Write x509 certificate into PEM formatted string in java?

Is there some high level way to write an X509Certificate into a PEM formatted string? Currently I'm doing x509cert.encode() to write it into a DER formatted string, then base 64 encoding it and appending the header and footer to create a PEM string, but it seems bad. Especially since I have to throw in line breaks too.

This is not bad. Java doesn't provide any functions to write PEM files. What you are doing is the correct way. Even KeyTool does the same thing,

BASE64Encoder encoder = new BASE64Encoder();
out.println(X509Factory.BEGIN_CERT);
encoder.encodeBuffer(cert.getEncoded(), out);
out.println(X509Factory.END_CERT);

If you use BouncyCastle, you can use PEMWriter class to write out X509 certificate in PEM.

Haven't seen anyone bring up Java 8's Base64.getMimeEncoder method yet - actually allows you to specify both the line length and line separator like so:

final Base64.Encoder encoder = Base64.getMimeEncoder(64, LINE_SEPARATOR.getBytes());

I looked to see if there was any difference with this ^ vs the standard encoder, and I couldn't find anything. The javadoc cites RFC 2045 for both BASIC and MIME encoders, with the addition of RFC 4648 for BASIC. AFAIK both of these standards use the same Base64 alphabet (tables look the same), so you should fine to use MIME if you need to specify a line length.

This means that with Java 8, this can be accomplished with:

import java.security.cert.Certificate;
import java.security.cert.CertificateEncodingException;
import java.util.Base64;

...

public static final String BEGIN_CERT = "-----BEGIN CERTIFICATE-----";
public static final String END_CERT = "-----END CERTIFICATE-----";
public final static String LINE_SEPARATOR = System.getProperty("line.separator");

...

public static String formatCrtFileContents(final Certificate certificate) throws CertificateEncodingException {
    final Base64.Encoder encoder = Base64.getMimeEncoder(64, LINE_SEPARATOR.getBytes());

    final byte[] rawCrtText = certificate.getEncoded();
    final String encodedCertText = new String(encoder.encode(rawCrtText));
    final String prettified_cert = BEGIN_CERT + LINE_SEPARATOR + encodedCertText + LINE_SEPARATOR + END_CERT;
    return prettified_cert;
}

Previous answer gives compatibility problems with 3de party software (like PHP), because PEM cert is not correctly chunked.

Imports:

import org.apache.commons.codec.binary.Base64;

Code:

protected static String convertToPem(X509Certificate cert) throws CertificateEncodingException {
 Base64 encoder = new Base64(64);
 String cert_begin = "-----BEGIN CERTIFICATE-----\n";
 String end_cert = "-----END CERTIFICATE-----";

 byte[] derCert = cert.getEncoded();
 String pemCertPre = new String(encoder.encode(derCert));
 String pemCert = cert_begin + pemCertPre + end_cert;
 return pemCert;
}

The following uses no big external libraries or possibly version-inconsistent sun.* libraries. It builds on judoman's answer, but it also chunks lines at 64 characters, as required by OpenSSL, Java, and others.

Import:

import javax.xml.bind.DatatypeConverter;
import java.security.cert.X509Certificate;
import java.io.StringWriter;

Code:

public static String certToString(X509Certificate cert) {
    StringWriter sw = new StringWriter();
    try {
        sw.write("-----BEGIN CERTIFICATE-----\n");
        sw.write(DatatypeConverter.printBase64Binary(cert.getEncoded()).replaceAll("(.{64})", "$1\n"));
        sw.write("\n-----END CERTIFICATE-----\n");
    } catch (CertificateEncodingException e) {
        e.printStackTrace();
    }
    return sw.toString();
}

(I would have just commented on judoman's answer, but I don't have enough reputation points to be allowed to comment, and my simple edit was rejected because it should have been a comment or an answer, so here's the answer.)

If you want to write straight to file, also import java.io.FileWriter and:

FileWriter fw = new FileWriter(certFilePath);
fw.write(certToString(myCert));
fw.close();