Class level, instance level and local ThreadLocals

Question

I understand how class level thread locals makes sense. Being associated with thread, we need thread locals to be shared among different instances and classes across that thread. So we need to make them class level. If we want to share thread local across different instances of same class, we can make them private static. If we want to share thread local across different classes, we can make them public static.

Q0. correct me if am wrong with above

My doubts are about instance scoped (non-static) thread locals and local (defined inside some method) thread locals:

Q1. Is there any valid use case for instance scoped (non-static) thread locals?
Q2. Is there any valid use case for local (defined inside some method) thread locals?
Q3. Are instance scoped (non-static) thread locals deleted when an instance is garbage collected?
Q4. Are local (defined inside some method) thread locals deleted when method returns?

Answer 1

ThreadLocal when implemented correctly as a static variable acts essentially as an instance variable for all threads that have access to it. Even though there's a single ThreadLocal variable, the mechanism makes it so that each thread has its own instance of the value in it.

Therefore

Q1. No, it doesn't make sense to have an instance scoped ThreadLocal. This doesn't mean you couldn't write code that would use an instance scoped TL, but you would need to keep track (in your developer mind) of both the instance and the thread being used for correct functionality, that even if you would find a use case that the code would solve, there would be a lot better way to handle it.

Q2. No. As a local variable can never have more than a single thread access it, it would not differ from a regular local variable.

Q3. The ThreadLocal<> wrapper becomes unreachable, but the actual variable is still contained in the thread's map, as you correctly said. This causes a resource/memory leak, as it can't be cleared until the thread stops.

Q4. Same as with Q3, if you lose the wrapper reference, it's an instant leak. If you assign the reference somewhere, it's just weird programming. A method local ThreadLocal variable would be extremely worrying code.

The class is not something you'd want to use too much anyway in modern code (or even older code), and it's not compatible with reactive programming, but if you do use it the usage is straight-forward. A single ThreadLocal most easily implemented as a class level variable.

Answer 2

Q2. Is there any valid use case for local (defined inside some method) thread locals?

First, lets's just be clear. If you say "a local Foobar" (for any class Foobar), then it's not entirely clear what you are talking about. Variables can be "class level" (i.e., static) or "instance level," or "local;" but a Foobar instance is not a variable. The variables in a Java program can only refer to Foobar instances that are allocated on the heap. It's very easy, and very common to have more than one variable in a program refer to the same instance.

ThreadLocal is a class, and instances of ThreadLocal are objects on the heap. The same ThreadLocal object could be referenced by a static ThreadLocal variable and also, at the same time, referenced by local variables in one or more threads.

When you say "a local ThreadLocal," you could be talking about a local variable that holds a reference to a ThreadLocal instance that is shared with other threads, -OR- you could be talking about a ThreadLocal instance that is only referenced by one local variable. The second case would not make any sense because that instance could not be shared by multiple threads.

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Q1. Is there any valid use case for instance scoped (non-static) thread locals?

Maybe so, but I would call it a "code smell." (That is, a reason to look closely at the code and see whether it could be better organized.) I personally would never use ThreadLocal in new code. The only times I have ever used it is, while porting older, single-threaded code into a multi-threaded system; and when I did it, the variables in question always were static (i.e., class level) variables.

I personally try never to use static in new code except in cases where some function is clearly labelled as returning a reference to a "singleton object."

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Q3., Q4. [...when are instances deleted...]?

An instance will be eligible to be deleted when there is no "live" variable in the program that refers to it. One way that can happen is if the only variable that refers to it is a local variable of some function, and that function returns. A second way it can happen is if the only variable that refers to the instance is assigned to refer to some other instance. A third way is if the only references to it are from instance variables of other objects, and all of those other objects are themselves, eligible to be deleted.

In most Java implementations, the instance will not be immediately deleted when it becomes eligible for deletion. The actual deletion will happen some time later. When, depends on the strategies employed by the JRE's garbage collector and on the patterns of object use by the program.