E:\AndroidProject\TheTestPro\NDKDemo\app\build\intermediates\javac\debug\classes>javap -s -p com.lwp.ndkdemo.MainActivity
Compiled from "MainActivity.java"
public class com.lwp.ndkdemo.MainActivity extends androidx.appcompat.app.AppCompatActivity {
public static java.lang.String TAG;
descriptor: Ljava/lang/String;
public java.lang.String s;
descriptor: Ljava/lang/String;
public static int count;
descriptor: I
private java.util.Date date;
descriptor: Ljava/util/Date;
public com.lwp.ndkdemo.MainActivity();
descriptor: ()V
public native java.lang.String accessFiled();
descriptor: ()Ljava/lang/String;
public native void accessStaticFiled();
descriptor: ()V
public native java.lang.String accessStaticMethod();
descriptor: ()Ljava/lang/String;
public static java.lang.String getUUID();
descriptor: ()Ljava/lang/String;
public native int accessMethod();
descriptor: ()I
public int genRandomInt(int);
descriptor: (I)I
public native long accessConstructor();
descriptor: ()J
protected void onCreate(android.os.Bundle);
descriptor: (Landroid/os/Bundle;)V
public native java.lang.String stringFromJNI();
descriptor: ()Ljava/lang/String;
public native java.lang.String stringFromJNITwo();
descriptor: ()Ljava/lang/String;
public native java.lang.String hehedaFromJNI();
descriptor: ()Ljava/lang/String;
static {};
descriptor: ()V
}
MainActivity
package com.lwp.ndkdemo;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;
import java.util.Date;
import java.util.Random;
import java.util.UUID;
public class MainActivity extends AppCompatActivity {
// Used to load the 'native-lib' library on application startup.
static {
System.loadLibrary("native-lib");
}
//***********************************************
public static String TAG = "MainActivity";
//待修改的属性，一会儿我们通过C++把它修改为super shadow
public String s = "shadow";
//这个方法就是触发点，告诉C/C++层代码去修改上面的成员属性s
public native String accessFiled();
//***********************************************
//静态属性
public static int count = 3;
public native void accessStaticFiled();
//触发native层调用静态方法
public native String accessStaticMethod();
//静态方法，用来产生一个随机的UUID字符串
public static String getUUID(){
return UUID.randomUUID().toString();
}
//触发native层调用非静态方法
public native int accessMethod();
////产生指定范围的随机数
public int genRandomInt(int max){
return new Random().nextInt(max);
}
private Date date;
//访问Date类的构造函数
public native long accessConstructor();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Example of a call to a native method
TextView tv = findViewById(R.id.sample_text);
// tv.setText(hehedaFromJNI());
//***********************************************
//通过打印log我们看一下修改是否成功
Log.e(TAG + "修改前s是：" ,s);
//调用native方法，让C代码修改成员变量s
accessFiled();
Log.e(TAG + "修改后s是：" ,s);
//***********************************************
Log.e(TAG + "修改前count是：" ,count + "");
//调用native方法，让C代码修改成员变量county
accessStaticFiled();
Log.e(TAG + "修改后count是：" ,count + "");
//触发native层 调用 静态方法
// tv.setText(accessStaticMethod());
//触发native层 调用 非静态方法
// tv.setText(accessMethod() + "");
//触发native层 调用 构造方法
tv.setText(accessConstructor() + "");
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
*/
public native String stringFromJNI();
public native String stringFromJNITwo();
public native String hehedaFromJNI();
}
heheheda.cpp
#include <jni.h>
#include <string>
extern "C"
JNIEXPORT jstring JNICALL
Java_com_lwp_ndkdemo_MainActivity_hehedaFromJNI(JNIEnv *env, jobject thiz) {
std::string hello = "heheda heheda heheda from C++";
return env->NewStringUTF(hello.c_str());
}
extern "C"
JNIEXPORT jstring JNICALL
Java_com_lwp_ndkdemo_MainActivity_accessFiled(JNIEnv *env, jobject thiz) {
//1.obj就代表MainActivity的实体对象，但我们这里需要的是MainActiviyt.class字节码文件，
// 通过以下方法可得到MainActiviy对应的字节码文件。
jclass cls = (env->GetObjectClass(thiz));
//2.找到属性对应的id-->jfieldID,函数三个参数：
// cls就是上面得到的 MainActiviyt.class字节码文件。
// s就是属性名称，这个是我们在MainActivity中自己定义的。
// 最后一个是属性签名，
jfieldID fid = env->GetFieldID(cls, "s", "Ljava/lang/String;");
//3.获取属性的值,
// 修改原来的属性shadow为super shadow.。
// 一般获取某个属性的值，用到的方法都是这种模式：Get<Type>Field
jstring jstr = static_cast<jstring>(env->GetObjectField(thiz, fid));
//4.将jni数据类型转为对应的C数据类型，jstring -> c字符串，
//方便下一步的字符串拼接操作，
//关于第二个参数，isCopy 是否赋值（true代表赋值，false不赋值），一般写为NULL或JNI_FALSE就可以
char *c_str = const_cast<char *>(env->GetStringUTFChars(jstr, JNI_FALSE));
//拼接得到新的字符串，下面这两句都是c语言的语法，首先定义了一个字符数组（也即字符串）
//然后把它和上面得到的字符串c_str拼接到一起
char text[20] = "super ";
strcat(text,c_str);
//5.再把C数据类型转回到jni对应的数据类型，c字符串 ->jstring
jstring new_jstr = env->NewStringUTF(text);
//6.修改属性，模式：Set<Type>Field.这个和上面的GetObjectField方法类似，只是多了一个参数
//这个参数就是我们修改后的属性的值。
env->SetObjectField(thiz, fid, new_jstr);
//只要使用了GetStringUTFChars或GetStringUTF函数，记得一定要去释放。
//释放GetStringUTFChars函数或GetStringUTF函数
env->ReleaseStringUTFChars(jstr,c_str);
//7.返回修改后的属性值
return new_jstr;
}
extern "C"
JNIEXPORT void JNICALL
Java_com_lwp_ndkdemo_MainActivity_accessStaticFiled(JNIEnv *env, jobject thiz) {
jclass cls = env->GetObjectClass(thiz);
jfieldID fid = env->GetStaticFieldID(cls, "count", "I");
jint count = env->GetStaticIntField(cls, fid);
//修改静态属性值
count++;
//SetStatic<Type>Field
env->SetStaticIntField(cls,fid,count);
}
extern "C"
JNIEXPORT jstring JNICALL
Java_com_lwp_ndkdemo_MainActivity_accessStaticMethod(JNIEnv *env, jobject thiz) {
//jclass
jclass cls = env->GetObjectClass(thiz);
//获取方法对应的id
jmethodID mid = env->GetStaticMethodID(cls, "getUUID", "()Ljava/lang/String;");
//调用java中的静态方法getUUID()，模式：CallStatic<Type>Method
jstring uuid = static_cast<jstring>(env->CallStaticObjectMethod(cls, mid));
return uuid;
}
extern "C"
JNIEXPORT jint JNICALL
Java_com_lwp_ndkdemo_MainActivity_accessMethod(JNIEnv *env, jobject thiz) {
jclass cls = env->GetObjectClass(thiz);
jmethodID mid = env->GetMethodID(cls, "genRandomInt", "(I)I");
//第三个参数为方法的形参，个数不限
//这里则是在Java层定义的genRandomInt(int max)中的max，即产生的随机数最大不能超过200
jint random = env->CallIntMethod(thiz, mid, 200);
return random;
}
extern "C"
JNIEXPORT jlong JNICALL
Java_com_lwp_ndkdemo_MainActivity_accessConstructor(JNIEnv *env, jobject thiz) {
//一般的，当我们可以通过一个类的对象拿到对应的jclass时，就用getObjectClass()方法
//当没有一个类的对象，我们就需要通过这个类来拿到其对应的jclass,这个时候就用findClass()
jclass cls = env->FindClass("java/util/Date");
//jmethodID,<init>就代表的是构造方法
// 这里"<init>"可能会爆红，不用管。。。直接运行，没问题的
jmethodID constructor_mid = env->GetMethodID(cls, "<init>", "()V");
//实例化一个Date对象
jobject date_obj = env->NewObject(cls, constructor_mid);
//通过得到的Date对象其调用getTime方法
jmethodID mid = env->GetMethodID(cls, "getTime", "()J");
jlong time = env->CallLongMethod(date_obj, mid);
return time;
}
native-lib.cpp
#include <jni.h>
#include <string>
extern "C" JNIEXPORT jstring JNICALL
Java_com_lwp_ndkdemo_MainActivity_stringFromJNI(
JNIEnv* env,
jobject /* this */) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}
extern "C"
JNIEXPORT jstring JNICALL
Java_com_lwp_ndkdemo_MainActivity_stringFromJNITwo(
JNIEnv *env,
jobject thiz) {
std::string hello = "Hehehehehehehehehehe from C++";
return env->NewStringUTF(hello.c_str());
}
CMakeLists.txt
# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html
# Sets the minimum version of CMake required to build the native library.
cmake_minimum_required(VERSION 3.4.1)
# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.
add_library( # Sets the name of the library.
native-lib
# Sets the library as a shared library.
SHARED
# Provides a relative path to your source file(s).
native-lib.cpp heheheda.cpp )
# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.
find_library( # Sets the name of the path variable.
log-lib
# Specifies the name of the NDK library that
# you want CMake to locate.
log )
# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.
target_link_libraries( # Specifies the target library.
native-lib
# Links the target library to the log library
# included in the NDK.
${log-lib} )
app.gradle
apply plugin: 'com.android.application'
android {
compileSdkVersion 29
buildToolsVersion "29.0.2"
defaultConfig {
applicationId "com.lwp.ndkdemo"
minSdkVersion 21
targetSdkVersion 29
versionCode 1
versionName "1.0"
testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
externalNativeBuild {
cmake {
cppFlags ""
}
}
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
}
}
externalNativeBuild {
cmake {
path "src/main/cpp/CMakeLists.txt"
version "3.10.2"
}
}
}
dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'androidx.test.ext:junit:1.1.1'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
}
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