Thread-local variables in Java (ctd)

On the previous page, we saw an example of using ThreadLocal to pool Calendar objects. On this page, we consider what are the general criteria for choosing ThreadLocal?

When to use ThreadLocal?

So what are other good candidates for object re-use via ThreadLocal? Basically, objects where:

• The objects are non-trivial to construct;
• An instance of the object is frequently needed by a given thread;
• The application pools threads, such as in a typical server (if every time the thread-local is used it is from a new thread, then a new object will still be created on each call!);
• It doesn't matter that Thread A will never share an instance with Thread B;
• It's not convenient to subclass Thread. If you can subclass Thread, you could add extra instance variables to your subclass instead of using ThreadLocal. But for example, if you are writing a servlet running in an off-the-shelf servlet runner such as Tomcat, you generally have no control over the class of created threads. Of course, even if you can subclass Thread, you may simply prefer the cleaner syntax of ThreadLocal.

That means that typical objects to use with ThreadLocal could be:

• Random number generators (provided a per-thread sequence was acceptable);
• Collators;
• native ByteBuffers (which in some environments cannot be destroyed once they're created);
• XML parsers or other cases where creating an instance involves going through slightly non-trival code to 'choose a registered service provider';
• Per-thread information such as profiling data which will be periodically collated.

Note that it is generally better not to re-use objects that are trivial to construct and finalize. (By "trivial to finalize", we mean objects that don't override finalize.) This is because recent garbage collector implementations are optimised for "temporary" objects that are constructed, trivially used and then fall out of scope without needing to be added to the finalizer queue. Pooling something trivial like a StringBuffer, Integer or small byte array can actually degrade performance on modern JVMs.

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