Introduction to Java I/O

Traditional Java I/O:

• InputStream and OutputStream classes.
• Blocking nature.

Limitations:

• Scalability issues in high concurrency environments.
• Thread per connection model.
• Introduction to Java NIO

Why NIO?

• Address limitations of traditional I/O.
• Scalability needs for modern applications.

Key Difference:

Behavior

• Blocking I/O blocks a thread until operation completes.
• NIO doesn't block the thread, it can still do other tasks

Resource Utilization

• Blocking I/O: inefficient resource usage with high concurrency, including memory size ( ~2Mb per thread) and CPU for context switching between threads
• NIO: better concurrency since one thread can handle many concurrent I/O operations

Complexity

• Blocking I/O is more simple to work with
• NIO is more complex (selectors, callback) to check or notified when an I/O operation completes.

Core components

• Under the hood, NIO leverages much of the underlying OS to provide non-blocking capabilities

Buffers

• Data container for I/O operations
• Types: ByteBuffer, CharBuffer, IntBuffer ...

Direct vs Non-direct buffer

• Direct buffers allocate memory in OS-level memory outside the usual JVM heap. They can be more expensive to allocate but can be passed directly to native code without copying.
• Non-direct buffers are stored inside the JVM heap and might need copying when passed to native code.

Structure

• Capacity: The number of elements a buffer can contain.
• Limit: The first element that shouldn't be read or written. You can't read or write data at positions greater than this limit.
• Position: The next element to be read or written. After reading/writing n elements, the position increases by n.

Operations

• flip
• rewind
• clear
• compact

Channels

• Represent open connections to I/O devices.

Sample

• FileChannel: to work with files
• SocketChannel, ServerSocketChannel: working with network connections

Properties

• Bidirectional: unlike streams (should be either input stream or output stream)
• Support asynchronous operations

Operations

• open / close
• read / write
• configureBlocking

Selectors

• Allows a single thread to handle multiple channels

Selection key

• A channel register with a selector for specific SelectionKey

SelectionKey key = channel.register(selector, SelectionKey.OP_READ);

• 4 types: READ, WRITE, CONNECT, ACCEPT

Operations

• select(): blocks until at least one channel is ready
• select(long timeout): blocks until timeout
• selectNow(): non-blocking

Non-blocking IO vs Asynchronous vs Reactive