JAVA Native Interface

Transcription

1 CSC System Development with Java JAVA Native Interface Department of Statistics and Computer Science

2 Java Native Interface Is a programming framework JNI functions written in a language other than Java They could be C, C++, or even Assembly Native methods are compiled into a dynamic link library (.dll,.so, etc.) Operating System loads and links the library into the process that is running the Java Virtual Machine 2

3 JNI Overview 3

4 Why Native Interface? Some functionality are not provided by java Low-level drives/devices specific to OS Increase performance by implementing rigorous tasks in native language Java is slow in general and may not be suitable for some functions (e.g., image compression and decompression) Try to use existing legacy library and could not afford to rewrite it in java JNI can be used as a wrapper of these legacy codes Can slowly migrate legacy code to a newer platform Improve efficiency of integration TCP/IP sockets can be used, but there are overhead in data transmission 4

5 Java Native Interface contd. Advantages Use the existing library that was previously written in another language. Use of windows API functions Increasing the speed of execution Invoke API functions from server product that is developed in C or C++ from a java client. Disadvantages Write Once Run Anywhere is not possible Run time errors debugging is difficult in native code An applet can not call a native method Security risk is potential 5

6 How JNI works? Native functions are implemented in separate.c or.cpp files JVM invokes the function it passes a JNIEnv pointer, a jobject pointer, and any Java arguments declared by the Java method The env pointer is a structure that contains the interface to the JVM Function format JNIEXPORT void JNICALL Java_ClassName_MethodName (JNIEnv *env, jobject obj) /*Implement Native Method Here*/ 6

7 JNI Development Steps 1. Declare native methods in the java class 2. Compile the java class 3. Generate C header for the java class 4. Create C file (for DLL) 5. Compile and Create DLL file 6. Copy DLL to Java Path 7. RUN the JAVA Program 7

8 Declare native methods Example : HelloWorld.java Native Method class HelloWorld public native void displayhelloworld(); static System.loadLibrary("hello"); public static void main(string[] args) new HelloWorld().displayHelloWorld(); Call hello.dll file Invoke native Method 8

9 Generate JNI Header Compile HelloWorld.java javac HelloWorld.java Generate HelloWorld.h javah HelloWorld 9

10 Sample: 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_displayHelloWorld(JNIEnv *env, jobject); #ifdef cplusplus #endif #endif The JVM reference The calling object 10

11 HelloWorld.C #include <jni.h> #include "HelloWorld.h" #include <stdio.h> JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj) printf("hello world!\n"); return; 11

12 Data types Java Type Native Type Size in bits boolean jboolean 8, unsigned byte jbyte 8 char jchar 16, unsigned short jshort 16 int jint 32 long jlong 64 float jfloat 32 double jdouble 64 void void n/a 12

13 JNI Data Mapping Example 01: class Prompt private native String getline(string prompt); JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *, jobject, jstring); Example 02: public native int jsum(int x, int y); JNIEXPORT jint JNICALL Java_JMaths_jsum(JNIEnv *env, jobject obj, jint x, jint y) return x + y; 13

14 Object Types 14

15 Create a DLL (mingw gcc) Type the following command (Command prompt) Gcc -Wall -D_JNI_IMPLEMENTATION_ -Wl,--kill-at -I HederLibariers -shared C_program -o output_file Example: gcc -Wall -D_JNI_IMPLEMENTATION_ -Wl,--kill-at -I C:/Program Files/Java/jdk1.6.0_02/include -I C:/Program Files/Java/jdk1.6.0_02/include/win32 -shared c:/budditha/helloword.c -o c:/budditha/hw.dll Input file Output file 15

16 Run the Program Command: java HelloWorld Result: Hello World! Possible exceptions: java.lang.unsatisfiedlinkerror: no hello in shared library path at java.lang.runtime.loadlibrary(runtime.java) at java.lang.system.loadlibrary(system.java) at java.lang.thread.init(thread.java) Copy hello.dll to: C:\Program Files\Java\jdk1.6.0_02\jre\bin\ 16

17 Example Create a.dll file named jm.dll Implement the following methods printmessage(): Print the message User define java native interface Jsum(int, int): Return the summation of the given two integer values Jmul(int, int): Return the multiplication of the given two integer values Create a java class named JMaths.java to implement the above Native methods Create a class named Calculator.java to run the program (use JMaths class) 17

18 Answer: JMaths.java class JMaths public native void printmessage(); public native int jsum(int x, int y); public native int jmul(int c, int g); public void calculate() System.out.println("JSUM " + jsum(2,4)); System.out.println("JMUL " + jmul(2,4)); static System.loadLibrary("jm"); Calculator.java public class Calculator public static void main(string[] args) JMaths j = new JMaths(); j.calculate(); 18

19 Generate a C header file Compile the JMaths.java Javac JMaths.java Create a header file Javah Jmaths /* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class JMaths */ #ifndef _Included_JMaths #define _Included_JMaths #ifdef cplusplus extern "C" #endif /* * Class: JMaths * Method: printmessage * Signature: ()V */ JNIEXPORT void JNICALL Java_JMaths_printMessage (JNIEnv *, jobject); /* * Class: JMaths * Method: jsum * Signature: (II)I */ JNIEXPORT jint JNICALL Java_JMaths_jsum (JNIEnv *, jobject, jint, jint); /* * Class: JMaths * Method: jmul * Signature: (II)I */ JNIEXPORT jint JNICALL Java_JMaths_jmul (JNIEnv *, jobject, jint, jint); #ifdef cplusplus #endif #endif 19

20 Answers JMaths.c #include <jni.h> #include <stdio.h> #include JMaths.h JNIEXPORT void JNICALL Java_JMaths_printMessage(JNIEnv *env, jobject obj) printf("java JNI Mathamatics!\n"); return; JNIEXPORT jint JNICALL Java_JMaths_jsum(JNIEnv *env, jobject obj, jint x, jint y) return x + y; JNIEXPORT jint JNICALL Java_JMaths_jmul(JNIEnv *env, jobject obj, jint x, jint y) return x * y; 20

21 Files Jmaths.c : Source files for the DLL Jmaths.java: Java class for native interface Jmaths.h: C Header file for the native interface Calculator.java: Calculator class Jm.dll: Created DLL file 21

22 Commands Javac Jmath.java Javah Jmaths Cd C:\Program Files\CodeBlocks\MinGW gcc -Wall -D_JNI_IMPLEMENTATION_ -Wl,--kill-at -I C:/Program Files/Java/jdk1.6.0_02/include -I C:/Program Files/Java/jdk1.6.0_02/include - shared c:/budditha/javamaths.c -o jm.dll Javac Calculator.java Java Calculator 22

23 Useful Tips for JNI developers Install Code::block for C programming Download: Install JDK Install Textpad for Java based programming Download: Use gcc compilers (Use MinGW gcc compiler on code::block) C:\Program Files\CodeBlocks\MinGW\bin 23