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Mar 10

A C++ Smart Pointer wrapper for use with JNI

Introduction

In this article, I'll propose a solution for managing native C++ resources from JNI using smart pointers.

While Smart Pointers can't be useful from Java, because of the limits of the Java memory management, it may be required by the native library to maintain allocated resources through shared_ptr or unique_ptr, for example because classes derive from std::enable_from_this.

Since there is a fixed pattern to maintain native objects in Java classes, a utility C++ class is proposed.

The problem of memory management when integrating Java and C++ code

Recently, I had to port a library I wrote for Linux to Android.

For instance, the library is aimed at implementing an OO interface to Bluetooth Low Energy service, using an USB dongle (BlueGiga BLED112), and avoiding the use of both Bluez and DBus.

Bluez's Low Energy support was not ready for production code when we started this project, and I found the DBus interface unnecessarily complex for use with C/C++ applications. But anyway, there were many missing features that we needed, so we switched to some hardware solution, and one was the BlueGiga dongle.

I proposed my customer to open source the library, so perhaps one day we'll release our solution. 

The library was entirely developed in C++11 under Linux on ARM platform, using very small dependency except for boost, needed to have some feature like atomic and futures on ARM.

As any modern C++ program should, it makes large use of smart pointers as shared and weak pointers, so the memory management is entirely automatically handled by the smart pointer logic.

Then one day my customer asked me to develop a version of this library to be used on Android.

I had two options: since the dongle is managed through a serial port, I could rewrite the logic that implemented the read/write to the serial port, and the protocol, the parsing and constructing of each packet, all the classes modelling the Input and Output endpoints (i.e. the Low Energy Characteristics), and so on. I also had to write all the tests, because no library comes without a set of supporting code to demonstrate the many different use case scenarios.

Or, simply recompile the C++ library under Android and build a Java library that used the native library through jni code.

I would like to write about many problems I had with this task, and I can't exclude, if I ever will find the time, to write here about.

But one of the most annoyng aspect of this task was the facts that Java and C++ (either "classic" and "modern") have two completely incompatible memory management system.

For those who don't catch the difference, the situation is this: C++ is very precise on objects lifecycle, either by letting the user to decide using the "classic" allocation/deallocation system through new/free calls, or by taking care of that with use of the smart pointers. When a smart pointer exits his scope, the pointed object is destroyed if there exists no other pointer that shares this object. So the object lifecycle is defined in a pretty predictabily way.

Java, on the other hand, isn't so precise. Objects are created when they are instantiated, but they are destroyed whenever the JVM decides their time has come. No predictability here.

Every Java programmer knows that this pones a lot of problems even with the most common operations on common objects like files, sockets, and so on. If the object needs to be deinitialized before disposing, user must manually call a proper operation. Java provides a `finalize()` function that's called when the system disposes the object, but it is up to the JVM, or more precisely to the Garbage Collector (GC) to decide when this happens.

This is the first aspect of the problem. You have a C++ library that's designed to use smart pointers to manage the relations between the classes, you have a Java Library that uses it but requires the user to manage the allocation/deallocation of objects, and you have to glue them together.

Note also that in the native library, many objects are instantiated during the use of a feature, and some objects are owned by the user, so their lifecycle becomes somehow independent from the object that created it.

For example, suppose we have an `adapter` object that creates a `protocol` object when needed, and this object can create one or many other instances of the class `characteristic`. All of them are passed through smart pointers. Adapter, Protocol and Characteristic can have different lifecycles, though it has not much sense for a Characteristic to survive his Adapter object.

In C++ this poses not much problem: the objects are kept alive untl all the smart pointers are valid, so either the children objects are keeping alive the father, or their reference is invalid, if they use a weak pointer. It is a user responsability to select the proper pointer and the proper strategy.

In Java we have something to take care: the native object reference management, and the management of their lifecycle.

Usually, if you have a native pointer to keep in a Java object, you use a Java long type, that is a jlong in jni terms.

So the JNI code must also keep track of the object type, because if you allocate an object instance in jni, this object must be kept in a jlong Java field and it must also be retrieved and deleted at proper time.

Another issue is, object owning must be carefully managed: suppose you have a native class AN that have a relation to the class BN, and the classes AJ and BJ must own them, how you manage their lifecycle in a safe way? If AJ exits the scope before BJ, then AN must be disposed as well, but it must not dispose BN until BJ is ready to be disposed.

If you are using Smart Pointer, perhaps the life is easier, perhaps no. Anyway, there's no such a thing like a smart pointer in JNI terms, you can keep a native pointer in a jlong variable, but std::shared_pointer is not a raw pointer, is an object with different internal fields.

The pattern on storing native pointers in java code is: allocate the pointer, cast it to jlong and store it in some long java field. To retrieve it, read the long java field, cast it back to the original pointer.

Additionally, when disposing the java object, you must retrieve the raw pointer from long java field, cast it back to the original type, delete it.

The following functions do exactly this:

#include <jni.h>

jfieldID inline getHandleField(JNIEnv *env, jobject obj)
{
    jclass c = env->GetObjectClass(obj);
    // J is the type signature for long:
    return env->GetFieldID(c, "nativeHandle", "J");
}

template <typename T>
T *getHandle(JNIEnv *env, jobject obj)
{
    jlong handle = env->GetLongField(obj, getHandleField(env, obj));
    return reinterpret_cast<T *>(handle);
}

template <typename T>
void setHandle(JNIEnv *env, jobject obj, T *t)
{
    jlong handle = reinterpret_cast<jlong>(t);
    env->SetLongField(obj, getHandleField(env, obj), handle);
}

 

The code should be self-explanatory. The getHandleField() function simply retrieve the jfieldId value from the java object passed as argument. The field has a fixed name, "nativeHandle" (an improvement is to make it codable).

getHandle and setHandle simply make the necessary cast. They are templatized so you can write

 

auto ptr = getHandle<MyObject>(env,object);

 

and you have your raw-pointer-to-object in ptr.

 But what for smart pointers? if your Object derives from std::enable_from_this, you must keep it in a smart_pointer, otherwise shared_from_this() will fail with a bad_weak_ptr exception.

So what you need is to allocate a smart pointer in the heap through a new. Or wrap it in a class, better if templatized:

#ifndef BTDROIDLIB_JNISMARTPOINTERWRAPPER_H
#define BTDROIDLIB_JNISMARTPOINTERWRAPPER_H

#include <memory>
#include "handle.h"
#include "jnihelpers.h"

/** @brief a Wrapper for smart pointers to be used in JNI code
 *
 * **Usage**
 * Instantiation:
 * SmartPointerWrapper<Object> obj = new SmartPointerWrapper<Object>(arguments);
 * obj->instantiate(env,instance);
 *
 * Recovery:
 * std::shared_ptr<Object> obj = SmartPointerWrapper<Object>::object(env,instance);
 *
 * or
 *
 * SmartPointerWrapper<Object> wrapper = SmartPointerWrapper<Object>::get(env,instance);
 * std::shared_ptr<Object> obj = wrapper->get();
 *
 * Dispose:
 * SmartPointerWrapper<Object> wrapper = SmartPointerWrapper<Object>::get(env,instance);
 * delete wrapper;
 *
 * or simpler
 *
 * SmartPointerWrapper<Object>::dispose(env,instance);
 */
template <typename T>
class SmartPointerWrapper {
    std::shared_ptr<T> mObject;
public:
    template <typename ...ARGS>
    explicit SmartPointerWrapper(ARGS... a) {
        mObject = std::make_shared<T>(a...);
    }

    explicit SmartPointerWrapper (std::shared_ptr<T> obj) {
        mObject = obj;
    }

    virtual ~SmartPointerWrapper() noexcept = default;

    void instantiate (JNIEnv *env, jobject instance) {
        setHandle<SmartPointerWrapper>(env, instance, this);
    }

    jlong instance() const {
        return reinterpret_cast<jlong>(this);
    }

    std::shared_ptr<T> get() const {
        return mObject;
    }

    static std::shared_ptr<T> object(JNIEnv *env, jobject instance) {
        return get(env, instance)->get();
    }

    static SmartPointerWrapper<T> *get(JNIEnv *env, jobject instance) {
        return getHandle<SmartPointerWrapper<T>>(env, instance);
    }

    static void dispose(JNIEnv *env, jobject instance) {
        auto obj = get(env,instance);
        delete obj;
        setHandle<SmartPointerWrapper>(env, instance, nullptr);
    }
};

 

Here it is. 

Use it this way: to instantiate, create the object and call instantiate(env,obj):

SmartPointerWrapper<Object> obj = new SmartPointerWrapper<Object>(arguments);
obj->instantiate(env,instance);

To recover the object smart pointer, use object():

SmartPointerWrapper<Object> wrapper = SmartPointerWrapper<Object>::get(env,instance);
std::shared_ptr<Object> obj = wrapper->get();

And to dispose/destroy:

SmartPointerWrapper<Object> wrapper = SmartPointerWrapper<Object>::get(env,instance);
delete wrapper;

// or

SmartPointerWrapper<Object>::dispose(env,instance);

 

Of course dispose must be called explicitly from java (do not use it from finalize()).

 

Happy Coding!

Permanent link to this article: http://www.studiofuga.com/2017/03/10/a-c-smart-pointer-wrapper-for-use-with-jni/

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