feat(internal/librarian/java): add README partials loader

Author: yangyzs

internal/librarian/java/readme.go

@@ -26,6 +26,12 @@ import (
 	"strings"
 	"unicode"
 	"unicode/utf8"
+
+	"github.com/googleapis/librarian/internal/yaml"
+)
+
+const (
+	readmePartialsFile = ".readme-partials.yaml"
 )
 
 var (
@@ -51,6 +57,9 @@ var (
 
 	// errEmptyFile indicates an empty file path was provided.
 	errEmptyFile = errors.New("file cannot be empty")
+
+	// errInvalidYAML indicates the YAML file syntax is invalid or cannot be unmarshaled.
+	errInvalidYAML = errors.New("invalid yaml syntax")
 )
 
 // codeSample represents a discovered Java code sample along with its derived title.
@@ -200,6 +209,35 @@ func parseRepoShortName(repo string) string {
 	return repo
 }
 
+// loadReadmePartials loads and camel-cases README partials from .readme-partials.yaml.
+func loadReadmePartials(dir string) (map[string]interface{}, error) {
+	if dir == "" {
+		return nil, errEmptyDir
+	}
+	partialsBytes, err := os.ReadFile(filepath.Join(dir, readmePartialsFile))
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("failed to read partials file: %w", err)
+	}
+	if len(partialsBytes) == 0 {
+		return nil, nil
+	}
+	rawPartials, err := yaml.Unmarshal[map[string]interface{}](partialsBytes)
+	if err != nil {
+		return nil, fmt.Errorf("%w: failed to unmarshal partials: %w", errInvalidYAML, err)
+	}
+	if rawPartials == nil || len(*rawPartials) == 0 {
+		return nil, nil
+	}
+	result := make(map[string]interface{}, len(*rawPartials))
+	for k, v := range *rawPartials {
+		result[toCamelCase(k)] = v
+	}
+	return result, nil
+}
+
 // collectSnippetFiles recursively scans dir/samples for Java and XML files containing snippets.
 func collectSnippetFiles(dir string) ([]string, error) {
 	samplesDir := filepath.Join(dir, "samples")

internal/librarian/java/readme_test.go

@@ -588,6 +588,103 @@ func TestParseRepoShortName(t *testing.T) {
 	}
 }
 
+func TestLoadReadmePartials(t *testing.T) {
+	for _, test := range []struct {
+		name       string
+		setupFiles func(t *testing.T, dir string)
+		want       map[string]interface{}
+	}{
+		{
+			name: "loads yaml partials with camel case conversion",
+			setupFiles: func(t *testing.T, dir string) {
+				path := filepath.Join(dir, readmePartialsFile)
+				content := `about_text: "Custom about"`
+				if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+					t.Fatal(err)
+				}
+			},
+			want: map[string]interface{}{"AboutText": "Custom about"},
+		},
+		{
+			name: "missing partials file returns nil",
+			setupFiles: func(t *testing.T, dir string) {
+				// No file written.
+			},
+			want: nil,
+		},
+		{
+			name: "empty partials file returns nil",
+			setupFiles: func(t *testing.T, dir string) {
+				path := filepath.Join(dir, readmePartialsFile)
+				if err := os.WriteFile(path, []byte(""), 0644); err != nil {
+					t.Fatal(err)
+				}
+			},
+			want: nil,
+		},
+		{
+			name: "partials file with only comments returns nil",
+			setupFiles: func(t *testing.T, dir string) {
+				path := filepath.Join(dir, readmePartialsFile)
+				if err := os.WriteFile(path, []byte("# only comments\n# no keys defined"), 0644); err != nil {
+					t.Fatal(err)
+				}
+			},
+			want: nil,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			dir := t.TempDir()
+			test.setupFiles(t, dir)
+			got, err := loadReadmePartials(dir)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if diff := cmp.Diff(test.want, got); diff != "" {
+				t.Errorf("mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestLoadReadmePartials_Error(t *testing.T) {
+	for _, test := range []struct {
+		name       string
+		dir        string
+		setupFiles func(t *testing.T, dir string)
+		wantErr    error
+	}{
+		{
+			name:    "empty directory parameter returns error",
+			dir:     "",
+			wantErr: errEmptyDir,
+		},
+		{
+			name: "invalid yaml syntax",
+			setupFiles: func(t *testing.T, dir string) {
+				path := filepath.Join(dir, readmePartialsFile)
+				content := `key: [unclosed list`
+				if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+					t.Fatal(err)
+				}
+			},
+			wantErr: errInvalidYAML,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			dir := test.dir
+			if test.setupFiles != nil {
+				dir = t.TempDir()
+				test.setupFiles(t, dir)
+			}
+			_, err := loadReadmePartials(dir)
+			if !errors.Is(err, test.wantErr) {
+				t.Errorf("loadReadmePartials() error = %v, wantErr %v", err, test.wantErr)
+			}
+		})
+	}
+}
+
 func TestCollectSnippetFiles(t *testing.T) {
 	for _, test := range []struct {
 		name       string