如何在JNI中访问从C ++返回java.lang.String的Java方法的返回值？ [英] How do I access return value of a Java method returning java.lang.String from C++ in JNI?

问题描述

我试图从一个从C ++调用的Java方法传回一个字符串。我无法找出我应该调用什么JNI函数来访问该方法并返回一个jstring值。

I am trying to pass back a string from a Java method called from C++. I am not able to find out what JNI function should I call to access the method and be returned a jstring value.

我的代码如下：

C ++ part

main() {
    jclass cls;
    jmethodID mid;
    jstring rv;

/** ... omitted code ... */

    cls = env->FindClass("ClassifierWrapper");
    mid = env->GetMethodID(cls, "getString","()Ljava/lang/String");

    rv = env->CallStatic<TYPE>Method(cls, mid, 0);
    const char *strReturn = env->GetStringUTFChars(env, rv, 0);

    env->ReleaseStringUTFChars(rv, strReturn);
}

Java代码

public class ClassifierWrapper {
    public String getString() { return "TEST";}
}

方法签名（来自javap -s Class）

The Method Signature (from "javap -s Class")

public java.lang.String getString();
  Signature: ()Ljava/lang/String;

推荐答案

你应该有

cls = env->FindClass("ClassifierWrapper"); 

然后你需要调用构造函数来获取一个新对象：

Then you need to invoke the constructor to get a new object:

jmethodID classifierConstructor = env->GetMethodID(cls,"<init>", "()V"); 
if (classifierConstructor == NULL) {
  return NULL; /* exception thrown */
}

jobject classifierObj = env->NewObject( cls, classifierConstructor);

您正在获取静态方法（即使方法名称错误）。但是你需要得到实例方法，因为getString（）不是静态的。

You are getting static method (even though the method name is wrong). But you need to get the instance method since getString() is not static.

jmethodID getStringMethod = env->GetMethodID(cls, "getString", "()Ljava/lang/String"); 

现在调用方法：

rv = env->CallObjectMethod(classifierObj, getStringMethod, 0); 
const char *strReturn = env->GetStringUTFChars(env, rv, 0);

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