{"id":12683,"date":"2013-11-05T12:53:00","date_gmt":"2013-11-05T11:53:00","guid":{"rendered":"https:\/\/touk.pl\/blog\/?guid=7434a17d85e634f43a8132cd4ab46eb8"},"modified":"2022-07-26T17:26:57","modified_gmt":"2022-07-26T15:26:57","slug":"log4j-and-mdc-in-grails","status":"publish","type":"post","link":"https:\/\/touk.pl\/blog\/2013\/11\/05\/log4j-and-mdc-in-grails\/","title":{"rendered":"Log4j and MDC in Grails"},"content":{"rendered":"
\n Log4j provides very useful feature: MDC<\/b> – mapped diagnostic context. It can be used to store data in context of current thread. It may sound scary a bit but idea is simple. My post is based on post\u00a0http:\/\/burtbeckwith.com\/blog\/?p=521<\/a>\u00a0from Burt Beckwith’s excellent blog, it’s definitely worth checking if you are interested in Grails.\n<\/div>\n
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Short background story…<\/h3>\n
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\n Suppose we want to do logging our brand new shopping system and we want to have in each log customer’s shopping basket number. And our system can be used at once by many users who can perform many transactions, actions like adding items and so on. How can we achieve that? Of course we can add\u00a0basket\u00a0number in every place where we do some logging but this task would be boring and error-prone.\n<\/div>\n
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\n Instead of this we can use MDC to store variable with\u00a0basket number in map.\n<\/div>\n
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\n In fact MDC can be treated as map of custom values for current thread that can be used by logger.\n<\/div>\n
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How to do that with Grails?<\/h3>\n
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\n Using MDC with Grails is quite simple. All we need to do is to create our own custom filter<\/b>\u00a0which works for given urls and puts our data in MDC.\n<\/div>\n
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\n Filters in Grails are classes in directory grails-app\/conf\/* <\/b>which names end with *Filters.groovy <\/b>postfix. We can create this class manually or use Grails command:\n<\/div>\n
grails create-filters info.rnowak.App.Basket\n \u00a0 \n<\/code><\/pre>\n
\n  In result class named BasketFilters<\/b> will be created in grails-app\/conf\/info\/rnowak\/UberApp<\/b>. Initially filter class looks a little bit empty: <\/p>\n
class BasketFilters {\n    def filters = {\n        all(controller:'*', action:'*') {\n            before = {\n\n            }\n            after = { Map model ->\n\n            }\n            afterView = { Exception e ->\n\n            }\n        }\n    }\n}\n<\/code><\/pre>\n
 All we need to do is fill empty closures, modify filter properties and put some data into MDC. <\/p>\n
  all<\/span> is the general name of our filter, as class BasketFilters<\/b> (plural!) can contain many various filters. You can name it whatever you want, for this post let assume it will be named basketFilter<\/span>.\u00a0<\/span>  <\/span>Another thing is change of filter parameters. According to official documentation (link<\/a>) we can customize our filter in many ways. You can specify controller to be filtered, its actions, filtered urls and so on. In our example you can stay with default option where filter is applied to every action of every controller. If you are interested in filtering only some urls, use <\/span>uri <\/span>parameter with expression describing desired urls to be filtered.<\/span>  <\/span>Three closures that are already defined in template have their function and they are started in these conditions:<\/span> <\/p>\n