Library mode

Use Java libraries in JavaScript

Library mode allows you to directly use Java methods, objects, and arrays from JavaScript. This API has been designed to take advantage of async/await to feel more natural to use, without sacrificing any flexibility.

Loading a library is as simple as calling cheerpjRunLibrary.

async function libraryModeTour() {
  await cheerpjInit();

  // Create a library mode object
  const lib = await cheerpjRunLibrary(""); // "" means standard library only

  // Resolve the Java classes we are going to use
  const ArrayList = await lib.java.util.ArrayList;
  const Point = await lib.java.awt.Point;
  const System = await lib.java.lang.System;

  // Create a new list object
  const points = await new ArrayList();

  // Create 4 point objects
  for (let i = 0; i < 4; i++) {
    // Allocate the point
    const p = await new Point(i, 0);

    // Add the point to the list
    await points.add(p);
  }

  // Convert to list to an Object[] array
  const a = await points.toArray();

  // Iterate on the array and set y = x
  for (let i = 0; i < a.length; i++) {
    // Fields can be read and written directly
    a[i].y = a[i].x;
  }

  // Convert all the elements to Strings
  for (let i = 0; i < a.length; i++) {
    // Java arrays can be read and written directly
    a[i] = await a[i].toString();
  }

  // Print them out
  for (let i = 0; i < a.length; i++) {
    // Static fields can be accessed too
    await System.out.println(a[i]);
  }
}

Library mode can be used to integrate powerful Java libraries into your Web application. As a practical example, these few lines of code make it possible to generate a PDF from JavaScript using the popular iText library:

async function iTextExample() {
  await cheerpjInit();

  const lib = await cheerpjRunLibrary("/app/itextpdf-5.5.13.3.jar");

  try {
    const Document = await lib.com.itextpdf.text.Document;
    const Paragraph = await lib.com.itextpdf.text.Paragraph;
    const PdfWriter = await lib.com.itextpdf.text.pdf.PdfWriter;
    const FileOutputStream = await lib.java.io.FileOutputStream;

    const document = await new Document();
    const writer = await PdfWriter.getInstance(
      document,
      await new FileOutputStream("/files/HelloIText.pdf"),
    );

    await document.open();
    await document.add(await new Paragraph("Hello World!"));
    await document.close();
    await writer.close();

    const blob = await cjFileBlob("/files/HelloIText.pdf");
    const url = URL.createObjectURL(blob);
    pdfDisplay.data = url;
  } catch (e) {
    const IOException = await lib.java.io.IOException;

    if (e instanceof IOException) console.log("I/O error");
    else console.log("Unknown error: " + (await e.getMessage()));
  }
}

Library mode

See also