Java Native Interface JNI

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Java Native Interface (JNI)

• SENG 609.04 Design Patterns
• Winter, 2003
• Presented by Guy Davis, Ming Zhou

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Agenda

• Overview
• Justification
• Hello World!
• Summary
• References
• QA

3
JNI Overview
4
Interactions with Native Code
Access to Java world from native code
Access to native code from Java
5
Justification

• Pros
• Reuse allows access to useful native code
• Efficiency use best language for the task
• Cons
• Applets doesn't work as they're mobile
• Portability native methods aren't portable
• Extra work javah, create shared native libs

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HelloWorld.java

• class HelloWorld
• public native void displayHelloWorld()
• static
• System.loadLibrary("hello")
• public static void main(String args)
• new HelloWorld().displayHelloWorld()

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HelloWorld.java

• class HelloWorld
• public native void displayHelloWorld()
• static
• System.loadLibrary("hello")
• public static void main(String args)
• new HelloWorld().displayHelloWorld()

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HelloWorld.java

• class HelloWorld
• public native void displayHelloWorld()
• static
• System.loadLibrary("hello")
• public static void main(String args)
• new HelloWorld().displayHelloWorld()

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HelloWorld.java

• class HelloWorld
• public native void displayHelloWorld()
• static
• System.loadLibrary("hello")
• public static void main(String args)
• new HelloWorld().displayHelloWorld()

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HelloWorld.h
include jni.h / Header for class HelloWorld
/ ifndef _Included_HelloWorld define
_Included_HelloWorld ifdef __cplusplus
extern C endif / Class HelloWorld
Method displayHelloWorld Signature ()V
/ JNIEXPORT void JNICALL Java_HelloWorld_display
HelloWorld(JNIEnv , jobject) ifdef
__cplusplus endif endif
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HelloWorld.h
include jni.h / Header for class HelloWorld
/ ifndef _Included_HelloWorld define
_Included_HelloWorld ifdef __cplusplus
extern C endif / Class HelloWorld
Method displayHelloWorld Signature ()V
/ JNIEXPORT void JNICALL Java_HelloWorld_display
HelloWorld(JNIEnv , jobject) ifdef
__cplusplus endif endif
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HelloWorld.h
include jni.h / Header for class HelloWorld
/ ifndef _Included_HelloWorld define
_Included_HelloWorld ifdef __cplusplus
extern C endif / Class HelloWorld
Method displayHelloWorld Signature ()V
/ JNIEXPORT void JNICALL Java_HelloWorld_display
HelloWorld(JNIEnv , jobject) ifdef
__cplusplus endif endif
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HelloWorldImp.c
include ltjni.hgt include "HelloWorld.h" include
ltstdio.hgt JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv env,
jobject obj) printf("Hello world!\n")
return
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HelloWorldImp.c
include ltjni.hgt include "HelloWorld.h" include
ltstdio.hgt JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv env,
jobject obj) printf("Hello world!\n")
return
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HelloWorldImp.c
include ltjni.hgt include "HelloWorld.h" include
ltstdio.hgt JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv env,
jobject obj) printf("Hello world!\n")
return
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HelloWorldImp.c
include ltjni.hgt include "HelloWorld.h" include
ltstdio.hgt JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv env,
jobject obj) printf("Hello world!\n")
return
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Create a Shared Library

• class HelloWorld
• . . .
• System.loadLibrary("hello")
• . . .
• Compile the native code into a shared library
• (example for MS Windows Visual C 4.0)
• cl -Ic\java\include -Ic\java\include\win32
• -LD HelloWorldImp.c -Fehello.dll

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Run the Program

• Command
• java HelloWorld
• Result
• Hello World!
• Possible exceptions
• java.lang.UnsatisfiedLinkError no hello in
  shared library path at java.lang.Runtime.loadLibra
  ry(Runtime.java) at java.lang.System.loadLibrary(S
  ystem.java) at java.lang.Thread.init(Thread.java)
  

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Summary

• Connect Java with native languages
• Code reuse
• Powerful
• Compromise of Java safety features,
  cross-platform and dynamic ability
• JNI call is slow and costful
• Not work well for applets

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References

• Glenn L. Vanderburg. et al., Tricks of the Java
  Programming Gurus, 1996, http//docs.rinet.ru8080
  /JaTricks/
• Beth Stearns, Trail Java Native Interface, Sun
  Microsystems, Inc., 2003, http//java.sun.com/docs
  /books/tutorial/native1.1/
• Roedy Green, JNI, The Java Native Interface,
  Canadian Mind Products, 2002, http//mindprod.com/
  jni.html
• Kong Wenyu, A Brief Introduction to Java Native
  Interface, http//www.geocities.com/kongwenyu/jni.
  html

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Questions?