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What it looks like

java.lang.ExceptionInInitializerError
Caused by: java.lang.NullPointerException

Extends LinkageError in java.lang.

What it means

When the JVM first uses a class it runs its static initialisers. If one throws an unchecked exception, the JVM wraps it in ExceptionInInitializerError and marks the class as unusable. Every later attempt to use that class fails with NoClassDefFoundError: Could not initialize class X, which hides the real cause. Always look at the first occurrence in the log and read its Caused by line.

Common causes

• A static field initialised from configuration, a file or the environment that is missing.
• A static block that opens a connection or reads a resource and fails.
• A singleton or utility class that builds its state in a static initialiser using something not yet ready.
• A static initialiser calling a method that throws, such as Pattern.compile with a bad regex.

How to fix it

1. Find the earliest log entry and read its Caused by chain; fix that exception.
2. Avoid doing risky work in static initialisers; use lazy initialisation or dependency injection.
3. Validate constants such as regular expressions with a unit test.
4. Restart the application: a failed class cannot be re-initialised in the same JVM.

Broken and fixed

private static final Pattern P = Pattern.compile("[a-z"); // invalid regex

private static final Pattern P = Pattern.compile("[a-z]+");

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