feat: Support writing directly into file in JsonRenderer (#388)

Previously, openapi-diff rendered the JSON output into a string and wrote the string representation into the designated output file.

This failed with an `OutOfMemoryError` for very large outputs.

This change set introduces a method in `JsonRenderer` to write the JSON output directly into a file and removes the verbatim copy of the old and new OpenAPI specs (`oldSpecOpenApi` and `newSpecOpenApi`) from the JSON output.

Author: eacolina

cli/src/main/java/org/openapitools/openapidiff/cli/Main.java

@@ -196,9 +196,8 @@ public static void main(String... args) {
       }
       if (line.hasOption("json")) {
         JsonRender jsonRender = new JsonRender();
-        String output = jsonRender.render(result);
         String outputFile = line.getOptionValue("json");
-        writeOutput(output, outputFile);
+        jsonRender.renderToFile(result, outputFile);
       }
       if (line.hasOption("state")) {
         System.out.println(result.isChanged().getValue());

core/src/main/java/org/openapitools/openapidiff/core/model/ChangedOpenApi.java

@@ -1,5 +1,6 @@
 package org.openapitools.openapidiff.core.model;
 
+import com.fasterxml.jackson.annotation.JsonIgnore;
 import io.swagger.v3.oas.models.OpenAPI;
 import java.util.List;
 import java.util.Objects;
@@ -8,8 +9,8 @@
 import org.openapitools.openapidiff.core.utils.EndpointUtils;
 
 public class ChangedOpenApi implements ComposedChanged {
-  private OpenAPI oldSpecOpenApi;
-  private OpenAPI newSpecOpenApi;
+  @JsonIgnore private OpenAPI oldSpecOpenApi;
+  @JsonIgnore private OpenAPI newSpecOpenApi;
   private List<Endpoint> newEndpoints;
   private List<Endpoint> missingEndpoints;
   private List<ChangedOperation> changedOperations;

core/src/main/java/org/openapitools/openapidiff/core/output/JsonRender.java

@@ -3,6 +3,8 @@
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import java.io.IOException;
+import java.nio.file.Paths;
 import org.openapitools.openapidiff.core.model.ChangedOpenApi;
 
 public class JsonRender implements Render {
@@ -17,4 +19,14 @@ public String render(ChangedOpenApi diff) {
       throw new RuntimeException("Could not serialize diff as JSON", e);
     }
   }
+
+  public void renderToFile(ChangedOpenApi diff, String file) {
+    try {
+      objectMapper.writeValue(Paths.get(file).toFile(), diff);
+    } catch (JsonProcessingException e) {
+      throw new RuntimeException("Could not serialize diff as JSON", e);
+    } catch (IOException e) {
+      throw new RuntimeException("Could not write to JSON file", e);
+    }
+  }
 }