feat: Add HealthRenderer for health-response JSON

Author: thced

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

@@ -0,0 +1,76 @@
+package com.retailsvc.http.internal;
+
+import com.retailsvc.http.Dependency;
+import com.retailsvc.http.HealthOutcome;
+import java.util.List;
+
+/**
+ * Hand-rolled JSON renderer for {@link HealthOutcome} responses.
+ *
+ * <p>Mirrors {@link ProblemDetailRenderer} — the library avoids pulling in a JSON writer for a
+ * handful of fixed fields with known shapes.
+ */
+public final class HealthRenderer {
+
+  /** Initial capacity sized for a typical health document with a handful of dependencies. */
+  private static final int INITIAL_BUFFER_CAPACITY = 128;
+
+  /** Codepoints below this value are control characters and must be unicode-escaped in JSON. */
+  private static final int FIRST_PRINTABLE_ASCII = 0x20;
+
+  private HealthRenderer() {}
+
+  public static String toJson(HealthOutcome outcome) {
+    StringBuilder out = new StringBuilder(INITIAL_BUFFER_CAPACITY);
+    out.append('{');
+    appendStringField(out, "outcome", outcome.outcome());
+    out.append(",\"dependencies\":[");
+    appendDependencies(out, outcome.dependencies());
+    out.append("]}");
+    return out.toString();
+  }
+
+  private static void appendDependencies(StringBuilder out, List<Dependency> deps) {
+    for (int i = 0; i < deps.size(); i++) {
+      if (i > 0) {
+        out.append(',');
+      }
+      Dependency d = deps.get(i);
+      out.append('{');
+      appendStringField(out, "id", d.id());
+      out.append(',');
+      appendStringField(out, "status", d.status());
+      out.append('}');
+    }
+  }
+
+  private static void appendStringField(StringBuilder out, String name, String value) {
+    out.append('"').append(name).append("\":\"");
+    appendEscaped(out, value);
+    out.append('"');
+  }
+
+  private static void appendEscaped(StringBuilder out, String value) {
+    for (int i = 0; i < value.length(); i++) {
+      char c = value.charAt(i);
+      switch (c) {
+        case '\\' -> out.append("\\\\");
+        case '"' -> out.append("\\\"");
+        case '\n' -> out.append("\\n");
+        case '\r' -> out.append("\\r");
+        case '\t' -> out.append("\\t");
+        case '\b' -> out.append("\\b");
+        case '\f' -> out.append("\\f");
+        default -> appendUnicodeOrLiteral(out, c);
+      }
+    }
+  }
+
+  private static void appendUnicodeOrLiteral(StringBuilder out, char c) {
+    if (c < FIRST_PRINTABLE_ASCII) {
+      out.append(String.format("\\u%04x", (int) c));
+    } else {
+      out.append(c);
+    }
+  }
+}

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

@@ -0,0 +1,53 @@
+package com.retailsvc.http.internal;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.retailsvc.http.Dependency;
+import com.retailsvc.http.HealthOutcome;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class HealthRendererTest {
+
+  private static final ObjectMapper MAPPER = new ObjectMapper();
+
+  @Test
+  void rendersOutcomeAndEmptyDependencies() {
+    String json = HealthRenderer.toJson(new HealthOutcome("Up", List.of()));
+    assertThat(json).isEqualTo("{\"outcome\":\"Up\",\"dependencies\":[]}");
+  }
+
+  @Test
+  void rendersOutcomeAndDependencies() throws Exception {
+    String json =
+        HealthRenderer.toJson(
+            new HealthOutcome(
+                "Down", List.of(new Dependency("jdbc", "Down"), new Dependency("cache", "Up"))));
+
+    JsonNode root = MAPPER.readTree(json);
+    assertThat(root.get("outcome").asText()).isEqualTo("Down");
+    assertThat(root.get("dependencies")).hasSize(2);
+    assertThat(root.get("dependencies").get(0).get("id").asText()).isEqualTo("jdbc");
+    assertThat(root.get("dependencies").get(0).get("status").asText()).isEqualTo("Down");
+    assertThat(root.get("dependencies").get(1).get("id").asText()).isEqualTo("cache");
+    assertThat(root.get("dependencies").get(1).get("status").asText()).isEqualTo("Up");
+  }
+
+  @Test
+  void escapesQuotesAndBackslashes() throws Exception {
+    String json =
+        HealthRenderer.toJson(new HealthOutcome("Up", List.of(new Dependency("a\"b\\c", "Up"))));
+    JsonNode root = MAPPER.readTree(json);
+    assertThat(root.get("dependencies").get(0).get("id").asText()).isEqualTo("a\"b\\c");
+  }
+
+  @Test
+  void escapesControlCharacters() throws Exception {
+    String id = "tab\there\nnextend";
+    String json = HealthRenderer.toJson(new HealthOutcome("Up", List.of(new Dependency(id, "Up"))));
+    JsonNode root = MAPPER.readTree(json);
+    assertThat(root.get("dependencies").get(0).get("id").asText()).isEqualTo(id);
+  }
+}