Filesystem Guide Rework #308

rijulshrestha · 2025-11-12T01:04:19Z

I've updated the structure, a bit of text and tried to make the code examples simpler and easy to follow.

… and Filesystem guide'.

rijulshrestha · 2025-11-13T12:48:08Z

Considering to change the "Mounting points overview" callout to normal bullet points.

epanholz

Added some comments. Overall it already looks way better but some parts can still be improved,

epanholz · 2025-11-13T12:59:47Z

pnpm-workspace.yaml

+  - sites/*
+  - packages/*
+
+ignoredBuiltDependencies:
+  - esbuild
+  - sharp


Can you explain why these changes are necessary?

I've forced build everything on my machine and it got committed on accident. I've reverted the changes.

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

epanholz · 2025-11-13T13:56:14Z

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

+## Getting a file from the Virtual File System to JavaScript (i.e local file system)

-## 2\. Getting Files from the Virtual Filesystem to the Local Filesystem (Java to JavaScript)
+When Java runs under CheerpJ, files are saved in the virtual filesystem (typically under **`/files/`**), not directly to the user’s disk. To make a file available to the page (for download, upload, or preview), expose it via a **Java native method** implemented in JavaScript. The JavaScript side reads the bytes from the VFS and then uses standard browser APIs (e.g., create a `Blob`, trigger a download, or upload it).


(typically under /files/) -> (defaults to /files/)

I would also rewrite this slightly:
To make a file available to the page (for download, upload, or preview), expose it via a Java native method implemented in JavaScript. The JavaScript side reads the bytes from the VFS and then uses standard browser APIs (e.g., create a Blob, trigger a download, or upload it).

Something like this:
CheerpJ provides APIs that are available through JavaScript to extract files from the virtual file system (link to blob api). To implement a more seamless file interaction you can utilize JavaScript native methods (link to docs) to handle the file after a filer interaction. This can look something like this: (Example flow you already have)

We should also add the cjFileBlob() api here as method 1, and then method 2 is with javascript natives.

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

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rijulshrestha · 2025-11-17T14:32:03Z

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

+Entries you create under **`/str/`** (from JavaScript) are readable from Java as regular files. Remember: `/str/` is **not persisted** and is **read-only** from Java.
+
+```java title="App.java"
+import java.nio.file.Files;
+import java.nio.file.Path;
+/*
+Rest of your Java code...
+*/
+String text = Files.readString(Path.of("/str/fileName.txt"));
+System.out.println(text);
+```


How does this code look? Do I need to add more structure and details like below;

import java.io.File; // Import the File class import java.io.FileNotFoundException; // Import this class to handle errors import java.util.Scanner; // Import the Scanner class to read text files public class ReadFile { public static void main(String[] args) { File myObj = new File("filename.txt"); // try-with-resources: Scanner will be closed automatically try (Scanner myReader = new Scanner(myObj)) { while (myReader.hasNextLine()) { String data = myReader.nextLine(); System.out.println(data); } } catch (FileNotFoundException e) { System.out.println("An error occurred."); e.printStackTrace(); } } }

This part looks good now.

rijulshrestha · 2025-11-17T14:34:21Z

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

- **`cheerpjRunLibrary("")`**: This creates a library mode object that allows you to use Java's `nio.file` APIs in JavaScript. You can read more about library mode in the [CheerpJ documentation](/docs/reference/cheerpjRunLibrary).
- **`Files.copy(source, target, [StandardCopyOption.REPLACE_EXISTING])`**: This copies the file from the `/app/` mount point to the `/files/` mount point. The `REPLACE_EXISTING` option ensures that if the target file already exists, it will be overwritten.
- **Accessing the File in Java**: After copying, your Java application can access the file simply by using `new File("notes_tmp.txt")`, without needing to specify the `/app/` prefix.
+When Java runs under CheerpJ, files are saved in the virtual filesystem (defaults under **`/files/`**), not directly to the user’s disk. Use the JavaScript-accessible [cjFileBlob API](/docs/reference/cjFileBlob) to extract files from the virtual file system. For a more seamless flow, define a [Native Methods](/docs/guides/implementing-native-methods) implemented in JavaScript to process the file. This can look like:


I've linked to the docs here and have removed the 'Notes' part in the end where the docs were initially linked.

That is okay. I originally suggested to add a Method 1 - Using cjFileBlob and a Method 2 - Using native methods to make it more align with the previous section and also a bit easier to read. cjFileBlob can be used to extract files from the virtual file system and having a very short stand alone example here would be nice.

epanholz

Requested a few more small changes, after that it should be ready for merging.

epanholz · 2025-11-17T15:38:23Z

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

+**Mounting points overview**

-CheerpJ's virtual filesystem has different **mounting points**, each with specific purposes:
+- **`/app/`** — Read-only mount mapping to your web server’s root (HTTP assets). Java can read the files using the '/app/' prefix.


Read-only mount mapping to your web server’s root (HTTP assets). Java can read the files using the '/app/' prefix. -> Read-only mount that points to the root of your web server. Java can read the files using the '/app/' prefix.

I think saying HTTP assets might confuse users, since they can pass any files they want to the application like this, like config files or similar.

epanholz · 2025-11-17T15:41:05Z

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

+To access local files, first import them into the virtual file system (see previous section). To save or download files to the user’s machine from Java, you can utilize a JavaScript native method again.

-// (Retain the native method declaration from Section 2)
+**Java Source Code: Essential `JFileChooser` and Download Logic**


Java Source Code: Essential JFileChooser and Download Logic -> Java Source Code: JFileChooser and download logic

epanholz · 2025-11-17T15:42:32Z

sites/cheerpj/src/content/docs/11-guides/filesystem.mdx

+    System.out.println("Triggered direct URL download: " + fileUrl.toString());
+  } catch (Exception ex) {
+    ex.printStackTrace();
+    JOptionPane.showMessageDialog(null, "Error downloading from URL: " + ex.getMessage(), "Error", JOptionPane.ERROR_MESSAGE);


I would remove this line, not need to add the reference to JOptionPane if it does not show in the code example.

rijulshrestha added 2 commits November 10, 2025 23:03

Removed filesystem details and added reference to the dedicated 'File…

a625d40

… and Filesystem guide'.

Updated the headings

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rijulshrestha requested a review from epanholz November 12, 2025 01:04

rijulshrestha self-assigned this Nov 12, 2025

rijulshrestha added the documentation Improvements or additions to documentation label Nov 12, 2025

rijulshrestha marked this pull request as ready for review November 12, 2025 01:13

rijulshrestha requested a review from GabrielaReyna as a code owner November 12, 2025 01:13

epanholz requested changes Nov 13, 2025

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rijulshrestha force-pushed the filesystem-guide-rework branch from 2859d5e to 24a37ef Compare November 13, 2025 15:59

rijulshrestha added 11 commits November 13, 2025 16:20

'JavaScript to VFS' section updated

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'VFS to JavaScript' section updated

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'JFileChooser' section updated

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Special Case section updated

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Prettier check implemented

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Added one line description for mounting points

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'JavaScript to VFS' section cleaned

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'VFS to JavaScript' section cleaned

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JFileChooser section cleaned

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'Special Case' section cleaned

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Prettier check implemented

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rijulshrestha force-pushed the filesystem-guide-rework branch from 24a37ef to 7f05140 Compare November 13, 2025 16:21

rijulshrestha added 4 commits November 13, 2025 17:07

Rewored introduction, VFS to virtual file system, added java code exa…

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…mple

Updated Method 2 for more clarity

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Implemented feedback

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Prettier check implemented

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rijulshrestha commented Nov 17, 2025

View reviewed changes

rijulshrestha requested a review from epanholz November 17, 2025 14:35

epanholz requested changes Nov 17, 2025

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Filesystem Guide Rework #308

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Filesystem Guide Rework #308

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rijulshrestha Nov 17, 2025 •

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