1Z0-829 Java I/O API and Securing Applications Practice Question
An architect is designing a microservice that reads large CSV files (up to 500 MB) from a shared filesystem and processes each row. The processing is CPU-bound and must not block the main thread. The service is deployed in a container with limited memory (512 MB heap). Which approach is most suitable?
⚠ Common exam trap
Many candidates assume memory-mapped files (FileChannel) are always efficient for large files, but they ignore the memory constraint and the need to keep the main thread responsive; Oracle tests whether you understand that bounded queues and separate threads are required for back-pressure and non-blocking processing in constrained environments.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a BufferedReader on a separate thread, reading lines into a bounded queue, and a thread pool to process rows from the queue.
It uses a bounded blocking queue to decouple I/O (reading lines from the CSV) from CPU-bound processing, preventing the main thread from being blocked while also avoiding memory exhaustion. The bounded queue acts as a back-pressure mechanism, ensuring that the limited 512 MB heap is not overwhelmed by the 500 MB file. Processing rows via a thread pool allows parallel CPU-bound work without blocking the reading thread.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use Files.readAllLines() to read the entire file into memory, then process in parallel using streams.
Why it's wrong here
readAllLines() loads the entire file into memory, causing OOM with large files and limited heap.
- ✓
Use a BufferedReader on a separate thread, reading lines into a bounded queue, and a thread pool to process rows from the queue.
Why this is correct
This approach reads line by line, using a bounded queue to decouple reading and processing.
- ✗
Use a FileChannel with a memory-mapped byte buffer of the entire file.
Why it's wrong here
Memory-mapping the entire file would also exceed the heap (maps are off-heap but still consume address space; could cause issues).
- ✗
Use a java.nio.file.FileSystem with a WatchService to detect changes to the file.
Why it's wrong here
WatchService is for monitoring directory changes, not for reading file contents.
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