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Native From Java: Java code example
|
JavaFAQ Home » Java Code Examples
Native call from Java: These samples working illustrate how to call a native library from within Java.
It also includes a sample that attempts a simple performance (speed) comparison between the pure native, pure Java and JNI implementation.
You can find two versions:
- Native call from Java program to C program and
- Native call from Java program to C++ program
C version:
JNISamples.c
| Code: |
#include <jni.h>
#include "JNISamples.h"
#include <stdio.h>
static void printString(JNIEnv *env, jstring jstr);
JNIEXPORT void JNICALL Java_JNISamples_syncNativeMethod (JNIEnv *env, jobject obj, jobject syncObj)
{
printf("Inside synchronized native method\n");
(*env)->MonitorEnter(env, syncObj);
printf("After synchronizing syncObj\n");
(*env)->MonitorExit(env, syncObj);
printf("After releasing syncObj\n");
/*
The functions Object.wait, Object.notify, and Object.notifyAll provide
another useful thread synchronization mechanism. While the JNI does not
directly support these functions, a native method can always follow the
JNI method call mechanism to invoke them.
*/
}
JNIEXPORT void JNICALL Java_JNISamples_nativeCatchThrow (JNIEnv *env, jobject obj)
{
jclass cls = (*env)->GetObjectClass(env, obj);
jmethodID mid = (*env)->GetMethodID(env, cls, "javaCatchThrow", "()V");
jthrowable exc;
jclass newException;
if(0 == mid)
{
printf("Error obtaining method id\n");
return;
}
(*env)->CallVoidMethod(env, obj, mid);
exc = (*env)->ExceptionOccurred(env);
if(exc)
{
/*
We don't do much with the exception, except that we print a debug message
using ExceptionDescribe, clear it, and throw a new exception.
*/
/*
It is extremely important to check, handle, and clear the pending exception
before calling subsequent JNI functions. Calling arbitrary JNI functions with
a pending exception may lead to unexpected results. You can safely call only
a small number of JNI functions when there is a pending exception. These
functions are ExceptionOccurred, ExceptionDescribe, and ExceptionClear.
*/
(*env)->ExceptionDescribe(env);
(*env)->ExceptionClear(env);
}
newException = (*env)->FindClass(env, "java/lang/IllegalArgumentException");
if (newException == 0)
{
/* Unable to find the new exception class, give up. */
printf("Error finding the new exception class\n");
return;
}
(*env)->ThrowNew(env, newException, "IllegalArgumentException thrown from C code");
}
JNIEXPORT void JNICALL Java_JNISamples_fieldAccess (JNIEnv *env, jobject obj)
{
jfieldID fid;
jstring jstr;
const char *str;
jint iStaticIntValue;
jclass cls = (*env)->GetObjectClass(env, obj);
fid = (*env)->GetStaticFieldID(env, cls, "iStaticIntFld", "I");
if(0 == fid)
{
printf("Error getting fid for iStaticIntFld\n");
return;
}
iStaticIntValue = (*env)->GetStaticIntField(env, cls, fid);
printf("iStaticIntValue=%d\n", iStaticIntValue);
(*env)->SetStaticIntField(env, cls, fid, 200);
fid = (*env)->GetFieldID(env, cls, "sStringFld", "Ljava/lang/String;");
if(0 == fid)
{
printf("Error getting fid for sStringFld\n");
return;
}
jstr = (*env)->GetObjectField(env, obj, fid);
printString(env, jstr);
jstr = (*env)->NewStringUTF(env, "NewString");
(*env)->SetObjectField(env, obj, fid, jstr);
}
JNIEXPORT void JNICALL Java_JNISamples_nativeMethod (JNIEnv *env, jobject obj, jint iDepth)
{
jclass cls = (*env)->GetObjectClass(env, obj);
static jmethodID method = 0;
/*
Method ID is valid only for as long as the class
from which it is derived is not unloaded.
Obtaining a method ID is a relatively expensive operation.
Use NewGlobalRef to create a global ref in native code to
prevent the class from being garbage collected.
You MUST use DeleteGlobalRef to delete the global reference.
Use DeleteLocalRef to delete a local ref to release mem immediately.
*/
if(0 == method)
{
method = (*env)->GetMethodID(env, cls, "callback", "(I)V");
}
if (method == 0)
{
printf("Could not get method callback(I)V\n");
return;
}
printf("In C, depth = %d, about to enter Java\n", iDepth);
(*env)->CallVoidMethod(env, obj, method, iDepth);
printf("In C, depth = %d, back from Java\n", iDepth);
}
JNIEXPORT jint JNICALL Java_JNISamples_stringArrAccess(JNIEnv *env, jobject obj, jobjectArray asArr)
{
int iIndex;
jsize len = (*env)->GetArrayLength(env, asArr);
for(iIndex=0; iIndex < len; iIndex++)
{
jstring jstr = (*env)->GetObjectArrayElement(env, asArr, iIndex);
printString(env, jstr);
/* explicitly releasing to assist garbage collection, though not required */
(*env)->DeleteLocalRef(env, jstr);
}
return len;
}
JNIEXPORT jintArray JNICALL Java_JNISamples_intArrAccess (JNIEnv *env, jobject obj, jintArray aiArr)
{
int i, sum = 0;
jintArray aiRet;
jint *retBody;
jsize len = (*env)->GetArrayLength(env, aiArr);
jint *body = (*env)->GetIntArrayElements(env, aiArr, 0);
/* to copy a region of the array only, use Get/Set<type>ArrayRegion functions */
for (i=0; i<len; i++)
{
sum += body[i];
body[i]++;
}
aiRet = (*env)->NewIntArray(env, len);
retBody = (*env)->GetIntArrayElements(env, aiRet, 0);
for (i=0; i<len; i++)
{
retBody[i] = body[i];
}
(*env)->ReleaseIntArrayElements(env, aiArr, body, 0);
(*env)->ReleaseIntArrayElements(env, aiRet, retBody, 0);
return aiRet;
}
JNIEXPORT jstring JNICALL Java_JNISamples_stringAccess (JNIEnv *env, jobject obj, jstring prompt)
{
char buf[128];
const char * str = (*env)->GetStringUTFChars(env, prompt, 0);
printf("Inside Native: %s\n", str);
sprintf(buf, "Hello [%s][%d][%d]",
str,
(*env)->GetStringUTFLength(env, prompt),
(*env)->GetStringLength(env, prompt));
(*env)->ReleaseStringUTFChars(env, prompt, str);
return (*env)->NewStringUTF(env, buf);
}
static void printString(JNIEnv *env, jstring jstr)
{
const char * str = (*env)->GetStringUTFChars(env, jstr, 0);
printf("%s\n", str);
(*env)->ReleaseStringUTFChars(env, jstr, str);
}
|
JNISamples.h
| Code: |
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JNISamples */
#ifndef _Included_JNISamples
#define _Included_JNISamples
#ifdef __cplusplus
extern "C" {
#endif
/* Inaccessible static: iStaticIntFld */
/*
* Class: JNISamples
* Method: fieldAccess
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_JNISamples_fieldAccess
(JNIEnv *, jobject);
/*
* Class: JNISamples
* Method: intArrAccess
* Signature: ([I)[I
*/
JNIEXPORT jintArray JNICALL Java_JNISamples_intArrAccess
(JNIEnv *, jobject, jintArray);
/*
* Class: JNISamples
* Method: nativeCatchThrow
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_JNISamples_nativeCatchThrow
(JNIEnv *, jobject);
/*
* Class: JNISamples
* Method: nativeMethod
* Signature: (I)V
*/
JNIEXPORT void JNICALL Java_JNISamples_nativeMethod
(JNIEnv *, jobject, jint);
/*
* Class: JNISamples
* Method: stringAccess
* Signature: (Ljava/lang/String;)Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_JNISamples_stringAccess
(JNIEnv *, jobject, jstring);
/*
* Class: JNISamples
* Method: stringArrAccess
* Signature: ([Ljava/lang/String;)I
*/
JNIEXPORT jint JNICALL Java_JNISamples_stringArrAccess
(JNIEnv *, jobject, jobjectArray);
/*
* Class: JNISamples
* Method: syncNativeMethod
* Signature: (Ljava/lang/Object;)V
*/
JNIEXPORT void JNICALL Java_JNISamples_syncNativeMethod
(JNIEnv *, jobject, jobject);
#ifdef __cplusplus
}
#endif
#endif
|
JNISamples.java
| Code: |
public class JNISamples
{
private native String stringAccess(String sStr);
private native int [] intArrAccess(int [] aiInp);
private native int stringArrAccess(String [] asInp);
private native void nativeMethod(int iDepth);
private synchronized native void syncNativeMethod(Object oSync);
private native void fieldAccess();
private native void nativeCatchThrow() throws IllegalArgumentException;
static
{
System.loadLibrary("JNISamples");
}
static int iStaticIntFld;
String sStringFld;
public static void main(String [] args)
{
JNISamples sample = new JNISamples();
System.out.println("String access test");
sample.doString();
System.out.println("int array test");
sample.doIntArray();
System.out.println("String array test");
sample.doStrArray();
System.out.println("Callback test");
sample.callback(0);
System.out.println("Field access test");
sample.doFldAccess();
System.out.println("Native exception catch and throw test");
try
{
sample.nativeCatchThrow();
}
catch(IllegalArgumentException iae)
{
iae.printStackTrace();
}
System.out.println("Synchronized Native Methods");
sample.syncNativeMethod(new Object());
System.out.println("Success!");
}
private void javaCatchThrow() throws NullPointerException
{
throw new NullPointerException("Null pointer exception thrown in javaCatchThrow");
}
private void doFldAccess()
{
iStaticIntFld = 100;
sStringFld = "sStringFld";
fieldAccess();
System.out.println("In Java");
System.out.println("iStaticIntFld = " + iStaticIntFld);
System.out.println("sStringFld = " + sStringFld);
}
private void doString()
{
String sRet = stringAccess("String test");
System.out.println(sRet);
}
private void doStrArray()
{
String [] asInp = new String[10];
int iIndex, iLen;
for(iIndex=0; iIndex<10; iIndex++)
{
asInp[iIndex] = "String: " + Integer.toString(iIndex);
}
iLen = stringArrAccess(asInp);
System.out.println("String array size: " + iLen);
}
private void doIntArray()
{
int [] aiInp = new int[10];
int iIndex;
for(iIndex=0; iIndex<10; iIndex++)
{
aiInp[iIndex] = iIndex;
}
int [] aiOut = intArrAccess(aiInp);
System.out.print("Int array: {");
for(iIndex=0; iIndex<10; iIndex++)
{
System.out.print(aiInp[iIndex] + ", ");
}
System.out.println("}");
}
private void callback(int iDepth)
{
System.out.println("In Java callback. Depth=" + iDepth);
if(5 > iDepth)
{
nativeMethod(iDepth + 1);
}
else
{
System.out.println("Stopping recursion");
}
}
}
|
makefile
| Code: |
#SOLARIS
#---------------
#INCFLAGS=-I/usr/java1.2/include -I/usr/java1.2/include/solaris
#CC=cc
#CXX=CC
#LINK=ld
#KPIC_COMPILE=-KPIC
#KPIC_LINK=-G
#DIGITAL OSF
#---------------
INCFLAGS=-I/usr/opt/java122/include -I/usr/opt/java122/include/alpha -pthread
CC=cc
CXX=cxx
LINK=cc
KPIC_COMPILE=
KPIC_LINK=-shared
all : libJNISamples.so JNISamples.class
echo "All done"
JNISamples.class : JNISamples.java
javac JNISamples.java
JNISamples.h : JNISamples.class
javah -jni JNISamples
libJNISamples.so : JNISamples.o
${LINK} ${KPIC_LINK} -o libJNISamples.so JNISamples.o
JNISamples.o : JNISamples.c JNISamples.h
${CC} ${KPIC_COMPILE} -c JNISamples.c ${INCFLAGS}
|
C++ version:
JNISamples.cpp
| Code: |
#include <jni.h>
#include "JNISamples.h"
#include <iostream.h>
static void printString(JNIEnv *env, jstring jstr);
JNIEXPORT void JNICALL Java_JNISamples_syncNativeMethod (JNIEnv *env, jobject obj, jobject syncObj)
{
cout << "Inside synchronized native method\n";
env->MonitorEnter(syncObj);
cout << "After synchronizing syncObj\n";
env->MonitorExit(syncObj);
cout << "After releasing syncObj\n";
/*
The functions Object.wait, Object.notify, and Object.notifyAll provide
another useful thread synchronization mechanism. While the JNI does not
directly support these functions, a native method can always follow the
JNI method call mechanism to invoke them.
*/
}
JNIEXPORT void JNICALL Java_JNISamples_nativeCatchThrow (JNIEnv *env, jobject obj)
{
jclass cls = env->GetObjectClass(obj);
jmethodID mid = env->GetMethodID(cls, "javaCatchThrow", "()V");
jthrowable exc;
jclass newException;
if(0 == mid)
{
cout << "Error obtaining method id\n";
return;
}
env->CallVoidMethod(obj, mid);
exc = env->ExceptionOccurred();
if(exc)
{
/*
We don't do much with the exception, except that we print a debug message
using ExceptionDescribe, clear it, and throw a new exception.
*/
/*
It is extremely important to check, handle, and clear the pending exception
before calling subsequent JNI functions. Calling arbitrary JNI functions with
a pending exception may lead to unexpected results. You can safely call only
a small number of JNI functions when there is a pending exception. These
functions are ExceptionOccurred, ExceptionDescribe, and ExceptionClear.
*/
env->ExceptionDescribe();
env->ExceptionClear();
}
newException = env->FindClass("java/lang/IllegalArgumentException");
if (newException == 0)
{
/* Unable to find the new exception class, give up. */
cout << "Error finding the new exception class\n";
return;
}
env->ThrowNew(newException, "IllegalArgumentException thrown from C code");
}
JNIEXPORT void JNICALL Java_JNISamples_fieldAccess (JNIEnv *env, jobject obj)
{
jfieldID fid;
jstring jstr;
const char *str;
jint iStaticIntValue;
jclass cls = env->GetObjectClass(obj);
fid = env->GetStaticFieldID(cls, "iStaticIntFld", "I");
if(0 == fid)
{
cout << "Error getting fid for iStaticIntFld\n";
return;
}
iStaticIntValue = env->GetStaticIntField(cls, fid);
cout << "iStaticIntValue=" << iStaticIntValue << "\n";
env->SetStaticIntField(cls, fid, 200);
fid = env->GetFieldID(cls, "sStringFld", "Ljava/lang/String;");
if(0 == fid)
{
cout << "Error getting fid for sStringFld\n";
return;
}
jstr = (jstring)env->GetObjectField(obj, fid);
printString(env, jstr);
jstr = env->NewStringUTF("NewString");
env->SetObjectField(obj, fid, jstr);
}
JNIEXPORT void JNICALL Java_JNISamples_nativeMethod (JNIEnv *env, jobject obj, jint iDepth)
{
jclass cls = env->GetObjectClass(obj);
static jmethodID method = 0;
/*
Method ID is valid only for as long as the class
from which it is derived is not unloaded.
Obtaining a method ID is a relatively expensive operation.
Use NewGlobalRef to create a global ref in native code to
prevent the class from being garbage collected.
You MUST use DeleteGlobalRef to delete the global reference.
Use DeleteLocalRef to delete a local ref to release mem immediately.
*/
if(0 == method)
{
method = env->GetMethodID(cls, "callback", "(I)V");
}
if (method == 0)
{
cout << "Could not get method callback(I)V\n";
return;
}
cout << "In C, depth = " << iDepth << "about to enter Java\n";
env->CallVoidMethod(obj, method, iDepth);
cout << "In C, depth = " << iDepth << ", back from Java\n";
}
JNIEXPORT jint JNICALL Java_JNISamples_stringArrAccess(JNIEnv *env, jobject obj, jobjectArray asArr)
{
int iIndex;
jsize len = env->GetArrayLength(asArr);
for(iIndex=0; iIndex < len; iIndex++)
{
jstring jstr = (jstring)env->GetObjectArrayElement(asArr, iIndex);
printString(env, jstr);
/* explicitly releasing to assist garbage collection, though not required */
env->DeleteLocalRef(jstr);
}
return len;
}
JNIEXPORT jintArray JNICALL Java_JNISamples_intArrAccess (JNIEnv *env, jobject obj, jintArray aiArr)
{
int i, sum = 0;
jintArray aiRet;
jint *retBody;
jsize len = env->GetArrayLength(aiArr);
jint *body = env->GetIntArrayElements(aiArr, 0);
/* to copy a region of the array only, use Get/Set<type>ArrayRegion functions */
for (i=0; i<len; i++)
{
sum += body[i];
body[i]++;
}
aiRet = env->NewIntArray(len);
retBody = env->GetIntArrayElements(aiRet, 0);
for (i=0; i<len; i++)
{
retBody[i] = body[i];
}
env->ReleaseIntArrayElements(aiArr, body, 0);
env->ReleaseIntArrayElements(aiRet, retBody, 0);
return aiRet;
}
JNIEXPORT jstring JNICALL Java_JNISamples_stringAccess (JNIEnv *env, jobject obj, jstring prompt)
{
char buf[128];
const char * str = env->GetStringUTFChars(prompt, 0);
cout << "Inside Native: " << str << "\n";
sprintf(buf, "Hello [%s][%d][%d]",
str,
env->GetStringUTFLength(prompt),
env->GetStringLength(prompt));
env->ReleaseStringUTFChars(prompt, str);
return env->NewStringUTF(buf);
}
static void printString(JNIEnv *env, jstring jstr)
{
const char * str = env->GetStringUTFChars(jstr, 0);
cout << str << "\n";
env->ReleaseStringUTFChars(jstr, str);
}
|
JNISamples.h
| Code: |
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class JNISamples */
#ifndef _Included_JNISamples
#define _Included_JNISamples
#ifdef __cplusplus
extern "C" {
#endif
/* Inaccessible static: iStaticIntFld */
/*
* Class: JNISamples
* Method: fieldAccess
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_JNISamples_fieldAccess
(JNIEnv *, jobject);
/*
* Class: JNISamples
* Method: intArrAccess
* Signature: ([I)[I
*/
JNIEXPORT jintArray JNICALL Java_JNISamples_intArrAccess
(JNIEnv *, jobject, jintArray);
/*
* Class: JNISamples
* Method: nativeCatchThrow
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_JNISamples_nativeCatchThrow
(JNIEnv *, jobject);
/*
* Class: JNISamples
* Method: nativeMethod
* Signature: (I)V
*/
JNIEXPORT void JNICALL Java_JNISamples_nativeMethod
(JNIEnv *, jobject, jint);
/*
* Class: JNISamples
* Method: stringAccess
* Signature: (Ljava/lang/String;)Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_JNISamples_stringAccess
(JNIEnv *, jobject, jstring);
/*
* Class: JNISamples
* Method: stringArrAccess
* Signature: ([Ljava/lang/String;)I
*/
JNIEXPORT jint JNICALL Java_JNISamples_stringArrAccess
(JNIEnv *, jobject, jobjectArray);
/*
* Class: JNISamples
* Method: syncNativeMethod
* Signature: (Ljava/lang/Object;)V
*/
JNIEXPORT void JNICALL Java_JNISamples_syncNativeMethod
(JNIEnv *, jobject, jobject);
#ifdef __cplusplus
}
#endif
#endif
|
JNISamples.java
| Code: |
public class JNISamples
{
private native String stringAccess(String sStr);
private native int [] intArrAccess(int [] aiInp);
private native int stringArrAccess(String [] asInp);
private native void nativeMethod(int iDepth);
private synchronized native void syncNativeMethod(Object oSync);
private native void fieldAccess();
private native void nativeCatchThrow() throws IllegalArgumentException;
static
{
System.loadLibrary("JNISamples");
}
static int iStaticIntFld;
String sStringFld;
public static void main(String [] args)
{
JNISamples sample = new JNISamples();
System.out.println("String access test");
sample.doString();
System.out.println("int array test");
sample.doIntArray();
System.out.println("String array test");
sample.doStrArray();
System.out.println("Callback test");
sample.callback(0);
System.out.println("Field access test");
sample.doFldAccess();
System.out.println("Native exception catch and throw test");
try
{
sample.nativeCatchThrow();
}
catch(IllegalArgumentException iae)
{
iae.printStackTrace();
}
System.out.println("Synchronized Native Methods");
sample.syncNativeMethod(new Object());
System.out.println("Success!");
}
private void javaCatchThrow() throws NullPointerException
{
throw new NullPointerException("Null pointer exception thrown in javaCatchThrow");
}
private void doFldAccess()
{
iStaticIntFld = 100;
sStringFld = "sStringFld";
fieldAccess();
System.out.println("In Java");
System.out.println("iStaticIntFld = " + iStaticIntFld);
System.out.println("sStringFld = " + sStringFld);
}
private void doString()
{
String sRet = stringAccess("String test");
System.out.println(sRet);
}
private void doStrArray()
{
String [] asInp = new String[10];
int iIndex, iLen;
for(iIndex=0; iIndex<10; iIndex++)
{
asInp[iIndex] = "String: " + Integer.toString(iIndex);
}
iLen = stringArrAccess(asInp);
System.out.println("String array size: " + iLen);
}
private void doIntArray()
{
int [] aiInp = new int[10];
int iIndex;
for(iIndex=0; iIndex<10; iIndex++)
{
aiInp[iIndex] = iIndex;
}
int [] aiOut = intArrAccess(aiInp);
System.out.print("Int array: {");
for(iIndex=0; iIndex<10; iIndex++)
{
System.out.print(aiInp[iIndex] + ", ");
}
System.out.println("}");
}
private void callback(int iDepth)
{
System.out.println("In Java callback. Depth=" + iDepth);
if(5 > iDepth)
{
nativeMethod(iDepth + 1);
}
else
{
System.out.println("Stopping recursion");
}
}
}
|
makefile
| Code: |
all : libJNISamples.so JNISamples.class
echo "All done"
JNISamples.class : JNISamples.java
javac JNISamples.java
JNISamples.h : JNISamples.class
javah -jni JNISamples
libJNISamples.so : JNISamples.o
cxx -shared -o libJNISamples.so JNISamples.o
JNISamples.o : JNISamples.cpp JNISamples.h
cxx -c JNISamples.cpp -I/usr/opt/java122/include -I/usr/opt/java122/include/alpha -pthread
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