JDT_LS_PATH env var to reference local JDT-LS build

Author: mickaelistria

CONTRIBUTING.md

@@ -176,6 +176,16 @@ While developing the language server and the extension, you don't need to deploy
 - You can modify `launch.json` to use a different port:
     - Modify `JDTLS_CLIENT_PORT` to specify the port VS Code should connect to.
 
+## C-3) Set location to a local copy of JDT-LS
+
+This approach is mainly targeting developers of JDT-LS who want to test their changes in VSCode without having to rebuild vscode-java and to deal with the npm and vsix toolchains.
+
+- Make sure a recent snapshot of vscode-java is installed in your VSCode instance (as of vscode-java 1.20.0)
+- (Optionally) clear the workspace state: `$ rm -r $HOME/.config/Code/User/workspaceStorage/0123456789abcdef01234567890abcdef/redhat.java/jdt_ws`
+- Start VSCode with the `JDT_LS_PATH` environment variable set to the local copy of JDT-LS you want to try: `$ JDT_LS_PATH=$HOME/git/eclipse.jdt.ls/org.eclipse.jdt.ls.product/target/repository code`
+
+Most other options such as `DEBUG_VSCODE_JAVA` are still usable with this approach.
+
 ## Sideloading
 
 You can create a binary that you can sideload to your VS Code installation.

src/javaServerStarter.ts

@@ -155,7 +155,7 @@ function prepareParams(requirements: RequirementsData, javaConfiguration, worksp
 		params.push('-noverify');
 	}
 
-	const serverHome: string = path.resolve(__dirname, '../server');
+	const serverHome: string = process.env.JDT_LS_PATH || path.resolve(__dirname, '../server');
 	const launchersFound: Array<string> = glob.sync('**/plugins/org.eclipse.equinox.launcher_*.jar', { cwd: serverHome });
 	if (launchersFound.length) {
 		params.push('-jar'); params.push(path.resolve(serverHome, launchersFound[0]));
@@ -171,12 +171,12 @@ function prepareParams(requirements: RequirementsData, javaConfiguration, worksp
 		configDir = isSyntaxServer ? 'config_ss_linux' : 'config_linux';
 	}
 	params.push('-configuration');
+	params.push(startedFromSources() || process.env.JDT_LS_PATH !== undefined ?
+		path.resolve(serverHome, configDir) : resolveConfiguration(context, configDir));
 	if (startedFromSources()) { // Dev Mode: keep the config.ini in the installation location
 		console.log(`Starting jdt.ls ${isSyntaxServer?'(syntax)' : '(standard)'} from vscode-java sources`);
-		params.push(path.resolve(__dirname, '../server', configDir));
-	} else {
-		params.push(resolveConfiguration(context, configDir));
 	}
+
 	params.push('-data'); params.push(workspacePath);
 	return params;
 }