feat: Add ContentTypeHeader helper for parsing media-type headers

Author: thced

src/main/java/com/retailsvc/http/internal/ContentTypeHeader.java

@@ -0,0 +1,50 @@
+package com.retailsvc.http.internal;
+
+import java.util.Locale;
+import java.util.Optional;
+
+/** Parses {@code Content-Type} header values. */
+public final class ContentTypeHeader {
+
+  private ContentTypeHeader() {}
+
+  /** Returns the bare media type, stripping parameters. {@code null} → {@code application/json}. */
+  public static String subtype(String header) {
+    if (header == null) {
+      return "application/json";
+    }
+    int semi = header.indexOf(';');
+    String bare = (semi < 0 ? header : header.substring(0, semi));
+    return bare.trim();
+  }
+
+  /** Returns the named parameter value (e.g. {@code charset}), or empty if absent. */
+  public static Optional<String> parameter(String header, String name) {
+    if (header == null) {
+      return Optional.empty();
+    }
+    String target = name.toLowerCase(Locale.ROOT);
+    int semi = header.indexOf(';');
+    if (semi < 0) {
+      return Optional.empty();
+    }
+    String[] parts = header.substring(semi + 1).split(";");
+    for (String p : parts) {
+      String trimmed = p.trim();
+      int eq = trimmed.indexOf('=');
+      if (eq <= 0) {
+        continue;
+      }
+      String key = trimmed.substring(0, eq).trim().toLowerCase(Locale.ROOT);
+      if (!key.equals(target)) {
+        continue;
+      }
+      String value = trimmed.substring(eq + 1).trim();
+      if (value.length() >= 2 && value.startsWith("\"") && value.endsWith("\"")) {
+        value = value.substring(1, value.length() - 1);
+      }
+      return Optional.of(value);
+    }
+    return Optional.empty();
+  }
+}

src/test/java/com/retailsvc/http/internal/ContentTypeHeaderTest.java

@@ -0,0 +1,56 @@
+package com.retailsvc.http.internal;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class ContentTypeHeaderTest {
+
+  @Test
+  void subtypeReturnsBareMediaType() {
+    assertThat(ContentTypeHeader.subtype("application/json")).isEqualTo("application/json");
+  }
+
+  @Test
+  void subtypeStripsParameters() {
+    assertThat(ContentTypeHeader.subtype("text/plain; charset=utf-8")).isEqualTo("text/plain");
+  }
+
+  @Test
+  void subtypeTrimsWhitespace() {
+    assertThat(ContentTypeHeader.subtype("  application/json  ")).isEqualTo("application/json");
+  }
+
+  @Test
+  void subtypeDefaultsToApplicationJsonWhenNull() {
+    assertThat(ContentTypeHeader.subtype(null)).isEqualTo("application/json");
+  }
+
+  @Test
+  void parameterReturnsValue() {
+    assertThat(ContentTypeHeader.parameter("text/plain; charset=iso-8859-1", "charset"))
+        .contains("iso-8859-1");
+  }
+
+  @Test
+  void parameterUnquotesValue() {
+    assertThat(ContentTypeHeader.parameter("text/plain; charset=\"utf-8\"", "charset"))
+        .contains("utf-8");
+  }
+
+  @Test
+  void parameterReturnsEmptyWhenMissing() {
+    assertThat(ContentTypeHeader.parameter("text/plain", "charset")).isEmpty();
+  }
+
+  @Test
+  void parameterNameMatchIsCaseInsensitive() {
+    assertThat(ContentTypeHeader.parameter("text/plain; CHARSET=utf-8", "charset"))
+        .contains("utf-8");
+  }
+
+  @Test
+  void parameterReturnsEmptyForNullHeader() {
+    assertThat(ContentTypeHeader.parameter(null, "charset")).isEmpty();
+  }
+}