mapbox/jni.hpp

jni.hpp is a modern, type-safe, header-only, C++14 wrapper for JNI (Java Native Interface). Its aim is to make calling Java from C++, or C++ from Java, convenient and safe, without sacrificing low-level control, as you commonly do with code generation approaches.

Two levels of wrappers are provided.

Low-level wrappers

The low-level wrappers match JNI, type-for-type and function-for-function, with modern C++ equivalents in a jni namespace. The matching uniformly follows a set of rules.

Rules for Names

• Everything in jni.hpp is in the jni namespace.
• When a name exists in JNI, the same name is used in the namespace.
• When the original name is a macro, a lowercase name is used instead, e.g. jni::jni_ok for JNI_OK.

Rules for Types

• Types corresponding to Java primitive types (jni::jboolean, jni::jint, jni::jdouble, etc.) are matched unchanged.
• Types corresponding to Java reference types (jni::jobject, jni::jclass, jni::jarray, etc.) are non-copyable and non-constructible. Their references and pointers automatically convert to base types as expected. (These types do not hide pointer-ness behind a confusing typedef, like the underlying JNI headers do.)
• jni::jsize is std::size_t, not a signed integer. jni.hpp checks for overflow in the necessary places.
• Ownership types are instantiations of unique_ptr, and using declarations are provided for each specific instantiation, e.g. jni::UniqueGlobalRef.
• Families of functions, such as Call*Method and CallStatic*Method, are matched with a single template function, such as jni::CallMethod<ResultType> and jni::CallStaticMethod<ResultType>.
• jni::jvalue and the Call*MethodV and Call*MethodA function families are made redundant by type safety, and omitted.

Rules for Parameters and Results

• Parameters are passed, and results returned, by value or reference, not by pointer. An exception is made for possibly-null jni::jobject parameters and results, which are pointers.
• When transferring ownership out of a function, the return type is an ownership type.
• When transferring ownership into a function, the parameter type is an ownership type.
• Output parameters are returned, not passed as pointer-to-pointer. When there are multiple outputs, they are returned as a std::tuple.
• Whenever a function receives paired "pointer to T and length" parameters, an overload is provided that accepts a statically-sized array, std::array<T>, std::vector<T>, or (if T is a character type) std::basic_string<T> argument. These overloads compute the length automatically.
• In string-related functions, char16_t replaces jchar.

Rules for Error Handling

• Errors are thrown, not returned or otherwise left to be explicitly checked.
• Pending Java exceptions are checked after every JNI call that may produce a pending Java exception, and thrown as jni::PendingJavaException.
• Invocation API errors are thrown as std::system_errors containing std::error_codes with a custom category.

High-level wrappers

The high-level wrappers provide additional syntactic convenience and type safety when working with non-primitive Java types. They are built around the concept of class tags. A class tag is a C++ type that provides a static Name method returning a fully-qualified Java class name, thus associating a unique C++ type with the corresponding Java class. For example:

struct Math { static constexpr auto Name() { return "java/lang/Math"; } };

The high-level wrappers consist of a set of classes templated on class tag:

• jni::Class<Tag>, a wrapper for a reference to the Java class associated with the tag.
• jni::Object<Tag>, a wrapper for a (possibly-null) reference to an instance of the Java class associated with the tag.
• jni::Array<E>, a wrapper for a (possibly-null) reference to a Java array. The element type E is a jni.hpp primitive type or Object<Tag>.
• jni::Constructor<Tag, Args...>, a wrapper for a constructor for the the Java class associated with the tag. The result type R and each argument type in Args is a jni.hpp primitive type or Object<Tag>.
• jni::Method<Tag, R (Args...)>, a wrapper for an instance method of the Java class associated with the tag. The result type R and each argument type in Args is a jni.hpp primitive type or Object<Tag>.
• jni::StaticMethod<Tag, R (Args...)>, a wrapper for a static method of the Java class associated with the tag. The result type R and each argument type in Args is a jni.hpp primitive type or Object<Tag>.
• jni::Field<Tag, T>, a wrapper for an instance field of the Java class associated with the tag, and providing Get and Set methods. The field type T is a jni.hpp primitive type or Object<Tag>.
• jni::StaticField<Tag, T>, a wrapper for a static field of the Java class associated with the tag, and providing Get and Set methods. The field type T is a jni.hpp primitive type or Object<Tag>.

Method Registration

Registering native methods is a central part of JNI, and jni.hpp provides several features that make this task safer and more convenient. The jni.hpp wrapper method jni::RegisterNatives has the following signature:

template < class... Methods >
void RegisterNatives(jni::JNIEnv& env, jni::jclass&, const Methods&... methods);

In other words, rather than receiving a length and pointer to an array of JNIMethods, it takes a variable number of variable types. This allows jni::RegisterNatives to type check the methods, ensuring that their type signatures are valid for JNI.

Use the helper function jni::MakeNativeMethod to construct method arguments for jni::RegisterNatives. jni::MakeNativeMethod wraps your method in a try / catch block that translates C++ exceptions to Java exceptions. It is overloaded on the following combinations of arguments:

• A const char * name, const char * signature, and lambda. The type of the first parameter of the lambda must be jni::JNIEnv&. The type of the second parameter must be either jni::jclass* (for a native static method) or jni::jobject* (for a native instance method). The result type must be a JNI primitive type or type convertable to jni::jobject*. (These requirements are type checked.)
• A const char * name, const char * signature, and function pointer. The function has the same parameter and return type requirements as the lambda. In order to guarantee a unique exception-handling wrapper for each unique function pointer, the function pointer must be provided as a template parameter rather than method parameter:

jni::MakeNativeMethod<decltype(myFunction), myFunction>(name, signature)

The repetition of myFunction with decltype is necessary because it is not possible to infer the function type from the non-type template parameter. You may wish to define and use a macro to avoid the repetition.

• A const char * name and lamba whose parameter and return types use high-level jni.hpp wrapper types. In this case, jni.hpp will compute the signature automatically.
• A const char * name and function pointer whose parameter and return types use high-level jni.hpp wrapper types. Again, jni.hpp will compute the signature automatically, and again, the function pointer must be provided as a template parameter rather than method parameter.

Finally, jni.hpp provides a mechanism for registering a "native peer": a long-lived native object corresponding to a Java object, usually created when the Java object is created and destroyed when the Java object's finalizer runs. Between creation and finalization, a pointer to the native peer is stored in a long field on the Java object. jni.hpp will take care of wrapping lambdas, function pointers, or member function pointers with code that automatically gets the value of this field, casts it to a pointer to the peer, and calls the member function (or passes a reference to the peer as an argument to the lambda or function pointer). See the example code for details.

Example code

Example code for both the low-level and high-level wrappers is provided in the examples subdirectory. This code shows the use of jni.hpp for:

• Registering native methods such that they can be called from Java.
• Calling back into Java methods from native methods.
• Native peer registration.

Prior art

• Many code generation approaches. SWIG, JavaCPP, and so on. But jni.hpp is explicitly not a binding generator.
• JniHelpers is similar in spirit. In comparison, jni.hpp takes advantage of modern C++ features such as unique_ptr and variadic templates, introduces no preprocessor macros, and is header-only -- no build step required.
• I hear that Boost.Python is similar, but I did not reference it in building this library.
• The low-level wrapping strategy is inspired by Lisa Lippincott's presentation "How to call C libraries from C++" from Cppcon 2014, and by her previous work "The Nitrogen Manifesto". However jni.hpp does not use "PlusPlus", the wrapping library abstraction mentioned in the presentation; I found it to be a poor economy compared to simply writing out the wrappers manually.