Replace ByteBufferReadable interface with ManagedBytes, which takes t…

…he responsibility of releasing resources after data access.

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

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2020 The gRPC Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.grpc;
+
+import java.nio.ByteBuffer;
+import java.util.List;
+import javax.annotation.Nullable;
+
+/**
+ * A logical representation of bytes with the responsibility of managing the resource lifecycle
+ * used to hold the data. Usually used for transferring data by directly hand over the ownership
+ * of the backing resources.
+ */
+public abstract class ManagedBytes {
+
+  /**
+   * Returns the data in a list of {@link ByteBuffer}s.
+   */
+  public abstract List<ByteBuffer> asByteBuffers();
+
+  /**
+   * Release the usage of the data. The underlying resource used to hold the data may be released
+   * and no more access should be attempted.
+   */
+  public void release() {
+    // no-op
+  }
+
+  /**
+   * A readable data source that supports the operation of reading its content into
+   * {@link ManagedBytes}.
+   */
+  public interface ManagedBytesReadable {
+
+    /**
+     * Reads up to a total of {@code length} bytes as {@link ManagedBytes}.
+     *
+     * @param length the maximum number of bytes to be read.
+     * @return {@link ManagedBytes} that contains the bytes being read or {@code null}
+     *     if no more bytes can be read.
+     */
+    @Nullable
+    ManagedBytes readManagedBytes(int length);
+  }
+}