Getting rid of null parameters with a simple spring aspect

What is the most hated and at the same time the most popular exception in the world? I bet it’s the NullPointerException.

NullPointerException can mean anything, from simple “ups, I didn’t think that can be null” to hours and days of debugging of third-party libraries (try using Dozer for complicated transformations, I dare you).

The funny thing is, it’s trivial to get rid of all the NullPointerExceptions in your code. This triviality is a side effect of a technique called “Design by Contract”.

I won’t go into much details about the theory, you can find everything you need on Wikipedia, but in the nutshell Design by Contract means:

  • each method has a precondition (what it expects before being called)
  • each method has a postcondition (what it guarantees, what is returned)
  • each class has an constraint on its state (class invariant)

So at the beginning of each method you check whether preconditions are met, at the end, whether postconditions and invariant are met, and if something’s wrong you throw an exception saying what is wrong.

Using Spring’s internal static methods that throw appropriate exceptions

(IllegalArgumentException), it can look something like this:

import static org.springframework.util.Assert.notNull;
import static org.springframework.util.StringUtils.hasText;

public class BranchCreator {
    public Story createNewBranch(Story story, User user, String title) {
        verifyParameters(story, user, title);
        Story branch = //... the body of the class returnig an object
        verifyRetunedValue(branch);
        return branch;
    }

    private void verifyParameters(Story story, User user, String title) {
        notNull(story);
        notNull(user);
        hasText(title);
    }

    private void verifyRetunedValue(Story branch) {
        notNull(branch);
    }
}

You can also use Validate class from apache commons instead of spring’s notNull/hasText.

Usually I just check preconditions and write tests for postconditions and constraints. But still, this is all boiler plate code. To move it out of your class, you can use many Design by Contract libraries, for example SpringContracts, or Contract4J. Either way you end up checking the preconditions on every public method.

And guess what? Except for Data Transfer Objects and some setters, every public method I write expects its parameters NOT to be null.

So to save us some writing of this boiler plate ocde, how about adding a simple aspect that will make it impossible in the whole application, to pass null to anything other than DTOs and setters? Without any additional libraries (I assume you are already using Spring Framework), annotations, and what else.

Why would I like to not allow for nulls in parameters? Because we have method overloading in modern languages. Seriously, how often do you want to see something like this:

Address address = AddressFactory.create(null, null, null, null);

And this is not much better either

Microsoft.Office.Interop.Excel.Workbook theWorkbook = ExcelObj.Workbooks.Open(openFileDialog.FileName, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);

The solution

So here is a simple solution: you add one class to your project and a few lines of spring IoC configuration.

The class (aspect) looks like this:

import org.aspectj.lang.JoinPoint;
import static org.springframework.util.Assert.notNull;

public class NotNullParametersAspect {
    public void throwExceptionIfParametersAreNull(JoinPoint joinPoint) {
        for(Object argument : joinPoint.getArgs()) {
            notNull(argument);
        }
    }
}

And the spring configuration is here (remember to change the namespace to your project).

The “&&” is no error, it’s just && condition escaped in xml. If you do not understand aspectj pointcut definition syntaxt, here is a little cheat sheet.

And here is a test telling us that the configuration is succesfull.

public class NotNullParametersAspectIntegrationTest extends AbstractIntegrationTest {
    @Resource(name = "userFeedbackFacade")
    private UserFeedbackFacade userFeedbackFacade;

    @Test(expected = IllegalArgumentException.class)
    public void shouldThrowExceptionIfParametersAreNull() {
        //when
        userFeedbackFacade.sendFeedback(null);

        //then exception is thrown
    }

    @Test
    public void shouldNotThrowExceptionForNullParametersOnDto() {
        //when
        UserBookmarkDto userBookmarkDto = new UserBookmarkDto();
        userBookmarkDto.withChapter(null);
        StoryAncestorDto ancestorDto = new StoryAncestorDto(null, null, null, null);

        //then no exception is thrown
    }
}

AbstractIntegrationTest is a simple class that starts the spring test context. You can use AbstractTransactionalJUnit4SpringContextTests with @ContextConfiguration(..) instead.

The catch

Ah yes, there is a catch. Since spring AOP uses either J2SE dynamic proxies basing on an interface or aspectj CGLIB proxies, every class will either need an interface (for simple proxy based aspect weaving) or a constructor without any parameters (for cglib weaving). The good news is that the constructor can be private.

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6. OAuth2 (login via Facebook)
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Securing web resources is all nice and cool, but in a well designed application it's more natural to secure methods (for example on backend facade or even domain objects). While we may get away with role-based authorization in many intranet business applications, nobody will ever handle assigning roles to users in a public, free to use Internet service. We need authorization based on rules described in our domain.

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The method for accepting chapters, looks like this:

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Log4j and MDC in Grails

Log4j provides very useful feature: MDC - 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 http://burtbeckwith.com/blog/?p=521 from Burt Beckwith's excellent blog, it's definitely worth checking if you are interested in Grails.

Short background story...


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 basket number in every place where we do some logging but this task would be boring and error-prone. 

Instead of this we can use MDC to store variable with basket number in map. 

In fact MDC can be treated as map of custom values for current thread that can be used by logger. 


How to do that with Grails?


Using MDC with Grails is quite simple. All we need to do is to create our own custom filter which works for given urls and puts our data in MDC.

Filters in Grails are classes in directory grails-app/conf/* which names end with *Filters.groovy postfix. We can create this class manually or use Grails command: 
grails create-filters info.rnowak.App.Basket

In result class named BasketFilters will be created in grails-app/conf/info/rnowak/UberApp.

Initially filter class looks a little bit empty:
class BasketFilters {
def filters = {
all(controller:'*', action:'*') {
before = {

}
after = { Map model ->

}
afterView = { Exception e ->

}
}
}
}
All we need to do is fill empty closures, modify filter properties and put some data into MDC.

all is the general name of our filter, as class BasketFilters (plural!) can contain many various filters. You can name it whatever you want, for this post let assume it will be named basketFilter

Another thing is change of filter parameters. According to official documentation (link) 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 uri parameter with expression describing desired urls to be filtered.

Three closures that are already defined in template have their function and they are started in these conditions:

  • before - as name says, it is executed before filtered action takes place
  • after - similarly, it is called after the action
  • afterView - called after rendering of the actions view
Ok, so now we know what are these mysterious methods and when they are called. But what can be done within them? In official Grails docs (link again) under section 7.6.3 there is a list of properties that are available to use in filter.

With that knowledge, we can proceed to implementing filter.

Putting something into MDC in filter


What we want to do is quite easy: we want to retrieve basket number from parameters and put it into MDC in our filter:
class BasketFilters {
def filters = {
basketFilter(controller:'*', action:'*') {
before = {
MDC.put("basketNumber", params.basketNumber ?: "")
}
after = { Map model ->
MDC.remove("basketNumber")
}
}
}
}

We retrieve basket number from Grails params map and then we put in map under specified key ("basketNumber" in this case), which will be later used in logger conversion pattern. It is important to remove custom value after processing of action to avoid leaks.

So we are putting something into MDC. But how make use of it in logs?


We can refer to custom data in MDC in conversion patter using syntax: %X{key}, where key is our key we used in filter to put data, like:
def conversionPattern = "%d{yyyy-MM-dd HH:mm:ss} %-5p %t [%c{1}] %X{basketNumber} - %m%n"


And that's it :) We've put custom data in log4j MDC and successfully used it in logs to display interesting values.