Extract the zip and convert to base 64 of each file

I am reading the list of attachment from a system witch returns the attached document in base 64 encoded string as a zip and My objective is to get the base 64 encoded string for each attached document.
Note:- I am trying below code where I am unzipping the zip and writing at my local file system.
. But in real I wanted to get the base 64 format for each file without writing the file in local file system.

public class UnzipUtility {
	  private static final int BUFFER_SIZE = 4096;
	
		 private static void extractFile(ZipInputStream zipIn, ZipEntry entry) throws IOException {
		
		
	        BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("D:/Project/"+File.separator+entry.getName()));
	        byte[] bytesIn = new byte[BUFFER_SIZE];
	        System.out.println("File Name  "+entry.getName());
	        int read = 0;
	        while ((read = zipIn.read(bytesIn)) != -1) {
	        	//Hear I dont not want to write the output stream insted I want to get the base64 data for each file.
              bos.write(bytesIn);
	        }
	        bos.close();
	    }
	 public static void main(String[] args) throws IOException {
	 String attachmentVariable="zip base 64 data"
		  byte[] bytedata = attachmentVariable.getBytes("UTF-8");
	     byte[] valueDecoded = Base64.decodeBase64(bytedata);
	     ZipInputStream zipIn = new ZipInputStream(new ByteArrayInputStream(valueDecoded));
	     ZipEntry entry = zipIn.getNextEntry();
	    
	        // iterates over entries in the zip file
	         while (entry != null) {	        		extractFile(zipIn,entry);
	                zipIn.closeEntry();
	                entry = zipIn.getNextEntry();
	                
	            
	      }       
	    
		}
}

To Base64 encode data as it is being written to an OutputStream, use the Encoder.wrap(OutputStream os) method.

By default, BufferedOutputStream will use a 8192-byte buffer, so if you increase BUFFER_SIZE to 8192, then you won't need the BufferedOutputStream.

You should use try-with-resources, and the newer NIO.2 API.

Which means your code should be:

private static final int BUFFER_SIZE = 8192;

private static void extractFile(ZipInputStream zipIn, ZipEntry entry) throws IOException {
	try ( OutputStream fos = Files.newOutputStream(Paths.get("D:/Project", entry.getName()));
	      OutputStream b64os = Base64.getEncoder().wrap(fos); ) {
		System.out.println("File Name  " + entry.getName());
		byte[] buf = new byte[BUFFER_SIZE];
		for (int len = 0; (len = zipIn.read(buf)) != -1; ) {
			b64os.write(buf, 0, len);
		}
	}
}

So, you have a Base64 encoded string with a zip file, and you want a Map<String, String>, where key is zip entry name and value is the Base64 encoded content.

In Java 9+, that is easily done like this:

String base64ZipFile = "zip base 64 data";
Map<String, String> base64Entries = new LinkedHashMap<>();
try (ZipInputStream zipIn = new ZipInputStream(new ByteArrayInputStream(Base64.getDecoder().decode(base64ZipFile)))) {
	Encoder encoder = Base64.getEncoder();
	for (ZipEntry entry; (entry = zipIn.getNextEntry()) != null; ) {
		base64Entries.put(entry.getName(), encoder.encodeToString(zipIn.readAllBytes()));
	}
}