NDK

Native Development Suite (NDK) is a set of tools that can be written in C and C++ code in Android applications for Java to Native communication.

The flow chart

The first step

Write the target class files Directory: app/SRC/main/com/zhyx/jnidemo1 / jniclass/JniExample Java

public class JniExample {
    //native
    public static native String getJniStr(a);
}
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The second step

Command to generate the corresponding header file

javac -h -jni_temp ./jniclass/JniExample.java
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Javac -h < directory > specifies the location of the native header file generated – jni_temp generated directory file called “- jni_temp”. / jniclass JniExample. Java target class file name

The third step

Write c/ C ++ concrete implementation in app/ SRC /main directory create a JNI directory



Create a.c filejniExample.cConcrete implementation:

#include <stdlib.h>
#include <stdio.h>
#include <jni.h>
#include "com_zhyx_jnidemo1_jniclass_JniExample.h"
JNIEXPORT jstring JNICALL Java_com_zhyx_jnidemo1_jniclass_JniExample_getJniStr
  (JNIEnv *env, jclass jcs) {
    char* tempStr = "I created a string from C.";
    return (*env)->NewStringUTF(env,tempStr);
 }
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The function name and return value can be copied directly from the contents of the generated. H file

The fourth step

(1) to createjni/Android.mkfile

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := jniExample
LOCAL_SRC_FILES := jniExample.c
include $(BUILD_SHARED_LIBRARY)
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  • LOCAL_PATCHThis variable represents the location of the source file in the development tree,my-dirThe path to the current directory (where the Android.mk file itself resides) is returned

  • CLEAR_VARSThe variable points to a special GNU Makefile that clears many LOCAL_XXX variables for you. Note that the GNU Makefile does not clear LOCAL_PATH. This variable must retain its value because the system parses all build control files in a single GNU Make execution context (where all variables are global). This variable must be declared (redeclared) before each module can be described

  • LOCAL_MODULEThe variable stores the name of the module you want to build. Use this variable once in each module of your application

  • LOCAL_SRC_FILESThe variable must contain a list of C and/or C++ source files to build into the module

  • BUILD_SHARED_LIBRARYThe variable points to a GNU Makefile script that collects all the information you have defined in the LOCAL_XXX variable since the most recent include. This script determines what to build and how

This is a basic must, see Android.mk for more

(2) to createjni/Application.mkfile

APP_ABI := all
APP_OPTM := debug
APP_STL := c++_static
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  • APP_ABIThis variable represents the supported Android architecture platformallMultiple platforms can be separated by Spaces, for example, APP_ABI := armeabi -v7A arm64-v8a

  • APP_OPTMDefine this optional variable asrelease 或 debug. By default, the publishing binaries are built. Publish mode enables optimization and may generate binaries that cannot be used with the debugger. Debug mode deactivates optimization so that the debugger can be used.
  • APP_STLThe C++ standard library for this application. By default, system STL is used. Other options include c++_shared, c++_static, and none.This may vary slightly depending on your current VERSION of the NDK, please check the usage.

Step 5

Enter the created jni directory and run the command to generate the corresponding. So file

Perform the NDK - buildCopy the code

So libraries that are then generated are automatically prefixed with ‘lib’ to automatically generate libs folders

Step 6

Configure the libs directory in app/build

defaultConfig {
    ...
    ndk {
        moduleName "jniExample" // This is the same as LOCAL_MODULE name
    }
}
sourceSets {
    main {
        jniLibs.srcDirs = ["src/main/libs"]}}Copy the code

Step 7

Use the C/C++ native method JniExample class file to load the library

public class JniExample {
    static {
        // The system loads the lib method
        System.loadLibrary("jniExample");
    }
    //native
    public static native String getJniStr(a);
}
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Step 8

As with normal method calls, use MainActivity directly:

findViewById<TextView>(R.id.text).text = JniExample.getJniStr()
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End