Android学习JNI，使用JNI实现字符串加密

//加密本地方法
public native String encodeFromC(String text, int length);
//解密本地方法
public native String decodeFromC(String text, int length);

    public void encode(View v)
    {
    	String passwd = et.getText().toString();
    	et.setText(encodeFromC(passwd, passwd.length()));
    }

    public void decode(View v)
    {
    	String passwd = et.getText().toString();
    	et.setText(decodeFromC(passwd, passwd.length()));
    }

#include <jni.h>
#include <String.h>

//把java的字符串转换成c的字符串,使用反射
char*   Jstring2CStr(JNIEnv*   env,   jstring   jstr)
{
	 char*   rtn   =   NULL;
	 //1:先找到字节码文件
	 jclass   clsstring   =   (*env)->FindClass(env,"java/lang/String");
	 jstring   strencode   =   (*env)->NewStringUTF(env,"GB2312");
	 //2:通过字节码文件找到方法ID
	 jmethodID   mid   =   (*env)->GetMethodID(env,clsstring,   "getBytes",   "(Ljava/lang/String;)[B");
	 //3:通过方法id，调用方法
	 jbyteArray   barr=   (jbyteArray)(*env)->CallObjectMethod(env,jstr,mid,strencode); // String .getByte("GB2312");
	 //4:得到数据的长度
	 jsize   alen   =   (*env)->GetArrayLength(env,barr);
	 //5：得到数据的首地址
	 jbyte*   ba   =   (*env)->GetByteArrayElements(env,barr,JNI_FALSE);
	 //6:得到C语言的字符串
	 if(alen   >   0)
	 {
	  rtn   =   (char*)malloc(alen+1);         //"\0"
	  memcpy(rtn,ba,alen);
	  rtn[alen]=0;
	 }
	 (*env)->ReleaseByteArrayElements(env,barr,ba,0);  //
	 return rtn;
}

/*
 * Class:     com_demo_passwd_MainActivity
 * Method:    encodeFromC
 * Signature: (Ljava/lang/String;I)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_demo_passwd_MainActivity_encodeFromC
  (JNIEnv *env, jobject obj, jstring passwd, jint length)
{
	//1:将java的字符串转化为c语言
	char* cstr = Jstring2CStr(env, passwd);
	int i = 0;
	//2:给C语言字符加1
	for(i = 0; i < length; i++)
	{
		*(cstr + i) += 1;
	}
	//3:将c语言字符串转化为java字符串
	return (*env)->NewStringUTF(env, cstr);
}

/*
 * Class:     com_demo_passwd_MainActivity
 * Method:    decodeFromC
 * Signature: (Ljava/lang/String;I)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_demo_passwd_MainActivity_decodeFromC
(JNIEnv *env, jobject obj, jstring passwd, jint length)
{
	//1:将java的字符串转化为c语言
	char* cstr = Jstring2CStr(env, passwd);
	int i = 0;
	//2:给C语言字符减1
	for(i = 0; i < length; i++)
	{
		*(cstr + i) -= 1;
	}
	//3:将c语言字符串转化为java字符串
	return (*env)->NewStringUTF(env, cstr);
}