HttpServerErrorException$InternalServerError: 500 with RestTemplate

[Alejandro Barrero]

I have a Spring Boot application trying to access a REST web service with RestTemplate.
The code in the client is:

package com.paperless.client;

import java.net.URL;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

import org.springframework.web.client.RestTemplate;

import com.paperless.Select_Conn;

public class SelectClient {

	public static void main(String[] args) {
		try {
			String selectStatement = "select * from user";
			SelectClient selectClient = new SelectClient();
			Connection conn = selectClient.getConnection();
			Select_Conn select_Conn = new Select_Conn(selectStatement, conn);
			ResultSet resultSet = getResultSet(select_Conn);
			while (resultSet.next()) {
				String firstName = resultSet.getString("first_name");
				String lastName = resultSet.getString("last_name");
				System.out.println("firstName = " + firstName + " - lastName = " + lastName);
			}
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

	public static ResultSet getResultSet(Select_Conn select_Conn) {
		RestTemplate restTemplate = new RestTemplate();
		ResultSet resultSet = restTemplate.getForObject("http://localhost:8080/resultSet/{select_Conn}", ResultSet.class, select_Conn);
		return resultSet;
	}

	private Connection getConnection() throws SQLException {
		URL url = getClass().getResource("PaperlessOffice.db");
		String filePath = url.getFile();
		String fullConnectionString = "jdbc:sqlite:" + filePath;
		Connection conn = DriverManager.getConnection(fullConnectionString);
		return conn;
	}
}

The code in the controller is:

ackage com.paperless.controler;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import com.paperless.Select_Conn;

@Controller
public class PaperlessController {

	@ResponseBody
	@RequestMapping(path = "/resultSet/{selectStatement}")
	@GetMapping()
	public ResultSet resultSet(@PathVariable Select_Conn Select_Conn) {
		Connection conn = null;
		ResultSet result = null;
		try {
			conn = Select_Conn.conn;
			Statement statement = conn.createStatement();
			result = statement.executeQuery(Select_Conn.selectStatement);
		} catch (SQLException e) {
			return null;
		} finally {
			try {
				if (conn != null)
					conn.close();
			} catch (SQLException e) {
				return null;
			}
		}
		return result;
	}
}

The problem is that i am getting the exception
Exception in thread "main" org.springframework.web.client.HttpServerErrorException$InternalServerError: 500 : [{"timestamp":"2020-08-25T19:30:13.149+00:00","status":500,"error":"Internal Server Error","message":"","path":"/resultSet/com.paperless.Select_Conn@4044fb95"}]
Digging in the debugger, it appears that the program is trying to do some JSON mapping, but that is just something I have noticed. I am an experienced Java developer, but I am learning how to program REST clients. I have spend the last two days browsing the net without success.

[Rob Spoor]

It looks as if you're trying to pass a connection to your database file through HTTP to the server. That is not going to work. The connection is local to one machine, and you cannot transfer it to another machine in any way. You can transfer the source, but not the connection itself.

Spring Boot doesn't tell you that clearly though. It tries to convert the connection to a string (for the URL), then back to a connection. The first part works using toString() (the com.paperless.Select_Conn@4044fb95 part), but converting it back fails.

Maybe you could tell us what you're trying to achieve here?

Alejandro Barrero wrote:

	@RequestMapping(path = "/resultSet/{selectStatement}")
	@GetMapping()

Just a side note: you can merge the contenet of @RequestMapping into the @GetMapping; @GetMapping is @RequestMapping with the method pre-filled to GET.

[Dave Tolls]

Looks like my post this morning (which said much the same thing) vanished...

There's also the inverse of sending the Connection, which is expecting a ResultSet back.
That won't work either.

Instances of these classes are not designed to be sent across a network.

[Alejandro Barrero]

Thanks for your quick response Rob and Dave. Originally I wasn't passing the connection; I wanted to return a ResultSet, but it doesn't work after the connection is closed. It appears I have to construct the list of objects from the ResultSet and return the list.

[Paul Clapham]

Alejandro Barrero wrote:Thanks for your quick response Rob and Dave. Originally I wasn't passing the connection; I wanted to return a ResultSet, but it doesn't work after the connection is closed. It appears I have to construct the list of objects from the ResultSet and return the list.

Well, yes, you can't pass a ResultSet between machines anyway. Constructing a list of objects from a ResultSet is the standard way to do what you're trying to do; it's even the standard way to extract data from a database and pass it along to something else in the same JVM.