如何通过JNI将HashMap从Java发送到C [英] How to send a HashMap from Java to C via JNI

问题描述

我有一个 Object ，它有一个 HashMap 字段。当 Object 传递给C时，我如何访问该字段？

对象的 Class 包含以下字段：

  private String hello; 
私人地图< String，String> params = new HashMap< String，String>（）; 
  

解决方案

您的问题的答案可以归结为为什么你想要将 Map 传递给C，而不是迭代Java中的 Map 并且将内容传递给C.但是，我是谁来质疑为什么？

你问如何访问 HashMap （在你提供的代码中， Map ）字段？在Java中为它写一个访问器方法，并在你传递容器 Object 时调用C的访问器方法。下面是一些简单的示例代码，展示了如何将Java中的 Map 传递给C，以及如何访问 size（） Map 的方法。从它，你应该能够推断如何调用其他方法。

容器对象：

  public class Container {
 
 private String hello; 
私人地图< String，String> parameterMap = new HashMap< String，String>（）; 
 
 public Map< String，String> getParameterMap（）{
 return parameterMap; 
 
 
 
 
 
 
 $ b 
将Container传递给JNI的主类： p> 
 
 
  public class MyClazz {
 
 public doProcess（）{
 
 Container container = new Container（）; 
 container.getParameterMap（）。put（foo，bar）; 
 
 manipulateMap（container）; 
} 
 
 public native void manipulateMap（Container container）; 
 
 $ / code> 
相关C函数：
 
  JNIEXPORT jint JNICALL Java_MyClazz_manipulateMap（JNIEnv * env，jobject selfReference，jobject jContainer）{
 
 //初始化Container类
 jclass c_Container =（* env） - > GetObjectClass（env，jContainer）; 
 
 //初始化Container类的Get Parameter Map方法
 jmethodID m_GetParameterMap =（* env） - > GetMethodID（env，c_Container，getParameterMap，（）Ljava / util /地图;）; 
 
 //调用所述方法将参数映射存储在jParameterMap中$ b $ jobject jParameterMap =（* env） - > CallObjectMethod（env，jContainer，m_GetParameterMap）; 
 
 //初始化Map接口
 jclass c_Map = env-> FindClass（java / util / Map）; 
 
 //初始化Map 
的Get Size方法jmethodID m_GetSize =（* env） - > GetMethodID（env，c_Map，size，（）I）; 
 
 //获取大小并将其存储在jSize中; jSize的值应该是1 
 int jSize =（* env） - > CallIntMethod（env，jParameterMap，m_GetSize）; 
 
 //定义您需要的其他方法。 
} 
  

值得注意的是，我并没有在方法中初始化methodID和类本身; 这个答案显示了如何将它们缓存以供重用。

I have an Object that has a HashMap field. When the Object is passed to C, how can I access the field?

The Object's Class has the following fields:

private String hello;
private Map<String, String> params = new HashMap<String, String>();

解决方案

The answer to your question really boils down to why you'd want to pass a Map to C rather than iterate your Map in Java and pass the contents to C. But, who am I to question why?

You ask how to access the HashMap (in your provided code, Map) field? Write an accessor method for it in Java and call that accessor method from C when you pass the container Object. Below is some bare-bones sample code showing how to pass a Map from Java to C, and how to access the size() method of the Map. From it, you should be able to extrapolate how to call other methods.

Container Object:

public class Container {

    private String hello;
    private Map<String, String> parameterMap = new HashMap<String, String>();

    public Map<String, String> getParameterMap() {
        return parameterMap;
    }
}

Master Class which passes a Container to JNI:

public class MyClazz {

    public doProcess() {

        Container container = new Container();
        container.getParameterMap().put("foo","bar");

        manipulateMap(container);
    }

    public native void manipulateMap(Container container);
}

Relevant C function:

JNIEXPORT jint JNICALL Java_MyClazz_manipulateMap(JNIEnv *env, jobject selfReference, jobject jContainer) {

    // initialize the Container class
    jclass c_Container = (*env)->GetObjectClass(env, jContainer);

    // initialize the Get Parameter Map method of the Container class
    jmethodID m_GetParameterMap = (*env)->GetMethodID(env, c_Container, "getParameterMap", "()Ljava/util/Map;");

    // call said method to store the parameter map in jParameterMap
    jobject jParameterMap =  (*env)->CallObjectMethod(env, jContainer, m_GetParameterMap);

    // initialize the Map interface
    jclass c_Map = env->FindClass("java/util/Map");

    // initialize the Get Size method of Map
    jmethodID m_GetSize = (*env)->GetMethodID(env, c_Map, "size", "()I");

    // Get the Size and store it in jSize; the value of jSize should be 1
    int jSize = (*env)->CallIntMethod(env, jParameterMap, m_GetSize);

    // define other methods you need here.
}

Of note, I'm not crazy about initializing methodIDs and classes in the method itself; this SO Answer shows you how to cache them for re-use.

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