API Programming with Java

- A Hello World Project -
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Contents

  1. Introduction
  2. Prerequisites
  3. Resources
  4. IDE Configurations
  5. Program a @GET API
  6. Program a @POST API
  7. Exercises
  8. Troubleshooting

Introduction

RESTful web services are built to work best on the Web (REST: Representational State Transfer). REST is an architectural style that specifies constraints such as uniform interface and enable services to work best on the Web. In REST architecture, data and functionality are considered resources, and accessed them by using Uniform Resource Identifiers (URIs). There are some motivations of using RESTful Web Services:

  • Open protocols and standards used for exchanging data between applications or systems;
  • High interoperability (e.g., between Java and Python);
  • Resource representation such as XML, JSON;
  • Data exchange via HTTP methods such as GET, POST, PUT, DELETE.

The objective of this tutorial is to help you to get familiar with programming APIs, especially understand the idea of microservices architecture.


Prerequisite

The examples discussed in this tutorial were programmed using the following software and tools:

  • Eclipse IDE for Enterprise Java Developers
  • Postman
  • Apache Tomcat 8.5
  • Apache XAMPP
  • Jersey 2


Resources

  • Sample Code with Dependencies/Libraries
  • Dependencies/Libraries only


IDE Configurations

We first need to configure Apache Tomcat by extracting it to your local disk after downloading the source file. We note that the path to Apache Tomcat directory should not contain any whitespace or special characters. Then, you should follow the steps below to configure Apache Tomcat in Eclipse IDE:

  • Open Windows → Preferences → Server → Runtime Environments;
  • Configure the Apache Tomcat installation directory as shown in below figures.
Fig 1A. Configure Server Runtime Environment (1)
Fig 1A. Configure Server Runtime Environment (1)
Fig 1B. Configure Server Runtime Environment (2)
Fig 1B. Configure Server Runtime Environment (2)
Fig 1C. Configure Server Runtime Environment (3)
Fig 1C. Configure Server Runtime Environment (3)


Program a @GET API

In this first example, we will show you how to program an API with GET method. We note that GET method is a kind of HTTP request methods to indicate the desired action to be performed for a given resource. Before beginning, we need to configure the project as follows:

  • Open Eclipse IDE and create a new Dynamic Web Project, as shown in below figures.
  • Fig 2. Create a Dynamic Web Project
    Fig 2. Create a Dynamic Web Project
    Fig 3. Configure the created Dynamic Web Project (1)
    Fig 3. Configure the created Dynamic Web Project (1)
    Fig 4. Configure the created Dynamic Web Project (2)
    Fig 4. Configure the created Dynamic Web Project (2)
  • After creating the project, you need to configure its build path, and the results should look like in Fig. 5.
    • Option 1 — Extract all JARs in jaxrs-ri/api, jaxrs-ri/ext, jaxrs-ri/lib from Jersey to src/main/webapp/WEB-INF/lib directory.
    • Option 2 — I have already bundled all necessary libraries and dependencies of this sample project, you just need to copy-and-paste them to src/main/webapp/WEB-INF/lib directory. Visit the Resources section to download the file.
  • Fig 5. Configure project build path
    Fig 5. Configure project build path
  • Next, create a package name com.soa.computing and a Calculator.java class, as shown in Fig. 6. According to the Oracle documentation, the @Path annotation identifies the URI path template to which the resource responds, and is specified at the class level of a resource. The @Path annotation's value is a partial URI path template relative to the base URI of the server on which the resource is deployed, the context root of the WAR, and the URL pattern to which the Jersey helper servlet responds. URI path templates are URIs with variables embedded within the URI syntax. These variables are substituted at runtime in order for a resource to respond to a request based on the substituted URI. Variables are denoted by curly braces. To obtain the value of the username variable, the @PathParam annotation may be used on the method parameter of a request method.
  • Fig 6. Configure the created Dynamic Web Project (2)
    Fig 6. Calculator.java file
  • Before deploying this project as a WebAPI, you need to configure web.xml file located in /WebContent/WEB-INF directory as follows:
  • Fig 7. Configure the web.xml file
    Fig 7. Configure the web.xml file
  • Now, you can deploy and run your project on Apache Tomcat server as shown in below figures. The first time when you run your project, Eclipse IDE will ask you to confirm the runtime environment, as we presented in IDE Configurations section. In the next step, we will present how to test (or call) your programmed API.
  • Fig 8. Deploy and run your project on an Apache Tomcat server
    Fig 8. Deploy and run your project on an Apache Tomcat server
    Fig 9. Configure Apache Tomcat server
    Fig 9. Configure Apache Tomcat server
  • After deploying your project (or APIs) to Apache Tomcat server, we can now request the implemented APIs and get corresponding responses. To do this, we use Postman software with the configurations as shown in below figure.
  • Fig 10. Postman interface for testing API (1)
    Fig 10. Postman interface for testing an API (1)
    Fig 11. Postman interface for testing API (2)
    Fig 11. Postman interface for testing an API (2)


Program a @POST API

  • In previous section, we implement a @GET method whose aim is to request a representation of the specified resource. In this section, we implement a @POST method to submits an entity to the specified resource, often causing a change in state or side effects on the server.
  • Before implementing a @POST method, we need to implement a Student.java class that represents a Student object in real-world. We assume a Student has four properties, i.e., id:String, name:String, gender:int, major:String, as shown in below figure.
  • Fig 12. Implementation of Student class
    Fig 12. Implementation of Student.java class
  • After implementing Student.java class, we can now implement a @POST method in Calculator.java, as shown in below figure. You can implement this method in another class. There are two nothworthy points in this method:
    • The @Consumes annotation is used to specify which MIME media types of representations a resource can accept, or consume, from the client. If @Consumes is applied at the class level, all the response methods accept the specified MIME types by default. If applied at the method level, @Consumes overrides any @Consumes annotations applied at the class level. If a resource is unable to consume the MIME type of a client request, the JAX-RS runtime sends back an HTTP 415 ("Unsupported Media Type") error. The value of @Consumes is an array of String of acceptable MIME types such as text/plain, text/html.
    • Mapping a JSON Object — The input of this method is a Student object that send from a client, and the question here is how this method can map an JSON object from client to a Java object (Student). POJO suppport represents the easiest way to convert your Java Objects to JSON and back which is based on the Jackson library. To use this approach, you will need to turn the JSONConfiguration.FEATURE_POJO_MAPPING feature on which could be done in web.xml using the following servlet init parameter, as shown in below figure. For further reading, you can refer to this article to learn more about Jersey JSON.
  • Fig 13. Implementation of a POST method
    Fig 13. Implementation of a @POST method
    Fig 14. Turn on Jersey-POJOMappingFeature in web.xml configuration
    Fig 14. Turn on Jersey-POJOMappingFeature in web.xml configuration
  • To test this new @POST API, we need to use Postman software with the configurations as shown in below figure. After submitting the request, the API will (1) return a message to client to inform the results, and (2) print the Student object on server's console window.
  • Fig 15. Configurations of Postman to test a @POST API.
    Fig 15. Configurations of Postman to test a @POST API.


Exercises

  1. Continue with the function GetDateTime from the sample project, you are required to modify this method that allows a client passes location information as a parameter such as America/Chicago, Europe/Bucharest, then the method return the current date-and-time of the given location. To implement this methods, you can take advantage of Joda-Time library which has been already bundled as presented in Resources section.
  2. Continue with the demo of Student object from the sample project, you are required to implement a method that inputs an id of a student and returns TRUE/FALSE where TRUE indicates the given id exists in the database, and vice versa. To implement this method, you do not need to construct a database, just declare an array/list of students.
  3. Continue with the demo of Student object from the sample project, based on the idea of SETTER method of OOP, you are required to implement a set of methods that allows a client change student's information. For example, public Response changeGender(int id, int gender) method does change the gender property of a student to the new one based on the given id information; then, the method returns HTTP response status codes to inform the results.
  4. Instead of returning Response objects, you are required to modify the previous methods to make them return JSON objects. For example, public boolean searchStudentById(String id) method is changed to public Student searchStudentById(String id) to return a JSON object that represents a student, otherwise, return NULL or a dumb object. You can read this article to know how to convert a Java object to/from JSON object by using Jackson 2 library.
  5. (*) Programming a Webpage that allows a client to interact with APIs and get responses from them.


Troubleshooting

  1. If you get the following error message SEVERE: Servlet [Jersey REST Service] in web application [PROJECT_NAME] threw load() exception java.lang.IllegalArgumentException, you should change Java Compiler version from 17 to 1.7 by performing the following steps: Right-click on project name → Properties → Java Compiler → Uncheck the Use compliance from execution environment 'JavaSE-17' on the 'Java Build Path' option → Select 1.7 from the drop-down list of Compiler compliance level → Then, click Apply and Close button to save the changes.
  2. Fig 16. Configure Java Compilier version from 17 to 1.7.
    Fig 16. Configure Java Compilier version from 17 to 1.7.
  3. Fixing 404 - Page Not Found error — When you run your project on Apache Tomcat server, it will automatically open the default webpage (index.html). By default, the index.html file is absent, it will thus return the 404 - Page Not Found error code, as shown in Fig. 17. To address this problem, we just create an index.html file in src/main/webapp/ directory. For example, the content of index.html file is presented in Fig. 18.
  4. Fig 12. 404 - Page Not Found error.
    Fig 17. 404 - Page Not Found error.
    Fig 18. Sample content of index.html file.
    Fig 18. Sample content of index.html file.
  5. If you get this error message - "Java compiler level does not match the version of the installed Java project facet." - you should re-configure the project facet (Fig. 19) as follows: Right-click on project name → Select Project Facets → Change Java version to 1.7 → Then, click Apply and Close to save changes.
  6. Fig 19. Configure Project Facets.
    Fig 19. Configure Project Facets.
  7. How to change Tomcat's port in Eclipse — By default, Apache Tomcat is configured to listen on 8005, 8080, 8009 for Tomcat admin port, HTTP/1.1, AJP/1.3, respectively. We can easily change the Tomcat's port by doing the following steps: Double-click on the server name in Servers view (as shown in Fig. 20) → A Tomcat's configuration page will be open and you can change the port to your desire number, as shown in Fig. 21 → Press Ctrl + S (MacOS: Cmd + S) to save changes → Next time when you start your Apache Tomcat server, you can spot the new port value as shown in Fig. 22.
  8. Fig 20. Server view in Eclipse IDE.
    Fig 20. Server view in Eclipse IDE.
    Fig 21. Apache Tomcat configuration page.
    Fig 21. Apache Tomcat configuration page.
    Fig 22. Configure Apache Tomcat's port
    Fig 22. Configure Apache Tomcat's port


Relaxing 🧘