fix: update file-upload example (#321)

- Fixed the file-upload example: it is necessary to declare at least one
  query, otherwise the graphql-java-tools throws exceptions during the
  schema parsing
- Replaced the return boolean type with a UploadedFile type containing
  the filename, type and the content of the textual file uploaded
- Added a README explaining how to run it

Refs: #94

file-upload/README.md

@@ -0,0 +1,31 @@
+# File Upload
+This Spring example shows how the upload operation works using GraphQL and the Apollo Upload type.
+
+## How to run
+- Create a file: `echo 'Hello World!' > hello-world.txt`
+- Run the example: `../gradlew bootRun`
+- Make a GraphQL request:
+```sh
+curl --location 'http://localhost:9000/graphql' \
+     --form 'operations="{\"query\": \"mutation upload($file:Upload){ upload(file: $file){filename type content}}\"}"' \
+     --form 'map="{\"blob\": [\"variables.file\"]}"' \
+     --form 'blob=@"./hello-world.txt"'
+```
+
+If the file is successfully uploaded, this is the response received and the log shown:
+```json
+{
+  "data": {
+    "upload": {
+      "filename": "hello-world.txt",
+      "type": "text/plain",
+      "content": "Hello World!\n"
+    }
+  }
+}
+
+```
+
+```log
+INFO 1584868 --- [nio-9000-exec-7] upload.UploadMutation: File uploaded: {type=text/plain, filename=hello-world.txt, content=Hello World!}
+```

file-upload/src/main/java/upload/Query.java

@@ -4,4 +4,10 @@
 import org.springframework.stereotype.Service;
 
 @Service
-class Query implements GraphQLQueryResolver {}
+class Query implements GraphQLQueryResolver {
+
+   // This query is necessary because the graphql-java-tool requires at least one query
+   public String getHello() {
+      return "Hello World";
+   }
+}

file-upload/src/main/java/upload/UploadMutation.java

@@ -2,6 +2,8 @@
 
 import graphql.kickstart.tools.GraphQLMutationResolver;
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.Map;
 import javax.servlet.http.Part;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.stereotype.Service;
@@ -10,9 +12,13 @@
 @Service
 class UploadMutation implements GraphQLMutationResolver {
 
-  boolean upload(Part part) throws IOException {
-    log.info("Part: {}", part.getSubmittedFileName());
-    part.write(part.getSubmittedFileName());
-    return true;
+  Map<String, String> upload(Part part) throws IOException {
+    Map<String, String> uploadedFile = Map.of(
+      "filename", part.getSubmittedFileName(),
+      "type", part.getContentType(),
+      "content", new String(part.getInputStream().readAllBytes(), StandardCharsets.UTF_8)
+    );
+    log.info("File uploaded: {}", uploadedFile);
+    return uploadedFile;
   }
 }

file-upload/src/main/resources/schema.graphqls

@@ -1,9 +1,16 @@
 scalar Upload
 
-type Query {
+type UploadedFile {
+    filename: String
+    type: String
+    content: String
+}
 
+# The hello query declaration is necessary since the graphql-java-tools requires at least one query
+type Query {
+    hello: String
 }
 
 type Mutation {
-    upload(file: Upload): Boolean
+    upload(file: Upload): UploadedFile
 }