Suppose that group of developers work in parallel on parts of big project - some developers are working on service implementation, while others are working on code using this service. Both groups agreed on service API, and started working separately, having in mind the API assumptions...

Do you think this story will have happy end? Well, ... - maybe :) - there are tools which can help achieve it :) - one of them is FindBugs, supported with JSR-305 (annotations for software defect detection).

Let's take a look at the service API contract:

package com.blogspot.vardlokkur.services;

import java.util.List;

import javax.annotation.CheckForNull;
import javax.annotation.Nonnull;

import com.blogspot.vardlokkur.entities.domain.Employer;

/**
* Defines the API contract for the employer service.
*
* @author Warlock
* @since 1.0
*/
public interface EmployerService {

    /**
* @param identifier the employer's identifier
* @return the employer having specified {@code identifier}, {@code null} if not found
*/
    @CheckForNull Employer withId(@Nonnull Long identifier);

    /**
* @param specification defines which employers should be returned
* @return the list of employers matching specification
*/
    @Nonnull List<Employer> thatAre(@Nonnull Specification specification);

}

As you see there are annotations like @Nonnull or @CheckForNull added to the service method signatures. The purpose of their usage is to define the requirements for the method parameters (ex. identifier parameter cannot be null), and the expectations for the values returned by methods (ex. service method result can be null and you should check it in your code).

So what? - you may ask - should I check them in the code by myself or trust the co-workers that they will use the guidelines defined by those annotations? Of course not :) - trust no one, use the tools which will verify the API assumptions, like FindBugs.

Suppose that we have following service API usage:

package com.blogspot.vardlokkur.test;

import org.junit.Before;
import org.junit.Test;

import com.blogspot.vardlokkur.services.EmployerService;
import com.blogspot.vardlokkur.services.impl.DefaultEmployerService;

/**
* Employer service test.
*
* @author Warlock
* @since 1.0
*/
public class EmployerServiceTest {

    private EmployerService employers;

    @Before
    public void before() {
        employers = new DefaultEmployerService();
    }

    @Test
    public void test01() {
        Long identifier = null;
        employers.withId(identifier);
    }

    @Test
    public void test02() {
        employers.withId(Long.valueOf(1L)).getBusinessName();
    }

    @Test
    public void test03() {
        employers.thatAre(null);
    }
}

 Let's try to verify the code against the service API assumptions:

Null passed for nonnull parameter

Possible null pointer dereference

Similar way, guys writing the service code may verify their work against defined API assumptions, for ex. if you run FindBugs for the very early version of service implementation:

package com.blogspot.vardlokkur.services.impl;

import java.util.List;

import com.blogspot.vardlokkur.entities.domain.Employer;
import com.blogspot.vardlokkur.services.EmployerService;
import com.blogspot.vardlokkur.services.Specification;

/**
* Default implementation of {@link EmployerService}.
*
* @author Warlock
* @since 1.0
*/
public class DefaultEmployerService implements EmployerService {

    /**
* {@inheritDoc}
*/
    public Employer withId(Long identifier) {
        return null;
    }

    /**
* {@inheritDoc}
*/
    public List<Employer> thatAre(Specification specification) {
        return null;
    }

}

Following error will be found:

• JSR-305: Annotations for Software Defect Detection
• JSR 305: a silver bullet or not a bullet at all?