Android Studio JNI development (2) - Java calls C method to output string

Android Studio JNI development (2) - Java calls C method to output string

1. Create a new Android project
   

2. Configure the ndk path in the local.properties file

ndk.dir=D\:\\Android\\SDK\\ndk-bundle

3. File ----> Project Structure ----> SDK Lcation配置Android NDK location。
   

4. The gradle.properties file configuration is compatible with the old ndk

android.useDeprecatedNdk=true

5. Create a new Java JNI class MyJni.java, and write the JNI native method.

package com.csu.ndkdemo;

public class MyJNI {
    
    

    public native String sayHello();

}

6. To generate the header file, enter the project path NdkDemo\app\src\main\java with cmd, and execute the command to generate the .h header file.

javah -classpath . -jni com.csu.ndkdemo.MyJNI

Generate a .h header file in the same directory as the MyJNI.java file (main\java\com.csu.ndkdemo) com_csu_ndkdemo_MyJNI.h.

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_csu_ndkdemo_MyJNI */

#ifndef _Included_com_csu_ndkdemo_MyJNI
#define _Included_com_csu_ndkdemo_MyJNI
#ifdef __cplusplus
extern "C" {
    
    
#endif
/*
 * Class:     com_csu_ndkdemo_MyJNI
 * Method:    sayHello
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_csu_ndkdemo_MyJNI_sayHello
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

7. Create a new jni directory in the same directory as java under main, copy the .h file generated by step6 to the jni directory, and then create a new Hello.c file to implement the method in the .h file.

#include <stdio.h>
#include <stdlib.h>
#include <jni.h>

JNIEXPORT jstring JNICALL Java_com_csu_ndkdemo_MyJNI_sayHello(JNIEnv *env, jobject thiz) {
    
    
    return (*env)->NewStringUTF(env, "I`m Str from jni libs!");
}

8. Create a new Android.mk file under the jni directory.
   

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := Hello
LOCAL_SRC_FILES := Hello.c

APP_ABI := all

include $(BUILD_SHARED_LIBRARY)

9. Enter the project path NdkDemo\app\src\main\java with cmd, execute the command to compile and generate the .so file.

ndk-build

Two folders libs and obj will be generated in the same directory of jni.

10. Copy the contents of the libs folder to the libs directory under the module project name.
    
11. Add a reference to libs in the android{} node of the module's build.gradle file:

sourceSets {
    main {
        jniLibs.srcDirs = ['libs']
    }
}

12. The .so file is loaded in the JNI Java file MyJni.java.

package com.csu.ndkdemo;

public class MyJNI {
    
    

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

    public native String sayHello();

}

13. Call the JNI method in the Activity and print out the string in C.

public class MainActivity extends AppCompatActivity {
    
    

    @Override
    protected void onCreate(Bundle savedInstanceState) {
    
    
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

		// 调用JNI方法
        String rs = new MyJNI().sayHello();
        Log.i("MainActivity", "onCreate: " + rs);
    }
}