xref: /aosp_15_r20/external/protobuf/javanano/README.md (revision 1b3f573f81763fcece89efc2b6a5209149e44ab8)

Protocol Buffers - Google's data interchange format
===================================================

[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf)

Copyright 2008 Google Inc.

This directory contains the Java Protocol Buffers Nano runtime library.

**Nano is no longer supported by protobuf team. We recommend Android users to
use protobuf lite runtime instead.**

Installation - With Maven
-------------------------

The Protocol Buffers build is managed using Maven.  If you would
rather build without Maven, see below.

1) Install Apache Maven if you don't have it:

     http://maven.apache.org/

2) Build the C++ code, or obtain a binary distribution of protoc.  If
   you install a binary distribution, make sure that it is the same
   version as this package.  If in doubt, run:

     $ protoc --version

   You will need to place the protoc executable in ../src.  (If you
   built it yourself, it should already be there.)

3) Run the tests:

     $ mvn test

   If some tests fail, this library may not work correctly on your
   system.  Continue at your own risk.

4) Install the library into your Maven repository:

     $ mvn install

5) If you do not use Maven to manage your own build, you can build a
   .jar file to use:

     $ mvn package

   The .jar will be placed in the "target" directory.

Installation - Without Maven
----------------------------

If you would rather not install Maven to build the library, you may
follow these instructions instead.  Note that these instructions skip
running unit tests.

1) Build the C++ code, or obtain a binary distribution of protoc.  If
   you install a binary distribution, make sure that it is the same
   version as this package.  If in doubt, run:

     $ protoc --version

   If you built the C++ code without installing, the compiler binary
   should be located in ../src.

2) Invoke protoc to build DescriptorProtos.java:

     $ protoc --java_out=src/main/java -I../src \
         ../src/google/protobuf/descriptor.proto

3) Compile the code in src/main/java using whatever means you prefer.

4) Install the classes wherever you prefer.

Nano version
------------

JavaNano is a special code generator and runtime library designed specially for
resource-restricted systems, like Android. It is very resource-friendly in both
the amount of code and the runtime overhead. Here is an overview of JavaNano
features compared with the official Java protobuf:

- No descriptors or message builders.
- All messages are mutable; fields are public Java fields.
- For optional fields only, encapsulation behind setter/getter/hazzer/
  clearer functions is opt-in, which provide proper 'has' state support.
- For proto2, if not opted in, has state (field presence) is not available.
  Serialization outputs all fields not equal to their defaults
  (see important implications below).
  The behavior is consistent with proto3 semantics.
- Required fields (proto2 only) are always serialized.
- Enum constants are integers; protection against invalid values only
  when parsing from the wire.
- Enum constants can be generated into container interfaces bearing
  the enum's name (so the referencing code is in Java style).
- CodedInputByteBufferNano can only take byte[] (not InputStream).
- Similarly CodedOutputByteBufferNano can only write to byte[].
- Repeated fields are in arrays, not ArrayList or Vector. Null array
  elements are allowed and silently ignored.
- Full support for serializing/deserializing repeated packed fields.
- Support  extensions (in proto2).
- Unset messages/groups are null, not an immutable empty default
  instance.
- toByteArray(...) and mergeFrom(...) are now static functions of
  MessageNano.
- The 'bytes' type translates to the Java type byte[].

The generated messages are not thread-safe for writes, but may be
used simultaneously from multiple threads in a read-only manner.
In other words, an appropriate synchronization mechanism (such as
a ReadWriteLock) must be used to ensure that a message, its
ancestors, and descendants are not accessed by any other threads
while the message is being modified. Field reads, getter methods
(but not getExtension(...)), toByteArray(...), writeTo(...),
getCachedSize(), and getSerializedSize() are all considered read-only
operations.

IMPORTANT: If you have fields with defaults and opt out of accessors

How fields with defaults are serialized has changed. Because we don't
keep "has" state, any field equal to its default is assumed to be not
set and therefore is not serialized. Consider the situation where we
change the default value of a field. Senders compiled against an older
version of the proto continue to match against the old default, and
don't send values to the receiver even though the receiver assumes the
new default value. Therefore, think carefully about the implications
of changing the default value. Alternatively, turn on accessors and
enjoy the benefit of the explicit has() checks.

IMPORTANT: If you have "bytes" fields with non-empty defaults

Because the byte buffer is now of mutable type byte[], the default
static final cannot be exposed through a public field. Each time a
message's constructor or clear() function is called, the default value
(kept in a private byte[]) is cloned. This causes a small memory
penalty. This is not a problem if the field has no default or is an
empty default.

Nano Generator options
----------------------

```
java_package           -> <file-name>|<package-name>
java_outer_classname   -> <file-name>|<package-name>
java_multiple_files    -> true or false
java_nano_generate_has -> true or false [DEPRECATED]
optional_field_style   -> default or accessors
enum_style             -> c or java
ignore_services        -> true or false
parcelable_messages    -> true or false
generate_intdefs       -> true or false
```

**java_package=\<file-name\>|\<package-name\>** (no default)

  This allows overriding the 'java_package' option value
  for the given file from the command line. Use multiple
  java_package options to override the option for multiple
  files. The final Java package for each file is the value
  of this command line option if present, or the value of
  the same option defined in the file if present, or the
  proto package if present, or the default Java package.

**java_outer_classname=\<file-name\>|\<outer-classname\>** (no default)

  This allows overriding the 'java_outer_classname' option
  for the given file from the command line. Use multiple
  java_outer_classname options to override the option for
  multiple files. The final Java outer class name for each
  file is the value of this command line option if present,
  or the value of the same option defined in the file if
  present, or the file name converted to CamelCase. This
  outer class will nest all classes and integer constants
  generated from file-scope messages and enums.

**java_multiple_files={true,false}** (no default)

  This allows overriding the 'java_multiple_files' option
  in all source files and their imported files from the
  command line. The final value of this option for each
  file is the value defined in this command line option, or
  the value of the same option defined in the file if
  present, or false. This specifies whether to generate
  package-level classes for the file-scope messages in the
  same Java package as the outer class (instead of nested
  classes in the outer class). File-scope enum constants
  are still generated as integer constants in the outer
  class. This affects the fully qualified references in the
  Java code. NOTE: because the command line option
  overrides the value for all files and their imported
  files, using this option inconsistently may result in
  incorrect references to the imported messages and enum
  constants.

**java_nano_generate_has={true,false}** (default: false)

  DEPRECATED. Use optional_field_style=accessors.

  If true, generates a public boolean variable has\<fieldname\>
  accompanying each optional or required field (not present for
  repeated fields, groups or messages). It is set to false initially
  and upon clear(). If parseFrom(...) reads the field from the wire,
  it is set to true. This is a way for clients to inspect the "has"
  value upon parse. If it is set to true, writeTo(...) will ALWAYS
  output that field (even if field value is equal to its
  default).

  IMPORTANT: This option costs an extra 4 bytes per primitive field in
  the message. Think carefully about whether you really need this. In
  many cases reading the default works and determining whether the
  field was received over the wire is irrelevant.

**optional_field_style={default,accessors,reftypes}** (default: default)

  Defines the style of the generated code for fields.

  * default

  In the default style, optional fields translate into public mutable
  Java fields, and the serialization process is as discussed in the
  "IMPORTANT" section above.

  * accessors

  When set to 'accessors', each optional field is encapsulated behind
  4 accessors, namely get\<fieldname\>(), set\<fieldname\>(), has\<fieldname\>()
  and clear\<fieldname\>() methods, with the standard semantics. The hazzer's
  return value determines whether a field is serialized, so this style is
  useful when you need to serialize a field with the default value, or check
  if a field has been explicitly set to its default value from the wire.

  In the 'accessors' style, required and nested message fields are still
  translated to one public mutable Java field each, repeated fields are still
  translated to arrays. No accessors are generated for them.

  IMPORTANT: When using the 'accessors' style, ProGuard should always
  be enabled with optimization (don't use -dontoptimize) and allowing
  access modification (use -allowaccessmodification). This removes the
  unused accessors and maybe inline the rest at the call sites,
  reducing the final code size.
  TODO(maxtroy): find ProGuard config that would work the best.

  * reftypes

  When set to 'reftypes', each proto field is generated as a public Java
  field. For primitive types, these fields use the Java reference types
  such as java.lang.Integer instead of primitive types such as int.

  In the 'reftypes' style, fields are initialized to null (or empty
  arrays for repeated fields), and their default values are not available.
  They are serialized over the wire based on equality to null.

  The 'reftypes' mode has some additional cost due to autoboxing and usage
  of reference types. In practice, many boxed types are cached, and so don't
  result in object creation. However, references do take slightly more memory
  than primitives.

  The 'reftypes' mode is useful when you want to be able to serialize fields
  with default values, or check if a field has been explicitly set to the
  default over the wire without paying the extra method cost of the
  'accessors' mode.

  Note that if you attempt to write null to a required field in the reftypes
  mode, serialization of the proto will cause a NullPointerException. This is
  an intentional indicator that you must set required fields.

  NOTE
  optional_field_style=accessors or reftypes cannot be used together with
  java_nano_generate_has=true. If you need the 'has' flag for any
  required field (you have no reason to), you can only use
  java_nano_generate_has=true.

**enum_style={c,java}** (default: c)

  Defines where to put the int constants generated from enum members.

  * c

  Use C-style, so the enum constants are available at the scope where
  the enum is defined. A file-scope enum's members are referenced like
  'FileOuterClass.ENUM_VALUE'; a message-scope enum's members are
  referenced as 'Message.ENUM_VALUE'. The enum name is unavailable.
  This complies with the Micro code generator's behavior.

  * java

  Use Java-style, so the enum constants are available under the enum
  name and referenced like 'EnumName.ENUM_VALUE' (they are still int
  constants). The enum name becomes the name of a public interface, at
  the scope where the enum is defined. If the enum is file-scope and
  the java_multiple_files option is on, the interface will be defined
  in its own file. To reduce code size, this interface should not be
  implemented and ProGuard shrinking should be used, so after the Java
  compiler inlines all referenced enum constants into the call sites,
  the interface remains unused and can be removed by ProGuard.

**ignore_services={true,false}** (default: false)

  Skips services definitions.

  Nano doesn't support services. By default, if a service is defined
  it will generate a compilation error. If this flag is set to true,
  services will be silently ignored, instead.

**parcelable_messages={true,false}** (default: false)

  Android-specific option to generate Parcelable messages.

**generate_intdefs={true,false}** (default: false)
  Android-specific option to generate @IntDef annotations for enums.

  If turned on, an '@IntDef' annotation (a public @interface) will be
  generated for each enum, and every integer parameter and return
  value in the generated code meant for this enum will be annotated
  with it. This interface is generated with the same name and at the
  same place as the enum members' container interfaces described
  above under 'enum_style=java', regardless of the enum_style option
  used. When this is combined with enum_style=java, the interface
  will be both the '@IntDef' annotation and the container of the enum
  members; otherwise the interface has an empty body.

  Your app must declare a compile-time dependency on the
  android-support-annotations library.

  For more information on how these @IntDef annotations help with
  compile-time type safety, see:
  https://sites.google.com/a/android.com/tools/tech-docs/support-annotations
  and
  https://developer.android.com/reference/android/support/annotation/IntDef.html


To use nano protobufs within the Android repo:
----------------------------------------------

- Set 'LOCAL_PROTOC_OPTIMIZE_TYPE := nano' in your local .mk file.
  When building a Java library or an app (package) target, the build
  system will add the Java nano runtime library to the
  LOCAL_STATIC_JAVA_LIBRARIES variable, so you don't need to.
- Set 'LOCAL_PROTO_JAVA_OUTPUT_PARAMS := ...' in your local .mk file
  for any command-line options you need. Use commas to join multiple
  options. In the nano flavor only, whitespace surrounding the option
  names and values are ignored, so you can use backslash-newline or
  '+=' to structure your make files nicely.
- The options will be applied to *all* proto files in LOCAL_SRC_FILES
  when you build a Java library or package. In case different options
  are needed for different proto files, build separate Java libraries
  and reference them in your main target. Note: you should make sure
  that, for each separate target, all proto files imported from any
  proto file in LOCAL_SRC_FILES are included in LOCAL_SRC_FILES. This
  is because the generator has to assume that the imported files are
  built using the same options, and will generate code that reference
  the fields and enums from the imported files using the same code
  style.
- Hint: 'include $(CLEAR_VARS)' resets all LOCAL_ variables, including
  the two above.

To use nano protobufs outside of Android repo:
----------------------------------------------

- Link with the generated jar file
  \<protobuf-root\>java/target/protobuf-java-2.3.0-nano.jar.
- Invoke with --javanano_out, e.g.:
```
./protoc '--javanano_out=\
    java_package=src/proto/simple-data.proto|my_package,\
    java_outer_classname=src/proto/simple-data.proto|OuterName\
  :.' src/proto/simple-data.proto
```

Contributing to nano:
---------------------

Please add/edit tests in NanoTest.java.

Please run the following steps to test:

- cd external/protobuf
- ./configure
- Run "make -j12 check" and verify all tests pass.
- cd java
- Run "mvn test" and verify all tests pass.
- cd ../../..
- . build/envsetup.sh
- lunch 1
- "make -j12 aprotoc libprotobuf-java-2.3.0-nano aprotoc-test-nano-params NanoAndroidTest" and
  check for build errors.
- Plug in an Android device or start an emulator.
- adb install -r out/target/product/generic/data/app/NanoAndroidTest.apk
- Run:
  "adb shell am instrument -w com.google.protobuf.nano.test/android.test.InstrumentationTestRunner"
  and verify all tests pass.
- repo sync -c -j256
- "make -j12" and check for build errors

Usage
-----

The complete documentation for Protocol Buffers is available via the
web at:

    https://developers.google.com/protocol-buffers/