Add Evaluate() method to maven package (#1268)

Useful for evaluating properties from pom files using the Maven evaluate plugin.

pkg/maven/maven.go

@@ -2,6 +2,7 @@ package maven
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"strings"
 
@@ -10,6 +11,7 @@ import (
 	"github.com/SAP/jenkins-library/pkg/piperutils"
 )
 
+// ExecuteOptions are used by Execute() to construct the Maven command line.
 type ExecuteOptions struct {
 	PomPath                     string   `json:"pomPath,omitempty"`
 	ProjectSettingsFile         string   `json:"projectSettingsFile,omitempty"`
@@ -30,6 +32,8 @@ type mavenExecRunner interface {
 
 const mavenExecutable = "mvn"
 
+// Execute constructs a mvn command line from the given options, and uses the provided
+// mavenExecRunner to execute it.
 func Execute(options *ExecuteOptions, command mavenExecRunner) (string, error) {
 	stdOutBuf, stdOut := evaluateStdOut(options)
 	command.Stdout(stdOut)
@@ -51,6 +55,27 @@ func Execute(options *ExecuteOptions, command mavenExecRunner) (string, error) {
 	return string(stdOutBuf.Bytes()), nil
 }
 
+// Evaluate constructs ExecuteOptions for using the maven-help-plugin's 'evaluate' goal to
+// evaluate a given expression from a pom file. This allows to retrieve the value of - for
+// example - 'project.version' from a pom file exactly as Maven itself evaluates it.
+func Evaluate(pomFile, expression string, command mavenExecRunner) (string, error) {
+	expressionDefine := "-Dexpression=" + expression
+	options := ExecuteOptions{
+		PomPath:      pomFile,
+		Goals:        []string{"org.apache.maven.plugins:maven-help-plugin:3.1.0:evaluate"},
+		Defines:      []string{expressionDefine, "-DforceStdout", "-q"},
+		ReturnStdout: true,
+	}
+	value, err := Execute(&options, command)
+	if err != nil {
+		return "", err
+	}
+	if strings.HasPrefix(value, "null object or invalid expression") {
+		return "", fmt.Errorf("expression '%s' in file '%s' could not be resolved", expression, pomFile)
+	}
+	return value, nil
+}
+
 func evaluateStdOut(config *ExecuteOptions) (*bytes.Buffer, io.Writer) {
 	var stdOutBuf *bytes.Buffer
 	var stdOut io.Writer

pkg/maven/maven_test.go

@@ -65,6 +65,27 @@ func TestExecute(t *testing.T) {
 	})
 }
 
+func TestEvaluate(t *testing.T) {
+	t.Run("should evaluate expression", func(t *testing.T) {
+		execMockRunner := mock.ExecMockRunner{}
+		execMockRunner.StdoutReturn = map[string]string{"mvn --file pom.xml -Dexpression=project.groupId -DforceStdout -q --batch-mode org.apache.maven.plugins:maven-help-plugin:3.1.0:evaluate": "com.awesome"}
+
+		result, err := Evaluate("pom.xml", "project.groupId", &execMockRunner)
+		if assert.NoError(t, err) {
+			assert.Equal(t, "com.awesome", result)
+		}
+	})
+	t.Run("should not evaluate expression", func(t *testing.T) {
+		execMockRunner := mock.ExecMockRunner{}
+		execMockRunner.StdoutReturn = map[string]string{"mvn --file pom.xml -Dexpression=project.groupId -DforceStdout -q --batch-mode org.apache.maven.plugins:maven-help-plugin:3.1.0:evaluate": "null object or invalid expression"}
+
+		result, err := Evaluate("pom.xml", "project.groupId", &execMockRunner)
+		if assert.EqualError(t, err, "expression 'project.groupId' in file 'pom.xml' could not be resolved") {
+			assert.Equal(t, "", result)
+		}
+	})
+}
+
 type mockDownloader struct {
 	shouldFail bool
 }