Cannot use express middleware in `BackendApplicationContribution.configure`

[bendavis78]

The BackendApplicationContribution leads one to believe they can inject custom functionality into the express server. However:

import { injectable, inject } from 'inversify';
import { ILogger } from '@theia/core';
import { BackendApplicationContribution } from '@theia/core/lib/node';
import * as express from 'express';

@injectable()
export class TheiaCorsBackendApplicationContribution implements BackendApplicationContribution {
    @inject(ILogger) private readonly logger: ILogger;

    configure(app: express.Application): void {
        this.logger.info('Setting up Middleware');
        app.use((req: any, res: any, next: any) => {
            this.logger.info('Middleware activated');
            next();
        });
    }
}

The above example will not work. The middleware function never gets called.

[debovema]

Have you added something like that:

    bind(TheiaCorsBackendApplicationContribution).toSelf().inSingletonScope();
    bind(BackendApplicationContribution).toService(TheiaCorsBackendApplicationContribution);

in your backend module ?

I did and it seems to work.

[Lythenas]

I'm running into the same issue. Registering custom endpoints with app.get seems to work. But the app.use middleware is only called for /favicon.ico (presumably because the file does not exist).

I suspect the reason for this is that some other middleware that was registered earlier handles the requests and does not call next.

Is there a way to register a middleware before anything else is configured in the express.Application?

[qianyiliushang]

I'm running into the same issue. Registering custom endpoints with app.get seems to work. But the app.use middleware is only called for /favicon.ico (presumably because the file does not exist).

I suspect the reason for this is that some other middleware that was registered earlier handles the requests and does not call next.

Is there a way to register a middleware before anything else is configured in the express.Application?

I'm running into the same issue,too, any updates?

[Lythenas]

Sorry for not posting here sooner. I actually found a hacky solution a while back:

    configure(app: express.Application): MaybePromise<void> {
        const middleware = this.expressMiddleware.bind(this);
        app.use(middleware);

        // Hacky way to add our middleware at the beginning.
        // This might break if any of the other other BackendApplicationContributions
        // also do something like this.
        const routerStack = app._router.stack as any[];

        const layer = routerStack.splice(routerStack.findIndex(r => r.handler === middleware), 1);
        routerStack.splice(routerStack.findIndex(r => r.name === "expressInit") + 1, 0, ...layer);
    }

This basically puts my middleware after the "expressInit" middleware (which I picked more or less randomly from what worked).

I expect that this can easily break with theia updates.

[qianyiliushang]

Hi, I want to know what expressMiddleware is in your code snipt, is that a shortcut for app.use((req,res,next))? Thank you

[Lythenas]

Yes, it is just a normal middleware function. You could also specify an arrow function there. But my middleware was a bit too big as an arrow function for my taste.