Friday, June 30, 2017

How to a Build Standalone REST Application in Java

There are many frameworks that allow you to build REST easily in Java. In this blog, however, I am going tho show you how to build a standalone REST application without any framework. We are going to use the following libraries:
  1. Jetty for web server
  2. Jersey for REST
  3. Jackson for JSON
  4. Swagger for API documentation

First we need the build script. I am going to use Gradle for this.
plugins {
    id "com.github.johnrengelman.shadow" version "2.0.1"
    id "java"

apply plugin: "java"
apply plugin: "com.github.johnrengelman.shadow"

defaultTasks = ["clean", "build", "shadowJar"]

repositories {

dependencies {
    // Jetty dependencies
    compile "org.eclipse.jetty:jetty-server:9.4.6.v20170531"
    compile "org.eclipse.jetty:jetty-servlet:9.4.6.v20170531"

    // Jersey dependencies
    compile "org.glassfish.jersey.core:jersey-server:2.25.1"
    compile ""
    compile "org.glassfish.jersey.containers:jersey-container-servlet:2.25.1"

    // Swagger dependencies
    compile "io.swagger:swagger-core:1.5.15"
    compile "io.swagger:swagger-annotations:1.5.15"
    compile "io.swagger:swagger-jersey2-jaxrs:1.5.15"
    // Required by BeanConfig
    runtime "org.codehaus.groovy:groovy:2.4.12"

    // Required by Swagger
    runtime "ch.qos.logback:logback-core:1.2.3"
    runtime "ch.qos.logback:logback-classic:1.2.3"

jar {
    manifest {
        attributes "Main-Class": ""

Second, we will write a typical JAX-RS application with JAX-RS, Jackson, and Swagger annotations.

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import io.swagger.annotations.Api;
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import io.swagger.annotations.ApiOperation;
import io.swagger.annotations.ApiResponse;


@Api(value = "/hello", description = "API for Hello")
public class HelloResource {
    public static class HelloRequest {
        @ApiModelProperty(value = "The name", required = true)
        private final String name;

        public HelloRequest(@JsonProperty("name") String name) {
   = name;

        public String getName() {
            return name;

    public static class HelloResponse {
        @ApiModelProperty(value = "The message", required = true)
        private final String message;

        public HelloResponse(@JsonProperty("message") String message) {
            this.message = message;

        public String getMessage() {
            return message;

    @ApiOperation(value = "Gets the message", response = HelloResponse.class)
    @ApiResponse(code = 200, message = "Successful operation")
    public Response getMessage(HelloRequest request) {
        return Response.ok(new HelloResponse("Hello, " + request.getName())).build();

Third, we will need to register our REST resource into Jetty and start the server.

import com.fasterxml.jackson.jaxrs.json.JacksonJsonProvider;
import io.swagger.jaxrs.config.BeanConfig;
import io.swagger.jaxrs.listing.ApiListingResource;
import io.swagger.jaxrs.listing.SwaggerSerializers;
import org.eclipse.jetty.server.Handler;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.server.handler.HandlerList;
import org.eclipse.jetty.server.handler.ResourceHandler;
import org.eclipse.jetty.servlet.ServletContextHandler;
import org.eclipse.jetty.servlet.ServletHolder;
import org.eclipse.jetty.util.resource.Resource;
import org.glassfish.jersey.server.ServerProperties;
import org.glassfish.jersey.servlet.ServletContainer;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import java.util.StringJoiner;

public class HelloServer {
    public static class Bootstrap extends HttpServlet {
        public void init(ServletConfig config) throws ServletException {

            BeanConfig beanConfig = new BeanConfig();
            beanConfig.setTitle("Hello API");
            beanConfig.setSchemes(new String[]{"http"});

    public static void main(String[] args) throws Exception {
        ServletContextHandler servletHandler = new ServletContextHandler(ServletContextHandler.SESSIONS);

        ServletHolder jerseyServlet = servletHandler.addServlet(ServletContainer.class, "/api/*");

            new StringJoiner(",")

        ResourceHandler staticHandler = new ResourceHandler();
        staticHandler.setWelcomeFiles(new String[]{"index.html"});

        HandlerList handlers = new HandlerList();
        // Add two handlers, one for static content and the other one for dynamic content.
        handlers.setHandlers(new Handler[]{staticHandler, servletHandler});

        // The mapping doesn't really matter here.
        ServletHolder swaggerServlet = servletHandler.addServlet(Bootstrap.class, "/");

        Server jettyServer = new Server(8080);

        try {
        } finally {

Few important things to note.
  1. Add the Jersey servlet container (org.glassfish.jersey.servlet.ServletContainer). This servlet implementation comes from Jersey library.
  2. Register some providers, such as our own REST provider (, Jackson provider (com.fasterxml.jackson.jaxrs.json.JacksonJsonProvider), and Swagger providers (io.swagger.jaxrs.listing.ApiListingResource and io.swagger.jaxrs.listing.SwaggerSerializers). The Swagger providers will create swagger.json specified by the servlet mapping path, which is api/swagger.json in this case.

Lastly, we need to add some Swagger UI so that we can display the API documentation in a nice format. What we need is basically download Swagger UI from and copy the dist directory into src/main/resources. The goal of this is to keep the Swagger UI in the JAR and is available in the classpath. Swagger UI is just a static content and we can tell Jetty to serve static content by using org.eclipse.jetty.server.handler.ResourceHandler. Make sure to modify the URL in Swagger's index.html from to http://localhost:8080/api/swagger.json since that's where our swagger.json file lives. To build and run the example, we can type the following command.
./gradlew && java -jar build/libs/java-rest-swagger-example-all.jar
Full source code can be found here.

Thursday, April 6, 2017

How to Shade a JAR and Relocate Classes in Maven

The example below shows how to shade org.fredy.myjar and guava JARS and rename the Guava classes that contain to
                <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>

Friday, March 3, 2017

How to Cancel a Goroutine in Go

The example below shows how to cancel a goroutine while is in the middle of executing a long-running task.
package main

import (

func main() {
    busyChan := make(chan bool)

    ctx, cancel := context.WithCancel(context.Background())
    go func(ctx context.Context) {
        for {
            select {
            case <-busyChan:
                fmt.Println("Pretending to be very busy")
                time.Sleep(5000 * time.Second)
            case <-ctx.Done():
    busyChan <- true
    fmt.Println("Doing some other things")
    time.Sleep(3 * time.Second)
    fmt.Println("Stopping the goroutine")
Prentending to be very busy
Doing some other things
Stopping the goroutine

Wednesday, February 22, 2017

How to Embed JavaScript in Java

Java 8 came with a JavaScript engine called Nashorn. This makes it easy to call some JavaScript code from Java. In this example, I'm going to show how to pass some Java object into JavaScript and vice-versa as well as calling creating a Java object from JavaScript.
import javax.script.Bindings;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.SimpleBindings;
import java.util.Map;

public class JsExample {
    public static class JavaClass {
        public String greet() {
            return "Hello from Java";

    public static void main(String[] args) throws Exception {
        final String script =
            "var File = Java.type(\"\");" +
            "print(new File(\".\").exists());" +
            "var map = {\"msg\": \"Hello from JS\"};" +
            "print(javaClass.greet());" +

        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        Bindings bindings = new SimpleBindings();
        bindings.put("javaClass", new JavaClass());

        Map<String, String> retVal = (Map<String, String>) engine.eval(script, bindings);
Hello from Java
Hello from JS

Wednesday, December 14, 2016

How to Use Jetty to Serve Static Content

  1. Download Jetty.
  2. Extract it to some location. We will call this JETTY_HOME.
  3. Set JETTY_BASE environment variable. This is a separate directory where your configurations live.
  4. Copy $JETTY_HOME/start.ini into $JETTY_BASE.
  5. Create $JETTY_BASE/webapps directory.
  6. Create $JETTY_BASE/webapps/scratch.xml with the content explained here.
  7. Start Jetty by running $JETTY_HOME/bin/ start.
  8. Stop Jetty by running $JETTY_HOME/bin/ stop.

Sunday, August 21, 2016

How to Use Vendoring in Go

Vendoring is a way to put dependencies in a Go project without having to mess with the GOPATH. The idea is simple, that is to put the dependencies in a directory called "vendor".
|-- foo (this directory contains a library, i.e. non-main package)
|   `-- foo.go
|-- main.go (this is the main program)
`-- vendor (this is where the third-party libs live)
    `-- goini
        |-- goini.go
        |-- goini_test.go
        |-- LICENSE
        `-- testdata
            |-- test_expected.ini
            `-- test.ini

The project structure above has the following benefits.
  1. It can be used to build a library.
  2. It can be used to build an executable.
  3. It is go-gettable.
This is an example of using it in a standard Go workspace.
`-- src
    `-- awesomego
        |-- foo
        |   `-- foo.go
        |-- main.go
        `-- vendor
            `-- goini
                |-- goini.go
                |-- goini_test.go
                |-- LICENSE
                `-- testdata
                    |-- test_expected.ini
                    `-- test.ini
To build it as a library:
GOPATH=`pwd` go install awesomego/foo
To build it as an executable:
GOPATH=`pwd` go install awesomego

Wednesday, August 3, 2016

Compile-Time Enum in Go

Unlike other languages, Go does not support enum. However, it is pretty straightforward to create something that behaves similar to enum. There are a lot of articles on the internet on how to create an enum in Go, which pretty much looks like below.
package main

import (

type myType string

const (
    Foo myType = "foo"
    Bar myType = "bar"

func doSomething(t myType) {

func main() {
    // baz := "baz"
    // This will result in compilation error:
    // "cannot use baz (type string) as type myType in argument to doSomething"
    // doSomething(baz)

    // However, this is allowed.
As you can see in the code above, calling doSomething("baz") does not result in a compilation error. To fix that, we can change the code to look like below.
package main

import (

type myType string

const (
    Foo myType = "foo"
    Bar myType = "bar"

func doSomething(t *myType) {

func main() {
    // This will now result in a compilation error.
    // doSomething("baz")

    baz := myType("baz")