Expand Gradle plugin's docs on setting bootRun's system properties

Closes gh-23578

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/running.adoc

@@ -92,9 +92,34 @@ See {gradle-api}/org/gradle/api/tasks/JavaExec.html#setArgsString-java.lang.Stri
 [[running-your-application-passing-system-properties]]
 === Passing System properties to your application
 Since `bootRun` is a standard `JavaExec` task, system properties can be passed to the application's JVM by specifying them in the build script.
-The values can be parameterized and passed as properties on the command line using the `-P` flag.
+To make that value of a system property to be configurable set its value using a {gradle-dsl}/org.gradle.api.Project.html#N14FE1[project property].
+To allow a project property to be optional, reference it using `findProperty`.
+Doing so also allows a default value to be provided using the `?:` Elvis operator, as shown in the following example:
 
-See {gradle-api}/org/gradle/api/tasks/JavaExec.html#systemProperty-java.lang.String-java.lang.Object-[the javadoc for `JavaExec.systemProperty`] for further details.
+[source,groovy,indent=0,subs="verbatim,attributes",role="primary"]
+.Groovy
+----
+include::../gradle/running/boot-run-system-property.gradle[tags=system-property]
+----
+
+[source,kotlin,indent=0,subs="verbatim,attributes",role="secondary"]
+.Kotlin
+----
+include::../gradle/running/boot-run-system-property.gradle.kts[tags=system-property]
+----
+
+The preceding example sets that `com.example.property` system property to the value of the `example` project property.
+If the `example` project property has not been set, the value of the system property will be `default`.
+
+Gradle allows project properties to be set in a variety of ways, including on the command line using the `-P` flag, as shown in the following example:
+
+[source,bash,indent=0,subs="verbatim,attributes"]
+----
+$ ./gradlew -Pexample=custom
+----
+
+The preceding example sets the value of the `example` project property to `custom`.
+`bootRun` will then use this as the value of the `com.example.property` system property.
 
 
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/running/boot-run-system-property.gradle

@@ -0,0 +1,18 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+// tag::system-property[]
+bootRun {
+	systemProperty 'com.example.property', findProperty('example') ?: 'default'
+}
+// end::system-property[]
+
+task configuredSystemProperties {
+	doLast {
+		bootRun.systemProperties.each { k, v -> 
+			println "$k = $v"
+		}
+	}
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/running/boot-run-system-property.gradle.kts

@@ -0,0 +1,20 @@
+import org.springframework.boot.gradle.tasks.run.BootRun
+
+plugins {
+	java
+	id("org.springframework.boot") version "{version}"
+}
+
+// tag::system-property[]
+tasks.getByName<BootRun>("bootRun") {
+	systemProperty("com.example.property", findProperty("example") ?: "default")
+}
+// end::system-property[]
+
+task("configuredSystemProperties") {
+	doLast {
+		tasks.getByName<BootRun>("bootRun").systemProperties.forEach { k, v -> 
+			println("$k = $v")
+		}
+	}
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/docs/RunningDocumentationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -68,4 +68,17 @@ class RunningDocumentationTests {
 				.build("optimizedLaunch").getOutput()).contains("false");
 	}
 
+	@TestTemplate
+	void bootRunSystemPropertyDefaultValue() throws IOException {
+		assertThat(this.gradleBuild.script("src/main/gradle/running/boot-run-system-property")
+				.build("configuredSystemProperties").getOutput()).contains("com.example.property = default");
+	}
+
+	@TestTemplate
+	void bootRunSystemPropetry() throws IOException {
+		assertThat(this.gradleBuild.script("src/main/gradle/running/boot-run-system-property")
+				.build("-Pexample=custom", "configuredSystemProperties").getOutput())
+						.contains("com.example.property = custom");
+	}
+
 }