Java Native Interface

Java Native Interface
JNI 小記

Overview
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Writing a Java Program with Native Methods

Step 1: Write the Java Code

class HelloWorld
{
  // Declare a Native Method
  public native void jasonmel1();
  public native void jasonmel2(int i, String s);

  // Load the Library
  static
  {
    System.loadLibrary("hello");
  }

  // Write the Main Method
  public static void main( String[] args )
  {
    new HelloWorld().jasonmel1();
    new HelloWorld().jasonmel2(123, new String("Test"));
  }
}

Step 2: Compile the Java Code
```
javac HelloWorld.java
```
Step 3: Create the .h File
```
javah -jni HelloWorld
```

Step 4: Write the Native Method Implementation HelloWorldImp.c:

#include "HelloWorld.h"

JNIEXPORT void JNICALL
Java_HelloWorld_jasonmel1(JNIEnv *env, jobject obj)
{
  printf("jasonmel1!\n");
  return;
}

JNIEXPORT void JNICALL
Java_HelloWorld_jasonmel2(JNIEnv *env, jobject obj, jint i, jstring s)
{
  const char *str = (*env)->GetStringUTFChars(env, s, 0);
  printf("jasonmel2: %d %s!\n", i, str);
  (*env)->ReleaseStringUTFChars(env, s, str);
  return;
}

Step 5: Create a Shared Library
On Solaris:

cc -G -I/usr/local/java/include -I/usr/local/java/include/solaris HelloWorldImp.c -o libhello.so

On MS Windows VC:

cl -Ic:\java\include -Ic:\java\include\win32 -LD HelloWorldImp.c -Fehello.dll

On Linux:

gcc -shared -I/usr/java/jdk1.5.0_07/include/ -I/usr/java/jdk1.5.0_07/include/linux/ HelloWorldImp.c -o libhello.so

On Mac OS X:

gcc -c -I/System/Library/Frameworks/JavaVM.framework/Headers/ HelloWorldImp.c
gcc -dynamiclib -o libhello.jnilib HelloWorldImp.o -framework JavaVM

Step 6: Run the Program
```
java HelloWorld
```
Remember to set up library path!
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