Reject host-less endpoints in EndpointUtil.validateEndpoint

exporters/common/src/main/java/io/opentelemetry/exporter/internal/EndpointUtil.java

@@ -30,6 +30,10 @@ public static URI validateEndpoint(String endpoint) {
       throw new IllegalArgumentException(
           "Invalid endpoint, must start with http:// or https://: " + uri);
     }
+    if (uri.getHost() == null) {
+      throw new IllegalArgumentException(
+          "Invalid endpoint, must start with http:// or https://: " + uri);
+    }
     return uri;
   }
 

exporters/common/src/test/java/io/opentelemetry/exporter/internal/EndpointUtilTest.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.exporter.internal;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.net.URI;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class EndpointUtilTest {
+
+  @ParameterizedTest
+  @MethodSource("validEndpoints")
+  void validateEndpoint_valid(String endpoint) {
+    assertThat(EndpointUtil.validateEndpoint(endpoint)).isEqualTo(URI.create(endpoint));
+  }
+
+  private static Stream<Arguments> validEndpoints() {
+    return Stream.of(
+        Arguments.argumentSet("http", "http://localhost:4318"),
+        Arguments.argumentSet("https", "https://localhost:4317"),
+        Arguments.argumentSet("path", "http://localhost:4318/v1/traces"),
+        Arguments.argumentSet("userinfo", "http://foo:bar@localhost:4317/path"));
+  }
+
+  @ParameterizedTest
+  @MethodSource("invalidEndpoints")
+  void validateEndpoint_invalid(String endpoint) {
+    assertThatThrownBy(() -> EndpointUtil.validateEndpoint(endpoint))
+        .isInstanceOf(IllegalArgumentException.class)
+        .hasMessageContaining("must start with http:// or https://");
+  }
+
+  private static Stream<Arguments> invalidEndpoints() {
+    return Stream.of(
+        Arguments.argumentSet("opaque, no host", "http:localhost:4317"),
+        Arguments.argumentSet("single slash, no host", "https:/foo"),
+        Arguments.argumentSet("no scheme", "localhost"),
+        Arguments.argumentSet("wrong scheme", "gopher://localhost"));
+  }
+}