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Protocol Buffers - Google's data interchange format
Copyright 2008 Google Inc.
This directory contains the Java Protocol Buffers runtime library.
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:
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 - 'Lite' Version - With Maven
Building the 'lite' version of the Java Protocol Buffers library is
the same as building the full version, except that all commands are
run using the 'lite' profile. (see
E.g. to install the lite version of the jar, you would run:
$ mvn install -P lite
The resulting artifact has the 'lite' classifier. To reference it
for dependency resolution, you would specify it as:
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
$ protoc --java_out=src/main/java -I../src \
3) Compile the code in src/main/java using whatever means you prefer.
4) Install the classes wherever you prefer.
Micro version
The runtime and generated code for MICRO_RUNTIME is smaller
because it does not include support for the descriptor and
reflection, and enums are generated as integer constants in
the parent message or the file's outer class, with no
protection against invalid values set to enum fields. Also,
not currently supported are packed repeated elements or
To create a jar file for the runtime and run tests invoke
"mvn package -P micro" from the <protobuf-root>/java
directory. The generated jar file is
If you wish to compile the MICRO_RUNTIME your self, place
the 7 files below, in <root>/com/google/protobuf and
create a jar file for use with your code and the generated
If you wish to change on the code generator it is located
in /src/google/protobuf/compiler/javamicro.
To generate code for the MICRO_RUNTIME invoke protoc with
--javamicro_out command line parameter. javamicro_out takes
a series of optional sub-parameters separated by commas
and a final parameter, with a colon separator, which defines
the source directory. Sub-parameters begin with a name
followed by an equal and if that sub-parameter has multiple
parameters they are seperated by "|". The command line options
opt -> speed or space
java_use_vector -> true or false
java_package -> <file-name>|<package-name>
java_outer_classname -> <file-name>|<package-name>
java_multiple_files -> true or false
opt={speed,space} (default: space)
This changes the code generation to optimize for speed or
space. When opt=speed this changes the code generation
for strings so that multiple conversions to Utf8 are
java_use_vector={true,false} (default: false)
This specifies the collection class for repeated elements.
If false, repeated elements use java.util.ArrayList<> and
the code must be compiled with Java 1.5 or above. If true,
repeated elements use java.util.Vector and the code can
be compiled with Java 1.3 or above. The 'source'
parameter of 'javac' may be used to control the version
of the source: "javac -source 1.3". You can also change
the <source> xml element for the maven-compiler-plugin.
Below is for 1.5 sources:
And below would be for 1.3 sources (note when changing
to 1.3 you must also set java_use_vector=true):
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
IMPORTANT: change of javamicro_out behavior:
In previous versions, if the outer class name has not been
given explicitly, javamicro_out would not infer the outer
class name from the file name, and would skip the outer
class generation. This makes the compilation succeed only
if the source file contains a single message and no enums,
and the generated class for that message is placed at the
package level. To re-align with java_out, javamicro_out
will now always generate the outer class, inferring its
name from the file name if not given, as a container of the
message classes and enum constants. To keep any existing
single-message source file from causing the generation of
an unwanted outer class, you can set the option
java_multiple_files to true, either in the file or as a
command line option.
Below are a series of examples for clarification of the
various parameters and options. Assuming this file:
package testprotobuf;
message SimpleData {
optional fixed64 id = 1;
optional string description = 2;
optional bool ok = 3 [default = false];
and the compiled protoc in the current working directory,
then a simple command line to compile this file would be:
./protoc --javamicro_out=. src/proto/simple-data-protos.proto
This will create testprotobuf/, which
has the following content (extremely simplified):
package testprotobuf;
public final class SimpleDataProtos {
public static final class SimpleData
extends MessageMicro {
The message SimpleData is compiled into the SimpleData
class, nested in the file's outer class SimpleDataProtos,
whose name is implicitly defined by the proto file name
The directory, aka Java package, testprotobuf is created
because on line 1 of simple-data-protos.proto is
"package testprotobuf;". If you wanted a different
package name you could use the java_package option in the
option java_package = "my_package";
or in command line sub-parameter:
./protoc '--javamicro_out=\
.' src/proto/simple-data-protos.proto
Here you see the new java_package sub-parameter which
itself needs two parameters the file name and the
package name, these are separated by "|". The value set
in the command line overrides the value set in the file.
Now you'll find in the my_package/
If you wanted to also change the optimization for
speed you'd add opt=speed with the comma seperator
as follows:
./protoc '--javamicro_out=\
.' src/proto/simple-data-protos.proto
If you also wanted a different outer class name you'd
do the following:
./protoc '--javamicro_out=\
.' src/proto/simple-data-protos.proto
Now you'll find my_package/ and the
message class SimpleData nested in it.
As mentioned java_package, java_outer_classname and
java_multiple_files may also be specified in the file.
In the example below we must define
java_outer_classname because otherwise the outer class
and one of the message classes will have the same name,
which is forbidden to prevent name ambiguity:
package testmicroruntime;
option java_package = "com.example";
option java_outer_classname = "SampleMessageProtos";
enum MessageType {
message SampleMessage {
required int32 id = 1;
required MessageType type = 2;
message SampleMessageContainer {
required SampleMessage message = 1;
This could be compiled using:
./protoc --javamicro_out=. src/proto/sample-message.proto
and the output will be:
package com.example;
public final class SampleMessageProtos {
public static final int SAMPLE = 1;
public static final int EXAMPLE = 2;
public static final class SampleMessage
extends MessageMicro {
public static final class SampleMessageContainer
extends MessageMicro {
As you can see the file-scope enum MessageType is
disassembled into two integer constants in the outer class.
In javamicro_out, all enums are disassembled and compiled
into integer constants in the parent scope (the containing
message's class or the file's (i.e. outer) class).
You may prefer the file-scope messages to be saved in
separate files. You can do this by setting the option
java_multiple_files to true, in either the file like this:
option java_multiple_files = true;
or the command line like this:
./protoc --javamicro_out=\
. src/proto/sample-message.proto
The java_multiple_files option causes javamicro to use a
separate file for each file-scope message, which resides
directly in the Java package alongside the outer class:
package com.example;
public final class SampleMessageProtos {
public static final int SAMPLE = 1;
public static final int EXAMPLE = 2;
package com.example;
public final class SampleMessage
extends MessageMicro {
package com.example;
public final class SampleMessageContainer
extends MessageMicro {
As you can see, the outer class now contains only the
integer constants, generated from the file-scope enum
"MessageType". Please note that message-scope enums are
still generated as integer constants in the message class.
Nano version
Nano is a special code generator and runtime library designed specially
for Android, and is very resource-friendly in both the amount of code
and the runtime overhead. An overview of Nano features:
- 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.
- If not opted in, has state is not available. Serialization outputs
all fields not equal to their defaults (see important implications
- Required fields 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 of serializing/deserializing repeated packed fields.
- Support of extensions.
- Unset messages/groups are null, not an immutable empty default
- toByteArray(...) and mergeFrom(...) are now static functions of
- 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
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
Same as Micro version.
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
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.
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
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:
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
- 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
- Invoke with --javanano_out, e.g.:
./protoc '--javanano_out=\
:.' src/proto/simple-data.proto
Contributing to nano:
Please add/edit tests in
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/
- lunch 1
- "make -j12 aprotoc libprotobuf-java-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/NanoAndroidTest.apk
- Run:
"adb shell am instrument -w"
and verify all tests pass.
- repo sync -c -j256
- "make -j12" and check for build errors
The complete documentation for Protocol Buffers is available via the
web at: