Fix repeated resolving of sender in configuration

jaeger-core/src/main/java/io/jaegertracing/Configuration.java

@@ -631,12 +631,10 @@ public SenderConfiguration withAuthPassword(String password) {
      * @return the sender passed via the constructor or a properly configured sender
      */
     public Sender getSender() {
-      // if we have a sender, that's the one we return
-      if (null != sender) {
-        return sender;
+      if (sender == null) {
+        sender = SenderResolver.resolve(this);
       }
-
-      return SenderResolver.resolve(this);
+      return sender;
     }
 
     /**

jaeger-core/src/test/java/io/jaegertracing/ConfigurationTest.java

@@ -23,6 +23,7 @@
 import io.jaegertracing.Configuration.CodecConfiguration;
 import io.jaegertracing.Configuration.ReporterConfiguration;
 import io.jaegertracing.Configuration.SamplerConfiguration;
+import io.jaegertracing.Configuration.SenderConfiguration;
 import io.jaegertracing.internal.JaegerSpanContext;
 import io.jaegertracing.internal.JaegerTracer;
 import io.jaegertracing.internal.metrics.InMemoryMetricsFactory;
@@ -183,6 +184,12 @@ public void testSenderBackwardsCompatibilityGettingAgentHostAndPort() {
         .getSenderConfiguration().getAgentPort());
   }
 
+  @Test
+  public void testSenderInstanceIsCached() {
+    SenderConfiguration senderConfiguration = SenderConfiguration.fromEnv();
+    assertEquals(senderConfiguration.getSender(), senderConfiguration.getSender());
+  }
+
   @Test
   public void testPropagationB3Only() {
     System.setProperty(Configuration.JAEGER_PROPAGATION, "b3");