "thread-safe," but they can still be used safely with the right precautions.
• Classes in java.util.concurrent are explicitly designed for multi-threaded use, not all other classes are inherently unsafe. You should carefully review the documentation, source code, and synchronization mechanisms used by a class to determine whether it is safe to use in a multi-threaded environment.