[Adroid NDK] Passing String JINI

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JNIEXPORT void JNICALLJava_MyJavaClass_printName(JNIEnv *env, jobject obj,  jstring name) {  const char *str= (*env)->GetStringUTFChars(env,name,0);  printf(“%s”, str);  //need to release this string when done with it in  //order to avoid memory leak  (*env)->ReleaseStringUTFChars(env, name, str); }

To convert a C-style string to a jstring , you can use the (*env)->NewStringUTF() function to create a new jstring from a C-style string. For example, a C function that needs to return a Java string could contain the following code:

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JNIEXPORT jstring JNICALLJava_MyJavaClass_getName(JNIEnv *env, jobject obj) {  return (*env)->NewStringUTF(env, “My String”); }

JNI String functions:

JNI Function	Description
GetStringChars ReleaseStringChars	Obtains or releases a pointer to the contents of a string in Unicode format. May return a copy of the string.
GetStringUTFChars ReleaseStringUTFChars	Obtains or releases a pointer to the contents of a string in UTF-8 format.
GetStringLength	Returns the number of Unicode characters in the string.
GetStringUTFLength	Returns the number of bytes needed to represent a string in the UTF-8 format.
NewString	Creates a java.lang.String instance that contains the same sequence of characters as the given Unicode C string.
NewStringUTF	Creates a java.lang.String instance that contains the same sequence of characters as the given UTF-8 encoded C string.
GetStringCritical ReleaseStringCritical	Obtains a pointer to the contents of a string in Unicode format. May return a copy of the string. Native code must not block between a pair of Get/ReleaseStringCritical calls.
GetStringRegion SetStringRegion	Copies the contents of a string to or from a preallocated C buffer in the Unicode format.
GetStringUTFRegion SetStringUTFRegion	Copies the content of a string to or from a preallocated C buffer in the UTF-8 format.

JNI String Example:

Let’s write a JNI application that passes a prompt message to a native method, which method prints the prompt message, reads the user input and sends it back to the application.

The Java code will be(NativePrompt.java):

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class NativePrompt {  private native String getInput(String prompt);  //native method  static   //static initializer code  {   System.loadLibrary("NativePrompt");  }  public static void main(String[] args)  {   NativePrompt NP = new NativePrompt();   String sName = NP.getInput("Enter your name: ");         System.out.println("Hello " + sName);  } }

Compile NativePrompt.java (javac NativePrompt.java) and generate hedder file(javah -jni NativePrompt).

The generated with javah header file NativePrompt.h will be:

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/* DO NOT EDIT THIS FILE - it is machine generated */ #include "jni.h" /* Header for class NativePrompt */ #ifndef _Included_NativePrompt #define _Included_NativePrompt #ifdef __cplusplus extern "C" { #endif /*  * Class:     NativePrompt  * Method:    getInput  * Signature: (Ljava/lang/String;)Ljava/lang/String;  */ JNIEXPORT jstring JNICALL Java_NativePrompt_getInput   (JNIEnv *, jobject, jstring); #ifdef __cplusplus } #endif #endif

The C++ implementation file NativePrompt.cpp will be:

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#include "NativePrompt.h" #include "jni.h" #include "string" #include "iostream" #include "vector" using namespace std; /*  * Class:     NativePrompt  * Method:    getInput  * Signature: (Ljava/lang/String;)Ljava/lang/String;  */ JNIEXPORT jstring JNICALL Java_NativePrompt_getInput  (JNIEnv *env, jobject obj, jstring prompt){  string sEntry;  const char *str;  str = env->GetStringUTFChars(prompt, NULL);  if (str == NULL) {   return env->NewStringUTF("");  }  else{  cout << str;   //Frees native string resources   env->ReleaseStringUTFChars(prompt, str);   //reads n-consecutive words from the   //keyboard and store them in string   getline(cin, sEntry);         return env->NewStringUTF(sEntry.c_str());  } }