JNI short tutorial

1 Write a Java class that uses C codes
CraftJNI.java
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public class CraftJNI {
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  static {
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    System.loadLibrary("hello"); // Load native library hello.dll (windows)
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                                 // or libhello.so (Unixes) at runtime.
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                                 // This library contains a native method sayHello()
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  }
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  // Declare an instance native method sayHello() which receives no parameter
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  // returns void
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  private native void sayHello();
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  // Test Driver
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  public static void main(String[] args) {
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    new CraftJNI().sayHello(); // Create an instance and invoke native method
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  }
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}
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2 Compile Java program CraftJNI.java
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javac -h . CraftJNI.java
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CraftJNI.h
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/* DO NOT EDIT THIS FILE - it is machine generated */
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#include <jni.h>
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/* Header for class CraftJNI */
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​
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#ifndef _Included_CraftJNI
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#define _Included_CraftJNI
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#ifdef __cplusplus
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extern "C" {
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#endif
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/*
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 * Class:     CraftJNI
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 * Method:    sayHello
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 * Signature: ()V
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 */
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JNIEXPORT void JNICALL Java_CraftJNI_sayHello
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  (JNIEnv *, jobject);
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​
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#ifdef __cplusplus
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}
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#endif
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#endif
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To know more about the generated class file
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javap -verbose CraftJNI.class
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3 Implement C program
CraftJNI.c
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#include <jni.h>   // Standard JDK provided header
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#include <stdio.h> // Standard C IO header
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#include <CraftJNI.h> // Generated header
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​
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JNIEXPORT void JNICALL Java_CraftJNI_sayHello(JNIEnv *env, jobject thisObj) {
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  printf("I am in C code \n");
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  return;
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}
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​
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4 Compile C program
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gcc -fPIC -I"." -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" \
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   -shared -o libhello.so CraftJNI.c
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> java -Djava.library.path=. CraftJNI
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I am in C code
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Troubleshooting
Error 1: class file has wrong version
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> javah CraftJNI.class
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Error: cannot access CraftJNI
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  bad class file: ./CraftJNI.class
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    class file has wrong version 55.0, should be 52.0
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Reason: javah is not available with Java 11
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> java -version
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openjdk version "11.0.9.1" 2020-11-04
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OpenJDK Runtime Environment (build 11.0.9.1+1-post-Debian-1deb10u2)
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OpenJDK 64-Bit Server VM (build 11.0.9.1+1-post-Debian-1deb10u2, mixed mode, sharing)
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​
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> javac -version
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javac 11.0.9.1
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​
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> javah -version
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javah version "1.8.0_252"
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​
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> whereis java
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java: /usr/bin/java /usr/share/java /usr/share/man/man1/java.1.gz
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​
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> ls /usr/share/man/man1 | grep java*
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java.1.gz
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javac.1.gz
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javadoc.1.gz
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javah.1.gz
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javap.1.gz
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Error 2:
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> java -Djava.library.path=. CraftJNI
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Exception in thread "main" java.lang.UnsatisfiedLinkError: no hello in java.library.path: [.]
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        at java.base/java.lang.ClassLoader.loadLibrary(ClassLoader.java:2670)
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        at java.base/java.lang.Runtime.loadLibrary0(Runtime.java:830)
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        at java.base/java.lang.System.loadLibrary(System.java:1873)
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        at CraftJNI.<clinit>(CraftJNI.java:4)
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References
1.JNI reference docs - https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/jniTOC.html​
2.Some of the code is shamelessly copied from https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html​

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