feat(java): add empty versions if pom.xml dependency versions can't be detected

docs/docs/coverage/language/java.md

@@ -69,6 +69,16 @@ The vulnerability database will be downloaded anyway.
 !!! Warning
     Trivy may skip some dependencies (that were not found on your local machine) when the `--offline-scan` flag is passed.
 
+### empty dependency version
+There are cases when Trivy cannot determine the version of dependencies:
+
+- Unable to determine the version from the parent because the parent is not reachable;
+- The dependency uses a [hard requirement][version-requirement] with more than one version.
+
+In these cases, Trivy uses an empty version for the dependency.
+
+!!! Warning
+    Trivy doesn't detect child dependencies for dependencies without a version.
 
 ### maven-invoker-plugin
 Typically, the integration tests directory (`**/[src|target]/it/*/pom.xml`) of [maven-invoker-plugin][maven-invoker-plugin] doesn't contain actual `pom.xml` files and should be skipped to avoid noise.
@@ -120,3 +130,4 @@ Make sure that you have cache[^8] directory to find licenses from `*.pom` depend
 [maven-pom-repos]: https://maven.apache.org/settings.html#repositories
 [sbt-dependency-lock]: https://stringbean.github.io/sbt-dependency-lock
 [detection-priority]: ../../scanner/vulnerability.md#detection-priority
+[version-requirement]: https://maven.apache.org/pom.html#dependency-version-requirement-specification

pkg/dependency/parser/java/pom/artifact.go

@@ -6,15 +6,22 @@ import (
 	"regexp"
 	"slices"
 	"strings"
+	"sync"
 
 	"github.com/samber/lo"
 
 	ftypes "github.com/aquasecurity/trivy/pkg/fanal/types"
 	"github.com/aquasecurity/trivy/pkg/log"
+	"github.com/aquasecurity/trivy/pkg/version/doc"
 )
 
 var (
-	varRegexp = regexp.MustCompile(`\${(\S+?)}`)
+	varRegexp        = regexp.MustCompile(`\${(\S+?)}`)
+	emptyVersionWarn = sync.OnceFunc(func() {
+		log.WithPrefix("pom").Warn("Dependency version is empty. Child dependencies will not be found.",
+			// e.g. https://aquasecurity.github.io/trivy/latest/docs/coverage/language/java/#empty-dependency-version
+			log.String("details", doc.URL("/docs/coverage/language/java/", "empty-dependency-version")))
+	})
 )
 
 type artifact struct {
@@ -42,7 +49,13 @@ func newArtifact(groupID, artifactID, version string, licenses []string, props m
 }
 
 func (a artifact) IsEmpty() bool {
-	return a.GroupID == "" || a.ArtifactID == "" || a.Version.String() == ""
+	if a.GroupID == "" || a.ArtifactID == "" {
+		return true
+	}
+	if a.Version.String() == "" {
+		emptyVersionWarn()
+	}
+	return false
 }
 
 func (a artifact) Equal(o artifact) bool {

pkg/dependency/parser/java/pom/parse_test.go

@@ -809,6 +809,17 @@ func TestPom_Parse(t *testing.T) {
 					Licenses:     []string{"Apache 2.0"},
 					Relationship: ftypes.RelationshipRoot,
 				},
+				{
+					ID:           "org.example:example-api",
+					Name:         "org.example:example-api",
+					Relationship: ftypes.RelationshipDirect,
+					Locations: []ftypes.Location{
+						{
+							StartLine: 28,
+							EndLine:   32,
+						},
+					},
+				},
 			},
 		},
 		{
@@ -1499,6 +1510,30 @@ func TestPom_Parse(t *testing.T) {
 				},
 			},
 		},
+		{
+			name:      "dependency without version",
+			inputFile: filepath.Join("testdata", "dep-without-version", "pom.xml"),
+			local:     true,
+			want: []ftypes.Package{
+				{
+					ID:           "com.example:dep-without-version:1.0.0",
+					Name:         "com.example:dep-without-version",
+					Version:      "1.0.0",
+					Relationship: ftypes.RelationshipRoot,
+				},
+				{
+					ID:           "org.example:example-api",
+					Name:         "org.example:example-api",
+					Relationship: ftypes.RelationshipDirect,
+					Locations: ftypes.Locations{
+						{
+							StartLine: 19,
+							EndLine:   22,
+						},
+					},
+				},
+			},
+		},
 		// [INFO] com.example:root-depManagement-in-parent:jar:1.0.0
 		// [INFO] \- org.example:example-dependency:jar:2.0.0:compile
 		// [INFO]    \- org.example:example-api:jar:1.0.1:compile

pkg/dependency/parser/java/pom/testdata/dep-without-version/pom.xml

@@ -0,0 +1,24 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <name>no-parent</name>
+    <description>Parent not found</description>
+
+    <parent>
+        <groupId>com.example</groupId>
+        <artifactId>wrong-parent</artifactId>
+        <version>1.0.0</version>
+    </parent>
+
+    <groupId>com.example</groupId>
+    <artifactId>dep-without-version</artifactId>
+    <version>1.0.0</version>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.example</groupId>
+            <artifactId>example-api</artifactId>
+        </dependency>
+    </dependencies>
+</project>

pkg/fanal/analyzer/language/java/pom/pom_test.go

@@ -147,6 +147,17 @@ func Test_pomAnalyzer_Analyze(t *testing.T) {
 								Licenses:     []string{"Apache-2.0"},
 								Relationship: types.RelationshipRoot,
 							},
+							{
+								ID:           "org.example:example-api",
+								Name:         "org.example:example-api",
+								Relationship: types.RelationshipDirect,
+								Locations: []types.Location{
+									{
+										StartLine: 21,
+										EndLine:   25,
+									},
+								},
+							},
 						},
 					},
 				},