Posts Tagged ‘Fuse’
[DevoxxBE2024] Project Panama in Action: Building a File System by David Vlijmincx
At Devoxx Belgium 2024, David Vlijmincx delivered an engaging session on Project Panama, demonstrating its power by building a custom file system in Java. This practical, hands-on talk showcased how Project Panama simplifies integrating C libraries into Java applications, replacing the cumbersome JNI with a more developer-friendly approach. By leveraging Fuse, virtual threads, and Panama’s memory management capabilities, David walked attendees through creating a functional file system, highlighting real-world applications and performance benefits. His talk emphasized the ease of integrating C libraries and the potential to build high-performance, innovative solutions.
Why Project Panama Matters
David began by addressing the challenges of JNI, which many developers find frustrating due to its complexity. Project Panama, part of OpenJDK, offers a modern alternative for interoperating with native C libraries. With a vast ecosystem of specialized C libraries—such as io_uring for asynchronous file operations or libraries for AI and keyboard communication—Panama enables Java developers to access functionality unavailable in pure Java. David demonstrated this by comparing file reading performance: using io_uring with Panama, he read files faster than Java’s standard APIs (e.g., BufferedReader or Channels) in just two nights of work, showcasing Panama’s potential for performance-critical applications.
Building a File System with Fuse
The core of David’s demo was integrating the Fuse (Filesystem in Userspace) library to create a custom file system. Fuse acts as a middle layer, intercepting commands like ls from the terminal and passing them to a Java application via Panama. David explained how Fuse provides a C struct that Java developers can populate with pointers to Java methods, enabling seamless communication between C and Java. This struct, filled with method pointers, is mounted to a directory (e.g., ~/test), allowing the Java application to handle file system operations transparently to the user, who sees only the terminal output.
Memory Management with Arenas
A key component of Panama is its memory management via arenas, which David used to allocate memory for passing strings to Fuse. He demonstrated using Arena.ofShared(), which allows memory sharing across threads and explicit lifetime control via try-with-resources. Other arena types, like Arena.ofConfined() (single-threaded) or Arena.global() (unbounded lifetime), were mentioned for context. David allocated a memory segment to store pointers to a string array (e.g., ["-f", "-d", "~/test"]) and used Arena.allocateFrom() to create C-compatible strings. This ensured safe memory handling when interacting with Fuse, preventing leaks and simplifying resource management.
Downcalls and Upcalls: Bridging Java and C
David detailed the process of making downcalls (Java to C) and upcalls (C to Java). For downcalls, he created a function descriptor mirroring the C method’s signature (e.g., fuse_main_real, returning an int and taking parameters like string arrays and structs). Using Linker.nativeLinker(), he generated a platform-specific linker to invoke the C method. For upcalls, he recreated Fuse’s struct in Java using MemoryLayout.structLayout, populating it with pointers to Java methods like getattr. Tools like JExtract simplified this by generating bindings automatically, reducing boilerplate code. David showed how JExtract creates Java classes from C headers, though it requires an additional abstraction layer for user-friendly APIs.
Implementing File System Operations
David implemented two file system operations: reading files and creating directories. For reading, he extracted the file path from a memory segment using MemorySegment.getString(), checked if it was a valid file, and copied file contents into a buffer with MemorySegment.reinterpret() to handle size constraints. For directory creation, he added paths to a map, demonstrating simplicity. Running the application mounted the file system to ~/test, where commands like mkdir and echo worked seamlessly, with Fuse calling Java methods via upcalls. David unmounted the file system, showing its clean integration. Performance tips included reusing method handles and memory segments to avoid overhead, emphasizing careful memory management.