Support of spring initializr meta-data v2.1

Update the `init` command to support the latest meta-data format. Recent
Spring Initializr version also supports Spring Boot CLI now and generates
a textual service capabilities when requested. The command no longer
generates the capabilities of the service unless said service does not
support it.

Closes gh-2515

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/init/InitializrService.java

@@ -48,6 +48,19 @@ class InitializrService {
 
 	private static final Charset UTF_8 = Charset.forName("UTF-8");
 
+	/**
+	 * Accept header to use to retrieve the json meta-data.
+	 */
+	public static final String ACCEPT_META_DATA =
+			"application/vnd.initializr.v2.1+json,application/vnd.initializr.v2+json";
+
+	/**
+	 * Accept header to use to retrieve the service capabilities of the service. If the
+	 * service does not offer such feature, the json meta-data are retrieved instead.
+	 */
+	public static final String ACCEPT_SERVICE_CAPABILITIES =
+			"text/plain," + ACCEPT_META_DATA;
+
 	/**
 	 * Late binding HTTP client.
 	 */
@@ -80,12 +93,7 @@ class InitializrService {
 		URI url = request.generateUrl(metadata);
 		CloseableHttpResponse httpResponse = executeProjectGenerationRequest(url);
 		HttpEntity httpEntity = httpResponse.getEntity();
-		if (httpEntity == null) {
-			throw new ReportableException("No content received from server '" + url + "'");
-		}
-		if (httpResponse.getStatusLine().getStatusCode() != 200) {
-			throw createException(request.getServiceUrl(), httpResponse);
-		}
+		validateResponse(httpResponse, request.getServiceUrl());
 		return createResponse(httpResponse, httpEntity);
 	}
 
@@ -97,6 +105,42 @@ class InitializrService {
 	 */
 	public InitializrServiceMetadata loadMetadata(String serviceUrl) throws IOException {
 		CloseableHttpResponse httpResponse = executeInitializrMetadataRetrieval(serviceUrl);
+		validateResponse(httpResponse, serviceUrl);
+		return parseJsonMetadata(httpResponse.getEntity());
+	}
+
+	/**
+	 * Loads the service capabilities of the service at the specified url.
+	 * <p>If the service supports generating a textual representation of the
+	 * capabilities, it is returned. Otherwhise the json meta-data as a
+	 * {@link JSONObject} is returned.
+	 * @param serviceUrl to url of the initializer service
+	 * @return the service capabilities (as a String) or the metadata describing the service
+	 * @throws IOException if the service capabilities cannot be loaded
+	 */
+	public Object loadServiceCapabilities(String serviceUrl) throws IOException  {
+		CloseableHttpResponse httpResponse = executeServiceCapabilitiesRetrieval(serviceUrl);
+		validateResponse(httpResponse, serviceUrl);
+		HttpEntity httpEntity = httpResponse.getEntity();
+		ContentType contentType = ContentType.getOrDefault(httpEntity);
+		if (contentType.getMimeType().equals("text/plain")) {
+			return  getContent(httpEntity);
+		} else {
+			return parseJsonMetadata(httpEntity);
+		}
+	}
+
+	private InitializrServiceMetadata parseJsonMetadata(HttpEntity httpEntity) throws IOException {
+		try {
+			return new InitializrServiceMetadata(getContentAsJson(httpEntity));
+		}
+		catch (JSONException ex) {
+			throw new ReportableException("Invalid content received from server ("
+					+ ex.getMessage() + ")", ex);
+		}
+	}
+
+	private void validateResponse(CloseableHttpResponse httpResponse, String serviceUrl) {
 		if (httpResponse.getEntity() == null) {
 			throw new ReportableException("No content received from server '"
 					+ serviceUrl + "'");
@@ -104,14 +148,6 @@ class InitializrService {
 		if (httpResponse.getStatusLine().getStatusCode() != 200) {
 			throw createException(serviceUrl, httpResponse);
 		}
-		try {
-			HttpEntity httpEntity = httpResponse.getEntity();
-			return new InitializrServiceMetadata(getContentAsJson(httpEntity));
-		}
-		catch (JSONException ex) {
-			throw new ReportableException("Invalid content received from server ("
-					+ ex.getMessage() + ")", ex);
-		}
 	}
 
 	private ProjectGenerationResponse createResponse(CloseableHttpResponse httpResponse,
@@ -139,11 +175,19 @@ class InitializrService {
 	 */
 	private CloseableHttpResponse executeInitializrMetadataRetrieval(String url) {
 		HttpGet request = new HttpGet(url);
-		request.setHeader(new BasicHeader(HttpHeaders.ACCEPT,
-				"application/vnd.initializr.v2+json"));
+		request.setHeader(new BasicHeader(HttpHeaders.ACCEPT, ACCEPT_META_DATA));
 		return execute(request, url, "retrieve metadata");
 	}
 
+	/**
+	 * Retrieves the service capabilities of the service at the specified URL
+	 */
+	private CloseableHttpResponse executeServiceCapabilitiesRetrieval(String url) {
+		HttpGet request = new HttpGet(url);
+		request.setHeader(new BasicHeader(HttpHeaders.ACCEPT, ACCEPT_SERVICE_CAPABILITIES));
+		return execute(request, url, "retrieve help");
+	}
+
 	private CloseableHttpResponse execute(HttpUriRequest request, Object url,
 			String description) {
 		try {
@@ -188,11 +232,15 @@ class InitializrService {
 	}
 
 	private JSONObject getContentAsJson(HttpEntity entity) throws IOException {
+		return new JSONObject(getContent(entity));
+	}
+
+	private String getContent(HttpEntity entity) throws IOException {
 		ContentType contentType = ContentType.getOrDefault(entity);
 		Charset charset = contentType.getCharset();
 		charset = (charset != null ? charset : UTF_8);
 		byte[] content = FileCopyUtils.copyToByteArray(entity.getContent());
-		return new JSONObject(new String(content, charset));
+		return new String(content, charset);
 	}
 
 	private String extractFileName(Header header) {

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/init/ServiceCapabilitiesReportGenerator.java

@@ -54,7 +54,15 @@ class ServiceCapabilitiesReportGenerator {
 	 * @throws IOException if the report cannot be generated
 	 */
 	public String generate(String url) throws IOException {
-		InitializrServiceMetadata metadata = this.initializrService.loadMetadata(url);
+		Object content = this.initializrService.loadServiceCapabilities(url);
+		if (content instanceof InitializrServiceMetadata) {
+			return generateHelp(url, (InitializrServiceMetadata) content);
+		} else {
+			return content.toString();
+		}
+	}
+
+	private String generateHelp(String url, InitializrServiceMetadata metadata) {
 		String header = "Capabilities of " + url;
 		StringBuilder report = new StringBuilder();
 		report.append(StringUtils.repeat("=", header.length()) + NEW_LINE);