Light4j Cookbook - Rest API, CORS and RDBMS

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Light 4j is a fast, lightweight and cloud-native microservices framework. To learn and know how Light4j works, read our previous article which will give some insight on Light4j and its architecture. In this article, we will see what and how hybrid framework works and integrate with RDMS databases like Mysql, also built in option of CORS handler for in-flight request.

Hybrid framework
Hybrid framework provides the facility of both monolithic and microservice options. This is happening because server is separate project and each service is built as individual project, then deploy by bundling the services on each server node or all services in one server as monolithic.

1. Hybrid server can be generated from the scaffold codegen-cli tool by providing the below command.

 java -jar light-codegen/codegen-cli/target/codegen-cli.jar -f light-hybrid-4j-server -o light-example-4j/hybrid/hello-world/server -c model-config/hybrid/hello-world/server/config.json

 Now go to the generated folder path light-example-4j/hybrid/hello-world/server and execute       

 mvn clean install exec:exec

Below will be console output.

 colferPath = /api/colfer
 jsonPath = /api/json
 formPath = /api/form
 HOST IP null
 Http port disabled.
 Https Server started on ip: Port:8443


2. Hybrid service1 can be generated from the codegen-cli tool by providing the below command. Config project has the project settings and schema for the rest api specifications. 

  java -jar light-codegen/codegen-cli/target/codegen-cli.jar -f light-hybrid-4j-service -o light-example-4j/hybrid/hello-world/service1 -m model-config/hybrid/hello-world/service1/schema.json -c model-config/hybrid/hello-world/service1/config.json

 Then build the project with

  mvn clean install

3. Now we can start the server with generated service with following command.   

   > cd networknt/light-example-4j/hello-world/server/target/
   >java -cp hello-1.0.0.jar;../../service1/target/service1-1.0.0.jar com.networknt.server.Server

The two services will be included in the server and it can be started.

Handler scanning package =
  RpcStartupHookProvider: handlers size 2
  RpcStartupHookProvider add id map to com.networ
  RpcStartupHookProvider add id map to com.netwo
  Unable to load config 'handler' with extension yml, yaml and json from external
  config, application config and module config. Please ignore this message if you
  are sure that your application is not using this config file.
4. Likewise we can create hello service 2 project and build the project like below.
 java -jar light-codegen/codegen-cli/target/codegen-cli.jar -f light-hybrid-4j-service -o light-example-4j/hybrid/hello-world/service2 -m model-config/hybrid/hello-world/service2/schema.json -c model-config/hybrid/hello-world/service2/config.json

 mvn clean install
5. We can start the server with both services 
   >java -cp hello-1.0.0.jar;../../service1/target/service1-1.0.0.jar;../../service2/target/service2-1.0.0.jar com.networknt.server.Server
Develop Hybrid rest services
Now we can develop the command rest API available in service2 project. Below code will receive three parameters, one is command and other two inputs for the command.
 public class Command implements Handler {
  public ByteBuffer handle(HttpServerExchange exchange, Object input)  {
   LinkedHashMap linkedHashMap = (LinkedHashMap) input;
   String command = linkedHashMap.get("command").toString();
   Integer input1 = Integer.parseInt(linkedHashMap.get("input1").toString());
   Integer input2 = Integer.parseInt(linkedHashMap.get("input2").toString());
   if (command.equals("add")) {
    Integer result = input1+input2;
    String finalOutput = String.format("Result of add two numbers (%d+%d) => %d", input1, input2, result );
    return NioUtils.toByteBuffer(finalOutput);
   return NioUtils.toByteBuffer("Executing command "+input.getClass());
We have to modify accordingly the schema json so that rest api receives the parameters accordingly.

   "":{"schema": {
        "title": "Service2",
        "type": "object",
        "properties": {
          "command": {
            "type": "string"
          "input1": {
            "type": "integer"
          "input2": {
            "type": "integer"
        "required": ["command", "input1"]

     "scope": "command.w"},"":{"schema": {
        "title": "Service2",
        "type": "object",
        "properties": {
          "command": {
            "type": "string"
          "input1": {
            "type": "string"
          "input2": {
            "type": "string"
        "required": ["command", "input1"]

  "scope": "command.w"}

Then build the project and start the server with service2. When we execute the service, it produces the addition value.
$ curl -k -X POST https://localhost:8443/api/json -d '{"host":"","service":"service2","action":"command","version":"0.1.1","data":{"command":"add","input1":"1","input2":"2"}}'
 % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current Dload  Upload   Total   Spent    Left  Speed
 100   168  100    36  100   132     74    272 --:--:-- --:--:-- --:--:--   347

Result of add two numbers (1+2) => 3 
Rest Database Access
In Hybrid server, add the datasource in service.yml
  - javax.sql.DataSource:
    - com.zaxxer.hikari.HikariDataSource:
         DriverClassName: com.mysql.jdbc.Driver
         jdbcUrl: jdbc:mysql://host:port/dbname?useSSL=false
         username: root
         maximumPoolSize: 10
         useServerPrepStmts: true
         cachePrepStmts: true
         cacheCallableStmts: true
         prepStmtCacheSize: 10
         prepStmtCacheSqlLimit: 2048
         connectionTimeout: 2000
Also add the corresponding dependencies in server pom.xml. Now in service 1 project, do the connection from data source and then query the datbase with prepraredstatement and respond back.


    <version> ${version.gson}</version>


  public class StudentModel {
    private int id;
    private String studentName;
    public StudentModel(int id, String studentName) {
      super(); = id;
      this.studentName = studentName;
    public int getId() {
      return id;
    public void setId(int id) { = id;
   public String getStudentName() {
     return studentName;
  public void setStudentName(String studentName) {
     this.studentName = studentName;
public class Query implements Handler {
    private static final DataSource ds = SingletonServiceFactory.getBean(DataSource.class);
    public ByteBuffer handle(HttpServerExchange exchange, Object input)  {
        String result = "";
        try {
            Connection connection = ds.getConnection();
            System.out.println("connected to database");

         List<StudentModel> studentList = new ArrayList<>();
            PreparedStatement statement = connection.prepareStatement("SELECT * FROM studentdb");

            ResultSet resultSet = statement.executeQuery();
            while ( {
                studentList.add(new StudentModel(resultSet.getInt("id"), resultSet.getString("studentname")));
        } catch (Exception exception){
            System.out.println("exception: "+exception);

        return NioUtils.toByteBuffer(gson.toJson(studentList));


So now when we hit the curl command for the query service, it will retrieve all the records from the database.(so please create table with dbname and have some records).

$  curl -k -X POST https://localhost:8443/api/json -d '{"host":"","service":"service1","action":"query","version":"0.1.0","data":{"param1":"value1","param2":"value2"}}'

 % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current  Dload  Upload   Total   Spent    Left  Speed
 100   385  100   261  100   124    341    162 --:--:-- --:--:-- --:--:--   503 



CORS handler

Cross Origin Resource Sharing handler handles the pre-flight OPTIONS request to enable CORS. Light4j provides flexibility of adding allowed origins in the configuration file cors.yml.

Generate the cors project from light codegen tool as shown below. It also works for simple rest api tutorial.

  java -jar light-codegen/codegen-cli/target/codegen-cli.jar -f openapi -o light-example-4j/rest/openapi/cors -m model-config/rest/openapi/cors/openapi.json -c model-config/rest/openapi/cors/config.json


Enable the cors handler by doing the below steps:

1. Add the maven dependency


2. Create cors.yml file in src/main/resources/config/

    description: Cors Http Handler
    enabled: true
     - GET
     - POST

3. Add the pathhandler provider

    # HandlerProvider implementation
     - com.networknt.handler.HandlerProvider:
     - com.networknt.cors.PathHandlerProvider under singletons.

4. Add the cors http handler in the middleware handler.

    - com.networknt.handler.MiddlewareHandler:
      #Cors handler to handler post/put pre-flight
      - com.networknt.cors.CorsHttpHandler

The above steps can be done in the ligh4j rest api project to enable CORS handler, only the path handler routing need to be added. Build and execute the below project.

    mvn clean install exec:exec
Now hit the pet store api with OPTIONS methods through curl command.
  curl -k -H "Origin:" -H "Access-Control-Request-Method: POST"  -H "Access-Control-Request-Headers: X-Requested-With"  -X OPTIONS --verbose https://localhost:8443/v1/postData
Following output will produce it.
 < HTTP/2 200 
 < access-control-allow-headers: X-Requested-With
 < server: L
 < access-control-allow-credentials: true
 < content-length: 0
 < access-control-allow-methods: GET
 < access-control-allow-methods: POST
 < access-control-max-age: 3600
 < date: Sat, 30 Mar 2019 15:14:31 GMT



Nagappan is a techie-geek and a full-stack senior developer having 10+ years of experience in both front-end and back-end. He has experience on front-end web technologies like HTML, CSS, JAVASCRIPT, Angular and expert in Java and related frameworks like Spring, Struts, EJB and RESTEasy framework. He hold bachelors degree in computer science and he is very passionate in learning new technologies.

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