feat(opa-gson): add MetricsTypeAdapterFactory mirroring opa-jackson #59

opa-gson/src/main/java/io/github/open_policy_agent/opa/gson/MetricsTypeAdapterFactory.java

@@ -0,0 +1,102 @@
+package io.github.open_policy_agent.opa.gson;
+
+import com.google.gson.Gson;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import io.github.open_policy_agent.opa.metrics.Metrics;
+import java.io.IOException;
+
+/**
+ * Gson {@link TypeAdapterFactory} that serializes {@link Metrics} types to match the JSON shape
+ * produced by OPA's decision logs ({@code timer_<name>_ns} convention).
+ *
+ * <ul>
+ *   <li>{@link Metrics.Timer} &rarr; nanoseconds (long)
+ *   <li>{@link Metrics.Counter} &rarr; integer value
+ *   <li>{@link Metrics.Histogram} &rarr; its {@link Metrics.Histogram.Values} object
+ * </ul>
+ *
+ * <p>Usage:
+ * <pre>{@code
+ * Gson gson = new GsonBuilder()
+ *     .registerTypeAdapterFactory(new MetricsTypeAdapterFactory())
+ *     .create();
+ * String json = gson.toJson(timer);
+ * }</pre>
+ */
+public class MetricsTypeAdapterFactory implements TypeAdapterFactory {
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    if (Metrics.Timer.class.isAssignableFrom(type.getRawType())) {
+      return (TypeAdapter<T>) new TimerAdapter();
+    }
+    if (Metrics.Counter.class.isAssignableFrom(type.getRawType())) {
+      return (TypeAdapter<T>) new CounterAdapter();
+    }
+    if (Metrics.Histogram.class.isAssignableFrom(type.getRawType())) {
+      return (TypeAdapter<T>) new HistogramAdapter(gson);
+    }
+    return null;
+  }
+
+  private static final class TimerAdapter extends TypeAdapter<Metrics.Timer> {
+    @Override
+    public void write(JsonWriter out, Metrics.Timer timer) throws IOException {
+      if (timer == null) {
+        out.nullValue();
+      } else {
+        out.value(timer.value().toNanos());
+      }
+    }
+
+    @Override
+    public Metrics.Timer read(JsonReader in) {
+      throw new UnsupportedOperationException("Deserialization of Metrics.Timer is not supported");
+    }
+  }
+
+  private static final class CounterAdapter extends TypeAdapter<Metrics.Counter> {
+    @Override
+    public void write(JsonWriter out, Metrics.Counter counter) throws IOException {
+      if (counter == null) {
+        out.nullValue();
+      } else {
+        out.value(counter.value());
+      }
+    }
+
+    @Override
+    public Metrics.Counter read(JsonReader in) {
+      throw new UnsupportedOperationException(
+          "Deserialization of Metrics.Counter is not supported");
+    }
+  }
+
+  private static final class HistogramAdapter extends TypeAdapter<Metrics.Histogram> {
+    private final TypeAdapter<Metrics.Histogram.Values> valuesAdapter;
+
+    HistogramAdapter(Gson gson) {
+      this.valuesAdapter = gson.getAdapter(Metrics.Histogram.Values.class);
+    }
+
+    @Override
+    public void write(JsonWriter out, Metrics.Histogram histogram) throws IOException {
+      if (histogram == null) {
+        out.nullValue();
+      } else {
+        valuesAdapter.write(out, histogram.value());
+      }
+    }
+
+    @Override
+    public Metrics.Histogram read(JsonReader in) {
+      throw new UnsupportedOperationException(
+          "Deserialization of Metrics.Histogram is not supported");
+    }
+  }
+}

opa-gson/src/test/java/io/github/open_policy_agent/opa/gson/MetricsTypeAdapterFactoryTest.java

@@ -0,0 +1,53 @@
+package io.github.open_policy_agent.opa.gson;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import io.github.open_policy_agent.opa.metrics.Metrics;
+import io.github.open_policy_agent.opa.metrics.SimpleMetrics;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Parallel to opa-jackson's MetricsModuleTest. Verifies that MetricsTypeAdapterFactory produces
+ * JSON matching OPA's decision log shape ({@code timer_<name>_ns} = nanoseconds as a long).
+ */
+class MetricsTypeAdapterFactoryTest {
+
+  private final Gson gson =
+      new GsonBuilder().registerTypeAdapterFactory(new MetricsTypeAdapterFactory()).create();
+
+  @Test
+  void timer_serializesAsNanoseconds() {
+    SimpleMetrics metrics = new SimpleMetrics();
+    Metrics.Timer timer = metrics.timer("rego_query_eval");
+    timer.start();
+    timer.stop();
+
+    String json = gson.toJson(timer, Metrics.Timer.class);
+
+    // Must be a plain number (nanoseconds), matching DecisionLogPlugin's timer.value().toNanos()
+    long nanos = timer.value().toNanos();
+    assertThat(json).isEqualTo(String.valueOf(nanos));
+  }
+
+  @Test
+  void timer_jsonShapeMatchesDirectNanosSerialization() {
+    SimpleMetrics metrics = new SimpleMetrics();
+    Metrics.Timer timer = metrics.timer("rego_query_eval");
+    timer.start();
+    timer.stop();
+
+    String timerJson = gson.toJson(timer, Metrics.Timer.class);
+    String nanosJson = gson.toJson(timer.value().toNanos());
+
+    // Serializing the Timer must produce exactly what serializing its nanosecond value produces.
+    assertThat(timerJson).isEqualTo(nanosJson);
+  }
+
+  @Test
+  void timer_null_serializesAsNull() {
+    String json = gson.toJson(null, Metrics.Timer.class);
+    assertThat(json).isEqualTo("null");
+  }
+}