Loading a self-signed SSL certificate .crt file and packaging it into a SSLSocketFactory for use with a HttpsURLConnection.

SelfSignedCrtCertificate.java

// Usage example...
HttpsURLConnection connection = (HttpsURLConnection) new URL("https://someurl.com").openConnection();
connection.setSSLSocketFactory(buildSslSocketFactory());

private static SSLSocketFactory buildSslSocketFactory(Context context) {
	// Add support for self-signed (local) SSL certificates
	// Based on http://developer.android.com/training/articles/security-ssl.html#UnknownCa
	try {
	  
		// Load CAs from an InputStream
		// (could be from a resource or ByteArrayInputStream or ...)
		CertificateFactory cf = CertificateFactory.getInstance("X.509");
		// From https://www.washington.edu/itconnect/security/ca/load-der.crt
		InputStream is = context.getResources().getAssets().openAsset("somefolder/somecertificate.crt");
		InputStream caInput = new BufferedInputStream(is);
		Certificate ca;
		try {
			ca = cf.generateCertificate(caInput);
			// System.out.println("ca=" + ((X509Certificate) ca).getSubjectDN());
		} finally {
			caInput.close();
		}

		// Create a KeyStore containing our trusted CAs
		String keyStoreType = KeyStore.getDefaultType();
		KeyStore keyStore = KeyStore.getInstance(keyStoreType);
		keyStore.load(null, null);
		keyStore.setCertificateEntry("ca", ca);

		// Create a TrustManager that trusts the CAs in our KeyStore
		String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
		TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
		tmf.init(keyStore);

		// Create an SSLContext that uses our TrustManager
		SSLContext context = SSLContext.getInstance("TLS");
		context.init(null, tmf.getTrustManagers(), null);
		return context.getSocketFactory();

	} catch (NoSuchAlgorithmException e) {
		e.printStackTrace();
	} catch (KeyStoreException e) {
		e.printStackTrace();
	} catch (KeyManagementException e) {
		e.printStackTrace();
	} catch (CertificateException e) {
		e.printStackTrace();
	} catch (IOException e) {
		e.printStackTrace();
	}
	return null;
	
}

It is solved:

put the file under "app/src/main/res/raw" with a name such as "mycertificate.crt".

        int identifier = context.getResources().getIdentifier("mycertificate","raw",context.getPackageName());
        InputStream is = context.getResources().openRawResource(identifier);

you can also solve it by :

InputStream is = context.getResources().getAssets().open("certs_server.crt");

where you have your *.crt file in src/main/assets folder.
In the 37th line of the above code, the "context" variable should be renamed to "sslContext"/other as it already exists or needed to call a static method from activity. Everything is just working fine. To understand all the detail and related security issues just follow this official documentation from where the above code snippet has come :

https://developer.android.com/training/articles/security-ssl.html#CommonProblems

how can you use crt file in swift code?

Usage says: connection.setSSLSocketFactory(buildSslSocketFactory());

private static SSLSocketFactory buildSslSocketFactory(Context context) {

what is Context ?