When You Need a Pool, Not a Thread Pool: SimpleJschPool

 A simple pool SimpleJschPool with no threads

The SimpleJschPool Project

The SimpleJschPool is a custom-built solution for managing JSch SFTP sessions and channels, designed to be more lightweight and targeted than general-purpose pooling libraries like Apache Commons Pool. It's built for a specific use case where a pool of reusable, non-thread-safe objects (JSch ChannelSftp) is needed without the overhead of thread management.

The project is composed of three core components that work together to provide a streamlined pooling mechanism:

• JschSessionProvider: This class is responsible for creating new JSch sessions. It acts as the factory for the resource being pooled. This is crucial because it encapsulates the logic for establishing a new SFTP connection, which can be a time-consuming operation.

• TicketGenerator: This component ensures that each request receives a unique identifier, commonly referred to as a "ticket." This ticket is then used to track and retrieve a specific ChannelSftp instance from the cache, ensuring that a single channel is never accessed by multiple requests simultaneously.

• JschChannelCache: This is the heart of the pooling logic, implemented using a Guava cache. It stores the ChannelSftp objects, mapping them to the unique tickets generated by the TicketGenerator. The cache handles the temporary storage and retrieval of the channels, making them available for reuse.

• Semaphore:  The Semaphore is used to regulate the number of active tickets, which in turn controls the number of ChannelSftp instances allocated at any given time. This is a critical design choice, as it ensures that the non-thread-safe ChannelSftp Objects are not shared between concurrent threads. The semaphore acts as a gatekeeper, granting access to a resource only when it's available.

Why this approach?

The key design decision behind this project is to avoid the complexity of a full-fledged thread pool. Instead, it focuses on creating a simple, efficient object pool. Using aSemaphore; pool provides a way to manage resource access without needing to manage a pool of threads. This makes the solution lightweight and perfectly suited for the task of delivering reusable ChannelSftp instances without the overhead of constant session creation and teardown. It provides a simple yet robust way to handle multiple SFTP transfers concurrently in a safe manner.