Avoid unsafe lazy-initialization for SSL sockets

This prevents calling HttpsURLConnection.getDefaultSSLSocketFactory() in
an unsafe manner due to the poorly implemented lazy-initialization on
the JDK.

When multiple threads call that method concurrently (calling
secureConnection()) the SSLSocketFactory is instantiated two times,
making one thread fail the check and overriding the custom socket
factory with the default one.

Author: ahaasler

core-client/src/main/java/org/glassfish/jersey/client/internal/HttpUrlConnector.java

@@ -71,6 +71,9 @@ public class HttpUrlConnector implements Connector {
 
     private static final Logger LOGGER = Logger.getLogger(HttpUrlConnector.class.getName());
     private static final String ALLOW_RESTRICTED_HEADERS_SYSTEM_PROPERTY = "sun.net.http.allowRestrictedHeaders";
+    // Avoid multi-thread uses of HttpsURLConnection.getDefaultSSLSocketFactory() because it does not implement a
+    // proper lazy-initialization. See https://github.com/jersey/jersey/issues/3293
+    private static final SSLSocketFactory DEFAULT_SSL_SOCKET_FACTORY = HttpsURLConnection.getDefaultSSLSocketFactory();
     // The list of restricted headers is extracted from sun.net.www.protocol.http.HttpURLConnection
     private static final String[] restrictedHeaders = {
             "Access-Control-Request-Headers",
@@ -299,7 +302,7 @@ protected void secureConnection(final JerseyClient client, final HttpURLConnecti
                 suc.setHostnameVerifier(verifier);
             }
 
-            if (HttpsURLConnection.getDefaultSSLSocketFactory() == suc.getSSLSocketFactory()) {
+            if (DEFAULT_SSL_SOCKET_FACTORY == suc.getSSLSocketFactory()) {
                 // indicates that the custom socket factory was not set
                 suc.setSSLSocketFactory(sslSocketFactory.get());
             }