What are immutable objects?

What is the relationship with thread-safety and immutable 开发者_如何学JAVAobjects? Does it makes easier to share a single resource among multiple threads? If immutable objects are stateless, can they be pooled in a container like a J2EE container?

thanks

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Immutable objects are objects that can not be changed. If an object can not be changed, then there is no concern that a competing thread will change the object state "behind the back" of the executing thread, and therefore immutable objects do not need to be protected via synchronization or some other technique.

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Threadsafe objects are objects which allow to be accessed concurrently by multiple threads. Their implementation guarantees (for example by lockings / synchronzized methods / ...) that they will not get into a invalid state. In addition, there should be no loss of data.

Immutable objects may not be altered after their creation. So: Yes, they are some kind of stateless.

As immutable objects can not be changed, there is no need for locking - reading access to objects is always threadsafe (when not modifying variables). Therefore, real immutable objects are always threadsafe.

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Immutable Object: An object that doesn't change its internal state.

The relationship with thread-safety: if an object cannot be mutated, it is safe to use it across threads i.e. no need to have locks or the like to ensure consistency across threads.