Question

我需要通过JNI从C ++调用一些Java方法。我的JNI实施基于
Is it possible to make a Java JNI which calls jdbc?

我的Java项目中有两个java文件。一种是定义一个类，另一种是包含c ++将调用的实际方法。

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Visual Studio 2010中创建了C ++ DLL。有JVM.cpp，JVM.h，JavaCalls.h和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 ++使用DLL调用Java methodA和methodB：

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，希望它可用于C ++是行不通的。但是我该怎么做才能将Java Object保存在C ++中并稍后将其传递给Java呢？我应该使用GetObjectField创建一个C ++结构并将Java Object字段映射到它吗？

Answer 1

我不太明白你的代码中需要void**的原因。如果要使界面不透明，只需使用void*即可。另外，不要忘记在返回的NewGlobalRef()上调用DeleteGlobalRef()和jobject - 这样可以防止垃圾收集器将其破坏：

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

有一个名为javah.exe的好助手程序。

编写您喜欢的代码，然后javah执行.class。它将为您创建一个完全兼容的.h文件。

你将成为这个：

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

您只需将callback调用到第3个c ++ - 参数。

JNI：将对象从Java返回到C ++，并将其传递回Java

JavaCalls.h

JavaCalls.cpp

2 个答案: