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This is a very complex topic and I recommend you check out a classic book on the topic.

Object Publishing

A reminder to everyone to please follow the proper rules for object publication - the sharing of an object between two threads. Even if those threads do not access the object concurrently, we must follow these rules. Concurrency is a topic we cannot meet half way.

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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.

Class Annotations

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

Efficiency

We should try and use concurrency strategies that are efficient. For example, see ConcurrentHashMap, ConcurrentHashMap#newKeySet.

Good Java Practices

When we make shortcuts, like violating encapsulation, we create situations that may be reasonable now, but cause problems later. We don't want to be pedantic, but many of the lessons from Object Oriented programming prevent problems down the line.

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