问题标签用户 C++ 调用Jni 返回json字符串时产生的问题，穿入参数后返回的字符串是空的

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问题标签用户 C++ 调用Jni 返回json字符串时产生的问题，穿入参数后返回的字符串是空的20

java代码如下：返回一个json字符串。

package com.iscas.test;
import java.util.ArrayList;
import com.google.gson.Gson;
import net.sf.json.JSONSerializer;
public class Test {
public String getItemList(int id,String name,float score)
{
ItemList list = new ItemList();
Item item = new Item();
item.id = id;
item.name = name;
item.score = score;

list.itemList = new ArrayList();
list.itemList.add(item);
list.itemList.add(item);
list.itemList.add(item);

list.num = 3;

Gson gson = new Gson();
String json = gson.toJson(list).toString();

return json;
}
}

穿入参数，返回json字符串，相应的C++ 调用jni代码如下：

#include <jni.h>
#include <iostream>
#include <string>
#include "windows.h"
using namespace std;

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#else
#define PATH_SEPARATOR ':'
#endif

jstring NewJString(JNIEnv *env, LPCTSTR str); 
string  JStringToCString (JNIEnv *env, jstring str); 


int main()
{
	JavaVMOption options[3];
	JNIEnv *env;
	JavaVM *jvm;
	JavaVMInitArgs vm_args;
	long status;
	jclass test,item,itemList;
	jmethodID mid;


	options[0].optionString = "-Djava.compiler=NONE"; 
	options[1].optionString = "-Djava.class.path=."; 
	options[2].optionString = "-verbose:jni";	//用于跟踪运行时的信息

	memset(&vm_args, 0, sizeof(vm_args));
	vm_args.version = JNI_VERSION_1_6;
	vm_args.nOptions = 1;
	vm_args.options = options;
	status = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);

	if (status != JNI_ERR)
	{
		test = env->FindClass("com/iscas/test/Test");
		item = env->FindClass("com/iscas/test/Item");
		itemList = env->FindClass("com/iscas/test/ItemList");

		if(test !=0 && item!=0)
		{   
			mid = env->GetMethodID(test, "getItemList", "(ILjava/lang/String;F)Ljava/lang/String;");

			if(mid != 0)
			{  
				
			       jobject obj_test = env->AllocObject(test);
			       jobject obj_item = env->AllocObject(item);
			       mid = env->GetMethodID(test,"getItemList","(ILjava/lang/String;F)Ljava/lang/String;);
				jint instance_id = 10;
				const char arg[] = "jom";
				jstring instance_name = NewJString(env, arg);
				jfloat instance_score = 98.5;

				
		                 jstring s = (jstring)env->CallObjectMethod(obj_test,mid,instance_id,instance_name,instance_score);
				cout<<s<<endl;
				string ss = JStringToCString(env,s);
				cout<<ss<<endl;


			}

		}

			jvm->DestroyJavaVM();
			return 0;
		}
	else
	{
		return -1;
	}

}


jstring NewJString(JNIEnv *env, LPCTSTR str)  
{  
  ......
}  

string  JStringToCString (JNIEnv *env, jstring str)// (jstring str, LPTSTR desc, int desc_len)  
{  
    ......
}

PS1：在工程目录下 com/iscas/test/ 路径下放置着 Test.class，Item.class，ItemList.class以及 gson-2.2.4.jar，jdom-2.0.5.jar，json-lib-2.4-jdk15.jar。
PS2：通过设置断点来看，
jstring s = (jstring)env->CallObjectMethod(obj_test,mid,instance_id,instance_name,instance_score);
这句话穿入的参数是正确的，返回的却为空。
还请各位不吝赐教！！