Java Generated Code

Compiler Invocation
Packages
Messages
Fields
Any
Oneof
Enumerations
Extensions
Services
Plugin Insertion Points

This page describes exactly what Java code the protocol buffer compiler generates for any given protocol definition. Any differences between proto2 and proto3 generated code are highlighted - note that these differences are in the generated code as described in this document, not the base message classes/interfaces, which are the same in both versions. You should read the proto2 language guide and/or proto3 language guide before reading this document.

Note that no Java protocol buffer methods accept or return nulls unless otherwise specified.

Compiler Invocation

The protocol buffer compiler produces Java output when invoked with the --java_out= command-line flag. The parameter to the --java_out= option is the directory where you want the compiler to write your Java output. For each .proto file input, the compiler creates a wrapper .java file containing a Java class which represents the .proto file itself.

If the .proto file contains a line like the following:

option java_multiple_files = true;

Then the compiler also creates a separate .java file for each top-level message, enumeration, and service declared in the .proto file.

Otherwise (that is, when the java_multiple_files option is false; which is the default), the aforementioned wrapper class is also used as an outer class, and the generated classes/enums for each top-level message, enumeration, and service declared in the .proto file are all nested within the outer wrapper class. Thus the compiler only generates a single .java file for the entire .proto file.

The wrapper class's name is chosen as follows. If the .proto file contains a line like the following:

option java_outer_classname = "Foo";

Then the wrapper class name will be Foo. Otherwise, the wrapper class name is determined by converting the .proto file base name to camel case. For example, foo_bar.proto will generate a class name of FooBar. If there's a service, enum, or message (including nested types) in the file with the same name, "OuterClass" will be appended to the wrapper class's name. Examples:

If foo_bar.proto contains a message called FooBar, the wrapper class will generate a class name of FooBarOuterClass.
If foo_bar.proto contains a service called FooService, and java_outer_classname is also set to the string FooService, then the wrapper class will generate a class name of FooServiceOuterClass.

In addition to any nested classes, the wrapper class itself will have the following API (assuming the wrapper class is named Foo and was generated from foo.proto):

public final class Foo {
  private Foo() {}  // Not instantiable.

  /** Returns a FileDescriptor message describing the contents of {@code foo.proto}. */
  public static com.google.protobuf.Descriptors.FileDescriptor getDescriptor();
  /** Adds all extensions defined in {@code foo.proto} to the given registry. */
  public static void registerAllExtensions(com.google.protobuf.ExtensionRegistry registry);
  public static void registerAllExtensions(com.google.protobuf.ExtensionRegistryLite registry);

  // (Nested classes omitted)
}

The Java package name is chosen as described under Packages, below.

The output file is chosen by concatenating the parameter to --java_out=, the package name (with .s replaced with /s), and the .java file name.

So, for example, let's say you invoke the compiler as follows:

protoc --proto_path=src --java_out=build/gen src/foo.proto

If foo.proto's Java package is com.example and it doesn't enable java_multiple_files and its outer classname is FooProtos, then the protocol buffer compiler will generate the file build/gen/com/example/FooProtos.java. The protocol buffer compiler will automatically create the build/gen/com and build/gen/com/example directories if needed. However, it will not create build/gen or build; they must already exist. You can specify multiple .proto files in a single invocation; all output files will be generated at once.

When outputting Java code, the protocol buffer compiler's ability to output directly to JAR archives is particularly convenient, as many Java tools are able to read source code directly from JAR files. To output to a JAR file, simply provide an output location ending in .jar. Note that only the Java source code is placed in the archive; you must still compile it separately to produce Java class files.

Packages

The generated class is placed in a Java package based on the java_package option. If the option is omitted, the package declaration is used instead.

For example, if the .proto file contains:

package foo.bar;

Then the resulting Java class will be placed in Java package foo.bar. However, if the .proto file also contains a java_package option, like so:

package foo.bar;
option java_package = "com.example.foo.bar";

Then the class is placed in the com.example.foo.bar package instead. The java_package option is provided because normal .proto package declarations are not expected to start with a backwards domain name.

Messages

Given a simple message declaration:

message Foo {}

The protocol buffer compiler generates a class called Foo, which implements the Message interface. The class is declared final; no further subclassing is allowed. Foo extends GeneratedMessage, but this should be considered an implementation detail. By default, Foo overrides many methods of GeneratedMessage with specialized versions for maximum speed. However, if the .proto file contains the line:

option optimize_for = CODE_SIZE;

then Foo will override only the minimum set of methods necessary to function and rely on GeneratedMessage's reflection-based implementations of the rest. This significantly reduces the size of the generated code, but also reduces performance.

Alternatively, if the Java output directory used by protoc is prefixed with lite: — for example --java_out=lite:project/protos — then Foo will include fast implementations of all methods, but will implement the MessageLite interface, which only contains a subset of the methods of Message. In particular, it does not support descriptors or reflection. However, in this mode, the generated code only needs to link against libprotobuf-lite.jar instead of libprotobuf.jar. The "lite" library is much smaller than the full library, and is more appropriate for resource-constrained systems such as mobile phones.

The Message interface defines methods that let you check, manipulate, read, or write the entire message. In addition to these methods, the Foo class defines the following static methods:

static Foo getDefaultInstance(): Returns the singleton instance of Foo. This instance's contents are identical to what you'd get if you called Foo.newBuilder().build() (so all singular fields are unset and all repeated fields are empty). Note that the default instance of a message can be used as a factory by calling its newBuilderForType() method.
static Descriptor getDescriptor(): Returns the type's descriptor. This contains information about the type, including what fields it has and what their types are. This can be used with the reflection methods of the Message, such as getField().
static Foo parseFrom(...): Parses a message of type Foo from the given source and returns it. There is one parseFrom method corresponding to each variant of mergeFrom() in the Message.Builder interface. Note that parseFrom() never throws UninitializedMessageException; it throws InvalidProtocolBufferException if the parsed message is missing required fields. This makes it subtly different from calling Foo.newBuilder().mergeFrom(...).build().
static Parser parser(): Returns an instance of the Parser, which implements various parseFrom() methods.
Foo.Builder newBuilder(): Creates a new builder (described below).
Foo.Builder newBuilder(Foo prototype): Creates a new builder with all fields initialized to the same values that they have in prototype. Since embedded message and string objects are immutable, they are shared between the original and the copy.

Builders

Message objects such as instances of the Foo class described above are immutable, just like a Java String. To construct a message object, you need to use a builder. Each message class has its own builder class – so in our Foo example, the protocol buffer compiler generates a nested class Foo.Builder which can be used to build a Foo. Foo.Builder implements the Message.Builder interface. It extends the GeneratedMessage.Builder class, but, again, this should be considered an implementation detail. Like Foo, Foo.Builder may rely on generic method implementations in GeneratedMessage.Builder or, when the optimize_for option is used, generated custom code that is much faster.

Foo.Builder does not define any static methods. Its interface is exactly as defined by the Message.Builder interface, with the exception that return types are more specific: methods of Foo.Builder that modify the builder return type Foo.Builder, and build() returns type Foo.

Methods that modify the contents of a builder – including field setters – always return a reference to the builder. That is, they "return this;". This allows multiple method calls to be chained together in one line. For example: builder.mergeFrom(obj).setFoo(1).setBar("abc").clearBaz();

Note that builders are not thread-safe, so Java synchronization should be used whenever it is necessary for multiple different threads to be modifying the contents of a single builder.

Sub-Builders

For messages containing sub-messages, the compiler also generates sub-builders. This allows you to repeatedly modify deep-nested sub-messages without rebuilding them. For example:

message Foo {
  optional int32 val = 1;
  // some other fields.
}

message Bar {
  optional Foo foo = 1;
  // some other fields.
}

message Baz {
  optional Bar bar = 1;
  // some other fields.
}

If you have a Baz message already, and want to change the deeply nested val in Foo. Instead of:

  baz = baz.toBuilder().setBar(
      baz.getBar().toBuilder().setFoo(
          baz.getBar().getFoo().toBuilder().setVal(10).build()
      ).build()).build();

You can write:

  Baz.Builder builder = baz.toBuilder();
  builder.getBarBuilder().getFooBuilder().setVal(10);
  baz = builder.build();

Nested Types

A message can be declared inside another message. For example: message Foo { message Bar { } }

In this case, the compiler simply generates Bar as an inner class nested inside Foo.

Fields

In addition to the methods described in the previous section, the protocol buffer compiler generates a set of accessor methods for each field defined within the message in the .proto file. The methods that read the field value are defined both in the message class and its corresponding builder; the methods that modify the value are defined in the builder only.

Note that method names always use camel-case naming, even if the field name in the .proto file uses lower-case with underscores (as it should). The case-conversion works as follows:

For each underscore in the name, the underscore is removed, and the following letter is capitalized.
If the name will have a prefix such as "get," the first letter is capitalized. Otherwise, it is lower-cased.

Thus, the field foo_bar_baz becomes fooBarBaz. If prefixed with get, it would be getFooBarBaz.

In a few special cases where a method name conflicts with reserved words in Java or methods already defined in the protobuf library, an extra underscore is appended. For instance, the getter for a field named class is getClass_ to avoid conflicting with the getClass method in java.lang.Object.

As well as accessor methods, the compiler generates an integer constant for each field containing its field number. The constant name is the field name converted to upper-case followed by _FIELD_NUMBER. For example, given the field optional int32 foo_bar = 5;, the compiler will generate the constant public static final int FOO_BAR_FIELD_NUMBER = 5;.

Singular Fields (proto2)

For any of these field definitions:

optional int32 foo = 1;
required int32 foo = 1;

The compiler will generate the following accessor methods in both the message class and its builder:

boolean hasFoo(): Returns true if the field is set.
int getFoo(): Returns the current value of the field. If the field is not set, returns the default value.

The compiler will generate the following methods only in the message's builder:

Builder setFoo(int value): Sets the value of the field. After calling this, hasFoo() will return true and getFoo() will return value.
Builder clearFoo(): Clears the value of the field. After calling this, hasFoo() will return false and getFoo() will return the default value.

For other simple field types, the corresponding Java type is chosen according to the scalar value types table. For message and enum types, the value type is replaced with the message or enum class.

Embedded Message Fields

For message types, setFoo() also accepts an instance of the message's builder type as the parameter. This is just a shortcut which is equivalent to calling .build() on the builder and passing the result to the method.

If the field is not set, getFoo() will return a Foo instance with none of its fields set (possibly the instance returned by Foo.getDefaultInstance()).

In addition, the compiler generates two accessor methods that allow you to access the relevant sub-builders for message types. The following method is generated in both the message class and its builder:

FooOrBuilder getFooOrBuilder(): Returns the builder for the field, if it already exists, or the message if not. Calling this method on builders will not create a sub-builder for the field.

The compiler generates the following method only in the message's builder.

Builder getFooBuilder(): Returns the builder for the field.

Singular Fields (proto3)

For this field definition:

int32 foo = 1;

The compiler will generate the following accessor method in both the message class and its builder:

int getFoo(): Returns the current value of the field. If the field is not set, returns the default value for the field's type.

The compiler will generate the following methods only in the message's builder:

Builder setFoo(int value): Sets the value of the field. After calling this, getFoo() will return value.
Builder clearFoo(): Clears the value of the field. After calling this, getFoo() will return the default value for the field's type.

For other simple field types, the corresponding Java type is chosen according to the scalar value types table. For message and enum types, the value type is replaced with the message or enum class.

Embedded Message Fields

For message field types, an additional accessor method is generated in both the message class and its builder:

boolean hasFoo(): Returns true if the field has been set.

setFoo() also accepts an instance of the message's builder type as the parameter. This is just a shortcut which is equivalent to calling .build() on the builder and passing the result to the method.

If the field is not set, getFoo() will return a Foo instance with none of its fields set (possibly the instance returned by Foo.getDefaultInstance()).

FooOrBuilder getFooOrBuilder(): Returns the builder for the field, if it already exists, or the message if not. Calling this method on builders will not create a sub-builder for the field.

The compiler generates the following method only in the message's builder.

Builder getFooBuilder(): Returns the builder for the field.

Enum Fields

For enum field types, an additional accessor method is generated in both the message class and its builder:

int getFooValue(): Returns the integer value of the enum.

The compiler will generate the following additional method only in the message's builder:

Builder setFooValue(int value): Sets the integer value of the enum.

In addition, getFoo() will return UNRECOGNIZED if the enum value is unknown - this is a special additional value added by the proto3 compiler to the generated enum type.

Repeated Fields

For this field definition:

repeated string foo = 1;

The compiler will generate the following accessor methods in both the message class and its builder:

int getFooCount(): Returns the number of elements currently in the field.
String getFoo(int index): Returns the element at the given zero-based index.
ProtocolStringList getFooList(): Returns the entire field as a ProtocolStringList. If the field is not set, returns an empty list.

The compiler will generate the following methods only in the message's builder:

Builder setFoo(int index, String value): Sets the value of the element at the given zero-based index.
Builder addFoo(String value): Appends a new element to the field with the given value.
Builder addAllFoo(Iterable<? extends String> value): Appends all elements in the given Iterable to the field.
Builder clearFoo(): Removes all elements from the field. After calling this, getFooCount() will return zero.

For other simple field types, the corresponding Java type is chosen according to the scalar value types table. For message and enum types, the type is the message or enum class.

Repeated Embedded Message Fields

For message types, setFoo() and addFoo() also accept an instance of the message's builder type as the parameter. This is just a shortcut which is equivalent to calling .build() on the builder and passing the result to the method. There is also an additional generated method:

Builder addFoo(int index, Field value): Inserts a new element at the given zero-based index. Shifts the element currently at that position (if any) and any subsequent elements to the right (adds one to their indices).

In addition, the compiler generates the following additional accessor methods in both the message class and its builder for message types, allowing you to access the relevant sub-builders:

FooOrBuilder getFooOrBuilder(int index): Returns the builder for the specified element, if it already exists, or the element if not. If this is called from a message class, it will always return a message rather than a builder. Calling this method on builders will not create a sub-builder for the field.
List<FooOrBuilder> getFooOrBuilderList(): Returns the entire field as an unmodifiable list of builders (if available) or messages if not. If this is called from a message class, it will always return an immutable list of messages rather than an unmodifiable list of builders.

The compiler will generate the following methods only in the message's builder:

Builder getFooBuilder(int index): Returns the builder for the element at the specified index.
Builder addFooBuilder(int index): Inserts and returns a builder for a default message instance at the specified index. The existing entries are shifted to higher indices to make room for the inserted builder.
Builder addFooBuilder(): Appends and returns a builder for a default message instance.
Builder removeFoo(int index): Removes the element at the given zero-based index.
List<Builder> getFooBuilderList(): Returns the entire field as an unmodifiable list of builders.

Repeated Enum Fields (proto3 only)

The compiler will generate the following additional methods in both the message class and its builder:

int getFooValue(int index): Returns the integer value of the enum at the specified index.
List<java.lang.Integer> getFooValueList(): Returns the entire field as a list of Integers.

The compiler will generate the following additional method only in the message's builder:

Builder setFooValue(int index, int value): Sets the integer value of the enum at the specified index.

Name Conflicts

If another non-repeated field has a name that conflicts with one of the repeated field’s generated methods, then both field names will have their protobuf field number appended to the end.

For these field definitions:

int32 foo_count = 1;
repeated string foo = 2;

The compiler will first rename them to the following:

int32 foo_count_1 = 1;
repeated string foo_2 = 2;

The accessor methods will subsequently be generated as described above.

Oneof Fields

For this oneof field definition:

oneof example_name {
    int32 foo = 1;
    ...
}

The compiler will generate the following accessor methods in both the message class and its builder:

boolean hasFoo() (proto2 only): Returns true if the oneof case is FOO.
int getFoo(): Returns the current value of example_name if the oneof case is FOO. Otherwise, returns the default value of this field.

The compiler will generate the following methods only in the message's builder:

Builder setFoo(int value): Sets example_name to this value and sets the oneof case to FOO. After calling this, hasFoo() will return true, getFoo() will return value and getExampleNameCase() will return FOO.
Builder clearFoo():
- Nothing will be changed if the oneof case is not FOO.
- If the oneof case is FOO, sets example_name to null and the oneof case to EXAMPLENAME_NOT_SET. After calling this, hasFoo() will return false, getFoo() will return the default value and getExampleNameCase() will return EXAMPLENAME_NOT_SET.

For other simple field types, the corresponding Java type is chosen according to the scalar value types table. For message and enum types, the value type is replaced with the message or enum class.

Map Fields

For this map field definition:

map<int32, int32> weight = 1;

The compiler will generate the following accessor methods in both the message class and its builder:

Map<Integer, Integer> getWeightMap();: Returns an unmodifiable Map.
int getWeightOrDefault(int key, int default);: Returns the value for key or the default value if not present.
int getWeightOrThrow(int key);: Returns the value for key or throws IllegalArgumentException if not present.
boolean containsWeight(int key);: Indicates if the key is present in this field.
int getWeightCount();: Returns the number of elements in the map.

The compiler will generate the following methods only in the message's builder:

Builder putWeight(int key, int value);: Add the weight to this field.
Builder putAllWeight(Map<Integer, Integer> value);: Adds all entries in the given map to this field.
Builder removeWeight(int key);: Removes the weight from this field.
Builder clearWeight();: Removes all weights from this field.
@Deprecated Map<Integer, Integer> getMutableWeight();: Returns a mutable Map. Note that multiple calls to this method may return different map instances. The returned map reference may be invalidated by any subsequent method calls to the Builder.

Any

Given an Any field like this:

import "google/protobuf/any.proto";

message ErrorStatus {
  string message = 1;
  google.protobuf.Any details = 2;
}

In our generated code, the getter for the details field returns an instance of com.google.protobuf.Any. This provides the following special methods to pack and unpack the Any's values:

class Any {
  // Packs the given message into an Any using the default type URL
  // prefix “type.googleapis.com”.
  public static Any pack(Message message);
  // Packs the given message into an Any using the given type URL
  // prefix.
  public static Any pack(Message message,
                         String typeUrlPrefix);

  // Checks whether this Any message’s payload is the given type.
  public <T extends Message> boolean is(class<T> clazz);

  // Unpacks Any into the given message type. Throws exception if
  // the type doesn’t match or parsing the payload has failed.
  public <T extends Message> T unpack(class<T> clazz)
      throws InvalidProtocolBufferException;
}

Oneofs

Given a oneof definition like this:

oneof example_name {
    int32 foo_int = 4;
    string foo_string = 9;
    ...
}

All the fields in the example_name oneof will use a single private field for their value. In addition, the protocol buffer compiler will generate a Java enum type for the oneof case, as follows:

public enum ExampleNameCase
        implements com.google.protobuf.Internal.EnumLite {
      FOO_INT(4),
      FOO_STRING(9),
      ...
      EXAMPLENAME_NOT_SET(0);
      ...
    };

The values of this enum type have the following special methods:

int getNumber(): Returns the object's numeric value as defined in the .proto file.
static ExampleNameCase forNumber(int value): Returns the enum object corresponding to the given numeric value (or null for other numeric values).

The compiler will also generate the following accessor method in both the message class and its builder:

ExampleNameCase getExampleNameCase(): Returns the enum indicating which field is set. Returns EXAMPLENAME_NOT_SET if none of them is set.

The compiler will generate the following method only in the message's builder:

Builder clearExampleName(): Resets the oneof's private field to null, and sets the oneof case to EXAMPLENAME_NOT_SET.

Enumerations

Given an enum definition like:

enum Foo {
  A = 0;
  B = 5;
  C = 1234;
}

The protocol buffer compiler will generate a Java enum type called Foo with the same set of values. If you are using proto3, it also adds the special value UNRECOGNIZED to the enum type. The values of the generated enum type have the following special methods:

int getNumber(): Returns the object's numeric value as defined in the .proto file.
EnumValueDescriptor getValueDescriptor(): Returns the value's descriptor, which contains information about the value's name, number, and type.
EnumDescriptor getDescriptorForType(): Returns the enum type's descriptor, which contains information about each defined value.

Additionally, the Foo enum type contains the following static methods:

static Foo forNumber(int value): Returns the enum object corresponding to the given numeric value. Returns null when there is no corresponding enum object.
static Foo valueOf(int value): Returns the enum object corresponding to the given numeric value. This method is deprecated in favor of forNumber(int value) and will be removed in an upcoming release.
static Foo valueOf(EnumValueDescriptor descriptor): Returns the enum object corresponding to the given value descriptor. May be faster than valueOf(int). In proto3 returns UNRECOGNIZED if passed an unknown value descriptor.
EnumDescriptor getDescriptor(): Returns the enum type's descriptor, which contains information about each defined value. (This differs from getDescriptorForType() only in that it is a static method.)

An integer constant is also generated with the suffix _VALUE for each enum value.

Note that the .proto language allows multiple enum symbols to have the same numeric value. Symbols with the same numeric value are synonyms. For example:

enum Foo {
  BAR = 0;
  BAZ = 0;
}

In this case, BAZ is a synonym for BAR. In Java, BAZ will be defined as a static final field like so:

static final Foo BAZ = BAR;

Thus, BAR and BAZ compare equal, and BAZ should never appear in switch statements. The compiler always chooses the first symbol defined with a given numeric value to be the "canonical" version of that symbol; all subsequent symbols with the same number are just aliases.

An enum can be defined nested within a message type. The compiler generates the Java enum definition nested within that message type's class.

Caution: when generating Java code, the maximum number of values in a protobuf enum may be surprisingly low -- in the worst case, the maximum is slightly over 1,700 values. This limit is due to per-method size limits for Java bytecode, and it varies across Java implementations, different versions of the protobuf suite, and any options set on the enum in the .proto file.

Extensions (proto2 only)

Given a message with an extension range:

message Foo {
  extensions 100 to 199;
}

The protocol buffer compiler will make Foo extend GeneratedMessage.ExtendableMessage instead of the usual GeneratedMessage. Similarly, Foo's builder will extend GeneratedMessage.ExtendableBuilder. You should never refer to these base types by name (GeneratedMessage is considered an implementation detail). However, these superclasses define a number of additional methods that you can use to manipulate extensions.

In particular Foo and Foo.Builder will inherit the methods hasExtension(), getExtension(), and getExtensionCount(). Additionally, Foo.Builder will inherit methods setExtension() and clearExtension(). Each of these methods takes, as its first parameter, an extension identifier (described below), which identifies an extension field. The remaining parameters and the return value are exactly the same as those for the corresponding accessor methods that would be generated for a normal (non-extension) field of the same type as the extension identifier.

Given an extension definition:

extend Foo {
  optional int32 bar = 123;
}

The protocol buffer compiler generates an "extension identifier" called bar, which you can use with Foo's extension accessors to access this extension, like so:

Foo foo =
  Foo.newBuilder()
     .setExtension(bar, 1)
     .build();
assert foo.hasExtension(bar);
assert foo.getExtension(bar) == 1;

(The exact implementation of extension identifiers is complicated and involves magical use of generics – however, you don't need to worry about how extension identifiers work to use them.)

Note that bar would be declared as a static field of the wrapper class for the .proto file, as described above; we have omitted the wrapper class name in the example.

Extensions can be declared inside the scope of another type to prefix their generated symbol names. For example, a common pattern is to extend a message by a field inside the declaration of the field's type:

message Baz {
  extend Foo {
    optional Baz foo_ext = 124;
  }
}

In this case, an extension with identifier foo_ext and type Baz is declared inside the declaration of Baz, and referring to foo_ext requires the addition of a Baz. prefix:

Baz baz = createMyBaz();
Foo foo =
  Foo.newBuilder()
     .setExtension(Baz.fooExt, baz)
     .build();
assert foo.hasExtension(Baz.fooExt);
assert foo.getExtension(Baz.fooExt) == baz;

When parsing a message that might have extensions, you must provide an ExtensionRegistry in which you have registered any extensions that you want to be able to parse. Otherwise, those extensions will be treated like unknown fields and the methods observing extensions will behave as if they don't exist.

  ExtensionRegistry registry = ExtensionRegistry.newInstance();
  registry.add(Baz.fooExt);
  Foo foo = Foo.parseFrom(input, registry);
  assert foo.hasExtension(Baz.fooExt);

  ExtensionRegistry registry = ExtensionRegistry.newInstance();
  Foo foo = Foo.parseFrom(input, registry);
  assert foo.hasExtension(Baz.fooExt) == false;

Services

If the .proto file contains the following line:

option java_generic_services = true;

Then the protocol buffer compiler will generate code based on the service definitions found in the file as described in this section. However, the generated code may be undesirable as it is not tied to any particular RPC system, and thus requires more levels of indirection than code tailored to one system. If you do NOT want this code to be generated, add this line to the file:

option java_generic_services = false;

If neither of the above lines are given, the option defaults to false, as generic services are deprecated. (Note that prior to 2.4.0, the option defaults to true)

RPC systems based on .proto-language service definitions should provide plugins to generate code approriate for the system. These plugins are likely to require that abstract services are disabled, so that they can generate their own classes of the same names. Plugins are new in version 2.3.0 (January 2010).

The remainder of this section describes what the protocol buffer compiler generates when abstract services are enabled.

Interface

Given a service definition:

service Foo {
  rpc Bar(FooRequest) returns(FooResponse);
}

The protocol buffer compiler will generate an abstract class Foo to represent this service. Foo will have an abstract method for each method defined in the service definition. In this case, the method Bar is defined as:

abstract void bar(RpcController controller, FooRequest request,
                  RpcCallback<FooResponse> done);

The parameters are equivalent to the parameters of Service.CallMethod(), except that the method argument is implied and request and done specify their exact type.

Foo subclasses the Service interface. The protocol buffer compiler automatically generates implementations of the methods of Service as follows:

getDescriptorForType: Returns the service's ServiceDescriptor.
callMethod: Determines which method is being called based on the provided method descriptor and calls it directly, down-casting the request message and callback to the correct types.
getRequestPrototype and getResponsePrototype: Returns the default instance of the request or response of the correct type for the given method.

The following static method is also generated:

static ServiceDescriptor getServiceDescriptor(): Returns the type's descriptor, which contains information about what methods this service has and what their input and output types are.

Foo will also contain a nested interface Foo.Interface. This is a pure interface that again contains methods corresponding to each method in your service definition. However, this interface does not extend the Service interface. This is a problem because RPC server implementations are usually written to use abstract Service objects, not your particular service. To solve this problem, if you have an object impl implementing Foo.Interface, you can call Foo.newReflectiveService(impl) to construct an instance of Foo that simply delegates to impl, and implements Service.

To recap, when implementing your own service, you have two options:

Subclass Foo and implement its methods as appropriate, then hand instances of your subclass directly to the RPC server implementation. This is usually easiest, but some consider it less "pure".
Implement Foo.Interface and use Foo.newReflectiveService(Foo.Interface) to construct a Service wrapping it, then pass the wrapper to your RPC implementation.

Stub

The protocol buffer compiler also generates a "stub" implementation of every service interface, which is used by clients wishing to send requests to servers implementing the service. For the Foo service (above), the stub implementation Foo.Stub will be defined as a nested class.

Foo.Stub is a subclass of Foo which also implements the following methods:

Foo.Stub(RpcChannel channel): Constructs a new stub which sends requests on the given channel.
RpcChannel getChannel(): Returns this stub's channel, as passed to the constructor.

The stub additionally implements each of the service's methods as a wrapper around the channel. Calling one of the methods simply calls channel.callMethod().

The Protocol Buffer library does not include an RPC implementation. However, it includes all of the tools you need to hook up a generated service class to any arbitrary RPC implementation of your choice. You need only provide implementations of RpcChannel and RpcController.

Blocking Interfaces

The RPC classes described above all have non-blocking semantics: when you call a method, you provide a callback object which will be invoked once the method completes. Often it is easier (though possibly less scalable) to write code using blocking semantics, where the method simply doesn't return until it is done. To accomodate this, the protocol buffer compiler also generates blocking versions of your service class. Foo.BlockingInterface is equivalent to Foo.Interface except that each method simply returns the result rather than call a callback. So, for example, bar is defined as:

abstract FooResponse bar(RpcController controller, FooRequest request)
                         throws ServiceException;

Analogous to non-blocking services, Foo.newReflectiveBlockingService(Foo.BlockingInterface) returns a BlockingService wrapping some Foo.BlockingInterface. Finally, Foo.BlockingStub returns a stub implementation of Foo.BlockingInterface that sends requests to a particular BlockingRpcChannel.

Plugin Insertion Points

Code generator plugins which want to extend the output of the Java code generator may insert code of the following types using the given insertion point names.

outer_class_scope: Member declarations that belong in the file's wrapper class.
class_scope:TYPENAME: Member declarations that belong in a message class. TYPENAME is the full proto name such as package.MessageType.
builder_scope:TYPENAME: Member declarations that belong in a message's builder class. TYPENAME is the full proto name such as package.MessageType.
enum_scope:TYPENAME: Member declarations that belong in an enum class. TYPENAME is the full proto enum name such as package.EnumType.
message_implements:TYPENAME: Class implementation declarations for a message class. TYPENAME is the full proto name such as package.MessageType.
builder_implements:TYPENAME: Class implementation declarations for a builder class. TYPENAME is the full proto name such as package.MessageType.

Generated code cannot contain import statements, as these are prone to conflict with type names defined within the generated code itself. Instead, when referring to an external class, you must always use its fully-qualified name.

The logic for determining output file names in the Java code generator is fairly complicated. You should probably look at the protoc source code, particularly java_headers.cc, to make sure you have covered all cases.

Do not generate code which relies on private class members declared by the standard code generator, as these implementation details may change in future versions of Protocol Buffers.

Utility Classes

Protocol buffer provides utility classes for message comparison, JSON conversion and working with well-known types (predefined protocol buffer messages for common use-cases).