DDevelopment & Design

SortedSet + Joda DateTime == danger

  • bympr
  • February 16, 2013
  • 2 minute read
It’s been quite a long time since I wrote something on this blog… Two things occurred that made me do this. Firstly, I’m going to talk at Java Developer’s Conference in Cairo and at Booster conference in Bergen next month, so I want to have some co…

It’s been quite a long time since I wrote something on this blog… Two things occurred that made me do this.
Firstly, I’m going to talk at Java Developer’s Conference in Cairo and at Booster conference in Bergen next month, so I want to have some content when I put a link at my slides ;)
Secondly, last week I encountered really weird situation. In fact it was endless loop.
Yep.
In was in rather critical place of our app and it was on semi-production environment so it was quite embarassing. What’s more, the code was working before, it was untouched for about half a year, and it had pretty good test coverage. It looked more or less like this (I’ve left some stuff out, so now it looks too complex for it’s task):

def findDates(dates:SortedSet[DateTime],a:List[DateTime])=
  if (dates.isEmpty || dates.head.toMilis < date) {
    (dates, a)
  } else {
    findDates(dates - dates.head, a+dates.head)
  }

Just simple tail recursion, how can it loop endlessly? It turns out it can. Actually, for some specific data dates – dates.head == dates.
Why? The reason is DateTime is not consistent with equals. If you look into Comparable definition, it says:

It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave “strangely” when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method.

What does this mean? That you should only use sorted collections for classes that satisfy following:if a.compareTo(b) == 0 then a.equals(b) == true And in joda’s DateTime javadoc you can read:

Compares this object with the specified object for ascending millisecond instant order. This ordering is inconsistent with equals, as it ignores the Chronology.

And it turns out that this was our case – in our data there were dates that were equal with respect to miliseconds, but in different timezones. What’s more, not every pair of such dates can lead to disaster. They have to cause some mess in underlying black-red tree… The solution was to introduce some wrapper (we used it anyway actually) that defined comparison consistent with equality…

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Need to make a quick json fixes – JSONPath for rescue

From time to time I have a need to do some fixes in my json data. In a world of flat files I do this with grep/sed/awk tool chain. How to handle it for JSON? Searching for a solution I came across the JSONPath. It quite mature tool (from 2007) but I haven't hear about it so I decided to share my experience with others.

First of all you can try it without pain online: http://jsonpath.curiousconcept.com/. Full syntax is described at http://goessner.net/articles/JsonPath/



But also you can download python binding and run it from command line:
$ sudo apt-get install python-jsonpath-rw
$ sudo apt-get install python-setuptools
$ sudo easy_install -U jsonpath

After that you can use inside python or with simple cli wrapper:
#!/usr/bin/python
import sys, json, jsonpath

path = sys.argv[1]

result = jsonpath.jsonpath(json.load(sys.stdin), path)
print json.dumps(result, indent=2)

… you can use it in your shell e.g. for json:
{
"store": {
"book": [
      {
"category": "reference",
"author": "Nigel Rees",
"title": "Sayings of the Century",
"price": 8.95
      },
      {
"category": "fiction",
"author": "Evelyn Waugh",
"title": "Sword of Honour",
"price": 12.99
      },
      {
"category": "fiction",
"author": "Herman Melville",
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"price": 8.99
      },
      {
"category": "fiction",
"author": "J. R. R. Tolkien",
"title": "The Lord of the Rings",
"isbn": "0-395-19395-8",
"price": 22.99
      }
    ],
"bicycle": {
"color": "red",
"price": 19.95
    }
  }
}

You can print only book nodes with price lower than 10 by:
$ jsonpath '$..book[?(@.price

Result:
[
  {
"category": "reference",
"price": 8.95,
"title": "Sayings of the Century",
"author": "Nigel Rees"
  },
  {
"category": "fiction",
"price": 8.99,
"title": "Moby Dick",
"isbn": "0-553-21311-3",
"author": "Herman Melville"
  }
]

Have a nice JSON hacking!From time to time I have a need to do some fixes in my json data. In a world of flat files I do this with grep/sed/awk tool chain. How to handle it for JSON? Searching for a solution I came across the JSONPath. It quite mature tool (from 2007) but I haven't hear about it so I decided to share my experience with others.