JNI in Android Studio--2--Writing C files

In the previous article, we wrote 2 methods in the native interface

Generate the corresponding. h file

At this time, we need to improve our own. C File

/* Don't EDIT this file-it are machine generated */
#include <jni.h>
#include <android/log.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>


/* Header for class LED_OKI_COM_JNIDEMO_APPJNI */

#ifndef _included_led_oki_com_jnidemo_appjni
#define _included_led_oki_com_jnidemo_appjni
#ifdef __cplusplus
extern "C" {
#endif

This section is constant, fixed

#define TAG    "jnidemo-jni-test"//This is the identity of the custom log  
#define LOGD (...)  

This section is primarily a custom log ID, equivalent to the LOG.D () method in Android


Method One: Getstringfromnative

Jniexport jstring Jnicall java_com_oki_led_jni_appjni_getstringfromnative
(JNIENV * env, Jobject jobj) {
   Logd ("Log string from NDK.");
   Return Env->newstringutf (env, "Hello from jni!");
}

This paragraph, corresponding to one of our native writing methods Getstringfromnative, the custom log use is using the custom method name LOGD ("Hint content")
This method means to return a string type of data that is the Hello from jni!
This string can be received on Android
Appjni Appjni = new Appjni ();
String msg = appjni.getstringfromnative ();
At this point, the MSG receives the data is the Hello from jni!


Method Two: Docoordtest (COORD COORD)

Jniexport Jobject Jnicall Java_com_oki_led_jni_appjni_docoordtest
(jnienv *env, Jobject obj, Jobject coord) {
Jclass Coordclass = (*env)->getobjectclass (env, coord);
if (Coordclass) {
Jboolean iscopy;
Jfieldid xId = (*env)->getfieldid (env, Coordclass, "X", "I");
Jint x = (int) (*ENV)->getintfield (env, coord, xId);
LOGD ("x =%d", x);
            
Jfieldid Yid = (*env)->getfieldid (env, Coordclass, "Y", "I");
Jint y = (int) (*ENV)->getintfield (env, coord, Yid);
Logd ("y =%d", y);
                        
Jfieldid Widthid = (*env)->getfieldid (env, Coordclass, "width", "I");
Jint width = (int) (*ENV)->getintfield (env, coord, Widthid);
LOGD ("width =%d", width);
            
Jfieldid Heightid = (*env)->getfieldid (env, Coordclass, "height", "I");
Jint height = (int) (*ENV)->getintfield (env, coord, Heightid);
LOGD ("height =%d", height);
}
     
Jclass cls = (*env)->findclass (env, "Com/oki/led/jni/nativeobj/coord");
Jmethodid id = (*env)->getmethodid (env, CLS, "<init>", "() V");
     
Jobject paramout = (*env)->newobjecta (env, CLS, ID, 0);
     
return paramout;
}

Where Coord is a Coord object class created in Android

public class COORD {
    public int x;
    public int y;
    public int width;
    public int height;
    
    /**
     * Set the initial value
* Why to set the initial value: prevent unnecessary problems such as data error or content overflow in C file, also reduce the judgment of data in C file, lazy method (can not be used)
     */
    Public COORD () {
        x = 0;
        y = 0;
        width = 0;
        Height = 0;
    }
}

If a class is passed in the native, the data in the C file should initially receive the Jobject type, converted to the Jclass type by the Jobject type, and the values in the class

Jclass Coordclass = (*env)->getobjectclass (env, coord);

Fragment 1: Taking a value

Take the value x ID in the class
Jfieldid xId = (*env)->getfieldid (env, Coordclass, "X", "I");
Converts the x ID to the corresponding type of x value
Jint x = (int) (*ENV)->getintfield (env, coord, xId);
Print x data to determine if the value is correct during Android operation
LOGD ("x =%d", x);

Fragment 2: Calling the initial method

Find Class class COORD
Jclass cls = (*env)->findclass (env, "Com/oki/led/jni/nativeobj/coord");
Call its initialization method on this class
Jmethodid id = (*env)->getmethodid (env, CLS, "<init>", "() V");
To convert data from a CLS class to a Jobject class
Jobject paramout = (*env)->newobjecta (env, CLS, ID, 0);
Return the data out
return paramout;

Fragment 3: Assigning values

If you need to return the value in fragment 1, you need to create a new Jobject method to host the data
Find Class class COORD
Jclass cls = (*env)->findclass (env, "Com/oki/led/jni/nativeobj/coord");
But you don't have to.
Jmethodid id = (*env)->getmethodid (env, CLS, "<init>", "() V");
To convert data from a CLS class to a Jobject class
Jobject paramout = (*env)->newobjecta (env, CLS, ID, 0);

Jfieldid xId = (*env)->getfieldid (env, Coordclass, "X", "I");
Assigns a XID to a value of 3
(*env)->setintfield (env, Paramout, XId, 3);

return paramout;

Basically, the question remains: is there a more convenient way to assign a value? Is there some code duplication inside?

JNI in Android Studio--2--Writing C files