feat: Add TextPlainParser for text/plain request bodies

Author: thced

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

@@ -0,0 +1,29 @@
+package com.retailsvc.http.internal;
+
+import java.nio.charset.Charset;
+import java.nio.charset.IllegalCharsetNameException;
+import java.nio.charset.StandardCharsets;
+import java.nio.charset.UnsupportedCharsetException;
+
+/** Decodes a text/plain request body using the charset declared on {@code Content-Type}. */
+public final class TextPlainParser {
+
+  public String parse(byte[] body, String contentTypeHeader) {
+    Charset charset = resolveCharset(contentTypeHeader);
+    return new String(body, charset);
+  }
+
+  private static Charset resolveCharset(String header) {
+    return ContentTypeHeader.parameter(header, "charset")
+        .map(TextPlainParser::safeCharset)
+        .orElse(StandardCharsets.UTF_8);
+  }
+
+  private static Charset safeCharset(String name) {
+    try {
+      return Charset.forName(name);
+    } catch (IllegalCharsetNameException | UnsupportedCharsetException _) {
+      return StandardCharsets.UTF_8;
+    }
+  }
+}

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

@@ -0,0 +1,36 @@
+package com.retailsvc.http.internal;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.nio.charset.StandardCharsets;
+import org.junit.jupiter.api.Test;
+
+class TextPlainParserTest {
+
+  private final TextPlainParser parser = new TextPlainParser();
+
+  @Test
+  void decodesUtf8ByDefault() {
+    String body = "hello världen";
+    assertThat(parser.parse(body.getBytes(StandardCharsets.UTF_8), null)).isEqualTo(body);
+  }
+
+  @Test
+  void respectsCharsetFromHeader() {
+    String body = "räksmörgås";
+    byte[] bytes = body.getBytes(StandardCharsets.ISO_8859_1);
+    assertThat(parser.parse(bytes, "text/plain; charset=iso-8859-1")).isEqualTo(body);
+  }
+
+  @Test
+  void emptyBodyDecodesToEmptyString() {
+    assertThat(parser.parse(new byte[0], null)).isEqualTo("");
+  }
+
+  @Test
+  void unknownCharsetFallsBackToUtf8() {
+    String body = "hello";
+    assertThat(parser.parse(body.getBytes(StandardCharsets.UTF_8), "text/plain; charset=bogus"))
+        .isEqualTo(body);
+  }
+}