Thread safety of static blocks in Java

Let’s say I have some Java code:

public class SomeClass {
    static {
        private final double PI = 3.14;
        private final double SOME_CONSTANT = 5.76;
        private final double SOME_OTHER_CONSTANT = 756.33;
    }

  //rest of class
}

If a thread is initializing SomeClass’s Class object and is in the middle of initializing the values in the static block when a second thread wants to load SomeClass’s Class again, what happens to the static block? Does the second thread ignore it assuming it’s already initialized even though the first thread is not done? Or does something else happen?

Answer

If the first thread hasn’t finished initializing SomeClass, the second thread will block.

This is detailed in the Java Language Specification in section 12.4.2.

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Answer