VSCode: Incremental build with maven reactors

.vscode_tasks.json

{
    // See https://go.microsoft.com/fwlink/?LinkId=733558
    // for the documentation about the tasks.json format
    "version": "0.1.0",
    "command": "gulp",
    "isShellCommand": true,
    "suppressTaskName": true,
    "tasks": [
        {
            "taskName": "compile-server",
            "args": [ "java:compile-server" ],
            "showOutput": "silent",
            "isBuildCommand": true,
            "isWatching": true,
            "problemMatcher": {
                "owner": "java",
                "fileLocation": "absolute",
                "pattern": [
                    {
                        "regexp": "^\\[(ERROR|WARNING|INFO)\\]\\s(.+\\.java):\\[(\\d+,\\d+)\\]\\s(.+)$",
                        "file": 2,
                        "location": 3,
                        "severity": 1,
                        "message": 4
                    }
                ]
            }
        }
    ]
}

gulpfile.js

const gulp = require('gulp');
const watch = require('gulp-watch');
const path = require('path');
const through2 = require('through2');
const spawn = require('child_process').spawn;

const File = require('vinyl');

// Map of project name to object:
// .last: last time it was compiled, to debounce the 'compile-server' logic
// .status: last know result (error/success)
// .running: if true, then a compile is running for this project.
const projects = {};

function getProjectName(file) {
	const segments = file.relative.split(path.sep);
	if (segments.length > 1) {
		return segments[0];
	} else {
		return null;
	}
}

function createProject(name) {
	return { last: 0, running: false, status: 'unknown' };
}

/**
 * Run a maven build for each input `pom.xml`
 * 
 * If the `pom.xml` is a module maven will be executed with `-pl <module>`, otherwise
 * it is assumed that this is the root project.
 * 
 * @param {boolean} [debounce] if `true` then skip silently if this has been requested recently already (ignored if this is a not a project)
 */
function runMaven(debounce = false) {
	return through2.obj(function(file, enc, cb) {
		if (path.basename(file.path) !== 'pom.xml') {
			// Ignore non-pom.xmls
			return cb();
		}

		const args = ['-q', '-B', 'compiler:compile', 'compiler:testCompile' ];

		let project;
		const projectName = getProjectName(file);
		if (projectName) {
			project = projects[projectName];
			if (project.running) {
				return cb();
			}

			const now = Date.now();
			if (!!debounce && now - project.last < 5000) {
				return cb();
			}
			project.running = true;
			project.last = now;
			args.push('-pl', projectName);
		} else {
			args.push('--fail-at-end');
		}

		const mvn = spawn('mvn', args, { shell: true });
		// FIXME: Maven prints the same error multiple times for reactor projects -- once in the project, and then in the summary.
		// We really only need the error once, until VSC dedups things on its own (problemCollectors.ts)
		mvn.stderr.pipe(process.stderr);
		mvn.stdout.pipe(process.stdout);
		mvn.on('close', (code) => {
			if (project) {
				project.running = false;
				project.status = code === 0 ? 'success' : 'failure';
			}
			return cb();
		});
	});
}

// Compile all files once
gulp.task('java:compile-initial', function() {
	function initializeProjects() {
		return through2.obj(function(file, enc, cb) {
			const projectName = getProjectName(file);
			projects[projectName] = createProject();
			return cb(null);
		}, function(cb) {
			this.push(new File({
			  cwd: process.cwd(),
				base: process.cwd(),
				path: "pom.xml",
			}));
			return cb(null);
		});
	}

	return gulp.src('*/pom.xml')
		.pipe(initializeProjects())
		.pipe(runMaven());
});

// Task that runs continously, and feeds changed files into the 'java:compile-project' task 
gulp.task('java:compile-server', [ 'java:compile-initial' ], function() {
    return watch('*/src/**/*.java', { read: false }, function(f) {
		// Determine the project
		const projectName = getProjectName(f);
		return gulp.src(path.join(projectName, 'pom.xml'), { read: false })
			.pipe(runMaven(true)); 
    });
});

package.json

{
  "name": "...",
  "version": "...",
  "devDependencies": {
    "gulp": "^3.9.1",
    "gulp-watch": "^4.3.8",
    "through2": "^2.0.1"
  }
}

Issues:

• severity markers seem to be matched case-sensitive, so only by luck do we end up with Severity.Error for 'ERROR'
• markers are duplicated, because maven produces the errors twice (once additionally at the end of the reactor), and VSCode doesn't deduplicate them
• markers are never cleared (unknown how to actually do that, likely also a bug-let in VSCode)
• somehow the 'column' part of the location doesn't work properly, might be that VSCode and/or java treat tabs as a fixed amount of spaces, and column is not a character index.