content: // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  package com.google.protobuf;
  
  import static com.google.protobuf.Internal.checkNotNull;
  
  import java.io.IOException;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentMap;
  
  /**
   * Main runtime interface for protobuf. Applications should interact with this interface (rather
   * than directly accessing internal APIs) in order to perform operations on protobuf messages.
   */
  @ExperimentalApi
  final class Protobuf {
    private static final Protobuf INSTANCE = new Protobuf();
  
    private final SchemaFactory schemaFactory;
  
    // TODO(nathanmittler): Consider using ClassValue instead.
    private final ConcurrentMap<Class<?>, Schema<?>> schemaCache =
        new ConcurrentHashMap<Class<?>, Schema<?>>();
  
    /** Gets the singleton instance of the Protobuf runtime. */
    public static Protobuf getInstance() {
      return INSTANCE;
    }
  
    /** Writes the given message to the target {@link Writer}. */
    public <T> void writeTo(T message, Writer writer) throws IOException {
      schemaFor(message).writeTo(message, writer);
    }
  
    /** Reads fields from the given {@link Reader} and merges them into the message. */
    public <T> void mergeFrom(T message, Reader reader) throws IOException {
      mergeFrom(message, reader, ExtensionRegistryLite.getEmptyRegistry());
    }
  
    /** Reads fields from the given {@link Reader} and merges them into the message. */
    public <T> void mergeFrom(T message, Reader reader, ExtensionRegistryLite extensionRegistry)
        throws IOException {
      schemaFor(message).mergeFrom(message, reader, extensionRegistry);
    }
  
    /** Marks repeated/map/extension/unknown fields as immutable. */
    public <T> void makeImmutable(T message) {
      schemaFor(message).makeImmutable(message);
    }
  
    /**
     * Checks if all required fields are set. TODO(xiaofeng): Make this package private when the tests
     * are moved to protobuf package.
     */
    public <T> boolean isInitialized(T message) {
      return schemaFor(message).isInitialized(message);
    }
  
    /** Gets the schema for the given message type. */
    public <T> Schema<T> schemaFor(Class<T> messageType) {
      checkNotNull(messageType, "messageType");
      @SuppressWarnings("unchecked")
      Schema<T> schema = (Schema<T>) schemaCache.get(messageType);
      if (schema == null) {
        schema = schemaFactory.createSchema(messageType);
        @SuppressWarnings("unchecked")
        Schema<T> previous = (Schema<T>) registerSchema(messageType, schema);
        if (previous != null) {
          // A new schema was registered by another thread.
          schema = previous;
        }
      }
      return schema;
    }
  
    /** Gets the schema for the given message. */
    @SuppressWarnings("unchecked")
    public <T> Schema<T> schemaFor(T message) {
      return schemaFor((Class<T>) message.getClass());
    }
  
    /**
     * Registers the given schema for the message type only if a schema was not already registered.
     *
     * @param messageType the type of message on which the schema operates.
     * @param schema the schema for the message type.
     * @return the previously registered schema, or {@code null} if the given schema was successfully
     *     registered.
     */
    public Schema<?> registerSchema(Class<?> messageType, Schema<?> schema) {
      checkNotNull(messageType, "messageType");
      checkNotNull(schema, "schema");
      return schemaCache.putIfAbsent(messageType, schema);
    }
  
    /**
     * Visible for testing only. Registers the given schema for the message type. If a schema was
     * previously registered, it will be replaced by the provided schema.
     *
     * @param messageType the type of message on which the schema operates.
     * @param schema the schema for the message type.
     * @return the previously registered schema, or {@code null} if no schema was registered
     *     previously.
     */
    public Schema<?> registerSchemaOverride(Class<?> messageType, Schema<?> schema) {
      checkNotNull(messageType, "messageType");
      checkNotNull(schema, "schema");
      return schemaCache.put(messageType, schema);
    }
  
    private Protobuf() {
      schemaFactory = new ManifestSchemaFactory();
    }
  
    int getTotalSchemaSize() {
      int result = 0;
      for (Schema<?> schema : schemaCache.values()) {
        if (schema instanceof MessageSchema) {
          result += ((MessageSchema) schema).getSchemaSize();
        }
      }
      return result;
    }
  }
  