Shulou(Shulou.com)06/01 Report--

This article will explain in detail how to use JNI in Linux system, Xiaobian thinks it is quite practical, so share it with you as a reference, I hope you can gain something after reading this article.

JNI stands for Java Native Interface, and by writing programs using Java Native Interface, you can ensure that your code is easily portable on different platforms. Since Java 1.1, JNI standards have been part of the Java platform, allowing Java code to interact with code written in other languages.

I'll explain in detail how native shared libraries work with Java on Linux, using Hello world as an example.

Define a Java class- Hello class public class Hello{ static { try {//here is the name of the link library where the local method is located System.loadLibrary("hello"); } catch(UnsatisfiedLinkError e) { System.err.println( "Cannot load hello library:\n " + e.toString(); } } public Hello() { }//declared local method public native void SayHello(String strName);}

Here are two caveats:

First: Write a native method declaration for each native method you want to use, except that you must specify the native keyword, as follows:

public native void SayHello(String strName);

Second: the native code base must be loaded explicitly. We need to load this library in a static block of the class (static libraries are called when the class loads)

Now let's edit hello.java to generate the hello.class file.

Generate Local Link Library

To generate a Java native interface header file for the class defined above, use javah, the Java compiler's javah function will generate the necessary declarations from the Hello class, this command will generate the Hello.h file

The Hello.h file generated looks like this:

#include /* Header for class Hello */#ifndef _Included_Hello#define _Included_Hello#ifdef __cplusplusextern "C" {#endif/** Class: Hello* Method: SayHello* Signature: (Ljava/lang/String; V*/JNIEXPORT void JNICALL Java_Hello_SayHello (JNIEnv *, jobject, jstring);#ifdef __cplusplus}#endif#endif Create a CPP file Hello.cpp in the same path as Hello.h

It reads as follows:

#include "Hello.h"#include //Same declaration as function in Hello.h JNIEXPORT void JNICALL Java_Hello_SayHello (JNIEnv * env, jobject arg, jstring instring){ //Get pointer to string UTF encoding from instring string const jbyte *str = (const jbyte *)env->GetStringUTFChars( instring, JNI_FALSE ); printf("Hello,%s\n",str); //Tells the VM that native code no longer needs to access Java strings via str. env->ReleaseStringUTFChars( instring, (const char *)str ); return;}

There are three parameters here, here is how to use them:

(1) All JNI calls use pointers of type JNIEnv *, customarily defined in CPP files as evn, which is the first parameter of any native method. The env pointer points to a table of function pointers that can be accessed directly with the "->" operator in VC. (2) jobject A handle to the Java object LocalFunction instantiated in this Java code, equivalent to this pointer. (3) The third parameter is the parameter passed in by Java program in the local call. In this case, there is only one String parameter. For string arguments, Java strings cannot be read directly in native code, but must be converted to C /C++ strings or Unicode.

Compiling generates shared libraries.

When using GCC, you must tell the compiler where to find the support files for this Java native method, and explicitly tell the compiler to generate location-independent code, compiled in my environment as follows:

gcc -I/home/jbuilder/jdk1.3.1/include -I/home/jbuilder/jdk1.3.1/include/linux -fPIC -c Hello.c

Hello.o

gcc -shared -Wl,-soname,libhello.so -o libhello.so Hello.o

Generate libhello.so (this is the file name format of the dynamic link library under linux, just like the.dll file suffix under windows)

Finally, notify the dynamic linker of the path to the shared file.

export LD_LIBRARY_PATH=`pwd`:$LD_LIBRARY_PATH Write a simple Java program to test our native methods

Save the following source code as A.java:

import Hello;import java.util.*; public class A{ public static void main(String argv[]) { A a = new A(); } public A() { Hello h = new Hello(); //invoke local method h.SayHello("Hello world"); }}

Compile A.java with javac to generate A. classUse java A as if you were executing a normal Java program and we'll see Hello world appear on the screen. OK, done!

About "how to use JNI in Linux system" this article is shared here, I hope the above content can be of some help to everyone, so that you can learn more knowledge, if you think the article is good, please share it for more people to see.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.