Protocol Buffers v3.0.0

@pherl pherl released this Jul 27, 2016

Version 3.0.0

This change log summarizes all the changes since the last stable release
(v2.6.1). See the last section about changes since v3.0.0-beta-4.


  • Introduced Protocol Buffers language version 3 (aka proto3).

    When protocol buffers was initially open sourced it implemented Protocol
    Buffers language version 2 (aka proto2), which is why the version number
    started from v2.0.0. From v3.0.0, a new language version (proto3) is
    introduced while the old version (proto2) will continue to be supported.

    The main intent of introducing proto3 is to clean up protobuf before pushing
    the language as the foundation of Google's new API platform. In proto3, the
    language is simplified, both for ease of use and to make it available in a
    wider range of programming languages. At the same time a few features are
    added to better support common idioms found in APIs.

    The following are the main new features in language version 3:

    1. Removal of field presence logic for primitive value fields, removal of
      required fields, and removal of default values. This makes proto3
      significantly easier to implement with open struct representations, as
      in languages like Android Java, Objective C, or Go.
    2. Removal of unknown fields.
    3. Removal of extensions, which are instead replaced by a new standard
      type called Any.
    4. Fix semantics for unknown enum values.
    5. Addition of maps (back-ported to proto2)
    6. Addition of a small set of standard types for representation of time,
      dynamic data, etc (back-ported to proto2)
    7. A well-defined encoding in JSON as an alternative to binary proto

    A new notion "syntax" is introduced to specify whether a .proto file
    uses proto2 or proto3:

    // foo.proto
    syntax = "proto3";
    message Bar {...}

    If omitted, the protocol buffer compiler generates a warning and "proto2" is
    used as the default. This warning will be turned into an error in a future

    We recommend that new Protocol Buffers users use proto3. However, we do not
    generally recommend that existing users migrate from proto2 from proto3 due
    to API incompatibility, and we will continue to support proto2 for a long

    Other significant changes in proto3.

  • Explicit "optional" keyword are disallowed in proto3 syntax, as fields are
    optional by default; required fields are no longer supported.

  • Removed non-zero default values and field presence logic for non-message
    fields. e.g. has_xxx() methods are removed; primitive fields set to default
    values (0 for numeric fields, empty for string/bytes fields) will be skipped
    during serialization.

  • Group fields are no longer supported in proto3 syntax.

  • Changed repeated primitive fields to use packed serialization by default in
    proto3 (implemented for C++, Java, Python in this release). The user can
    still disable packed serialization by setting packed to false for now.

  • Added well-known type protos (any.proto, empty.proto, timestamp.proto,
    duration.proto, etc.). Users can import and use these protos just like
    regular proto files. Additional runtime support are available for each

  • Proto3 JSON is supported in several languages (fully supported in C++, Java,
    Python and C# partially supported in Ruby). The JSON spec is defined in the
    proto3 language guide:

    We will publish a more detailed spec to define the exact behavior of
    proto3-conformant JSON serializers and parsers. Until then, do not rely
    on specific behaviors of the implementation if it’s not documented in
    the above spec.

  • Proto3 enforces strict UTF-8 checking. Parsing will fail if a string
    field contains non UTF-8 data.


  • Introduced new language implementations (C#, JavaScript, Ruby, Objective-C)
    to proto3.

  • Added support for map fields (implemented in both proto2 and proto3).
    Map fields can be declared using the following syntax:

    message Foo {
      map<string, string> values = 1;

    The data of a map field is stored in memory as an unordered map and
    can be accessed through generated accessors.

  • Added a "reserved" keyword in both proto2 and proto3 syntax. Users can use
    this keyword to declare reserved field numbers and names to prevent them
    from being reused by other fields in the same message.

    To reserve field numbers, add a reserved declaration in your message:

    message TestMessage {
      reserved 2, 15, 9 to 11, 3;

    This reserves field numbers 2, 3, 9, 10, 11 and 15. If a user uses any of
    these as field numbers, the protocol buffer compiler will report an error.

    Field names can also be reserved:

    message TestMessage {
      reserved "foo", "bar";
  • Added a deterministic serialization API (currently available in C++). The
    deterministic serialization guarantees that given a binary, equal messages
    will be serialized to the same bytes. This allows applications like
    MapReduce to group equal messages based on the serialized bytes. The
    deterministic serialization is, however, NOT canonical across languages; it
    is also unstable across different builds with schema changes due to unknown
    fields. Users who need canonical serialization, e.g. persistent storage in
    a canonical form, fingerprinting, etc, should define their own
    canonicalization specification and implement the serializer using reflection
    APIs rather than relying on this API.

  • Added a new field option "json_name". By default proto field names are
    converted to "lowerCamelCase" in proto3 JSON format. This option can be
    used to override this behavior and specify a different JSON name for the

  • Added conformance tests to ensure implementations are following proto3 JSON


  • Added arena allocation support (for both proto2 and proto3).

    Profiling shows memory allocation and deallocation constitutes a significant
    fraction of CPU-time spent in protobuf code and arena allocation is a
    technique introduced to reduce this cost. With arena allocation, new
    objects are allocated from a large piece of preallocated memory and
    deallocation of these objects is almost free. Early adoption shows 20% to
    50% improvement in some Google binaries.

    To enable arena support, add the following option to your .proto file:

    option cc_enable_arenas = true;

    The protocol buffer compiler will generate additional code to make the generated
    message classes work with arenas. This does not change the existing API
    of protobuf messages and does not affect wire format. Your existing code
    should continue to work after adding this option. In the future we will
    make this option enabled by default.

    To actually take advantage of arena allocation, you need to use the arena
    APIs when creating messages. A quick example of using the arena API:

      google::protobuf::Arena arena;
      // Allocate a protobuf message in the arena.
      MyMessage* message = Arena::CreateMessage<MyMessage>(&arena);
      // All submessages will be allocated in the same arena.
      if (!message->ParseFromString(data)) {
        // Deal with malformed input data.
      // Must not delete the message here. It will be deleted automatically
      // when the arena is destroyed.

    Currently arena allocation does not work with map fields. Enabling arenas in a .proto
    file containing map fields will result in compile errors in the generated
    code. This will be addressed in a future release.

  • Added runtime support for the Any type. To use Any in your proto file, first
    import the definition of Any:

    // foo.proto
    import "google/protobuf/any.proto";
    message Foo {
      google.protobuf.Any any_field = 1;
    message Bar {
      int32 value = 1;

    Then in C++ you can access the Any field using PackFrom()/UnpackTo()

    Foo foo;
    Bar bar = ...;
    if (foo.any_field().IsType<Bar>()) {
  • In text format, the entries of a map field will be sorted by key.

  • Introduced new utility functions/classes in the google/protobuf/util

    • MessageDifferencer: compare two proto messages and report their
    • JsonUtil: support converting protobuf binary format to/from JSON.
    • TimeUtil: utility functions to work with well-known types Timestamp
      and Duration.
    • FieldMaskUtil: utility functions to work with FieldMask.
  • Introduced a deterministic serialization API in
    CodedOutputStream::SetSerializationDeterministic(bool). See the notes about
    deterministic serialization in the General section.


  • Introduced a new util package that will be distributed as a separate
    artifact in maven. It contains:
    • JsonFormat: convert proto messages to/from JSON.
    • Timestamps/Durations: utility functions to work with Timestamp and Duration.
    • FieldMaskUtil: utility functions to work with FieldMask.
  • Introduced an ExperimentalApi annotation. Annotated APIs are experimental
    and are subject to change in a backward incompatible way in future releases.
  • Introduced zero-copy serialization as an ExperimentalApi
    • Introduction of the ByteOutput interface. This is similar to
      OutputStream but provides semantics for lazy writing (i.e. no
      immediate copy required) of fields that are considered to be immutable.
    • ByteString now supports writing to a ByteOutput, which will directly
      expose the internals of the ByteString (i.e. byte[] or ByteBuffer)
      to the ByteOutput without copying.
    • CodedOutputStream now supports writing to a ByteOutput. ByteString
      instances that are too large to fit in the internal buffer will be
      (lazily) written to the ByteOutput directly.
    • This allows applications using large ByteString fields to avoid
      duplication of these fields entirely. Such an application can supply a
      ByteOutput that chains together the chunks received from
      CodedOutputStream before forwarding them onto the IO system.
  • Other related changes to CodedOutputStream
    • Additional use of sun.misc.Unsafe where possible to perform fast
      access to byte[] and ByteBuffer values and avoiding unnecessary
      range checking.
    • ByteBuffer-backed CodedOutputStream now writes directly to the
      ByteBuffer rather than to an intermediate array.
  • Performance optimizations for String fields serialization.
  • The static PARSER in each generated message is deprecated, and it will
    be removed in a future release. A static parser() getter is generated
    for each message type instead.
  • File option "java_generate_equals_and_hash" is now deprecated. equals() and
    hashCode() methods are generated by default.


  • Python has received several updates, most notably support for proto3
    semantics in any .proto file that declares syntax="proto3".
    Messages declared in proto3 files no longer represent field presence
    for scalar fields (number, enums, booleans, or strings). You can
    no longer call HasField() for such fields, and they are serialized
    based on whether they have a non-zero/empty/false value.
  • One other notable change is in the C++-accelerated implementation.
    Descriptor objects (which describe the protobuf schema and allow
    reflection over it) are no longer duplicated between the Python
    and C++ layers. The Python descriptors are now simple wrappers
    around the C++ descriptors. This change should significantly
    reduce the memory usage of programs that use a lot of message
  • Added map support.
    • maps now have a dict-like interface (msg.map_field[key] = value)
    • existing code that modifies maps via the repeated field interface
      will need to be updated.
  • Added proto3 JSON format utility. It includes support for all field types and a few well-known types.
  • Added runtime support for Any, Timestamp, Duration and FieldMask.
  • "[ ]" is now accepted for repeated scalar fields in text format parser.
  • Removed legacy Python 2.5 support.
  • Moved to a single Python 2.x/3.x-compatible codebase


  • We have added proto3 support for Ruby via a native C/JRuby extension.

    For the moment we only support proto3. Proto2 support is planned, but not
    yet implemented. Proto3 JSON is supported, but the special JSON mappings
    for the well-known types are not yet implemented.

    The Ruby extension itself is included in the ruby/ directory, and details on
    building and installing the extension are in ruby/ The extension
    is also be published as a Ruby gem. Code generator support is included as
    part of protoc with the --ruby_out flag.

    The Ruby extension implements a user-friendly DSL to define message types
    (also generated by the code generator from .proto files). Once a message
    type is defined, the user may create instances of the message that behave in
    ways idiomatic to Ruby. For example:

    • Message fields are present as ordinary Ruby properties (getter method
      foo and setter method foo=).
    • Repeated field elements are stored in a container that acts like a native
      Ruby array, and map elements are stored in a container that acts like a
      native Ruby hashmap.
    • The usual well-known methods, such as #to_s, #dup, and the like, are

    Unlike several existing third-party Ruby extensions for protobuf, this
    extension is built on a "strongly-typed" philosophy: message fields and
    array/map containers will throw exceptions eagerly when values of the
    incorrect type are inserted.

    See ruby/ for details.


  • Objective-C includes a code generator and a native objective-c runtime
    library. By adding “--objc_out” to protoc, the code generator will generate
    a header(.pbobjc.h) and an implementation file(.pbobjc.m) for each proto

    In this first release, the generated interface provides: enums, messages,
    field support(single, repeated, map, oneof), proto2 and proto3 syntax
    support, parsing and serialization. It’s compatible with ARC and non-ARC
    usage. In addition, users can access it via the swift bridging header.


  • C# support is derived from the project at, which is now in maintenance mode.
  • The primary differences between the previous project and the proto3 version are that
    message types are now mutable, and the codegen is integrated in protoc
  • There are two NuGet packages: Google.Protobuf (the support library) and
    Google.Protobuf.Tools (containing protoc)
  • Target platforms now .NET 4.5, selected portable subsets and .NET Core.
  • Null values are used to represent "no value" for message type fields, and for wrapper
    types such as Int32Value which map to C# nullable value types.
  • Proto3 semantics supported; proto2 files are prohibited for C# codegen.
  • Enum values are PascalCased, and if there's a prefix which matches the
    name of the enum, that is removed (so an enum COLOR with a value
    COLOR_LIGHT_GRAY would generate a value of just LightGray).


  • Added proto2/proto3 support for JavaScript. The runtime is written in pure
    JavaScript and works in browsers and in Node.js. To generate JavaScript
    code for your proto, invoke protoc with "--js_out". See js/
    for more build instructions.
  • JavaScript has support for binary protobuf format, but not proto3 JSON.
    There is also no support for reflection, since the code size impacts from this
    are often not the right choice for the browser.
  • There is support for both CommonJS imports and Closure goog.require().


  • Supported Proto3 lite-runtime in Java for mobile platforms.
    A new "lite" generator parameter was introduced in the protoc for C++ for
    Proto3 syntax messages. Example usage:

    ./protoc --cpp_out=lite:$OUTPUT_PATH foo.proto

    The protoc will treat the current input and all the transitive dependencies
    as LITE. The same generator parameter must be used to generate the

    In Proto3 syntax files, "optimized_for=LITE_RUNTIME" is no longer supported.

    For Java, --javalite_out code generator is supported as a separate compiler
    plugin in a separate branch.

  • Performance optimizations for Java Lite runtime on Android:
    - Reduced allocations
    - Reduced method overhead after ProGuarding
    - Reduced code size after ProGuarding

  • Java Lite protos now implement deep equals/hashCode/toString

Compatibility Notice

  • v3.0.0 is the first API stable release of the v3.x series. We do not expect
    any future API breaking changes.
  • For C++, Java Lite and Objective-C, source level compatibility is
    guaranteed. Upgrading from v3.0.0 to newer minor version releases will be
    source compatible. For example, if your code compiles against protobuf
    v3.0.0, it will continue to compile after you upgrade protobuf library to
  • For other languages, both source level compatibility and binary level
    compatibility are guaranteed. For example, if you have a Java binary built
    against protobuf v3.0.0. After switching the protobuf runtime binary to
    v3.1.0, your built binary should continue to work.
  • Compatibility is only guaranteed for documented API and documented
    behaviors. If you are using undocumented API (e.g., use anything in the C++
    internal namespace), it can be broken by minor version releases in an
    undetermined manner.

Changes since v3.0.0-beta-4


  • When you assign a string field a.string_field = “X”, we now call
    #encode(UTF-8) on the string and freeze the copy. This saves you from
    needing to ensure the string is already encoded as UTF-8. It also prevents
    you from mutating the string after it has been assigned (this is how we
    ensure it stays valid UTF-8).
  • The generated file for foo.proto is now foo_pb.rb instead of just
    foo.rb. This makes it easier to see which imports/requires are from
    protobuf generated code, and also prevents conflicts with any foo.rb file
    you might have written directly in Ruby. It is a backward-incompatible
    change: you will need to update all of your require statements.
  • For package names like foo_bar, we now translate this to the Ruby module
    FooBar. This is more idiomatic Ruby than what we used to do (Foo_bar).


  • Scalar fields like numbers and boolean now return defaults instead of
    undefined or null when they are unset. You can test for presence
    explicitly by calling hasFoo(), which we now generate for scalar fields in

Java Lite

  • Java Lite is now implemented as a separate plugin, maintained in the
    javalite branch. Both lite runtime and protoc artifacts will be available
    in Maven.


  • Target platforms now .NET 4.5, selected portable subsets and .NET Core.
  • legacy_enum_values option is no longer supported.