Consider time in BuildInfo up-to-date checks and allow it to be set

Closes gh-12111
Closes gh-12266

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/integrating-with-actuator.adoc

@@ -38,6 +38,9 @@ By default, the generated build information is derived from the project:
 | `build.version`
 | The version of the project
 
+| `build.time`
+| The time at which the project is being built
+
 |===
 
 The properties can be customized using the DSL:
@@ -47,6 +50,13 @@ The properties can be customized using the DSL:
 include::../gradle/integrating-with-actuator/build-info-custom-values.gradle[tags=custom-values]
 ----
 
+The default value for `build.time` is the instant at which the project is being built. A
+side-effect of this is that the task will never be up-to-date and, therefore, builds will
+take slighly longer as more tasks will have to be executed. Another side-effect is that
+the task's output will always change and, therefore, the build will not be truly
+repeatable. If you value build performance or repeatability more highly than the accuracy
+of the `build.time` property, set `time` to `null` or a fixed value.
+
 Additional properties can also be added to the build information:
 
 [source,groovy,indent=0,subs="verbatim"]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/buildinfo/BuildInfo.java

@@ -61,7 +61,8 @@ public class BuildInfo extends ConventionTask {
 											? "unspecified"
 											: this.properties.getArtifact(),
 									this.properties.getVersion(),
-									this.properties.getName(), coerceToStringValues(
+									this.properties.getName(), this.properties.getTime(),
+									coerceToStringValues(
 											this.properties.getAdditional())));
 		}
 		catch (IOException ex) {

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/buildinfo/BuildInfoProperties.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.gradle.tasks.buildinfo;
 
 import java.io.Serializable;
+import java.time.Instant;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -40,10 +41,13 @@ public class BuildInfoProperties implements Serializable {
 
 	private String name;
 
+	private Instant time;
+
 	private Map<String, Object> additionalProperties = new HashMap<>();
 
 	BuildInfoProperties(Project project) {
 		this.project = project;
+		this.time = Instant.now();
 	}
 
 	/**
@@ -121,6 +125,23 @@ public class BuildInfoProperties implements Serializable {
 		this.name = name;
 	}
 
+	/**
+	 * Returns the value used for the {@code build.time} property. Defaults to
+	 * {@link Instant#now} when the {@code BuildInfoProperties} instance was created.
+	 * @return the time
+	 */
+	public Instant getTime() {
+		return this.time;
+	}
+
+	/**
+	 * Sets the value used for the {@code build.time} property.
+	 * @param time the build time
+	 */
+	public void setTime(Instant time) {
+		this.time = time;
+	}
+
 	/**
 	 * Returns the additional properties that will be included. When written, the name of
 	 * each additional property is prefixed with {@code build.}.
@@ -152,6 +173,7 @@ public class BuildInfoProperties implements Serializable {
 		result = prime * result + ((this.group == null) ? 0 : this.group.hashCode());
 		result = prime * result + ((this.name == null) ? 0 : this.name.hashCode());
 		result = prime * result + ((this.version == null) ? 0 : this.version.hashCode());
+		result = prime * result + ((this.time == null) ? 0 : this.time.hashCode());
 		return result;
 	}
 
@@ -207,6 +229,14 @@ public class BuildInfoProperties implements Serializable {
 		else if (!this.version.equals(other.version)) {
 			return false;
 		}
+		if (this.time == null) {
+			if (other.time != null) {
+				return false;
+			}
+		}
+		else if (!this.time.equals(other.time)) {
+			return false;
+		}
 		return true;
 	}