Show more informative error message on parsing error

If the API documentation JSON file is incorrect, it only shows GSON's
error message, which is quite difficult to understand at first.

Here is the old error message:

Caused by: com.google.gson.JsonSyntaxException:
Expected a com.google.gson.JsonObject but was com.google.gson.JsonPrimitive

src/main/kotlin/org/springdoc/openapi/gradle/plugin/OpenApiGeneratorTask.kt

@@ -2,6 +2,7 @@ package org.springdoc.openapi.gradle.plugin
 
 import com.google.gson.GsonBuilder
 import com.google.gson.JsonObject
+import com.google.gson.JsonSyntaxException
 import org.awaitility.Durations
 import org.awaitility.core.ConditionTimeoutException
 import org.awaitility.kotlin.*
@@ -95,7 +96,12 @@ open class OpenApiGeneratorTask : DefaultTask() {
 
     private fun prettifyJson(response: String): String {
         val gson = GsonBuilder().setPrettyPrinting().create()
-        val googleJsonObject = gson.fromJson(response, JsonObject::class.java)
-        return gson.toJson(googleJsonObject)
+        try {
+            val googleJsonObject = gson.fromJson(response, JsonObject::class.java)
+            return gson.toJson(googleJsonObject)
+        } catch (e: RuntimeException) {
+            throw JsonSyntaxException("Failed to parse the API docs response string. " +
+                    "Please ensure that the response is in the correct format. response=$response", e)
+        }
     }
 }

src/test/kotlin/org/springdoc/openapi/gradle/plugin/OpenApiGradlePluginTest.kt

@@ -11,8 +11,11 @@ import org.gradle.testkit.runner.GradleRunner
 import org.gradle.testkit.runner.TaskOutcome
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertNotNull
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
+import org.slf4j.Logger
+import org.slf4j.LoggerFactory
 import java.io.File
 import java.io.FileReader
 import java.nio.file.Files
@@ -47,6 +50,10 @@ class OpenApiGradlePluginTest {
         }
     """.trimIndent()
 
+    companion object {
+        val logger: Logger = LoggerFactory.getLogger(OpenApiGradlePluginTest::class.java)
+    }
+
     @BeforeEach
     fun createTemporaryAcceptanceProjectFromTemplate() {
         File(javaClass.classLoader.getResource("acceptance-project")!!.path).copyRecursively(projectTestDir)
@@ -278,6 +285,26 @@ class OpenApiGradlePluginTest {
         assertOpenApiJsonFile(2, outputJsonFileNameGroupB)
     }
 
+    @Test
+    fun `using invalid doc url`() {
+        buildFile.writeText(
+            """$baseBuildGradle
+            openApi{
+                apiDocsUrl = "http://localhost:8080/hello/world"
+            }
+        """.trimMargin()
+        )
+
+        try {
+            openApiDocsTask(runTheBuild())
+        } catch (e: RuntimeException) {
+            logger.error(e.message)
+            assertNotNull(e.message?.lines()?.find { it.contains(
+                "Failed to parse the API docs response string. " +
+                    "Please ensure that the response is in the correct format.") })
+        }
+    }
+
     private fun runTheBuild(vararg additionalArguments: String = emptyArray()) = GradleRunner.create()
         .withProjectDir(projectTestDir)
         .withArguments("clean", "generateOpenApiDocs", *additionalArguments)

src/test/resources/acceptance-project/src/main/java/com/example/demo/endpoints/HelloWorldController.java

@@ -1,12 +1,16 @@
 package com.example.demo.endpoints;
 
 import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.ResponseBody;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController("/hello")
+@RequestMapping("/hello")
 public class HelloWorldController {
 
     @GetMapping("/world")
+    @ResponseBody
     public String helloWorld() {
         return "Hello World!";
     }