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What it looks like

java.util.concurrent.ExecutionException: java.lang.IllegalStateException: boom

Extends Exception in java.util.concurrent.

What it means

When you call Future.get() and the task threw an exception, Java wraps it in ExecutionException so you can tell a failed task from your own thread being interrupted. The actual problem is in getCause(). In logs it appears as the first Caused by line, so debugging means reading the cause, not the wrapper. CompletableFuture has an equivalent, CompletionException, thrown by join().

Common causes

• Any unchecked exception inside a Callable or Runnable submitted to an executor.
• A checked exception declared by the Callable.
• A failed stage in a CompletableFuture chain.
• An Error such as OutOfMemoryError inside the task.

How to fix it

1. Catch ExecutionException and inspect or rethrow e.getCause().
2. Log the cause with its stack trace, not only the wrapper message.
3. Use exceptionally or handle on CompletableFutures to recover.
4. Unwrap and rethrow the real exception type when your method signature allows it.

Broken and fixed

try { future.get(); }
catch (ExecutionException e) { log.error("failed"); } // cause lost

try { future.get(); }
catch (ExecutionException e) {
    log.error("task failed", e.getCause());
    throw new RuntimeException(e.getCause());
}

🔎 Unravel the failing code 🔎 Unravel the fix

In Spring and enterprise code

Async methods in Spring (@Async returning Future) surface failures this way; void ones go to the AsyncUncaughtExceptionHandler.

Debug it faster

Paste the whole stack trace into the stack trace beautifier to hide framework frames, jump to the root cause and find the first line of your own code. It runs in your browser, so the trace never leaves your machine.

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