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Initializing the object via a static initializer.
Using the final keyword  on a field to ensure that other threads see a fully constructed object after the constructor returns. The value of the field must be effectively immutable or thread safe. Use Collections.unmodifiable and the like to ensure immutability.
Using the volatile keyword to ensure that threads read the most up to date value. The volatile keyword can be tricky, but is very cheap when update are rare.
Guarding the object via another memory barrier like synchronized or a Lock.

(32bit primitives can be slightly different?)

Class Annotations

Solr uses two annotations to indicate to developers whether a class is thread safe or intended for single threaded use. TODO

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