Learning JNI(一）:
one good webpage:http://marakana.com/bookshelf/java_fundamentals_tutorial/_java_native_interface_jni.html
I'm using eclipse.
So first I need to add javah for eclipse.
run -> external tools - > external tools configuarations -> Program.
Add Name:javah_JNI_gen
Location: /usr/lib/jvm/java-1.6.0-openjdk/bin/javah
Working Directory:
${workspace_loc:/${project_name}/bin}
Arguments:
-classpath ${project_classpath} -v -d ${workspace_loc:/${project_name}/src} ${java_type_name}
Then write some java code, such as:
package simple;
public class Hello {
public native void sayHi(String who, int times); //1
static { System.loadLibrary("HelloImpl"); } //2
public static void main (String[] args) {
Hello hello = new Hello();
hello.sayHi("Ying", 2); //3
}
}
Choose Run -> external tools -> javah***, then it will create a file simple_Hello.h in bin/
Now you can write a simple_Hello.c
#include <stdio.h>
#include "simple_Hello.h"
JNIEXPORT void JNICALL Java_simple_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);
}
}
compile:
gcc -o libHelloImpl.so -lc -shared \
-I/usr/lib/jvm/java-6-openjdk/include \
-I/usr/lib/jvm/java-6-openjdk/include/linux simple_Hello.c -fPIC
Note that in my case, nod adding -fPIC will have compile mistakes.
Now you can run the java program :)
When you use the *.so file for other project, add it into eclipse Project -> Properties -> Libraries -> choose the bottom JRE System Library,
native library location -> Edit, add the path of the so file, such as:
/home/ying/study/tools/SVM-Light-1.5-to-be-released/jniLib
How to Load a Java Native/Shared Library (.so)
from http://www.chilkatsoft.com/java-loadLibrary-Linux.aspThere are several ways to make it possible for the Java runtime to find and load a native shared library (.so) at runtime. I will list them briefly here, followed by examples with more explanation below.
- Call System.load to load the .so from an explicitly specified absolute path.
- Copy the shared library to one of the paths already listed in java.library.path
- Modify the LD_LIBRARY_PATH environment variable to include the directory where the shared library is located.
- Specify the java.library.path on the command line by using the -D option.
1. Call System.load to load the shared library from an explicitly specified absolute path.
This choice removes all uncertainty, but embeds a hard-coded path within your Java application. Example:import com.chilkatsoft.CkZip;
public class Test {
static {
try {
System.load("/home/joe/chilkatJava/libchilkat.so");
} catch (UnsatisfiedLinkError e) {
System.err.println("Native code library failed to load.\n" + e);
System.exit(1);
}
}
public static void main(String argv[])
{
CkZip zip = new CkZip();
System.out.println(zip.version());
}
}
2. Copy the shared library to one of the paths already listed in java.library.path
To view the paths listed in java.library.path, run this Java code:String property = System.getProperty("java.library.path");
StringTokenizer parser = new StringTokenizer(property, ";");
while (parser.hasMoreTokens()) {
System.err.println(parser.nextToken());
}
Note: The java.library.path is initialized from the LD_LIBRARY_PATH environment variable.
The loadLibrary method may be used when the directory containing the shared library is in java.library.path. To load "libchilkat.so", call System.loadLibrary("chilkat"), as shown below.
import com.chilkatsoft.CkZip;
public class Test {
static {
try {
System.loadLibrary("chilkat");
} catch (UnsatisfiedLinkError e) {
System.err.println("Native code library failed to load.\n" + e);
System.exit(1);
}
}
public static void main(String argv[])
{
CkZip zip = new CkZip();
System.out.println(zip.version());
}
}
3. Modify the LD_LIBRARY_PATH environment variable to include the path where the Chilkat shared library is located.
For Bourne Shell, K Shell or Bash, type:export LD_LIBRARY_PATH=/home/joe/chilkatJava-9.1.1-x86_64-linux:$LD_LIBRARY_PATHFor C Shell, type:
setenv LD_LIBRARY_PATH "/home/joe/chilkatJava-9.1.1-x86_64-linux:$LD_LIBRARY_PATH"
4. Specify the java.library.path on the command line by using the -D option.
For example:java -Djava.library.path=".:/home/joe/chilkatJava-9.1.1-x86_64-linux" TestApp
