Java JNI Quick Start tutorial (recommended) _java

1. JNI Introduction

JNI is the English abbreviation for the Java Native interface, meaning the Java local interface.

Problem Source: Because Java writing the bottom of the application is more difficult to implement, in some real-time requirements very high part of Java is more difficult to do (real-time requirements of the place is not yet involved, real-time such topics to be studied).

Workaround: Java can use JNI to invoke existing local libraries (C + + to develop any and system-related programs and class libraries), greatly flexible Java development.

2. JNI Quick Learning Tutorial

2.1 Questions:

Use JNI to write a piece of code that implements the String_java_test_helloworld (jnienv *env, Jclass cls, jstring j_str) function, which is implemented in front of the string J_str ("World") plus Hello, and returns.

2.2 Problem Solving Process:

I. Preparation of the Test.java category:

public class test{
  //native interface public
  native string HelloWorld (string text);
  
  public static void Main (string[] args) {
    //Load Dynamic Library
    system.loadlibrary ("Test2");
    Test ts = new test ();
    String Text = Ts.helloworld ("World");
    System.out.println (text);
  }

Note:

1, Load dynamic Class Library: System.loadlibrary ("Test2"); "Under Windows Load is Test2.dll,linux load is test2.so"

Ii. Compiling Test.java files

Enter cmd input command > Javac Test.java

Iii. Generating Test.h files

Enter cmd input command > Javah Test

The contents of the Test.h file are as follows:

/* Do isn't EDIT this file-it is machine generated/*
#include <jni.h>/
* Header for class Test/*

#ifnd EF _included_test
#define _included_test
#ifdef __cplusplus
extern "C" {
#endif
 * * Class:   Test
 * method:  HelloWorld
 * Signature: (ljava/lang/string;) ljava/lang/string;
 * *
jniexport jstring jnicall java_test_helloworld
 (jnienv *, Jobject, jstring);

#ifdef __cplusplus
}
#endif
#endif

Note:

1, function declaration, fixed format: jniexport; return type: Jstring;jni Call: Jnicall;java_ Full class Name _ Method name: Java_test_helloworld;

2, function parameters: Call jni.h encapsulated function pointer: Jnienv;java class itself: Jobject,java file incoming parameters: Jstring.

Iv. writing the C language file test2.c implement the function of the test class to invoke the dynamic link library:

#include "Test.h"
#include <string.h>

jniexport jstring jnicall java_test_helloworld
 (jnienv *env, Jobject obj, jstring string) {
  const char* str = (*env)->getstringutfchars (env,string,0);
  Char cap[128];
  Cap[0] = ' h ';
  CAP[1] = ' e ';
  CAP[2] = ' l ';
  CAP[3] = ' l ';
  Cap[4] = ' o ';
   
  strcat (CAP,STR);
   
  (*env)->releasestringutfchars (env,string,0);
  Return (*env)->newstringutf (ENV,CAP);
 }

Note:

1, because Java itself uses double-byte characters, C language itself are single-byte characters, so need to use (*env)->getstringutfchars () to convert between Java and C string;

2, Getstringutfchars () and Newstringutf (), the first is the conversion from UTF8 to C encoding format, the second is based on the string of C to return a UTF8 string;

3, Releasestringutfchars () is used to release objects, there are virtual machines in Java garbage collection, but in the C language these objects must be manually recycled, otherwise it may cause memory leaks.

V. Compiling and running

Compile:

Enter cmd input command > gcc-i "D:\Program files\java\jdk1.8.0_45\include"-i "D:\Program files\java\jdk1.8.0_45\include\win32" --share Test2.c-o Test2.dll

Run:

Enter cmd input command > Java Test

The results of the operation are as follows:

HelloWorld

3. Summary:

Step one: Write a Java Class (Test.java) with the native method, and use the Javac tool to compile the Java class (Generate Test.class);

Step two: Use Javah to generate the header file (Test.h) corresponding to the native method;

The third step is to implement the corresponding header file (test2.c) and compile it as a dynamic-link library (test2.so) using C + +.

This article runs the environment: Windows 64-bit operating system, JDK 1.8 version, MINGW64 (GCC).

The above Java JNI QuickStart Tutorial (recommended) is a small series to share all the content, hope to give you a reference, but also hope that we support the cloud habitat community.