Use default JavaExec error message when main class is not set (#1725)

Author: Goooler

docs/changes/README.md

@@ -16,6 +16,7 @@
   and `META-INF/groovy` will be merged into `META-INF/groovy/org.codehaus.groovy.runtime.ExtensionModule`.
 - Move injecting `Class-Path` manifest attr logic from `doFirst` into `copy`. ([#1720](https://github.com/GradleUp/shadow/pull/1720))
 - Deprecate `InheritManifest`. ([#1722](https://github.com/GradleUp/shadow/pull/1722))
+- Use default `JavaExec` error message when main class is not set. ([#1725](https://github.com/GradleUp/shadow/pull/1725))
 
 ### Fixed
 

src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/ApplicationPluginTest.kt

@@ -177,7 +177,8 @@ class ApplicationPluginTest : BasePluginTest() {
     val result = runWithFailure(runShadowPath)
 
     assertThat(result.output).contains(
-      "The main class must be specified and not left empty in `application.mainClass` or manifest attributes.",
+      "Error: Could not find or load main class",
+      "Caused by: java.lang.ClassNotFoundException:",
     )
   }
 

src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowApplicationPlugin.kt

@@ -132,12 +132,9 @@ public abstract class ShadowApplicationPlugin : Plugin<Project> {
     tasks.shadowJar.configure { task ->
       task.inputs.property("mainClassName", mainClassName)
       task.doFirst("Set $mainClassAttributeKey attribute in the manifest") {
+        val realClass = mainClassName.orNull
         // Inject the attribute if it is not already present.
-        if (!task.manifest.attributes.contains(mainClassAttributeKey)) {
-          val realClass = mainClassName.orNull
-          if (realClass.isNullOrEmpty()) {
-            error("The main class must be specified and not left empty in `application.mainClass` or manifest attributes.")
-          }
+        if (!task.manifest.attributes.contains(mainClassAttributeKey) && !realClass.isNullOrEmpty()) {
           task.manifest.attributes[mainClassAttributeKey] = realClass
         }
       }