java JNI 字符串返回值
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JNI String return value
提问by Salvatore
I have a Java instance method which returns a String and I'm calling this method through JNI in C++. I have written the following code:
我有一个 Java 实例方法，它返回一个字符串，我在 C++ 中通过 JNI 调用这个方法。我编写了以下代码：
const char *DiagLayerContainer_getDESC(JNIEnv *env, jobject diagLayer) {
jclass diagLayerClass = env->FindClass(PARSER_CLASS);
jmethodID getDESCDiagLayerMethodID = env->GetMethodID(diagLayerClass, "getDESCDiagLayer", "(Ljava/lang/Object;)Ljava/lang/String;");
jstring returnString = (jstring) env->CallObjectMethod(diagLayer, getDESCDiagLayerMethodID);
return env->GetStringUTFChars(returnString, JNI_FALSE);
}
How do I get the string and convert it to a const char *?
如何获取字符串并将其转换为 const char *？
My program crashes on the last line with access violation to 0x00000000. returnString is not NULL.
我的程序在最后一行崩溃，访问冲突为 0x00000000。returnString 不是 NULL。
回答by Olaf Dietsche
According to GetStringUTFChars, the last parameter is a pointer to jboolean.
根据GetStringUTFChars，最后一个参数是指向 的指针jboolean。
Change
改变
return env->GetStringUTFChars(returnString, JNI_FALSE);
to
到
return env->GetStringUTFChars(returnString, NULL);
Or better yet, return a std::string
或者更好的是，返回一个 std::string
std::string DiagLayerContainer_getDESC(...) {
...
const char *js = env->GetStringUTFChars(returnString, NULL);
std::string cs(js);
env->ReleaseStringUTFChars(returnString, js);
return cs;
}
I've built a similar simple example and the code as is, seems fine so far.
我已经构建了一个类似的简单示例和代码，到目前为止看起来还不错。
Although, there are two possible error sources.
虽然，有两个可能的错误来源。
The first one is the method signature. Try "()Ljava/lang/String;"instead of "(Ljava/lang/Object;)Ljava/lang/String;".
第一个是方法签名。尝试"()Ljava/lang/String;"代替"(Ljava/lang/Object;)Ljava/lang/String;".
The second one is in the java source itself. If the java method returns a null string, CallObjectMethod()will return a NULL jstringand GetStringUTFChars()fails.
第二个在 java 源代码中。如果 java 方法返回空字符串，CallObjectMethod()将返回 NULLjstring并GetStringUTFChars()失败。
Add a
添加一个
if (returnString == NULL)
return NULL;
after CallObjectMethod().
之后CallObjectMethod()。
So look into the java source and see, whether the method getDESCDiagLayer()might return a null string.
因此，查看 java 源代码并查看该方法是否getDESCDiagLayer()可能返回空字符串。
