Java Blocking I/O vs non-Blocking I/O

1.. JDBC (Database Access)

java

복사편집

String name = jdbcTemplate.queryForObject("SELECT name FROM users WHERE id = ?", String.class, 1);

🔒 Blocking behavior:

• The current thread sends the SQL query over the network to the DB
• The thread waits (blocks) until the DB returns a response
• No other work happens on that thread until the DB reply is received

📌 JDBC is a blocking API, always

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🔸 2. RestTemplate (HTTP API calls)

java

복사편집

String response = restTemplate.getForObject("https://api.example.com/data", String.class);

🔒 Blocking behavior:

• Opens an HTTP connection
• Sends the request
• Waits (blocks) until the remote server responds
• During this time, the thread is not doing anything else

📌 RestTemplate is also blocking (as it wraps classic HttpURLConnection or Apache client)

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🔸 3. File I/O (Files.readAllBytes, FileInputStream, etc.)

java

복사편집

byte[] data = Files.readAllBytes(Paths.get("data.txt"));

🔒 Blocking behavior:

• Calls into OS via read() syscall
• If the data isn’t immediately ready, the thread is put to sleep
• OS wakes it up when the disk read is done

📌 Java’s java.io.* is blocking I/O by design

 

 

 

TaskJava (classic)Node.js (libuv)Java (NIO/WebFlux)

File read	Blocking	Async callback (uses thread pool)	Async callback
Socket	Blocking	Non-blocking (epoll)	Non-blocking (selector)
DB Query	Blocking (JDBC)	Non-blocking (with some drivers)	Non-blocking (R2DBC)

 

 

 

ConceptJava NIONode.js / libuv

Multiplexer abstraction	Selector	libuv internal poll mechanism
Register a file/socket	channel.register(selector, ...)	uv_poll_t, uv_read_start, etc.
Wait for I/O readiness	selector.select()	epoll_wait() inside poll phase
I/O readiness event	SelectionKey (e.g., OP_READ)	Callback to JS handler (socket.on('data'))
Event loop processor	Your thread calling .select()	libuv's event loop (uv_run())
Underlying OS call	epoll, kqueue, IOCP	epoll, kqueue, IOCP (same!)

 

 

 

 

Java NIO is designed to be cross-platform, so it chooses the appropriate OS-level mechanism depending on the platform you're running on.

 

Under the hood: What Java NIO uses

OSJava NIO uses...Notes

Linux	✅ epoll	Efficient, scalable, edge-triggered
macOS / BSD	✅ kqueue	High-perf polling system
Windows	✅ IOCP (I/O Completion Port)	Uses special selector provider
Solaris	✅ /dev/poll or poll	Older but still functional

 

How does Java know what to use?

When you call:

java

복사편집

Selector selector = Selector.open();

Internally, the JVM uses a platform-specific SelectorProvider:

 

 

 

 

 

• For BIO, all methods are blocking and directly call OS syscalls (read, write, accept, etc.)
• For NIO, methods are non-blocking, and readiness is monitored via:
  • epoll on Linux
  • kqueue on macOS
  • IOCP on Windows
• FileChannel.read() is blocking, even though it’s part of java.nio — because files don’t support readiness

 

Let’s build the table like this:

 

I/O Task Category	Java BIO Method	OS Call (BIO)	Java NIO Method	OS Call (NIO)	epoll Applicable?
File Read	FileInputStream.read()	read()	FileChannel.read()	read()	❌ No (regular files always "ready")
File Write	FileOutputStream.write()	write()	FileChannel.write()	write()	❌ No
Memory Mapping	❌ N/A	❌ N/A	FileChannel.map()	mmap()	❌ No (direct memory access)
File Open	new FileInputStream()	open() / openat()	FileChannel.open()	open() / openat()	❌ No
Socket Accept	ServerSocket.accept()	accept()	ServerSocketChannel.accept()	accept() via epoll	✅ Yes
Socket Read	Socket.read()	read()	SocketChannel.read()	read() via epoll	✅ Yes
Socket Write	Socket.write()	write()	SocketChannel.write()	write() via epoll	✅ Yes
Socket Connect	Socket.connect()	connect()	SocketChannel.connect()	connect() via epoll	✅ Yes
UDP Send/Recv	DatagramSocket.receive()	recvfrom()	DatagramChannel.read()	recvfrom() via epoll	✅ Yes
Pipe Read	PipedInputStream.read()	read()	Pipe.SourceChannel.read()	read() via epoll	✅ Yes (on Linux)
Pipe Write	PipedOutputStream.write()	write()	Pipe.SinkChannel.write()	write() via epoll	✅ Yes (on Linux)