Show how to publish custom ShadowJar outputs (#1283)

* Test `publishCustomShadowJar`

* Doc "Publish Custom Shadowed JAR with the Custom ShadowJar Task"

https://docs.gradle.org/current/userguide/publishing_maven.html#publishing_maven:publications
https://docs.gradle.org/current/userguide/publishing_customization.html#sec:publishing_custom_artifacts_to_maven
https://docs.gradle.org/current/dsl/org.gradle.api.publish.maven.MavenPublication.html#org.gradle.api.publish.maven.MavenPublication:artifact(java.lang.Object)

Author: Goooler

src/docs/publishing/README.md

@@ -90,3 +90,42 @@ publishing {
   }
 }
 ```
+
+
+## Publish Custom ShadowJar Task Outputs
+
+It is possible to publish a custom `ShadowJar` task's output via the [`MavenPublication.artifact(java.lang.Object)`](https://docs.gradle.org/current/dsl/org.gradle.api.publish.maven.MavenPublication.html#org.gradle.api.publish.maven.MavenPublication:artifact(java.lang.Object)) method. 
+
+```groovy
+// Publishing a Shadow JAR with the Maven-Publish Plugin
+plugins {
+  id 'java'
+  id 'maven-publish'
+  id 'com.gradleup.shadow'
+}
+
+def testShadowJar = tasks.register('testShadowJar', com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar) {
+  group = com.github.jengelman.gradle.plugins.shadow.ShadowBasePlugin.GROUP_NAME
+  description = "Create a combined JAR of project and test dependencies"
+  archiveClassifier = "tests"
+  from sourceSets.test.output
+  configurations = [project.configurations.testRuntimeClasspath]
+}
+
+dependencies {
+  testImplementation 'junit:junit:3.8.2'
+}
+
+publishing {
+  publications {
+    shadow(MavenPublication) {
+      artifact(testShadowJar)
+    }
+  }
+  repositories {
+    maven {
+      url = "https://repo.myorg.com"
+    }
+  }
+}
+```

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

@@ -9,6 +9,7 @@ import assertk.assertions.isEqualTo
 import assertk.assertions.single
 import com.github.jengelman.gradle.plugins.shadow.ShadowJavaPlugin.Companion.SHADOW_RUNTIME_ELEMENTS_CONFIGURATION_NAME
 import com.github.jengelman.gradle.plugins.shadow.internal.classPathAttributeKey
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 import com.github.jengelman.gradle.plugins.shadow.util.GradleModuleMetadata
 import com.github.jengelman.gradle.plugins.shadow.util.Issue
 import com.github.jengelman.gradle.plugins.shadow.util.JarPath
@@ -140,6 +141,37 @@ class PublishingTest : BasePluginTest() {
     assertShadowVariantCommon(gmmAdapter.fromJson(repoPath("my/maven/1.0/maven-1.0.module")))
   }
 
+  @Test
+  fun publishCustomShadowJar() {
+    projectScriptPath.appendText(
+      publishConfiguration(
+        projectBlock = """
+          def testShadowJar = tasks.register('testShadowJar', ${ShadowJar::class.java.name}) {
+            group = com.github.jengelman.gradle.plugins.shadow.ShadowBasePlugin.GROUP_NAME
+            description = "Create a combined JAR of project and test dependencies"
+            archiveClassifier = "tests"
+            from sourceSets.test.output
+            configurations = [project.configurations.testRuntimeClasspath]
+          }
+        """.trimIndent(),
+        dependenciesBlock = """
+          testImplementation 'junit:junit:3.8.2'
+        """.trimIndent(),
+        publicationsBlock = """
+          shadow(MavenPublication) {
+            artifact testShadowJar
+          }
+        """.trimIndent(),
+      ),
+    )
+
+    publish()
+
+    assertThat(repoJarPath("my/maven/1.0/maven-1.0-tests.jar")).useAll {
+      containsEntries(*junitEntries)
+    }
+  }
+
   @ParameterizedTest
   @ValueSource(booleans = [false, true])
   fun publishShadowedGradlePlugin(legacy: Boolean) {