Calling CompletableFuture.get() can be dangerous in production environments if used incorrectly or in the wrong context. While get() is a convenient way to retrieve the result of an asynchronous computation, it comes with several risks that can hurt performance, responsiveness, and reliability of your application, especially in highly concurrent or reactive systems.

Below is a detailed explanation of why CompletableFuture.get() can be problematic in production and what you should do instead.

1. Blocking the Current Thread

Problem:
CompletableFuture.get() is a blocking call. It causes the calling thread to wait indefinitely until the computation completes or an exception is thrown.

Why it’s dangerous:

• In web applications, blocking an I/O thread (e.g., Tomcat’s or Netty’s) can cause thread pool exhaustion.
• In UI applications, blocking the UI thread (like in JavaFX or Swing) causes the interface to freeze.
• In microservices, blocking threads can reduce throughput and lead to degraded performance or timeouts.

Example:

String result = completableFuture.get(); // blocks until the result is available

If this is called in a REST controller or an event loop thread, it may severely impact scalability.

2. Potential for Deadlocks

Problem:
Blocking operations like get() can introduce deadlocks, especially if you are trying to combine multiple futures or call get() within an async callback.

Why it’s dangerous:

• If a thread holding a lock calls get() and waits for another task that needs the same lock, a deadlock can occur.
• If you accidentally create circular dependencies between futures, the application can hang permanently.

Example:

CompletableFuture<String> cf1 = ...;
CompletableFuture<String> cf2 = cf1.thenApply(data -> {
    return "Processed: " + data;
});

String result = cf2.get(); // if cf1 is waiting for cf2, deadlock

Such indirect dependencies can create subtle and hard-to-debug deadlocks in production.

3. Unbounded Waiting

Problem:
get() waits forever by default if the future never completes.

Why it’s dangerous:

• If the asynchronous task never finishes (e.g., stuck network call, infinite loop, exception), get() will hang indefinitely.
• Threads will accumulate and get blocked forever, potentially leading to OutOfMemoryError or thread pool exhaustion.

Safer alternative:

String result = completableFuture.get(5, TimeUnit.SECONDS);

Using a timeout protects your application from hanging forever and allows graceful error handling.

4. Poor Error Handling

Problem:
If the task fails, get() throws an ExecutionException, which wraps the actual exception thrown during computation.

Why it’s dangerous:

• Developers often forget to handle the wrapped exception properly.
• Catching the ExecutionException without unwrapping it can obscure the root cause.

Example:

try {
    String result = completableFuture.get();
} catch (ExecutionException e) {
    Throwable cause = e.getCause(); // important to retrieve real exception
}

If you ignore e.getCause(), you might miss meaningful error messages or stack traces.

5. Not Idiomatic for Asynchronous Programming

Problem:
Using get() defeats the purpose of asynchronous programming.

Why it’s dangerous:

• It turns asynchronous code into synchronous, negating all benefits of using CompletableFuture.
• If your architecture relies on non-blocking, event-driven execution (like in reactive programming), using get() introduces bottlenecks.

Better alternative:

completableFuture.thenAccept(result -> {
    // process result asynchronously
});

This approach is non-blocking and allows better utilization of CPU and threads.

6. Uncaught InterruptedException

Problem:
get() can throw InterruptedException, which many developers either ignore or mishandle.

Why it’s dangerous:

• Failing to properly handle thread interruption can lead to unpredictable behavior.
• Best practice is to restore the thread’s interrupted status if you catch InterruptedException.

Correct handling:

try {
    completableFuture.get();
} catch (InterruptedException e) {
    Thread.currentThread().interrupt(); // preserve interrupt status
}

Ignoring it can violate the contract of cooperative cancellation in Java concurrency.

Summary of Why get() Can Be Dangerous in Production

Risk	Explanation
Blocks current thread	Wastes CPU and reduces concurrency
Can lead to deadlocks	Especially in poorly chained or nested futures
Unbounded waiting	If the task never completes, thread hangs forever
Poor error transparency	Exceptions are wrapped and often mishandled
Anti-pattern in async systems	Synchronous wait defeats async benefits
Interrupts often mishandled	Can break thread interruption protocol