Guard constraint resolution by a property since it isn't really possi…

README.md

@@ -186,6 +186,21 @@ tasks.withType<DependencyUpdatesTask> {
 
 </details>
 
+#### Constraints
+
+If you use constraints, for example to define a BOM using the [`java-platform`](https://docs.gradle.org/current/userguide/java_platform_plugin.html) 
+plugin or to [manage](https://docs.gradle.org/current/userguide/managing_transitive_dependencies.html#sec:dependency_constraints) 
+transitive dependency versions, you can enable checking of constraints by specifying the `checkConstraints`
+attribute of the `dependencyUpdates` task.
+
+```groovy
+tasks {
+  dependencyUpdates {
+    checkConstraints = true
+  }
+}
+```
+
 #### Kotlin DSL
 
 If using Gradle's [kotlin-dsl][kotlin_dsl], you could configure the `dependencyUpdates` like this:

src/main/groovy/com/github/benmanes/gradle/versions/updates/DependencyUpdates.groovy

@@ -45,6 +45,7 @@ class DependencyUpdates {
   String reportfileName
   boolean checkForGradleUpdate
   String gradleReleaseChannel
+  boolean checkConstraints
 
   /** Evaluates the dependencies and returns a reporter. */
   DependencyUpdatesReporter run() {
@@ -65,7 +66,7 @@ class DependencyUpdates {
   private Set<DependencyStatus> resolveProjects(Map<Project, Set<Configuration>> projectConfigs) {
     projectConfigs.keySet().collect { proj ->
       Set<Configuration> configurations = projectConfigs.get(proj)
-      Resolver resolver = new Resolver(proj, resolutionStrategy)
+      Resolver resolver = new Resolver(proj, resolutionStrategy, checkConstraints)
       configurations.collect { Configuration config ->
         resolve(resolver, proj, config)
       }.flatten() as Set<DependencyStatus>

src/main/groovy/com/github/benmanes/gradle/versions/updates/DependencyUpdatesTask.groovy

@@ -57,6 +57,9 @@ class DependencyUpdatesTask extends DefaultTask {
   @Input
   boolean checkForGradleUpdate = true
 
+  @Input
+  boolean checkConstraints = false
+
   Object outputFormatter = 'plain'
 
   Closure resolutionStrategy = null;
@@ -80,7 +83,8 @@ class DependencyUpdatesTask extends DefaultTask {
     }
 
     def evaluator = new DependencyUpdates(project, resolutionStrategyAction, revisionLevel(),
-      outputFormatterProp(), outputDirectory(), getReportfileName(), checkForGradleUpdate, gradleReleaseChannelLevel())
+      outputFormatterProp(), outputDirectory(), getReportfileName(), checkForGradleUpdate, gradleReleaseChannelLevel(),
+      checkConstraints)
     DependencyUpdatesReporter reporter = evaluator.run()
     reporter?.write()
   }

src/main/groovy/com/github/benmanes/gradle/versions/updates/Resolver.groovy

@@ -57,12 +57,15 @@ class Resolver {
   final Action<? super ResolutionStrategyWithCurrent> resolutionStrategy
   final boolean useSelectionRules
   final boolean collectProjectUrls
+  final boolean checkConstraints
   final ConcurrentMap<ModuleVersionIdentifier, ProjectUrl> projectUrls
 
-  Resolver(Project project, Action<? super ResolutionStrategyWithCurrent> resolutionStrategy) {
+  Resolver(Project project, Action<? super ResolutionStrategyWithCurrent> resolutionStrategy,
+           boolean checkConstraints) {
     this.projectUrls = new ConcurrentHashMap<>()
     this.resolutionStrategy = resolutionStrategy
     this.project = project
+    this.checkConstraints = checkConstraints;
 
     useSelectionRules = new VersionComparator(project)
       .compare(project.gradle.gradleVersion, '2.2') >= 0
@@ -118,7 +121,8 @@ class Resolver {
       createQueryDependency(dependency, revision)
     }
 
-    // Common use case for dependency constraints is a java-platform BOM project.
+    // Common use case for dependency constraints is a java-platform BOM project or to control
+    // version of transitive dependency.
     if (supportsConstraints(configuration)) {
       configuration.dependencyConstraints.each { dependency ->
         latest.add(createQueryDependency(dependency, revision))
@@ -240,7 +244,11 @@ class Resolver {
     if (supportsConstraints(copy)) {
       for (DependencyConstraint constraint : copy.dependencyConstraints) {
         Coordinate coordinate = Coordinate.from(constraint)
-        coordinates.put(coordinate.key, declared.get(coordinate.key))
+        // Only add a constraint to the report if there is no dependency matching it, this means it
+        // is targeting a transitive dependency or is part of a platform.
+        if (!coordinates.containsKey(coordinate.key)) {
+          coordinates.put(coordinate.key, declared.get(coordinate.key))
+        }
       }
     }
 
@@ -360,11 +368,11 @@ class Resolver {
     return null
   }
 
-  private static boolean supportsConstraints(Configuration configuration) {
-    return configuration.metaClass.respondsTo(configuration, "getDependencyConstraints");
+  private boolean supportsConstraints(Configuration configuration) {
+    return checkConstraints && configuration.metaClass.respondsTo(configuration, "getDependencyConstraints");
   }
 
-  private static List<Coordinate> getResolvableDependencies(Configuration configuration) {
+  private List<Coordinate> getResolvableDependencies(Configuration configuration) {
     List<Coordinate> coordinates = configuration.dependencies.findAll { dependency ->
       dependency instanceof ExternalDependency
     }.collect { dependency ->

src/test/groovy/com/github/benmanes/gradle/versions/ConstraintsSpec.groovy

@@ -0,0 +1,131 @@
+package com.github.benmanes.gradle.versions
+
+import org.gradle.testkit.runner.GradleRunner
+import org.junit.Rule
+import org.junit.rules.TemporaryFolder
+import spock.lang.Specification
+
+import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
+
+final class ConstraintsSpec extends Specification {
+
+  @Rule TemporaryFolder testProjectDir = new TemporaryFolder()
+  private File buildFile
+
+  def "Show updates for an api dependency constraint"() {
+    given:
+    def mavenRepoUrl = getClass().getResource('/maven/').toURI()
+    buildFile = testProjectDir.newFile('build.gradle')
+    buildFile <<
+      """
+        plugins {
+          id 'java-library'
+          id 'com.github.ben-manes.versions'
+        }
+
+        tasks.dependencyUpdates {
+          checkConstraints = true
+        }
+
+        repositories {
+          maven {
+            url '${mavenRepoUrl}'
+          }
+        }
+
+        dependencies {
+          constraints {
+            api 'com.google.inject:guice:2.0'
+          }
+        }
+      """.stripIndent()
+
+    when:
+    def result = GradleRunner.create()
+      .withProjectDir(testProjectDir.root)
+      .withArguments('dependencyUpdates')
+      .withPluginClasspath()
+      .build()
+
+    then:
+    result.output.contains('com.google.inject:guice [2.0 -> 3.1]')
+    result.task(':dependencyUpdates').outcome == SUCCESS
+  }
+
+  def "Does not override explicit dependency with constraint"() {
+    given:
+    def mavenRepoUrl = getClass().getResource('/maven/').toURI()
+    buildFile = testProjectDir.newFile('build.gradle')
+    buildFile <<
+      """
+        plugins {
+          id 'java-library'
+          id 'com.github.ben-manes.versions'
+        }
+
+        tasks.dependencyUpdates {
+          checkConstraints = true
+        }
+
+        repositories {
+          maven {
+            url '${mavenRepoUrl}'
+          }
+        }
+
+        dependencies {
+          api 'com.google.inject:guice:3.0'
+          constraints {
+            api 'com.google.inject:guice:2.0'
+          }
+        }
+      """.stripIndent()
+
+    when:
+    def result = GradleRunner.create()
+      .withProjectDir(testProjectDir.root)
+      .withArguments('dependencyUpdates')
+      .withPluginClasspath()
+      .build()
+
+    then:
+    result.output.contains('com.google.inject:guice [3.0 -> 3.1]')
+    result.task(':dependencyUpdates').outcome == SUCCESS
+  }
+
+  def "Does not show updates for an api dependency constraint when disabled"() {
+    given:
+    def mavenRepoUrl = getClass().getResource('/maven/').toURI()
+    buildFile = testProjectDir.newFile('build.gradle')
+    buildFile <<
+      """
+        plugins {
+          id 'java-library'
+          id 'com.github.ben-manes.versions'
+        }
+
+        repositories {
+          maven {
+            url '${mavenRepoUrl}'
+          }
+        }
+
+        dependencies {
+          constraints {
+            api 'com.google.inject:guice:2.0'
+          }
+        }
+      """.stripIndent()
+
+    when:
+    def result = GradleRunner.create()
+      .withProjectDir(testProjectDir.root)
+      .withArguments('dependencyUpdates')
+      .withPluginClasspath()
+      .build()
+
+    then:
+    result.output.contains('No dependencies found.')
+    result.task(':dependencyUpdates').outcome == SUCCESS
+  }
+}

src/test/groovy/com/github/benmanes/gradle/versions/JavaLibrarySpec.groovy

@@ -46,40 +46,4 @@ final class JavaLibrarySpec extends Specification {
     result.output.contains('com.google.inject:guice [2.0 -> 3.1]')
     result.task(':dependencyUpdates').outcome == SUCCESS
   }
-
-  def "Show updates for an api dependency constraint in a java-library project"() {
-    given:
-    def mavenRepoUrl = getClass().getResource('/maven/').toURI()
-    buildFile = testProjectDir.newFile('build.gradle')
-    buildFile <<
-      """
-        plugins {
-          id 'java-library'
-          id 'com.github.ben-manes.versions'
-        }
-
-        repositories {
-          maven {
-            url '${mavenRepoUrl}'
-          }
-        }
-
-        dependencies {
-          constraints {
-            api 'com.google.inject:guice:2.0'
-          }
-        }
-      """.stripIndent()
-
-    when:
-    def result = GradleRunner.create()
-      .withProjectDir(testProjectDir.root)
-      .withArguments('dependencyUpdates')
-      .withPluginClasspath()
-      .build()
-
-    then:
-    result.output.contains('com.google.inject:guice [2.0 -> 3.1]')
-    result.task(':dependencyUpdates').outcome == SUCCESS
-  }
 }