[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);
}
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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”);
}
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JNI String functions:
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JNI Function
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Description
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GetStringChars
ReleaseStringChars
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Obtains or releases a
pointer to the contents of a string in Unicode format. May return a copy of
the string.
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GetStringUTFChars
ReleaseStringUTFChars
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Obtains or releases a
pointer to the contents of a string in UTF-8 format.
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GetStringLength
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Returns the number of
Unicode characters in the string.
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GetStringUTFLength
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Returns the number of
bytes needed to represent a string in the UTF-8 format.
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NewString
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Creates a
java.lang.String instance that contains the same sequence of characters as
the given Unicode C string.
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NewStringUTF
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Creates a
java.lang.String instance that contains the same sequence of characters as
the given UTF-8 encoded C string.
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GetStringCritical
ReleaseStringCritical
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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.
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GetStringRegion
SetStringRegion
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Copies the contents of
a string to or from a preallocated C buffer in the Unicode format.
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GetStringUTFRegion
SetStringUTFRegion
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Copies the content of
a string to or from a preallocated C buffer in the UTF-8 format.
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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);
}
}
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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
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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());
}
}
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