Add new function CreateJar

* for now used in spring-boot buildpack

crush/crush.go

@@ -103,6 +103,86 @@ func CreateTarGz(destination io.Writer, source string) error {
 	return CreateTar(gz, source)
 }
 
+// CreateJar heavily inspired by: https://gosamples.dev/zip-file/
+// Be aware that this function does not create a MANIFEST.MF file, not does it strictly enforce jar format
+// in regard to elements that need to be STORE'd versus other that need to be DEFLATE'd; here everything is STORE'd
+// Finally, source path must end with a trailing "/"
+func CreateJar(source, target string) error {
+
+	// 1. Create a ZIP file and zip.Writer
+	f, err := os.Create(target)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	writer := zip.NewWriter(f)
+	defer writer.Close()
+
+	// 2. Go through all the files of the source
+	return filepath.Walk(source, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		absolutePath := ""
+
+		if info.Mode()&os.ModeSymlink == os.ModeSymlink {
+			if absolutePath, err = filepath.EvalSymlinks(path); err != nil {
+				return fmt.Errorf("unable to eval symlink %s\n%w", absolutePath, err)
+			}
+			if file, err := os.Open(absolutePath); err != nil {
+				return fmt.Errorf("unable to open %s\n%w", absolutePath, err)
+			} else {
+				if info, err = file.Stat(); err != nil {
+					return fmt.Errorf("unable to stat %s\n%w", absolutePath, err)
+				}
+			}
+		}
+
+		// 3. Create a local file header
+		header, err := zip.FileInfoHeader(info)
+		if err != nil {
+			return err
+		}
+
+		// set compression
+		header.Method = zip.Store
+		// 4. Set relative path of a file as the header name
+		header.Name, err = filepath.Rel(source, path)
+		if err != nil {
+			return err
+		}
+		if info.IsDir() {
+			header.Name += "/"
+		}
+
+		// 5. Create writer for the file header and save content of the file
+		headerWriter, err := writer.CreateHeader(header)
+		if err != nil {
+			return err
+		}
+
+		if info.IsDir() {
+			return nil
+		}
+
+		if absolutePath != "" {
+			path = absolutePath
+		}
+
+		f, err := os.Open(path)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+
+		_, err = io.Copy(headerWriter, f)
+		writer.Flush()
+		return err
+	})
+
+}
+
 // Extract decompresses and extract source files to a destination directory or path. For archives, an arbitrary number of top-level directory
 // components can be stripped from each path.
 func Extract(source io.Reader, destination string, stripComponents int) error {

crush/crush_test.go

@@ -95,6 +95,32 @@ func testCrush(t *testing.T, context spec.G, it spec.S) {
 			Expect(filepath.Join(testPath, "dirA", "fileC.txt")).To(BeARegularFile())
 			Expect(os.Readlink(filepath.Join(testPath, "dirA", "fileD.txt"))).To(Equal(filepath.Join(path, "dirA", "fileC.txt")))
 		})
+
+		it("writes a JAR", func() {
+			cwd, _ := os.Getwd()
+			Expect(os.MkdirAll(filepath.Join(path, "META-INF"), 0755)).To(Succeed())
+			Expect(os.WriteFile(filepath.Join(path, "META-INF", "MANIFEST.MF"), []byte(`
+	Spring-Boot-Version: 3.3.1
+	Spring-Boot-Classes: BOOT-INF/classes
+	Spring-Boot-Lib: BOOT-INF/lib
+	`), 0644)).To(Succeed())
+			Expect(os.MkdirAll(filepath.Join(path, "BOOT-INF"), 0755)).To(Succeed())
+			Expect(os.MkdirAll(filepath.Join(path, "BOOT-INF", "classes"), 0755)).To(Succeed())
+			Expect(os.WriteFile(filepath.Join(path, "BOOT-INF", "classes", "OtherClass.class"), []byte(""), 0644)).To(Succeed())
+			Expect(os.WriteFile(filepath.Join(path, "BOOT-INF", "classes", "YetOther.class"), []byte(""), 0644)).To(Succeed())
+			Expect(os.MkdirAll(filepath.Join(path, "BOOT-INF", "lib"), 0755)).To(Succeed())
+			os.Symlink(filepath.Join(cwd, "testdata", "spring-cloud-bindings-1.2.3.jar"), filepath.Join(path, "BOOT-INF", "lib", "spring-cloud-bindings-1.2.3.jar"))
+
+			Expect(crush.CreateJar(path+"/", out.Name()+".jar")).To(Succeed())
+
+			in, err := os.Open(out.Name() + ".jar")
+			Expect(err).NotTo(HaveOccurred())
+
+			Expect(crush.Extract(in, testPath, 0)).To(Succeed())
+			Expect(filepath.Join(testPath, "BOOT-INF", "classes", "OtherClass.class")).To(BeARegularFile())
+			Expect(filepath.Join(testPath, "META-INF", "MANIFEST.MF")).To(BeARegularFile())
+			Expect(filepath.Join(testPath, "BOOT-INF", "lib", "spring-cloud-bindings-1.2.3.jar")).To(BeARegularFile())
+		})
 	})
 
 	context("Extract", func() {