Java调用C（Linux下实现Java本地方法），linuxjava

Java调用C（Linux下实现Java本地方法），linuxjava

一、Java类中声明本地方法，使用native关键字

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、生成.class文件，并使用javah命令，将.class文件生成.h文件

javac MyNative.class javah -jni MyNative

生成的.h文件如下

/* 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、将刚刚生成的.h文件包含在头文件中，并用C语言实现这些本地方法，代码如下

#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; // 更改偶数数组元素 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、书写makefile文件，编译刚才所写的C代码，生成.so文件，Linux环境下（windows，DLL文件）

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”为JDK安装目录

“libMyNative.so”是Java类中

    static {
        System.loadLibrary("MyNative");
    }

静态方法中参数＋lib

五、修改.bash_profile文件，配置环境变量，由于生成的.so文件在当前目录下，故

.bash_profile文件设置如下

export LD_LIBRARY_PATH=./

 

六、执行java MyNative命令，结果如下

Linux下实现Java本地方法（Java调用C）完成。

 

参考资料：

http://www.ibm.com/developerworks/cn/java/jnimthds/index.html