java jni挪用c函數實例分享(java挪用c函數)

從C/C++到Java，再從Java回到C/C++，明天終究無機會懂得了銜接Java、C/C++的橋梁——JNI。哈哈！分享一下！

1、簡介
JNI是Java native interface的簡寫，可以譯作Java原生接口。Java可以經由過程JNI挪用C/C++的庫，這關於那些對機能請求比擬高的Java法式或許Java沒法處置的義務無疑是一個很好的方法。

2、目標：Java代碼中挪用C/C++代碼
3、完成：假定我們的Java法式為J2C.java, C法式為J2C.c, Java與C之間的通訊函數名為write2proc;
              那末write2proc的聲明位於J2C.java，完成位於J2C.c;
4、操作
1. 編寫並編譯Java法式

    javac J2C.java => J2C.class
2. 生成C/C++頭文件

    javah J2C => J2C.h （裝置JDK後，$JAVA_HOME應當已參加$PATH, 不然應用相對途徑，例如/usr/bin/javah）

3. 編寫對應的C/C++法式：J2C.c

4. 生成C/C++目的文件
    gcc -I/usr/lib/jvm/java-6-openjdk-amd64/include -I/usr/lib/jvm/java-6-openjdk-amd64/include/linux -fPIC -c J2C.c => J2C.o
5. 生成C/C++同享庫

    gcc -shared -Wl,-soname,libj2c.so.1 -o libj2c.so.1.0 J2C.o => libj2c.so.1.0

6. 重定名cp libj2c.so.1.0 libj2c.so => libj2c.so

7. 將同享庫參加靜態鏈接庫的途徑(此例為以後目次)
    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH：.

8. 履行Java法式，完成跨說話通訊
     java J2C

5、詳細進程

1. 編寫並編譯J2C.java

import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;

public class J2C
{
     static
     {
          try{
               // 此處即為當地辦法地點鏈接庫名
               System.loadLibrary("j2c");
          } catch(UnsatisfiedLinkError e)
          {
               System.err.println( "Cannot load J2C library:\n " +
               e.toString() );
          }
     }

     //聲明的當地辦法
     public static native int write2proc(int pid);

     public static void main(String[] args){

          //獲得本過程(即主線程)的pid
          final RuntimeMXBean runtime = ManagementFactory.getRuntimeMXBean();
          final String info = runtime.getName();
          final int index = info.indexOf("@");

          if (index != -1) {
               final int pid = Integer.parseInt(info.substring(0, index));
               System.out.println(info);
               System.out.println(pid);

               write2proc(pid);
          }

          try{
               Thread.sleep(8000);
          } catch(InterruptedException e){
               e.printStackTrace();
          }
     }
}

note：Java法式中System.loadLibrary參數名表現要載入的C/C++同享庫，第6步生成的同享庫名必需與該參數分歧，即System.loadLibrary(Name) 對應同享庫名libName.so (同享庫名必需以lib開首)

2. 生成C頭文件J2C.h：javah J2C

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class J2C */

#ifndef _Included_J2C
#define _Included_J2C
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class: J2C
 * Method: write2proc
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_J2C_write2proc
     (JNIEnv *, jclass, jint);

#ifdef __cplusplus
}
#endif
#endif

note：1. 頭文件主動生成，不要修正它；

2. 函數JNIEXPORT jint JNICALL Java_J2C_write2proc(JNIEnv *, jclass, jint);

 依照正文的解釋是在J2C.java文件的類J2C的辦法write2proc處界說，故C法式的完成函數必需與該處簽名分歧；

3. 編寫C法式J2C.c

#include <stdio.h>

#include "J2C.h"

JNIEXPORT int JNICALL Java_J2C_write2proc(JNIEnv * env, jobject arg, jint pid)
{

     printf("current pid is %d\n", pid);

     return 0;

}

4. 編譯C法式

由於C法式裡#include "J2C.h"而J2C.h又#include <jni.h>, 而gcc外面默許情況其實不曉得jni.h是甚麼器械，故編譯時須要告知編譯器jni.h的地位( jni.h在jdk 的$JAVA_HOME/include上面），所以才有了下面的編譯參數；

由於應用gcc編譯獲得靜態庫，在jni挪用的時刻，某些情形會有異常, 可測驗考試改用g++。

總結

1. Java中辦法的原型聲明與C/C++對應的完成文件界說必需分歧（可以經由過程主動生成的C/C++頭文件來比擬），特別是類名和辦法名；

2. Java中System.loadLibrary()載入的同享庫名必需與前面C/C++生成的同享庫名分歧。

2，保留為文件

public boolean logWeight(Intent batteryChangeIntent) {
            Log.i(TAG, "logBattery");
            if (batteryChangeIntent == null)
                return false;
            try {
                FileWriter out = null;
                if (mWeightLogFile != null) {
                    try {
                        out = new FileWriter(mWeightLogFile, true);
                    }
                    catch (Exception e) {}
                }
                if (out == null) {
                    File root = Environment.getExternalStorageDirectory();
                    if (root == null)
                        throw new Exception("external storage dir not found");
                    mWeightLogFile = new File(root,WeightService.LOGFILEPATH);
                    boolean fileExists = mWeightLogFile.exists();
                    if (!fileExists) {
                        if(!mWeightLogFile.getParentFile().mkdirs()){
                            Toast.makeText(this, "create file failed", Toast.LENGTH_SHORT).show();
                        }
                        mWeightLogFile.createNewFile();
                    }
                    if (!mWeightLogFile.exists()) {
                        Log.i(TAG, "out = null");
                        throw new Exception("creation of file '"+mWeightLogFile.toString()+"' failed");
                    }
                    if (!mWeightLogFile.canWrite())
                        throw new Exception("file '"+mWeightLogFile.toString()+"' is not writable");
                    out = new FileWriter(mWeightLogFile, true);
                    if (!fileExists) {
                        String header = createHeadLine();
                        out.write(header);
                        out.write('\n');
                    }
                }
                Log.i(TAG, "out != null");
                String extras = createBatteryInfoLine(batteryChangeIntent);
                out.write(extras);
                out.write('\n');
                out.flush();
                out.close();
                return true;
            } catch (Exception e) {
                Log.e(TAG,e.getMessage(),e);
                return false;
            }
        }