package com.sample.util;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.util.Base64;
import java.util.Base64.Encoder;

public class CertificateUtil {
private static final int BUFFER_SIZE = 65535;

/**
  * 
  * 
  * @param certificateFilePath
  * @return the base64 encoded array from given certificate content.
  * 
  * @throws Exception
  */
public static byte[] convertCertificateToByteArray(String certificateFilePath) throws Exception {
if (certificateFilePath == null || certificateFilePath.isEmpty()) {
throw new Exception("certificateFilePath should not be null or empty");
}

  File file = new File(certificateFilePath);

if (!file.exists()) {
throw new Exception("File not exist : " + file.getAbsolutePath());
}

if (file.isDirectory()) {
throw new Exception("File shouldn't be a directory : " + file.getAbsolutePath());
}

try (InputStream inputStream = new FileInputStream(file)) {

   ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
byte[] buffer = new byte[BUFFER_SIZE];
int numberOfReadBytes;

while ((numberOfReadBytes = inputStream.read(buffer)) > 0) {
    byteArrayOutputStream.write(buffer, 0, numberOfReadBytes);
}

byte[] certificateInBytes = byteArrayOutputStream.toByteArray();

   Encoder encoder = Base64.getEncoder();
byte[] encodedBytes = encoder.encode(certificateInBytes);
return encodedBytes;

}
}

/**
  * Convert the certificate into base64 encoded string.
  * 
  * @param certificateFilePath
  * @return
  * @throws Exception
  */
public static String convertCertificateToString(String certificateFilePath) throws Exception {

byte[] encodedBytes = convertCertificateToByteArray(certificateFilePath);
  String str = new String(encodedBytes);

return str;

}
}

package com.sample.test;

import com.sample.util.CertificateUtil;

public class Test {
public static void main(String args[]) throws Exception {
  String certificateFilePath = "C:\\Users\\krishna\\myCert.cer";

  String certificateInfo = CertificateUtil.convertCertificateToString(certificateFilePath);

  System.out.println(certificateInfo);
}
}