JNI:将对象从java返回C++，并将其传递回java

Question

<>我有一些java方法，我需要通过JNI从C++调用。我的JNI实现基于
Is it possible to make a Java JNI which calls jdbc?

我的java项目中有两个java文件。一种是定义类，另一种包含C++调用的实际方法。p>

public class MyObject {
    private static int no;
    private static LocalDateTime time;
    private static String status;
    // getters, setters and toString
}

public class ObjectHandler {
    public static MyObject objectReturnToC;

    public static Object methodA (type1 arg1, type2 arg2, type3 arg3) {
        objectReturnToC = new MyObject();
        // setting fields in returnObject according to passed-in parameters
        return objectReturnToC;
    }
    public static void methodB(Object objectReturnedFromC) {
        // access fields in objectReturnedFromC, do computation and store in
    }
}

我在VisualStudio 2010中创建了C++ DLL。有JVM。cpp，JVM。h、 JavaCalls。h、 还有JavaCalls。cpp

JavaCalls。h

#ifdef JAVACALLSDLL_EXPORTS
#define JAVACALLSDLL_API __declspec(dllexport) 
#else
#define JAVACALLSDLL_API __declspec(dllimport) 
#endif

namespace JavaCalls
{
    class JavaCalls
    {
    public:
        static JAVACALLSDLL_API void *javaMethodA(type1, type2, type3);
        static JAVACALLSDLL_API string toString(void **javaObject);
        static JAVACALLSDLL_API void javaMethodB(void **javaObject);
    };
}

JavaCalls。cpp

namespace JavaCalls 
{
    void *JavaCalls::javaMethodA(type1 arg1, type2 arg2, type3 arg3) 
    {
        // invoke JVM
        // Find class, methodID
        jobject javaObject = CallStaticObjectMethod(jMain, "methodA",...);
        return javaObject;
    }
    void JavaCalls::javaMethodB(void** javaObject) {
        // invoke JVM
        // Find class, methodID
        CallStaticVoidMethod(jMain, "methodB",...);
    }
}

C++调用java方法A和方法B，使用dll：< /p>

int main() 
{
    void* a = JavaCalls::JavaCalls::javaMethodA(arg1, arg2, arg3);
    // doing other stuff and updating fields in a
    JavaCalls::JavaCalls::javaMethodB(static_cast<void**>(a));
}

显然，传递指针，希望它对C++是可用的，不起作用。但是我应该如何做才能将java对象保存在C++中，并在以后传递给java呢？是否应该使用GETObjeCdfield创建一个C++结构体并将java对象字段映射到它？p>

Answer 1

我不太明白为什么在代码中需要void**。如果想让界面不透明，只需使用void*。另外，不要忘记对返回的jobject调用NewGlobalRef()和DeleteGlobalRef()——这将防止垃圾收集器破坏它：

void *JavaCalls::javaMethodA(type1 arg1, type2 arg2, type3 arg3) 
{
    jobject javaObject = CallStaticObjectMethod(jMain, "methodA",...);
    return NewGlobalRef(jMain, javaObject);
}

void JavaCalls::javaMethodB(void* javaObject) {
     CallStaticVoidMethod(jMain, "methodB", static_cast<jobject>(javaObject));
}

// add this method - it should be called when you finish using the object
void JavaCalls::ReleaseObject(void* javaObject) {
     DeleteGlobalRef(jMain, static_cast<jobject>(javaObject));
}

Answer 2

有一个很棒的助手程序叫做javah.exe

像编写代码一样编写代码，并执行javah到.class。它将为您创建一个完全兼容的.h文件

你会因此变得：

public class Test {

    public class MyObject {
        private int no;
        private String status;
    }

    public class Callback {
        public void callback(MyObject afterCpp) {

        }
    }

    public native void register(Callback v, MyObject beforeCpp);
}

此标题：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Test */

#ifndef _Included_Test
#define _Included_Test
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Test
 * Method:    register
 * Signature: (LTest/Callback;LTest/MyObject;)V
 */
JNIEXPORT void JNICALL Java_Test_register
  (JNIEnv *, jobject, jobject, jobject);

#ifdef __cplusplus
}
#endif
#endif

只需调用callback第三个c++参数即可