Build a rest service from the command line, as simple as “every request has a response.”

Following with Handmade Series with Java, this is the 3rd chapter. I’ll make a rest service, as always, step by step, without IDEs.

I'll use the  Spark library to getting a “pong” to every “ping” request.

I declare a simple class called PingPongGame with main method inside.

public class PingPongGame {
    public static void main(String[] args) 
			//nothing inside yet
    }
}

Inside of the main method, call a static method named get , provided by Spark library, that contains:

• A url path /ping.
• GET verb http.
• Receiving a req and res object from request.
• Which finally return a response, a simple string “pong”.

I add the current import from Spark library spark.Spark.get, getting the code above:

import static spark.Spark.get;

public class PingPongGame {
    public static void main(String[] args) {
        get("/ping", (req, res) -> "pong!");
    }
}

Before executing it, download the spark library, from the maven repository.

After downloading it, place it inside PingPongGame class folder, it runs the next instruction through the command line

java -cp .\spark-core-2.9.3.jar .\PingPongGame.java
#it prints
Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
        at spark.Service.<clinit>(Service.java:58)
        at spark.Spark$SingletonHolder.<clinit>(Spark.java:54)
        at spark.Spark.getInstance(Spark.java:58)
        at spark.Spark.<clinit>(Spark.java:64)
        at HelloWorld.main(HelloWorld.java:5)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
        at spark.Service.<clinit>(Service.java:58)
        at spark.Spark$SingletonHolder.<clinit>(Spark.java:54)
        at spark.Spark.getInstance(Spark.java:58)
        at spark.Spark.<clinit>(Spark.java:64)
        at HelloWorld.main(HelloWorld.java:5)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at jdk.compiler/com.sun.tools.javac.launcher.Main.execute(Main.java:404)
        at jdk.compiler/com.sun.tools.javac.launcher.Main.run(Main.java:179)
        at jdk.compiler/com.sun.tools.javac.launcher.Main.main(Main.java:119)

Cause Spark library by itself can’t run this instance, it depends on others libraries. The required jar libraries list:

• slf4j-api
• spark-core
• javax.servlet-api
• jetty-server
• jetty-util
• jetty-http
• jetty-io
• log4j-slf4j-impl
• log4j-api
• log4j-core

After downloading them, run the long command line above:

java -cp "spark-core-2.9.3.jar;slf4j-api-1.7.25.jar;
javax.servlet-api-3.1.0.jar;jetty-server-9.4.31.v20200723.jar;
jetty-util-9.4.31.v20200723.jar;jetty-http-9.4.31.v20200723.jar;
jetty-io-9.4.31.v20200723.jar;log4j-slf4j-impl-2.17.2.jar;
log4j-api-2.17.2.jar;log4j-core-2.17.2.jar" .\PingPongGame.java
#It prints nothing because the process has started to run, leaving the terminal in a waiting state.

So, at this point, I need to verify if the rest service is “up & running” by executing this command:

curl http://localhost:4567/ping
#it should prints
pong!

Again, simple and effective, perhaps not too simple cause of the amount of added libraries, but still isn't to worry taking in count it made without IDEs.

That’s all!

1. Pong to my ping needed.
2. Creating a standar rest services.
3. Using a method of a knowing library.
4. Add it and test it.
5. Problem happened(missing libraries).
6. Adding missing libraries.
7. Problem fixed.
8. I got my pong trought rest service.

Command and options dictionary:

• java run .class and .java files.
• -cpindicate compiled folders directory(classpath).
• curl command to execute transfer to or from a server.

Tech stack

• Java 11.
• Windows 10.
• Spark-java.

Bonus track

Spark-java is a “microframework” as springboot but only based on functional programming with a simple implementation, I invited you to take in count in your read list the next article, to a deeper understanding.

Web microservices development in Java that will Spark joy