原文：Java調用C（Linux下實現Java本地方法）
一、Java類中聲明本地方法，使用native關鍵字
- public class MyNative {
- public void showParms(String s, int i, boolean b) {
- showParms0(s, i, b);
- }
- public int hypotenuse(int a, int b) {
- return hypotenuse0(a, b);
- }
- public void setArray(boolean[] ba) {
- for (int i = 0; i < ba.length; i++)
- ba[i] = true;
- setArray0(ba);
- }
- public void showStrings(String[] sa) {
- showStrings0(sa);
- }
- public String[] getStrings() {
- return getStrings0();
- }
- private native void showParms0(String s, int i, boolean b);
- private native int hypotenuse0(int a, int b);
- private native void setArray0(boolean[] ba);
- private native void showStrings0(String[] sa);
- private native String[] getStrings0();
- static {
- System.loadLibrary("MyNative");
- }
- public static void main(String[] args) {
- MyNative obj = new MyNative();
- obj.showParms("Hello", 23, true);
- obj.showParms("World", 34, false);
- System.out.println(obj.hypotenuse(3, 4));
- System.out.println(obj.hypotenuse(9, 12));
- boolean[] ba = new boolean[5];
- obj.setArray(ba);
- for (int i = 0; i < ba.length; i++)
- System.out.println(ba[i]);
- String[] sa = new String[] { "Hello,", "world!", "JNI", "is", "fun." };
- obj.showStrings(sa);
- obj.showStrings(obj.getStrings());
- }
- }
loadLibrary
public static void loadLibrary(String libname)
-
Loads the system library specified by the
libname argument. The manner in which a library name is mapped to the actual system library is system dependent.
The call System.loadLibrary(name) is effectively equivalent to the call
Runtime.getRuntime().loadLibrary(name)
-
-
Parameters:
-
libname - the name of the library.
-
Throws:
-
SecurityException - if a security manager exists and its checkLink method doesn't allow loading of the specified dynamic library
-
UnsatisfiedLinkError - if the library does not exist.
-
NullPointerException - if libname is null
-
See Also:
-
Runtime.loadLibrary(java.lang.String) , SecurityManager.checkLink(java.lang.String)
二、產生.class、.h檔案
- javac MyNative.class
- javah -jni MyNative
產生的.h檔案如下
- /* DO NOT EDIT THIS FILE - it is machine generated */
- #include <jni.h>
- /* Header for class MyNative */
- #ifndef _Included_MyNative
- #define _Included_MyNative
- #ifdef __cplusplus
- extern "C" {
- #endif
- /*
- * Class: MyNative
- * Method: getStrings0
- * Signature: ()[Ljava/lang/String;
- */
- JNIEXPORT jobjectArray JNICALL Java_MyNative_getStrings0
- (JNIEnv *, jobject);
- /*
- * Class: MyNative
- * Method: showStrings0
- * Signature: ([Ljava/lang/String;)V
- */
- JNIEXPORT void JNICALL Java_MyNative_showStrings0
- (JNIEnv *, jobject, jobjectArray);
- /*
- * Class: MyNative
- * Method: showParms0
- * Signature: (Ljava/lang/String;IZ)V
- */
- JNIEXPORT void JNICALL Java_MyNative_showParms0
- (JNIEnv *, jobject, jstring, jint, jboolean);
- /*
- * Class: MyNative
- * Method: hypotenuse0
- * Signature: (II)I
- */
- JNIEXPORT jint JNICALL Java_MyNative_hypotenuse0
- (JNIEnv *, jobject, jint, jint);
- /*
- * Class: MyNative
- * Method: setArray0
- * Signature: ([Z)V
- */
- JNIEXPORT void JNICALL Java_MyNative_setArray0
- (JNIEnv *, jobject, jbooleanArray);
- #ifdef __cplusplus
- }
- #endif
- #endif
三、用C語言實現.h檔案中聲明的方法
將剛剛產生的.h檔案包含在標頭檔中，並用C語言實現這些本地方法，代碼如下
- #include <stdio.h>
- #include <math.h>
- #include "MyNative.h"
- JNIEXPORT void JNICALL Java_MyNative_showParms0
- (JNIEnv *env, jobject obj, jstring s, jint i, jboolean b)
- {
- const char* szStr = (*env)->GetStringUTFChars( env, s, 0 );
- printf( "String = [%s]/n", szStr );
- printf( "int = %d/n", i );
- printf( "boolean = %s/n", (b==JNI_TRUE ? "true" : "false") );
- (*env)->ReleaseStringUTFChars( env, s, szStr );
- }
- JNIEXPORT jint JNICALL Java_MyNative_hypotenuse0
- (JNIEnv *env, jobject obj, jint a, jint b)
- {
- int rtn = (int)sqrt( (double)( (a*a) + (b*b) ) );
- return (jint)rtn;
- }
- JNIEXPORT void JNICALL Java_MyNative_setArray0
- (JNIEnv *env, jclass cls, jbooleanArray ba)
- {
- jboolean* pba = (*env)->GetBooleanArrayElements( env, ba, 0 );
- jsize len = (*env)->GetArrayLength(env, ba);
- int i=0;
- // 更改偶數數組元素
- for( i=0; i < len; i+=2 )
- pba[i] = JNI_FALSE;
- (*env)->ReleaseBooleanArrayElements( env, ba, pba, 0 );
- }
- JNIEXPORT void JNICALL Java_MyNative_showStrings0
- (JNIEnv *env, jclass cls, jobjectArray sa)
- {
- int len = (*env)->GetArrayLength( env, sa );
- int i=0;
- for( i=0; i < len; i++ )
- {
- jobject obj = (*env)->GetObjectArrayElement(env, sa, i);
- jstring str = (jstring)obj;
- const char* szStr = (*env)->GetStringUTFChars( env, str, 0 );
- printf( "%s ", szStr );
- (*env)->ReleaseStringUTFChars( env, str, szStr );
- }
- printf( "/n" );
- }
- JNIEXPORT jobjectArray JNICALL Java_MyNative_getStrings0
- (JNIEnv *env, jclass cls)
- {
- jstring str;
- jobjectArray args = 0;
- jsize len = 5;
- char* sa[] = { "Hello,", "world!", "JNI", "is", "fun" };
- int i=0;
- args = (*env)->NewObjectArray(env, len, (*env)->FindClass(env, "java/lang/String"), 0);
- for( i=0; i < len; i++ )
- {
- str = (*env)->NewStringUTF( env, sa[i] );
- (*env)->SetObjectArrayElement(env, args, i, str);
- }
- return args;
- }
四、編譯C語言檔案
書寫makefile檔案，編譯剛才所寫的C代碼，產生.so檔案，Linux環境下（windows，DLL檔案）
- libMyNative.so:MyNative.o makefile
- gcc -Wall -rdynamic -shared -o libMyNative.so MyNative.o
- MyNative.o:MyNative.c MyNative.h
- gcc -Wall -c MyNative.c -I./ -I/sandbox/JAVA2S/jdk1.6.0_16/include -I/sandbox/JAVA2S/jdk1.6.0_16/include/linux
- cl:
- rm -rf *.o *.so
其中，“/sandbox/JAVA2S/jdk1.6.0_16”為JDK安裝目錄
“libMyNative.so”是Java類中
static {
System.loadLibrary("MyNative");
}
靜態方法中參數＋lib
五、配置環境變數
修改.bash_profile檔案，配置環境變數，由於產生的.so檔案在目前的目錄下，故
.bash_profile檔案設定如下
export LD_LIBRARY_PATH=./
六、執行目標檔案
執行java MyNative命令，結果如下
Linux下實現Java本地方法（Java調用C）完成。
參考資料：
http://www.ibm.com/developerworks/cn/java/jnimthds/index.html
Tags:
Java和C、C++語言混合使用，Java如何調用C語言，Java如何調用C++語言，Java JNI的實現，Java本地調用方法。
轉自：http://blog.csdn.net/kangkanglou/archive/2010/08/13/5807787.aspx
作者：kangkang_kevin
