Resolve dependency constraints.

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

@@ -18,6 +18,7 @@ package com.github.benmanes.gradle.versions.updates
 import groovy.transform.EqualsAndHashCode
 import groovy.transform.TypeChecked
 import org.gradle.api.artifacts.Dependency
+import org.gradle.api.artifacts.DependencyConstraint
 import org.gradle.api.artifacts.ModuleVersionIdentifier
 import org.gradle.api.artifacts.ModuleVersionSelector
 import org.gradle.api.artifacts.component.ModuleComponentIdentifier
@@ -61,6 +62,10 @@ class Coordinate implements Comparable<Coordinate> {
     return new Coordinate(dependency.group, dependency.name, dependency.version)
   }
 
+  static Coordinate from(DependencyConstraint dependency) {
+    return new Coordinate(dependency.group, dependency.name, dependency.version)
+  }
+
   static Coordinate from(ModuleVersionIdentifier identifier) {
     return new Coordinate(identifier.group, identifier.name, identifier.version)
   }

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

@@ -25,6 +25,7 @@ import org.gradle.api.artifacts.ComponentMetadata
 import org.gradle.api.artifacts.ComponentSelection
 import org.gradle.api.artifacts.Configuration
 import org.gradle.api.artifacts.Dependency
+import org.gradle.api.artifacts.DependencyConstraint
 import org.gradle.api.artifacts.ExternalDependency
 import org.gradle.api.artifacts.LenientConfiguration
 import org.gradle.api.artifacts.ModuleVersionIdentifier
@@ -117,6 +118,15 @@ class Resolver {
       createQueryDependency(dependency, revision)
     }
 
+    // Common use case for dependency constraints is a java-platform BOM project.
+    try {
+      configuration.dependencyConstraints.each { dependency ->
+        latest.add(createQueryDependency(dependency, revision))
+      }
+    } catch (MissingPropertyException e) {
+      // Skip if constraints not supported
+    }
+
     Configuration copy = configuration.copyRecursive().setTransitive(false)
     // https://github.com/ben-manes/gradle-versions-plugin/issues/127
     if (copy.metaClass.respondsTo(copy, "setCanBeResolved", Boolean)) {
@@ -149,6 +159,19 @@ class Resolver {
     }
   }
 
+  /** Returns a variant of the provided dependency used for querying the latest version. */
+  @TypeChecked(SKIP)
+  private Dependency createQueryDependency(DependencyConstraint dependency, String revision) {
+    String versionQuery = useSelectionRules ? '+' : "latest.${revision}"
+
+    // If no version was specified then use 'none' to pass it through.
+    String version = dependency.version == null ? 'none' : versionQuery
+
+    return project.dependencies.create("${dependency.group}:${dependency.name}:${version}") {
+      transitive = false
+    }
+  }
+
   /** Adds a revision filter by rejecting candidates using a component selection rule. */
   @TypeChecked(SKIP)
   private void addRevisionFilter(Configuration configuration, String revision) {
@@ -184,12 +207,11 @@ class Resolver {
 
   /** Returns the coordinates for the current (declared) dependency versions. */
   private Map<Coordinate.Key, Coordinate> getCurrentCoordinates(Configuration configuration) {
-    Map<Coordinate.Key, Coordinate> declared = configuration.dependencies.findAll { dependency ->
-      dependency instanceof ExternalDependency
-    }.collectEntries {
-      Coordinate coordinate = Coordinate.from(it)
-      return [coordinate.key, coordinate]
-    }
+    Map<Coordinate.Key, Coordinate> declared =
+      getResolvableDependencies(configuration).collectEntries {
+        return [it.key, it]
+      }
+
     if (declared.isEmpty()) {
       return Collections.emptyMap()
     }
@@ -217,6 +239,15 @@ class Resolver {
       coordinates.put(coordinate.key, declared.get(coordinate.key))
     }
 
+    try {
+      for (DependencyConstraint constraint : copy.dependencyConstraints) {
+        Coordinate coordinate = Coordinate.from(constraint)
+        coordinates.put(coordinate.key, declared.get(coordinate.key))
+      }
+    } catch (MissingPropertyException e) {
+      // Skip if constraints not supported
+    }
+
     // Ignore undeclared (hidden) dependencies that appear when resolving a configuration
     coordinates.keySet().retainAll(declared.keySet())
 
@@ -333,6 +364,24 @@ class Resolver {
     return null
   }
 
+  private static List<Coordinate> getResolvableDependencies(Configuration configuration) {
+    List<Coordinate> coordinates = configuration.dependencies.findAll { dependency ->
+      dependency instanceof ExternalDependency
+    }.collect { dependency ->
+      Coordinate.from(dependency)
+    }
+
+    try {
+      configuration.dependencyConstraints.each {
+        coordinates.add(Coordinate.from(it))
+      }
+    } catch (MissingPropertyException e) {
+      // Skip
+    }
+
+    return coordinates
+  }
+
   private static final class ProjectUrl {
     boolean isResolved
     String url

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

@@ -46,4 +46,40 @@ 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
+  }
 }