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What it looks like

java.lang.IllegalStateException: Cannot call sendError() after the response has been committed

Extends RuntimeException in java.lang.

What it means

The arguments may be fine, but the object is not ready for this call: a closed resource used again, an iterator advanced after the end, a response modified after it was sent, a builder used before it is configured. Frameworks use it heavily to report wrong ordering or lifecycle mistakes, so the message is often the most useful part.

Common causes

• Using a resource after it was closed, such as a Scanner, a stream or a database connection.
• A servlet or controller writing to the response after it was already committed, often after an earlier error handler.
• Calling Iterator.remove() twice or before next().
• Spring context problems: "Failed to load ApplicationContext", or a bean accessed while the context is shutting down.

How to fix it

1. Find which object is in the wrong state; the message and stack trace name it.
2. Reorder calls so the setup happens before the use, and close resources only once.
3. For response-committed errors, make sure only one place writes the response and that you return after redirecting or sending an error.
4. Use try-with-resources so lifecycle is handled by the language.

Broken and fixed

Scanner in = new Scanner(System.in);
in.close();
in.nextLine(); // IllegalStateException: Scanner closed

try (Scanner in = new Scanner(System.in)) {
    String line = in.nextLine();
}

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