java/src/main/java/com/google/protobuf/nano/InternalNano.java

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// http://code.google.com/p/protobuf/
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package com.google.protobuf.nano;

import java.io.UnsupportedEncodingException;

/**
 * The classes contained within are used internally by the Protocol Buffer
 * library and generated message implementations. They are public only because
 * those generated messages do not reside in the {@code protobuf} package.
 * Others should not use this class directly.
 *
 * @author kenton@google.com (Kenton Varda)
 */
public class InternalNano {
  /**
   * Helper called by generated code to construct default values for string
   * fields.
   * <p>
   * The protocol compiler does not actually contain a UTF-8 decoder -- it
   * just pushes UTF-8-encoded text around without touching it.  The one place
   * where this presents a problem is when generating Java string literals.
   * Unicode characters in the string literal would normally need to be encoded
   * using a Unicode escape sequence, which would require decoding them.
   * To get around this, protoc instead embeds the UTF-8 bytes into the
   * generated code and leaves it to the runtime library to decode them.
   * <p>
   * It gets worse, though.  If protoc just generated a byte array, like:
   *   new byte[] {0x12, 0x34, 0x56, 0x78}
   * Java actually generates *code* which allocates an array and then fills
   * in each value.  This is much less efficient than just embedding the bytes
   * directly into the bytecode.  To get around this, we need another
   * work-around.  String literals are embedded directly, so protoc actually
   * generates a string literal corresponding to the bytes.  The easiest way
   * to do this is to use the ISO-8859-1 character set, which corresponds to
   * the first 256 characters of the Unicode range.  Protoc can then use
   * good old CEscape to generate the string.
   * <p>
   * So we have a string literal which represents a set of bytes which
   * represents another string.  This function -- stringDefaultValue --
   * converts from the generated string to the string we actually want.  The
   * generated code calls this automatically.
   */
  public static final String stringDefaultValue(String bytes) {
    try {
      return new String(bytes.getBytes("ISO-8859-1"), "UTF-8");
    } catch (UnsupportedEncodingException e) {
      // This should never happen since all JVMs are required to implement
      // both of the above character sets.
      throw new IllegalStateException(
          "Java VM does not support a standard character set.", e);
    }
  }

  /**
   * Helper called by generated code to construct default values for bytes
   * fields.
   * <p>
   * This is a lot like {@link #stringDefaultValue}, but for bytes fields.
   * In this case we only need the second of the two hacks -- allowing us to
   * embed raw bytes as a string literal with ISO-8859-1 encoding.
   */
  public static final byte[] bytesDefaultValue(String bytes) {
    try {
      return bytes.getBytes("ISO-8859-1");
    } catch (UnsupportedEncodingException e) {
      // This should never happen since all JVMs are required to implement
      // ISO-8859-1.
      throw new IllegalStateException(
          "Java VM does not support a standard character set.", e);
    }
  }

  /**
   * Helper function to convert a string into UTF-8 while turning the
   * UnsupportedEncodingException to a RuntimeException.
   */
  public static final byte[] copyFromUtf8(final String text) {
    try {
      return text.getBytes("UTF-8");
    } catch (UnsupportedEncodingException e) {
      throw new RuntimeException("UTF-8 not supported?");
    }
  }
}