Month: May 2017

Skip certain file extensions from Morgan HTTP logger for Express.js

Morgan is a useful HTTP request logger middleware for Express.js, which plugs in nicely to Node.js and the MEAN stack. More info on Morgan at https://github.com/expressjs/morgan

One useful feature is to add a filter to skip certain files that you don’t want logged. For example you may not want a log of every single get of an image file.

First you define a filter function that returns a boolean for certain file extensions types:

function skipLog (req, res) {
  var url = req.url;
  if(url.indexOf('?')>0)
    url = url.substr(0,url.indexOf('?'));
  if(url.match(/(js|jpg|png|ico|css|woff|woff2|eot)$/ig)) {
    return true;
  }
  return false;
}

The function above will return true for any files with the extension .js, .jpg, .png, (and so on…). Note: you’ll want to return true for skips because you want to evaluate it to skip=true. Also note that the code extracts out the filename from the URL in case there are request parameters attached to it.

Then to use it, you would initiate Morgan like so when setting it up in express.js:

var morgan = require('morgan');
var express = require('express');
var app = express();
//...
app.use(morgan('combined', {stream: accessLogStream, skip: skipLog}));

And that’s it!

Redirecting all stdout and stderr to Logger in Java

This would seem obvious, but it wasn’t to me, so I thought I’d write about it to help out anyone else attempting to accomplish the same. It’s pretty straight forward actually.

Let’s say you have a java.util.logging.Logger object that you’ve initialized, and you want to redirect all stderr (exceptions, System.err.print()’s) and stdout (System.out.print()) to it. You’ll want to use System.setErr and System.setOut to a custom java.io.PrintStream object which writes to your Logger object.

Let’s first define a class to do this for us, and then I’ll explain how it works:

class CustomOutputStream extends OutputStream 
{
	Logger logger;
	Level level;
	StringBuilder stringBuilder;
	
	public CustomOutputStream(Logger logger, Level level)
	{
		this.logger = logger;
		this.level = level;
		stringBuilder = new StringBuilder();
	}
	
	@Override
	public final void write(int i) throws IOException 
	{
		char c = (char) i;
		if(c == '\r' || c == '\n')
		{
			if(stringBuilder.length()>0)
			{
				logger.log(level,stringBuilder.toString());
				stringBuilder = new StringBuilder();
			}
		}
		else
			stringBuilder.append(c);
	}
}

The way this works is by extending OutputStream and overriding the write() method. But write() only takes one character at a time, so essentially you want to buffer each character into a String Builder, to build up the whole line, until you encounter a \r or \n (carriage return, new line), and then submit it to the logger.

To attach CustomOutputStream to your logger:

Logger logger = Logger.getLogger(...);
//...
System.setErr(
		new PrintStream(
			new CustomOutputStream(logger,Level.SEVERE) //Or whatever logger level you want
			)
		);  
System.setOut(
		new PrintStream(
				new CustomOutputStream(logger,Level.FINE) //Or whatever logger level you
			)
		);

Note: if you’ve configured your logger to always include the class/method with the log message, a side effect of this is that the output will not include your original method that wrote the log message to stderr or stdout , but instead your.package.CustomOutputStream.write().

Happy logging!