如何通过C结构来回的Java code在JNI？ [英] How to pass C structs back and forth to Java code in JNI?

问题描述

我已经得到了它，我通过JNI调用一些C函数其中一个指针指向的结构，以及其他一些功能，这将分配/释放一个指向相同类型的结构，使其更容易一点处理我的包装。出人意料的是，JNI文件说，甚少如何处理C结构。

I've got some C functions which I am calling through JNI which take a pointer to a structure, and some other functions which will allocate/free a pointer to the same type of structure so that it is a bit easier to deal with my wrapper. Surprisingly, the JNI documentation says very little about how to deal with C structures.

我的C头文件看起来像这样：

My C header file looks like so:

typedef struct _MyStruct {
  float member;
} MyStruct;

MyStruct* createNewMyStruct();
void processData(int *data, int numObjects, MyStruct *arguments);

相对应的JNI C包装文件包含：

The corresponding JNI C wrapper file contains:

JNIEXPORT jobject JNICALL
Java_com_myorg_MyJavaClass_createNewMyStruct(JNIEnv *env, jobject this) {
  return createNewMyStruct();
}

JNIEXPORT void JNICALL
Java_com_myorg_MyJavaClass_processData(JNIEnv *env, jobject this, jintArray data,
                                       jint numObjects, jobject arguments) {
  int *actualData = (*env)->GetIntArrayElements(env, data, NULL);
  processData(actualData, numObjects, arguments);
  (*env)->ReleaseIntArrayElements(env, data, actualData, NULL);
}

...最后，相应的Java类：

...and finally, the corresponding Java class:

public class MyJavaClass {
  static { System.loadLibrary("MyJniLibrary"); }

  private native MyStruct createNewMyStruct();
  private native void processData(int[] data, int numObjects, MyStruct arguments);

  private class MyStruct {
    float member;
  }

  public void test() {
    MyStruct foo = createNewMyStruct();
    foo.member = 3.14159f;
    int[] testData = new int[10];
    processData(testData, 10, foo);
  }
}

不幸的是，code击中后立即崩溃的JVM createNewMyStruct（）。我是一个有点新的JNI，而且不知道这个问题可能是。

Unfortunately, this code crashes the JVM right after hitting createNewMyStruct(). I'm a bit new to JNI and have no idea what the problem could be.

修改：我应该注意到，C code是非常香草C，是良好的测试，并从工作iPhone项目被移植。此外，该项目采用了Android NDK框架，它可以让你从Android项目从内部JNI运行原生的C code。不过，我不认为这是严格意义上的NDK问题......好像我的一个JNI设置/初始化错误。

Edit: I should note that the C code is very vanilla C, is well-tested and was ported from a working iPhone project. Also, this project is using the Android NDK framework, which lets you run native C code from an Android project from within JNI. However, I don't think that this is strictly an NDK issue... it seems like a JNI setup/initialization error on my part.

推荐答案

您需要创建具有相同成员的C结构的Java类，并通过方法在C code'地图'他们env-> GetIntField ，env-> SetIntField，env-> GetFloatField，env-> SetFloatField，等等 - 总之，大量的手工劳动，希望有已经存在，可以自动做计划：JNAerator（的 HTTP：//$c$c.google.com/p/jnaerator ）和SWIG（的 http://www.swig.org/ ）。两者都有自己的利弊，选择权在你。

You need to create a Java class with the same members as C struct, and 'map' them in the C code via methods env->GetIntField, env->SetIntField, env->GetFloatField, env->SetFloatField, and so on - in short, lots of manual labor, hopefully there already exist programs that do it automatically: JNAerator (http://code.google.com/p/jnaerator) and SWIG (http://www.swig.org/). Both have their pros and cons, the choice is up to you.

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