Marko Gargenta
Marakana, Inc.
Member since Jan 19, 2007
Location: San Francisco
Posts: 95
Location: San Francisco
Posts: 95
JNI is an interface that allows Java to interact with code written in another language. Motivation for JNI is code reusability and performance. WIth JNI, you can reuse existing/legacy code with Java (mostly C/C++). In terms of performance, native code used to be up to 20 times faster than Java, when running in interpreted mode. Modern JIT compilers (HotSpot) make this a moot point.
JNI can also be used to invoke Java code from within natively-written applications - such as those written in C/C++.
In fact, the
JNI Components
JNI data type mapping in variables
JNI Development - Java Part
1. Create a Java class with native method(s):
2. Load the library which implements the method:
3. Invoke the native method from Java
For example, our Java code could look like this:
The method
JNI Development - C Part
1. We use the JDK
2. We then create
The file
The file
Compiling
We are now ready to compile our program and run it. The compilation is system-dependent. This will create
For example, to compile Hello.c on Linux do:
Then set the
Finally, run your application:
Common mistakes resulting in
JNI can also be used to invoke Java code from within natively-written applications - such as those written in C/C++.
In fact, the
java command-line utility is an example of one such application, that launches Java code in a Java Virtual Machine.
JNI Components
javah is a JDK tool that builds C-style header files from a given Java class that includes native methods. It adapts Java method signatures to native function prototypesjni.h is a C/C++ header file included with the JDK that maps Java types to their native counterparts. javah automatically includes this file in the application header files.JNI data type mapping in variables
Code:
boolean jboolean
byte jbyte
char jchar
double jdouble
float jfloat
int jint
long jlong
short jshort
void void
JNI Development - Java Part
1. Create a Java class with native method(s):
public native void sayHi(String who, int times);2. Load the library which implements the method:
System.loadLibrary("HelloImpl");3. Invoke the native method from Java
For example, our Java code could look like this:
Code:
public class Hello {
public native void sayHi(String who, int times);
static { System.loadLibrary("HelloImpl"); }
public static void main (String[] args) {
Hello hello = new Hello();
hello.sayHi(args[0], Integer.parseInt(args[1]));
}
}
The method
sayHi will be implemented in C/C++ in separate file(s), which will be compiled into a library. The library filename will be called libHelloImpl.so (on Unix) or HelloImpl.dll (on Windows), but when loaded in Java, the library has to be loaded as HelloImpl.JNI Development - C Part
1. We use the JDK
javah utility to generate the header file Hello.h with a function prototype for sayHi method:
Code:
javac Hello.java
javah -jni Hello
2. We then create
Hello.c to implement the sayHi function.The file
Hello.h looks like:
Code:
...
#include <jni.h>
...
JNIEXPORT void JNICALL Java_Hello_sayHi (JNIEnv *, jobject, jstring, jint);
...
The file
Hello.c looks like:
Code:
#include <stdio.h>
#include "Hello.h" // generated by javah
JNIEXPORT void JNICALL Java_Hello_sayHi
(JNIEnv *env, jobject obj, jstring who, jint times) {
jint i;
jboolean iscopy;
const char *name;
name = (*env)->GetStringUTFChars(env, who, &iscopy);
for (i = 0; i < times; i++) {
printf("Hello %s\n", name);
}
}
Compiling
We are now ready to compile our program and run it. The compilation is system-dependent. This will create
libHelloImpl.so or HelloImpl.dll (depending on the O/S). Set LD_LIBRARY_PATH to point to the directory where the compiled library is stored. Run your Java application.For example, to compile Hello.c on Linux do:
Code:
gcc -o libHelloImpl.so -lc -shared \
-I/usr/local/jdk1.6.0_03/include \
-I/usr/local/jdk1.6.0_03/include/linux Hello.c
Then set the
LD_LIBRARY_PATH to the current working directory: export LD_LIBRARY_PATH=.Finally, run your application:
Code:
java Hello Student 5
Hello Student
Hello Student
Hello Student
Hello Student
Hello Student
Common mistakes resulting in
java.lang.UnsatisfiedLinkError usually come from incorrect naming of the shared library (O/S- dependent), the library not being in the search path, or wrong library being loaded by Java code.
Edited one time. Last edit by Marko Gargenta on Jun 14, 2010 at 10:41:38 PM (about 11 weeks ago).
