How to - Use files

All the functions to read and write files are organized under the object “IO” that can easily be use throw the variable “io”.

Read files

Read a text file to a string 

var a_string = io.readFileString("abc123.txt");

All the contents of the “abc123.txt” file will assigned to the variable “a_string”.

If the file is in a zip file you can use the shortcut (‘::’):

var a_string = io.readFileString("aZipFile.zip::abc123.txt")

Read a text file as an array of strings 

var an_array = io.readFileAsArray("abc123.txt");
print(an_array[0]); // Print the first line
print(an_array[1]); // Print the second line

The variable “an_array” will have an entry for each line on the file “abc123.txt”.

Read a json file 

var a_json = io.readFile("abc123.json");
print(a_json.something); // prints the element 'something' from the json object

If the file is in a zip file you can use the shortcut (‘::’):

var a_json = io.readFile("aZipFile.zip::abc123.txt");

Read a yaml file 

var a_json = io.readFileYAML("abc123.yaml");
print(a_json.something); // prints the element 'something' from the json object

Reads a json object from a YAML file.

Write files

Write a string to a text file 

var a_string = "The first line\nThe second line";
io.writeFileString("abc123.txt", a_string);

If you want to append lines instead of rewriting the entire file:

io.writeFileString("abc123.log", "Another line\n", void 0, true);

Write a text array to a text file 

var lines = [];
lines.push("The first line");
lines.push("The second line");
io.writeFileAsArray("abc123.txt", lines);

Write a json object to a text file 

var obj = { x: -1, y: 1 };
io.writeFile("abc123.json", obj);

Write a json object to a yaml file 

var obj = { x: -1, y: 1 };
io.writeFileYAML("abc123.yaml", obj);

Handle newline delimited JSON files

Read Newline delimited JSON from a text file 

io.readLinesNDJSON("abc123.json", (line) => { sprint(line); });

Write a Newline delimited JSON to a text file 

var logline = { date: now(), level: "INFO", message: "a log message" };
io.writeLineNDJSON("abc123.json", logline);

Work with LZ4 compressed files

OpenAF v20250721 adds native helpers for LZ4 compression, making it easy to exchange compressed payloads without external tools:

  • io.lz4(data) compresses a map, array, string, byte array or stream using the LZ4 frame format and returns the compressed bytes.
  • io.unlz4(bytes) performs the inverse operation, returning the original map/string when the input was compressed with io.lz4.
  • io.writeFileLZ4Stream(path) opens an output stream that writes LZ4-compressed data directly to disk. It is ideal when you want to compress large files on the fly.
  • io.readFileLZ4Stream(path) yields an input stream that decompresses LZ4 content while you read it, avoiding the need to load the entire file in memory.

Example usage:

// Compress an object and persist the bytes
var payload = { id: 1, values: [1, 2, 3] };
var compressed = io.lz4(payload);
io.writeFileBytes("payload.lz4", compressed);

// Later, decompress it back
var restored = io.unlz4(io.readFileBytes("payload.lz4"));

// Write compressed content incrementally
var outStream = io.writeFileLZ4Stream("huge.log.lz4");
try {
   logs.forEach(line => outStream.write((line + "\n").getBytes("UTF-8")));
} finally {
   outStream.close();
}

// Read compressed content as a stream
var inStream = io.readFileLZ4Stream("huge.log.lz4");
// ...use the Java InputStream as needed...
inStream.close();