194 lines
6 KiB
Markdown
194 lines
6 KiB
Markdown
# Application Packaging
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To mitigate [issues](https://github.com/joyent/node/issues/6960) around long
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path names on Windows, slightly speed up `require` and conceal your source code
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from cursory inspection, you can choose to package your app into an [asar][asar]
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archive with little changes to your source code.
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Most users will get this feature for free, since it's supported out of the box
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by [`electron-packager`][electron-packager], [`electron-forge`][electron-forge],
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and [`electron-builder`][electron-builder]. If you are not using any of these
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tools, read on.
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## Generating `asar` Archives
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An [asar][asar] archive is a simple tar-like format that concatenates files
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into a single file. Electron can read arbitrary files from it without unpacking
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the whole file.
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Steps to package your app into an `asar` archive:
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### 1. Install the asar Utility
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```sh
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$ npm install -g asar
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```
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### 2. Package with `asar pack`
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```sh
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$ asar pack your-app app.asar
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```
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## Using `asar` Archives
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In Electron there are two sets of APIs: Node APIs provided by Node.js and Web
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APIs provided by Chromium. Both APIs support reading files from `asar` archives.
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### Node API
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With special patches in Electron, Node APIs like `fs.readFile` and `require`
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treat `asar` archives as virtual directories, and the files in it as normal
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files in the filesystem.
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For example, suppose we have an `example.asar` archive under `/path/to`:
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```sh
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$ asar list /path/to/example.asar
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/app.js
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/file.txt
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/dir/module.js
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/static/index.html
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/static/main.css
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/static/jquery.min.js
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```
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Read a file in the `asar` archive:
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```javascript
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const fs = require('fs')
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fs.readFileSync('/path/to/example.asar/file.txt')
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```
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List all files under the root of the archive:
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```javascript
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const fs = require('fs')
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fs.readdirSync('/path/to/example.asar')
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```
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Use a module from the archive:
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```javascript
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require('./path/to/example.asar/dir/module.js')
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```
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You can also display a web page in an `asar` archive with `BrowserWindow`:
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```javascript
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const { BrowserWindow } = require('electron')
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const win = new BrowserWindow()
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win.loadURL('file:///path/to/example.asar/static/index.html')
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```
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### Web API
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In a web page, files in an archive can be requested with the `file:` protocol.
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Like the Node API, `asar` archives are treated as directories.
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For example, to get a file with `$.get`:
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```html
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<script>
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let $ = require('./jquery.min.js')
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$.get('file:///path/to/example.asar/file.txt', (data) => {
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console.log(data)
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})
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</script>
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```
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### Treating an `asar` Archive as a Normal File
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For some cases like verifying the `asar` archive's checksum, we need to read the
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content of an `asar` archive as a file. For this purpose you can use the built-in
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`original-fs` module which provides original `fs` APIs without `asar` support:
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```javascript
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const originalFs = require('original-fs')
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originalFs.readFileSync('/path/to/example.asar')
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```
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You can also set `process.noAsar` to `true` to disable the support for `asar` in
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the `fs` module:
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```javascript
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const fs = require('fs')
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process.noAsar = true
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fs.readFileSync('/path/to/example.asar')
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```
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## Limitations of the Node API
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Even though we tried hard to make `asar` archives in the Node API work like
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directories as much as possible, there are still limitations due to the
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low-level nature of the Node API.
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### Archives Are Read-only
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The archives can not be modified so all Node APIs that can modify files will not
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work with `asar` archives.
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### Working Directory Can Not Be Set to Directories in Archive
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Though `asar` archives are treated as directories, there are no actual
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directories in the filesystem, so you can never set the working directory to
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directories in `asar` archives. Passing them as the `cwd` option of some APIs
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will also cause errors.
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### Extra Unpacking on Some APIs
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Most `fs` APIs can read a file or get a file's information from `asar` archives
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without unpacking, but for some APIs that rely on passing the real file path to
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underlying system calls, Electron will extract the needed file into a
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temporary file and pass the path of the temporary file to the APIs to make them
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work. This adds a little overhead for those APIs.
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APIs that requires extra unpacking are:
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* `child_process.execFile`
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* `child_process.execFileSync`
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* `fs.open`
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* `fs.openSync`
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* `process.dlopen` - Used by `require` on native modules
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### Fake Stat Information of `fs.stat`
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The `Stats` object returned by `fs.stat` and its friends on files in `asar`
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archives is generated by guessing, because those files do not exist on the
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filesystem. So you should not trust the `Stats` object except for getting file
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size and checking file type.
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### Executing Binaries Inside `asar` Archive
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There are Node APIs that can execute binaries like `child_process.exec`,
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`child_process.spawn` and `child_process.execFile`, but only `execFile` is
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supported to execute binaries inside `asar` archive.
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This is because `exec` and `spawn` accept `command` instead of `file` as input,
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and `command`s are executed under shell. There is no reliable way to determine
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whether a command uses a file in asar archive, and even if we do, we can not be
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sure whether we can replace the path in command without side effects.
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## Adding Unpacked Files to `asar` Archives
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As stated above, some Node APIs will unpack the file to the filesystem when
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called. Apart from the performance issues, various anti-virus scanners might
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be triggered by this behavior.
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As a workaround, you can leave various files unpacked using the `--unpack` option.
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In the following example, shared libraries of native Node.js modules will not be
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packed:
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```sh
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$ asar pack app app.asar --unpack *.node
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```
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After running the command, you will notice that a folder named `app.asar.unpacked`
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was created together with the `app.asar` file. It contains the unpacked files
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and should be shipped together with the `app.asar` archive.
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[asar]: https://github.com/electron/asar
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[electron-packager]: https://github.com/electron/electron-packager
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[electron-forge]: https://github.com/electron-userland/electron-forge
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[electron-builder]: https://github.com/electron-userland/electron-builder
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