Example of using ExceptionHandler in Spring Boot: a controller method that returns either binary data or error JSON

PdfOrErrorController.java

package com.company.project.controllers;

import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

import java.util.Random;

/**
 * Example of a Spring Boot controller method that can return either:
 *  - 200 OK with binary data (application/pdf)
 *  - 503 Service Unavailable with JSON error message
 * 
 * (Everything defined as static nested classes just for readability of this example.) 
 *
 * @author Joni Karppinen
 * @since 2015-10-28
 */
@RestController
public class PdfOrErrorController {

    /**
     * Successful case, returns byte[] (fake PDF data)
     * Content type will be application/pdf
     */
    @RequestMapping(value = "/test", method = RequestMethod.GET)
    public ResponseEntity<byte[]> test() {
        // Set Content-Type header (+ content disposition, etc, if you want)
        // (Not using "produces", because that depends on request's Accept header including 
        // "application/pdf" and otherwise returns 406 Not Acceptable.)
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.parseMediaType("application/pdf"));
        String filename = "output.pdf";
        headers.setContentDispositionFormData(filename, filename);

        return new ResponseEntity<>(getDataOrThrowException(), headers, HttpStatus.OK);
    }

    /**
     * Error case, returns ErrorResponse which Spring automatically converts to JSON (using Jackson)
     * Content type will be application/json
     */
    @ExceptionHandler(OriginalExceptionFromAnotherApi.class)
    @ResponseStatus(HttpStatus.SERVICE_UNAVAILABLE)
    public ErrorResponse handle(OriginalExceptionFromAnotherApi e) {
        return new ErrorResponse(e.getMessage()); // use message from the original exception
    }


    private Random random = new Random();

    /**
     * Fake "service layer"
     */
    private byte[] getDataOrThrowException() {
        if (random.nextBoolean()) {
            return getBinaryData();
        }
        else {
            throw new OriginalExceptionFromAnotherApi("Failed because the other API is down!");
        }
    }

    private byte[] getBinaryData() {
        byte[] b = new byte[20];
        random.nextBytes(b);
        return b;
    }

    /**
     * Defines the JSON output format of error responses
     */
    private static class ErrorResponse {
        public String message;

        public ErrorResponse(String message) {
            this.message = message;
        }
    }

    private static class OriginalExceptionFromAnotherApi extends RuntimeException {
        public OriginalExceptionFromAnotherApi(String message) {
            super(message);
        }
    }

}