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However, this solution requires the assignment of a new Helper instance to helper from Foo's constructor. According to The Java Language Specification, §17.5.2, "Reading Final Fields During Construction" [JLS 2015]:
A read of a final field of an object within the thread that constructs that object is ordered with respect to the initialization of that field within the constructor by the usual happens-before rules. If the read occurs after the field is set in the constructor, it sees the value the final field is assigned; otherwise, it sees the default value.
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Because the Helper class is declared public, it uses a private lock to handle synchronization in conformance with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code.
Exceptions
TSM03-J-EX0: Classes that prevent partially initialized objects from being used may publish partially initialized objects. This could be implemented, for example, by setting a volatile Boolean flag in the last statement of the initializing code and checking whether the flag is set before allowing class methods to execute.
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