Djinni

Djinni is a tool for generating cross-language type declarations and interface bindings. It's designed to connect C++ with either Java or Objective-C.

We at Dropbox use Djinni to interface cross-platform C++ library code with platform-specific Java and Objective-C on Android and iOS.

We announced Djinni at CppCon 2014. Access the slides from https://bit.ly/djinnitalk and video from https://www.youtube.com/watch?v=ZcBtF-JWJhM

Main Features

• Generates parallel C++, Java and Objective-C type definitions from a single interface description file.
• Supports the intersection of the three core languages' primitive types, and user-defined enums, records, and interfaces.
• Generates interface code allowing bidirectional calls between C++ and Java (with JNI) or Objective-C (with Objective-C++).
• Can autogenerate comparator functions (equality, ordering) on data types.

Getting Started

Types

Djinni generates code based on interface definitions in an IDL file. An IDL file can contain three kinds of declaration: enums, records, and interfaces.

• Enums become C++ enum classes, Java enums, or ObjC NS_ENUMs.
• Records are pure-data value objects.
• Interfaces are objects with defined methods to call (in C++, passed by shared_ptr). Djinni produces code allowing an interface implemented in C++ to be transparently used from ObjC or Java, and vice versa.

IDL Files

Djinni's input is an interface description file. Here's an example:

# Multi-line comments can be added here. This comment will be propagated
# to each generated definition.
my_enum = enum {
    option1;
    option2;
    option3;
}

my_record = record {
    id: i32;
    info: string;
    store: set<string>;
    hash: map<string, i32>;

    values: list<another_record>;

    # Comments can also be put here

    # Constants can be included
    const string_const: string = "Constants can be put here";
    const min_value: another_record = {
        key1 = 0,
        key2 = ""
    };
}

another_record = record {
    key1: i32;
    key2: string;
} deriving (eq, ord)

# This interface will be implemented in C++ and can be called from any language.
my_cpp_interface = interface +c {
    method_returning_nothing(value: i32);
    method_returning_some_type(key: string): another_record;
    static get_version(): i32;

    # Interfaces can also have constants
    const version: i32 = 1;
}

# This interface will be implemented in Java and ObjC and can be called from C++.
my_client_interface = interface +j +o {
    log_string(str: string): bool;
}

Djinni files can also include each other. Adding the line:

@import "relative/path/to/filename.djinni"

at the beginning of a file will simply include another file. Child file paths are relative to the location of the file that contains the @import. Two different djinni files cannot define the same type. @import behaves like #include with #pragma once in C++, or like ObjC's #import: if a file is included multiple times through different paths, then it will only be processed once.

Generate Code

When the Djinni file(s) are ready, from the command line or a bash script you can run:

src/run \
   --java-out JAVA_OUTPUT_FOLDER \
   --java-package com.example.jnigenpackage \
   --java-cpp-exception DbxException \ # Choose between a customized C++ exception in Java and java.lang.RuntimeException (the default).
   --ident-java-field mFooBar \ # Optional, this adds an "m" in front of Java field names
   \
   --cpp-out CPP_OUTPUT_FOLDER \
   \
   --jni-out JNI_OUTPUT_FOLDER \
   --ident-jni-class NativeFooBar \ # This adds a "Native" prefix to JNI class
   \
   --objc-out OBJC_OUTPUT_FOLDER \
   --objc-type-prefix DB \ # Apple suggests Objective-C classes have a prefix for each defined type.
   \
   --objcpp-out OBJC_OUTPUT_FOLDER \
   \
   --idl MY_PROJECT.djinni

Some other options are also available, such as --cpp-namespace that put generated C++ code into the namespace specified. For a list of all options, run src/run --help

Sample generated code is in the example/generated-src/ and test-suite/generated-src/ directories of this distribution.

Note that if a language's output folder is not specified, that language will not be generated. For more information, run run --help to see all command line arguments available.

Use Generated Code in Your Project

Java / JNI / C++ Project

Includes & Build target

The following headers / code will be generated for each defined type:

Type	C++ header	C++ source	Java	JNI header	JNI source
Enum	my_enum.hpp		MyEnum.java	NativeMyEnum.hpp	NativeMyEnum.cpp
Record	my_record[_base].hpp	my_record[_base].cpp (+)	MyRecord[Base].java	NativeMyRecord.hpp	NativeMyRecord.cpp
Interface	my_interface.hpp	my_interface.cpp (+)	MyInterface.java	NativeMyInterface.hpp	NativeMyInterface.cpp

(+) Generated only for types that contain constants.

Add all generated source files to your build target, as well as the contents of support-lib/java.

Our JNI approach

JNI stands for Java Native Interface, an extension of the Java language to allow interop with native (C/C++) code or libraries. Complete documentation on JNI is available at: http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/jniTOC.html

For each type, built-in (list, string, etc.) or user-defined, Djinni produces a translator class with a toJava and fromJava function to translate back and forth.

Application code is responsible for the initial load of the JNI library. Add a static block somewhere in your code:

System.loadLibrary("YourLibraryName");
// The name is specified in Android.mk / build.gradle / Makefile, depending on your build system.

When a native library is called, JNI calls a special function called JNI_OnLoad. If you use Djinni for all JNI interface code, include support_lib/jni/djinni_main.cpp; if not, you'll need to add calls to your own JNI_OnLoad and JNI_OnUnload functions. See support-lib/jni/djinni_main.cpp for details.

Objective-C / C++ Project

Includes & Build Target

Generated file for Objective-C / C++ is as follows (assuming prefix is DB):

Type	C++ header	C++ source	Objective-C files	Objective-C++ files
Enum	my_enum.hpp		DBMyEnum.h
Record	my_record[_base].hpp	my_record[_base].cpp (+)	DBMyRecord[Base].h	DBMyRecord[Base]+Private.h
			DBMyRecord[Base].mm (++)	DBMyRecord[Base]+Private.mm
Interface	my_interface.hpp	my_interface.cpp (+)	DBMyInterface.h	DBMyInterface+Private.h
				DBMyInterface+Private.mm

(+) Generated only for types that contain constants. (++) Generated only for types with derived operations and/or constants. These have .mm extensions to allow non-trivial constants.

Add all generated files to your build target, as well as the contents of support-lib/objc. Note that +Private files can only be used with ObjC++ source (other headers are pure ObjC) and are not required by Objective-C users of your interface.

Details of Generated Types

Enum

Enums are translated to C++ enum classes with underlying type int, ObjC NS_ENUMs with underlying type NSInteger, and Java enums.

Record

Records are data objects. In C++, records contain all their elements by value, including other records (so a record cannot contain itself).

Data type

The available data types for a record are:

• Boolean (bool)
• Primitives (i8, i16, i32, i64, f64).
• Strings (string)
• Binary (binary). This is implemented as std::vector<uint8_t> in C++, byte[] in Java, and NSData in Objective-C.
• List (list<type>). This is vector<T> in C++, ArrayList in Java, and NSArray in Objective-C. Primitives in a list will be boxed in Java and Objective-C.
• Set (set<type>). This is set<T> in C++, TreeSet in Java, and NSSet in Objective-C. Primitives in a set will be boxed in Java and Objective-C.
• Map (map<typeA, typeB>). This is unordered_map<K, V> in C++, HashMap in Java, and NSDictionary in Objective-C. Primitives in a map will be boxed in Java and Objective-C.
• Enumerations
• Optionals (optional<typeA>). This is std::experimental::optional<T> in C++11, object / boxed primitive reference in Java (which can be null), and object / NSNumber strong reference in Objective-C (which can be nil).
• Other record types. This is generated with a by-value semantic, i.e. the copy method will deep-copy the contents.

Extensions

To support extra fields and/or methods, a record can be "extended" in any language. To extend a record in a language, you can add a +c (C++), +j (Java), or +o (ObjC) flag after the record tag. The generated type will have a Base suffix, and you should create a derived type without the suffix that extends the record type.

The derived type must be constructible in the same way as the Base type. Interfaces will always use the derived type.

Derived methods

For record types, Haskell-style "deriving" declarations are supported to generate some common methods. Djinni is capable of generating equality and order comparators, implemented as operator overloading in C++ and standard comparison functions in Java / Objective-C.

Things to note:

• All fields in the record are compared in the order they appear in the record declaration. If you need to add a field later, make sure the order is correct.
• Ordering comparison is not supported for collection types, optionals, and booleans.
• To compare records containing other records, the inner record must derive at least the same types of comparators as the outer record.

Interface

Exception Handling

When an interface implemented in C++ throws a std::exception, it will be translated to a java.lang.RuntimeException in Java or an NSException in Objective-C. The what() message will be translated as well.

Constants

Constants can be defined within interfaces and records. In Java and C++ they are part of the generated class; and in Objective-C, constant names are globals with name of the interface/record prefixed. Example:

record_with_const = record +c +j +o { const const_value: i32 = 8; }

will be RecordWithConst::CONST_VALUE in C++, RecordWithConst.CONST_VALUE in Java, and RecordWithConstConstValue in Objective-C.

Miscellaneous

Record constructors / initializers

Djinni does not permit custom constructors for records or interfaces, since there would be no way to implement them in Java except by manually editing the autogenerated file. Instead, use extended records or static functions.

Identifier Format

Djinni supports overridable formats for most generated filenames and identifiers. The complete list can found by invoking Djinni with --help. The format is specified by formatting the word FooBar in the desired style:

• FOO_BAR -> GENERATED_IDENT
• mFooBar -> mGeneratedIdent
• FooBar -> GeneratedIdent

Integer types

In Djinni, i8 through i64 are all used with fixed length. The C++ builtin int, long, etc and Objective-C NSInteger are not used because their length varies by architecture. Unsigned integers are not included because they are not available in Java.

Test Suite

Run make in the test-suite directory to invoke the test suite.

Authors

• Kannan Goundan
• Tony Grue
• Derek He
• Steven Kabbes
• Jacob Potter

Contacts

Jacob Potter - [email protected]