Android's JNI "hands-on tutorials" 3⃣️--java call C code __java

Read an article on:
http://blog.csdn.net/github_33304260/article/details/62891083
We should already be able to build the NDK project,
Let's take a look at Java call C code: Java invoke C code to perform addition operations

Java code:

    /**
     * through JNI simple plastic addition Operation
     * @param a
     * @param b
     * @return/public
    static native int addint (int A, T b);

. h Code:

 * * Class:     com_libin_factory_ndk_ndk
 * method:    addint
 * Signature: (II) I
* * Jniexport jint jnicall java_com_libin_factory_ndk_ndk_addint
  (jnienv *, Jclass, Jint, Jint);

C + + code:

Jniexport jint jnicall java_com_libin_factory_ndk_ndk_addint
        (jnienv *env, Jclass type, jint A, Jint b) {

    retur n A + b;
}

Call:

Toast.maketext (Jniactivity.this, "Add results:" + ndk.addint (2), Toast.length_short). Show ();

Show:

Description

Java_ Full Class Name _ Method name
Java_com_libin_factory_ndk_ndk_addint Java invoke C code to perform string concatenation operations

Java code:

    /**
     * Simple string concatenation via JNI
     * @param s
     * @return/public
    static native string addstring (string s);

. h Code:

 * * Class:     com_libin_factory_ndk_ndk
 * method:    addstring
 * Signature: (ljava/lang/string ;) ljava/lang/string;
 * *
jniexport jstring jnicall java_com_libin_factory_ndk_ndk_addstring
  (jnienv *, Jclass, jstring);

C + + code:

/**
 * Simple string concatenation via JNI
 * @param s
 * @return/
jniexport jstring jnicall java_com_libin_factory_ Ndk_ndk_addstring
        (jnienv *env, Jclass type, jstring s_) {

    char *text = Jstringtochar (env, s_);

    Char temp[20] = "    /  I am from C";

    strcat (text, temp);

    Return chartostring (env, text);

}

The following two are tool classes

/** * String converted to char * @param env * @param jstr * @return */char* Jstringtochar (jnienv* env, jstring jstr) {Char
    * Rtn = NULL;
    Jclass clsstring = Env->findclass ("java/lang/string");
    Jstring Strencode = Env->newstringutf ("Utf-8");
    Jmethodid mid = Env->getmethodid (clsstring, "GetBytes", "(ljava/lang/string;) [B");
    Jbytearray barr= (Jbytearray) Env->callobjectmethod (Jstr, Mid, Strencode);
    Jsize alen = Env->getarraylength (Barr);

    jbyte* ba = env->getbytearrayelements (Barr, Jni_false);
        if (Alen > 0) {rtn = (char*) malloc (alen + 1);
        memcpy (RTN, BA, Alen);
    Rtn[alen] = 0;
    } env->releasebytearrayelements (Barr, BA, 0);
return RTN;
    /** * Char is converted to String * @param env * @param chr * @return */jstring chartostring (jnienv *env, const char *CHR) {
    Logi ("chartostring:%s\n", CHR);
    Jclass strclass = Env->findclass ("java/lang/string"); Jmethodid strconstruct = Env->getmethodid (strClass, "<init>", "([bljava/lang/string;) V");
    Jbytearray bytes = Env->newbytearray (strlen (CHR));
    Env->setbytearrayregion (bytes, 0, strlen (CHR), (jbyte*) CHR);

    jstring encoding = ENV-&GT;NEWSTRINGUTF ("Utf-8");
Return (jstring) env->newobject (strclass, strconstruct, Bytes, encoding); }

Call:

Toast.maketext (Jniactivity.this, "JNI results:" + ndk.addstring ("I am from Java"), Toast.length_short). Show ();

Show:
Java calls C code executes each element plus ten

Java code:

    /**
     * Let C code give you each element plus a
     * @param intarray
     * @return
    /public static native int[] Increasearrayeles ( Int[] intarray);

. h Code:

 * * Class:     com_libin_factory_ndk_ndk
 * method:    increasearrayeles
 * Signature: ([i) [I
 */
Jniexport Jintarray jnicall java_com_libin_factory_ndk_ndk_increasearrayeles
  (jnienv *, Jclass, JintArray);

C + + code:

/**
* Let C code give you each element plus
* @param intarray
* @return * * *
jniexport jintarray jnicall
java_com_ Libin_factory_ndk_ndk_increasearrayeles (jnienv *env, Jclass type, Jintarray intarray_) {
    //Get the length of the array
    jsize size = Env->getarraylength (Intarray_);
    Gets the array element
    jint *array = env->getintarrayelements (Intarray_, jni_false);//jni_false the same part does not open up new space
    / Iterate over the array for each element by adding a for
    (int i = 0; i < size; i++) {
        * (array + i) + +;
        LOGE ("Array%d =%d", I, * (array + i));

    }
    Env->releaseintarrayelements (Intarray_, array, 0);
    Returns the result return
    Intarray_;

}

Call:

int a[] = {1,2,3,4,5,6,7};
int[] INTs = Ndk.increasearrayeles (a);
for (int i = 0; i< ints.length;i++) {
     logger.e (a[i]+ "");
}

Show:
Java calls C code execution detect password is correct

Java code:

    /**
     * Application: Detect password is correct, correct return 200, error return
     * @param pwd
     * @return
     /public
    static native int Checkpwd ( String pwd);

. h Code:

 * * Class:     com_libin_factory_ndk_ndk
 * method:    checkpwd
 * Signature: (ljava/lang/string;) I
 */
Jniexport jint jnicall java_com_libin_factory_ndk_ndk_checkpwd
  (jnienv *, Jclass, jstring);

C + + code:

/**
* Application: Detection password is correct, correct return 200, error return to
* @param pwd * @return * * * *
jniexport jint jnicall
Java_com_libin_factory_ndk_ndk_checkpwd (jnienv *env, Jclass type, jstring pwd_) {
    //Assume server password 123456
    char * Otigin = "123456";
    Char *fromuser = Jstringtochar (env, pwd_);
    Function comparison string is
    the same int code = strcmp (Otigin, fromuser);
    if (code = = 0) {return
        ;
    }

Call:

Toast.maketext (jniactivity.this, "password checksum" + ndk.checkpwd ("123456"), Toast.length_short). Show ();

Show:

Below we can look at the C call Java:
http://blog.csdn.net/github_33304260/article/details/71213921