如何在 Android NDK 上使用 JNI 在 C 和 Java 之间传递

问题描述

我的 Android 应用程序的 C 代码中有一个复杂的结构，我想在 Java 端使用它.我在 google 和 stackoverflow 中进行了一些研究，所以我从我的 C 结构创建了 java 类，但现在如何在 Java 中获取它.

I have a complex strucutre in the C code on my Android application and I would like to use it in the Java side. I've done some research with google and in stackoverflow, so I've created the java class from my C strucutre, but now how to get it in Java.

我找到了这些信息，关于在类中创建一个指针并在 C 端使用它:

I've found these informations, about making a pointer in the class and use this on the C side :

Get the field ID : (*env)->GetFieldID(...)
Get the pointer : (*env)->GetLongField(...)
Set the pointer : (*env)->SetLongField(...)

但我不明白它是如何工作的......

But I don't understand how it really works ...

在上面，你可以找到我到现在为止所做的......不是那么多！C端:

Above, you can find what I've done until now ... not so much ! On the C side :

ComplexStructure Java_com_main_MainActivity_listenUDP(JNIEnv* env, jclass clazz)
{
    int i,taille;
    ComplexStructure myStruct;    
    taille = -1;    
    taille = recvfrom(socket, &myStruct, sizeof(ComplexStructure ), 0, &rcvAddr, &sizeOfSock);
    if(taille != -1)
    {   
        return myStruct;
    }
    return NULL;
}

在 Java 方面:

public void getFromUDP() {

    ComplexClass myClass = new ComplexClass();
    myClass = listenUDP();              
}

@Override
public void run() {
    initUDP();
    getFromUDP();
}


public static native ComplexClass listenUDP();
public static native void initUDP();
public static native void closeUDP();

/** Load jni .so on initialization */
static {
     System.loadLibrary("native-interface");
}

我想补充一点，我的结构非常复杂:

EDIT : I want to add that my structure is very complex like that :

typedef struct{
  TYPE_A myStructA;
  TYPE_B myStructB;
  TYPE_C myStructC;
  TYPE_D myStructD;
}ComplexStructure;

typedef struct{
  float rad;
  int size;
  bool isEmpty;
}TYPE_A;

typedef struct{
  float rad;
  bool isEmpty;
  float color;
  int temp;
}TYPE_B;

typedef struct{
  int temp;
  float rain;
  bool isEmpty;
}TYPE_C;

typedef struct{
  float rad;
  int idPerson;
  bool isOnTime;
}TYPE_D;

更复杂，只是一个例子来告诉你它是怎么回事！

Even more complex, just an example to show you how it is !

推荐答案

您不能将原始 C 结构传递给 Java 并期望它将这些结构视为类.您需要为您的结构创建一个类.我看到你已经这样做了，所以你唯一需要做的就是将此结构转换为类的实例.

You cannot pass raw C structs into Java and expect it to treat these structs as classes. You need to create a class for your struct. I see you already did that, so the only thing you need to do is to convert this struct into an instance of the class.

Java端代码:

public static native ComplexClass listenUDP();

将转换为:

JNIEXPORT jobject JNICALL Java_com_main_MainActivity_listenUDP(JNIEnv *env, jclass);

在该 C 代码中，您需要使用 env->FindClass(); 函数加载 ComplexClass.然后要创建该类的新实例(如果您有零参数构造函数，它会简化问题)，您需要加载构造函数方法签名并在 env->NewObject() 方法.完整代码:

In that C code, you need to load the ComplexClass using the env->FindClass(); function. Then to create a new instance of that class (it simplifies matters if you have zero-parameter constructor), you need to load a constructor method signature and "invoke" it in the env->NewObject() method. Full code:

jclass complexClass = env->FindClass("/com/main/ComplexClass");
jmethod constructor = env->GetMethodId(complexClass, "<init>", "()com/main/ComplexClass"); //The name of constructor method is "<init>"
jobject instance = env->NewObject(complexClass, constructor);

那么你需要使用env->setXXXField();来设置这个类的字段.如果您有更多对象作为字段并想创建它们，则对另一个对象重复上述过程.

Then you need to set the fields of this class using env->setXXXField();. If you have more objects as fields and want to alse create them, then repeat the above process for another object.

这看起来很复杂，但这就是在托管 Java 代码中使用原生 C 的代价.

This looks very complicated, but that's the price for using native C in managed Java code.

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