CheerpJ provides full filesystem support for converted Java applications.

Read only and read/write filesystems are exposed in Java and can be used to read, write and manipulate files as normally when running on a JVM.

Note: CheerpJ provides access to a virtualized filesystem, which does not correspond to the local computer. Accessing local files from the browser it’s forbidden for security reasons.

File Systems in CheerpJ

CheerpJ implements three main filesystem concepts:

  1. A read-only, HTTP-based filesystem
  2. A read/write, IndexedDB-based, persistent filesystem
  3. A read-only, memory based filesystem

CheerpJ filesystems are implemented as UNIX-style virtual filesystems with multiple mount points. The default mount points are defined as follows:

  1. /app/ → An HTTP-based read-only filesystem, used to access JARs and data from your local server.
  2. /files/ → An IndexedDB-based, persistent read-write file system
  3. /lt/ → Another HTTP-based read-only filesystem, pointing to the CheerpJ runtime
  4. /str/ → A read-only filesystem to easily share JavaScript Strings or binary data (an Uint8Array) with Java code

/app/ mount point

The /app/ mount point corresponds to a virtual read-only, HTTP-based filesystem. /app/ is used to access JAR files and data from your local server.

The /app/ directory is virtual, and only exists inside of CheerpJ. It is required to distinguish files from the local server from runtime files and files stored in the browser database.

The /app/ directory refers to the root of your web server. So, assuming that your web server is available at, here are some example file mappings:

  • /app/example.jarhttp://
  • /app/subdirectory/example.txthttp://

/files/ mount point

The /files/ mount point corresponds to a virtual read-write, IndexedDB-based filesystem. /files/ is used to store persistent data on the browser client.

The /files/ directory is a virtual concept used by CheerpJ to store and refer to files.

/str/ mount point

The /str/ mount point is a simple read-only filesystem that can be populated from JavaScript to share data with Java code.

From JavaScript you can add files into the filesystem using the cheerpjAddStringFile API. Example:

cheerpjAddStringFile("/str/fileName.txt", "Some text in a JS String");

You can access this data from Java, for example:


FileReader f = new FileReader("/str/fileName.txt")

The cheerpjAddStringFile API can be used with JavaScript Strings or Uint8Arrays. Uint8Arrays may be useful to provide binary data to the Java application, for example a user selected file coming from an HTML5 <input type="file"> tag.