Support for lazily serialized values in Metadata.

api/src/main/java/io/grpc/InternalMetadata.java

@@ -18,6 +18,7 @@
 
 import com.google.common.io.BaseEncoding;
 import io.grpc.Metadata.AsciiMarshaller;
+import io.grpc.Metadata.BinaryStreamMarshaller;
 import io.grpc.Metadata.Key;
 import java.nio.charset.Charset;
 
@@ -82,4 +83,46 @@ public static byte[][] serialize(Metadata md) {
   public static int headerCount(Metadata md) {
     return md.headerCount();
   }
+
+  /**
+   * Serializes all metadata entries, leaving some values as {@link InputStream}s.
+   *
+   * <p>Produces serialized names and values interleaved. result[i*2] are names, while
+   * result[i*2+1] are values.
+   *
+   * <p>Names are byte arrays as described according to the {@link Metadata#serialize}
+   * method. Values are either byte arrays or {@link InputStream}s.
+   */
+  @Internal
+  public static Object[] serializePartial(Metadata md) {
+    return md.serializePartial();
+  }
+
+  /**
+   * Creates a holder for a pre-parsed value read by the transport.
+   *
+   * @param marshaller The {@link Metadata#BinaryStreamMarshaller} associated with this value.
+   * @param value The value to store.
+   * @return an object holding the pre-parsed value for this key.
+   */
+  @Internal
+  public static <T> Object parsedValue(BinaryStreamMarshaller<T> marshaller, T value) {
+    return new Metadata.LazyValue<>(marshaller, value);
+  }
+
+  /**
+   * Creates a new {@link Metadata} instance from serialized data,
+   * with some values pre-parsed. Metadata will mutate the passed in array.
+   *
+   * @param usedNames The number of names used.
+   * @param namesAndValues An array of interleaved names and values,
+   *     with each name (at even indices) represented as a byte array,
+   *     and each value (at odd indices) represented as either a byte
+   *     array or an object returned by the {@link #parsedValue}
+   *     method.
+   */
+  @Internal
+  public static Metadata newMetadataWithParsedValues(int usedNames, Object[] namesAndValues) {
+    return new Metadata(usedNames, namesAndValues);
+  }
 }