Get java array from c++ via JNI

Question

So, I want to obtain resultNative from c++ but resultNative is in Java. can anybody please show me how to do that via JNI? I am not so familiar with c++ and have searched google for a long time but didn't find any answer. thank you so much. the nativeResult is here in Java.

public String[] searchDatabase()
{
    String result[]=new String[6];
    String nativeResult[]=new String[2];
    tName=this.getTargetName();

    result=da.SearchKorea(tName,ctx);
    tType=result[2];
    tTrans=result[3];
    tImage=result[4];
    tFave=result[5];

    nativeResult[1]= tImage+" - "+tName;
    nativeResult[2]= tTrans+" ["+tType+"]";
    return nativeResult;
}

You will have to do a call from C++. Create an instance of a JVM, find searchDatabase, and call it. This is the only way for C++ to CALL a java function. Otherwise, you'll need to call the C++ function from java and pass it the result of searchDatabase. — Brandon
– Brandon, Commented Jan 11, 2014 at 23:51
It is not clear from your text that you want to receive an array of Strings from Java for use in C code; would you like to edit your question? Actually, there are two different scenarios: provide String[] parameter to a C function from Java, or use searchDatabase() method as a callback from C code. — Alex Cohn
– Alex Cohn, Commented Jan 19, 2014 at 9:13

Mark Vincze · Accepted Answer · 2014-01-19 08:19:46Z

First you have to get a reference to the class providing the method. Let's say your class is called MyClass and it is in package p. You get the reference to the class like this:

// You get the JNIEnv* pointer when calling a native function.
jclass myClass = env->FindClass("p/MyClass");

Or if you have a reference to the java object then you can also use GetObjectClass:

jclass myClass = env->GetObjectClass(javaObject);

Then you have to get the ID of the method you'd like to call by providing the name of the method and a string describing the signature of the method.

"()[java/lang/String;" describes a method expecting no arguments and returning a string array.
jmethodID methodID = env->GetMethodID(myClass , "searchDatabase", "()[java/lang/String;");

Then you have to call the method with JNIEnv::CallObjectMethod, and here you have to pass the referenc to the java object.

jobjectarray strings = env->CallObjectMethod(javaObject, methodID);

Then you can get an element of the array with GetObjectArrayElement.

int index = 0;
jstring string = env->GetObjectArrayElement(strings, index);

And then you can get the native string from it in various ways.

const char* nativeChars = env->GetStringUTFChars(string, nullptr);

You can find more information about JNI here, and details about JNI type signatures here.

martyj · Accepted Answer · 2014-01-11 17:31:13Z

0

I believe you could do it by passing back a jobjectArray.

int elements = 5;
jobjectArray returnArray = (jobjectArray)env->NewObjectArray(elements, env->FindClass("java/lang/string"), env->NewStringUTF(""));
for(int i = 0; i < elements; i++)
{
    char str[12];
    sprintf(str, "%i", i);
    env->SetObjectArrayElment(returnArray, i, env->NewStringUTF(str));
}

return returnArray;

I've only done this with a byte array

answered Jan 11, 2014 at 17:31

martyj

141 bronze badge

2 Comments

user3158109 Over a year ago

I mean, my c++ needs the nativeResult from java, how can I make a function in c++ to get the nativeResult from java?

martyj Over a year ago

Your JNI code would take in a jobjectArray as one of its parameters. You can use the env->GetObjectArrayElement to get a single jobject item. Cast it to a jString and convert it to a c string with env->GetStringUTFChars(env, STRING_VAR_HERE);

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