FileChannel

FileChannel is a component of Java's NIO (java.nio.channels) that provides a channel-based view of a file for reading, writing, and manipulating file data. It supports random access, selective locking, and memory-mapped I/O, making it suitable for large files or performance-sensitive applications. FileChannel is an abstract class that can be obtained either by opening a path with FileChannel.open(Path, OpenOption...) or by obtaining a channel from a file stream or a RandomAccessFile via getChannel().

Opening and basic use

A FileChannel can be opened with Path and a varargs set of OpenOption values such as READ, WRITE, CREATE, TRUNCATE_EXISTING, and APPEND. It can also be created from existing streams or random access files. Once opened, it provides methods to read and write data using ByteBuffer instances. The channel supports both single-buffer operations and gathering/scattering across multiple buffers, enabling flexible I/O patterns.

Core capabilities

- Position and size: You can query and adjust the channel’s current position and query its size, enabling random-access edits.

- I/O operations: read and write transfer data between the channel and ByteBuffer(s). Transfer can also be done between channels via transferTo and transferFrom for potentially zero-copy transfers.

- Memory-mapped I/O: The map method allows creating a MappedByteBuffer by mapping a region of the file into memory with MapMode.READ_ONLY, MapMode.READ_WRITE, or MapMode.PRIVATE.

- Locking: The lock and tryLock methods provide file-region locking through FileLock, enabling coordinated access across processes.

- Persistence: force(boolean) forces updates to be written to the storage device, with an option to include metadata.

Notes

FileChannel is typically blocking and may depend on the underlying filesystem. It is commonly used for random-access file editing, large data processing, and efficient I/O via memory mapping or zero-copy transfers. Proper resource management is important; channels should be closed when no longer needed, often via try-with-resources.

Related concepts include FileLock, ByteBuffer, MapMode, and RandomAccessFile.