Guarding from null host in Apache legacy HTTP client (#1746)

Author: eyalkoren

CHANGELOG.asciidoc

@@ -68,6 +68,7 @@ Even when matching on the main class name or on system properties,
 * Fix apm-log4j1-plugin and apm-log4j2-plugin dependency on slf4j - {pull}1723[#1723]
 * Avoid systematic `MessageNotWriteableException` error logging, now only visible in `debug` - {pull}1715[#1715]
 * Fix rounded number format for non-english locales - {pull}1728[#1728]
+* Fix `NullPointerException` on legacy Apache client instrumentation when host is `null` - {pull}1746[#1746]
 
 [[release-notes-1.x]]
 === Java Agent version 1.x

apm-agent-plugins/apm-apache-httpclient-plugin/src/main/java/co/elastic/apm/agent/httpclient/LegacyApacheHttpClientInstrumentation.java

@@ -56,7 +56,7 @@ public class LegacyApacheHttpClientInstrumentation extends BaseApacheHttpClientI
  public static class LegacyApacheHttpClientAdvice {
  @Nullable
  @Advice.OnMethodEnter(suppress = Throwable.class, inline = false)
- public static Object onBeforeExecute(@Advice.Argument(0) HttpHost host,
+ public static Object onBeforeExecute(@Advice.Argument(0) @Nullable HttpHost host,
  @Advice.Argument(1) HttpRequest request) {
  final AbstractSpan<?> parent = tracer.getActive();
  if (parent == null) {
@@ -66,7 +66,8 @@ public static Object onBeforeExecute(@Advice.Argument(0) HttpHost host,
  return null;
  }
  HttpUriRequest uriRequest = (HttpUriRequest) request;
- Span span = HttpClientHelper.startHttpClientSpan(parent, uriRequest.getMethod(), uriRequest.getURI(), host.getHostName());
+ String hostName = (host != null) ? host.getHostName() : null;
+ Span span = HttpClientHelper.startHttpClientSpan(parent, uriRequest.getMethod(), uriRequest.getURI(), hostName);
  if (span != null) {
  span.activate();
  span.propagateTraceContext(request, RequestHeaderAccessor.INSTANCE);