How to create and download large data set as CSV file from rest services by implementing StreamingOutput

Question

I have 20k records in db, I need to export all data into CSV file by implementing StreamingOutput from rest service. I have no idea about, how to implement StreamingOutput for downloading csv file.

Please help me..

Thanks in advance 🙏🙏

Answer 1

You have to read the records from the database, convert each row into CSV and use something like the following to use StreamingOutput :

package de.demo.services;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.ws.rs.core.StreamingOutput;
import java.io.*;

@Path( "/demo" )
public class DemoService
{
   @GET
   @Produces(MediaType.TEXT_PLAIN)
   @Path("/api")
   public Response getCsv() {
       StreamingOutput stream = new StreamingOutput() {
          public void write(OutputStream os) throws ... {
             Writer writer = new BufferedWriter(new OutputStreamWriter(os));
             writer.write( /* CSV data */ );            
             writer.flush();
          }
       };

       return Response.ok(stream).build();
   }
}

Be careful not to read all your 20k rows from the database at once but read the results based on a cursor by method setFetchSize of the JDBC statement.

Use the following dependencies in Maven:

    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>3.1.0</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>javax.ws.rs</groupId>
        <artifactId>javax.ws.rs-api</artifactId>
        <version>2.0</version>
    </dependency>
    <dependency>
        <groupId>org.glassfish.jersey.core</groupId>
        <artifactId>jersey-server</artifactId>
        <version>2.23.1</version>
    </dependency>
    <dependency>
        <groupId>org.glassfish.jersey.containers</groupId>
        <artifactId>jersey-container-servlet</artifactId>
        <version>2.23.1</version>
    </dependency>
    <dependency>
        <groupId>org.glassfish.jersey.containers</groupId>
        <artifactId>jersey-container-servlet-core</artifactId>
        <version>2.23.1</version>
    </dependency>

Version 2.23.1 of Jersey is not the latest version you can get.

Answer 2

I am using the same approach, like what u suggested, but control is not going inside the StreamingOutput implementation only..Here is the code....

@GET
@Produces(MediaType.TEXT_PLAIN)
@Path("/allEmployeeReport")
public Response exportAllEmployee()
{ 
   StreamingOutput stream = new  
   StreamingOutput() { 
      public void write(OutputStream os)
      throws IOException, Web...{
      Writer writer = new    
      BufferedWriter(new
     OutputStreamWriter(os));
      for(Employee employee : 
     repository.findAll()){
     writer.write(employee.getFName());
     writer.write(",");
     writer.write(employee.getLName());
     writer.write(",");
     writer.write(employee.getEmail()); 
      writer.write(",");
      writer.write(employee.getMobile());
     writer.write(",");
     writer.write(employee.getDOB());
     writer.write("\n");

     } 
    writer.flush();
     writer.close();
  }; 
  return Response.ok(stream).build(); 
 }