本文簡介:
最近看了一些文章,看到大部分關於Android jni的配置都還是ndkBuild模式的;有的則是說怎麼引用已經編譯好的os文件。本文主要以一個小dome爲列,記錄Android jni cmake模式下的配置。
一、環境配置
Android 開發的環境配置網上已經很多了,這裏就不重複。我們做jni開發的,需要用到snk,所以我們第一步,在Android studio 上引入ndk
這裏我們主要需要勾選NDK、CMake。爲啥要勾選CMake因爲我們自己常常用傳統方法生成,配置太過於麻煩,所以我們這裏用CMake,直接生成一個標準的例子,進行修改。這樣就會減少很多問題的產生,節省我們的時間。
二、項目創建
(一)首先創建新項目,選擇創建NativityC++項目
(二)填寫項目信息。
選擇對c++標準,這裏選擇的默認。
(三)創建成功後項目架構是如下圖
三、與Android項目有些差別及說明
(一)app下的build.gradle
這裏多了些關於編譯ndk的入口配置數據
externalNativeBuild {
cmake {
cppFlags ""
}
}
這裏的cppFlags 值爲引用的其他類庫比如:
cppFlags "-fexceptions"
編譯snk異常捕獲的
externalNativeBuild {
cmake {
path "src/main/cpp/CMakeLists.txt"
version "3.10.2"
}
}
path :指定編譯的目標配置文件地址
version:編譯版本的
(二)結構上:
main下面多了cpp的包,裏面有多了一個CMakeLists.txt和native-lib.cpp
這兩個文件,CMakeLists.txt是關於cmake的一些基礎配置和編譯ndk的文件關聯配置
native-lib.cpp則是C++將要編譯的代碼文件
1、首先看CMakeLists.txt的內容:
# 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)
# 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})
add_library 下面配置的os類庫對應類。native-lib爲類庫名,是生成os文件的名稱,也是我們java加載java的名稱。
native-lib.cpp 則是對應java方法生成的對C/C++的目錄地址。該文件地址與CMakeLists.txt文件位置相對
2、native-lib.cpp 查看其內容:
#include <jni.h>
#include <string>
extern "C" JNIEXPORT jstring JNICALL
Java_com_toy_key_cjni_MainActivity_stringFromJNI(
JNIEnv *env,
jobject /* this */) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}
這是一個C/C++的代碼方法,起作用就是調用stringFromJNI該方法,返回"Hello form C++"的字符串
(三)java中MainActivity
public class MainActivity extends AppCompatActivity {
// Used to load the 'native-lib' library on application startup.
static {
System.loadLibrary("native-lib");
}
@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(stringFromJNI());
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
*/
public native String stringFromJNI();
}
代碼中多了 static { System.loadLibrary("native-lib"); } 靜態代碼塊,作用將native-lib庫加載到系統庫中,這裏native-lib對應的就是CMakeLists.txt配置文件中設置的庫名。
public native String stringFromJNI();代碼聲明瞭native映射C/C++的方法。在native-lib.cpp中實現
(四)、os庫編譯
選AndroidStudio的build-Rebuild Project,進行編譯。
編譯成功後可以從項目app/build/intermediates/cmake/debug/obj/下面查看到對應不同處理器的C庫了
(五)項目運行效果
這裏已經基本講述完整個項目的關鍵地方,不過這些都是項目自己生成的,不自己動手不容易變成自己的知識,接下來我們嘗試去修改,從而掌握理解
四、修改項目
1、創建一個JNIHelper類來聲明管理接口,將MainActivity中關於調用C庫的代碼移動到該類,
public class JNIHelper {
// Used to load the 'native-lib' library on application startup.
//加載到系統 自定義的JNILibrary 庫
static {
System.loadLibrary("JNILibrary");
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
* 聲明JNILibrary庫中獲取String的方法
*/
public static native String stringFromJNI();
}
2、MainActivity代碼內容是:
public class MainActivity extends AppCompatActivity {
@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(JNIHelper.stringFromJNI());
}
}
這樣移動後項目會報錯,會讓你重新編譯,暫時不去理會他,當修改完全後從新編譯就不會出問題
3、CMakeLists.txt的修改後
# 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.
aux_source_directory(./ DIR_SRCS)
add_library( # Sets the name of the library.
JNILibrary
# Sets the library as a shared library.
SHARED
# Provides a relative path to your source file(s).
${DIR_SRCS})
# 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.
JNILibrary
# Links the target library to the log library
# included in the NDK.
${log-lib})
這裏將add_library 方法和target_link_libraries方法中的 native-lib 修改爲了自定義JNILibrary庫名,這裏庫名任意改動但是要與JNIHelper中的引用庫名對應一致。
此外這裏我將指定的C/C++庫文件做了調整。在上面聲明瞭一個字典變量 aux_source_directory(./ DIR_SRCS)
其中./是對於CMakeLists.txt 路徑的C/C++源碼文件的相對路徑。DIR_SRCS 則是引用庫名文件對應的C/C++源文件名
比如這裏的${DIR_SRCS} = JNILibrary.cpp
修改完CMakeLists.txt後我們去修改C/C++源文件
在對應的cpp目錄下創建一個NILibrary.cpp的文件(注意創建的文件名與自己定義庫名保持一致,不然無法編譯通過)
創建成功後可以看到cpp目錄下
然後往JNILibrary.cpp 實現對應JNIHelper類聲明的方法,
//
// Created by MZJ on 2020-04-22.
//
#include <jni.h>
#include <string>
extern "C"
JNIEXPORT jstring JNICALL
Java_com_toy_key_cjni_JNIHelper_stringFromJNI(JNIEnv *env, jobject thiz) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}
這裏要注意的是實現的方法和你的JNIHelper有關聯,可以看到方法名 Java_com_toy_key_cjni_JNIHelper_stringFromJNI
及規則爲Java+包名+方法名 以_隔開。
當然自己寫比較麻煩你也可以讓Android Studio自動幫你生成。
創建完JNILibrary.cpp 不去寫C/C++代碼,然後進行build project。這是我們可以看到java的JNIHelper中未實現的方法名爲紅色
選中方法按 option+回車鍵提示,選擇創建jni對應方法
創建成功後是這樣
方法的實現需要自己進行代碼完善,添加返回想要返回的字符。
注意:這裏通過提示生成的可能會生成是.c文件這是默認生成C語言的文件,這個時候實現的第一個方法只能自己手工打對應方法了。後面的再有其他方法纔會映射到這個JNILibrary.cpp文件自動生成。
然後再在方法裏實現對應的邏輯。我上面的則是直接按照dome的返回字符寫的
好這裏算是修改完成了,運行後如上面的運行的結果一樣則說明修改成功了。我再舉一反三。修改一個回調方法出來。
五、修改一個回調方法
1、在java裏創建一個NativeListener接口類裏面有個修改Text的方法
public interface NativeListener {
void changeText(String msg);
}
2、在JNIHelper裏聲明一個綁定NativeListener的方法
package com.toy.key.cjni;
public class JNIHelper {
// Used to load the 'native-lib' library on application startup.
//加載到系統 自定義的JNILibrary 庫
static {
System.loadLibrary("JNILibrary");
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
* 聲明JNILibrary庫中獲取String的方法
*/
public static native String stringFromJNI();
/**
* 註冊NativeListener監聽
*/
public static native void registerListener(NativeListener listener);
}
按option+回車鍵提示,並創建 jni 方法,然後在JNILibrary.cpp 會生成對應方法
//
// Created by MZJ on 2020-04-22.
//
#include <jni.h>
#include <string>
extern "C"
JNIEXPORT jstring JNICALL
Java_com_toy_key_cjni_JNIHelper_stringFromJNI(JNIEnv *env, jobject thiz) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}
extern "C"
JNIEXPORT void JNICALL
Java_com_toy_key_cjni_JNIHelper_registerListener(JNIEnv *env, jclass clazz, jobject listener) {
// TODO: implement registerListener()
}
3、編輯JNILibrary.cpp的邏輯
//
// Created by MZJ on 2020-04-22.
//
#include <jni.h>
#include <string>
//聲明變量
jobject _listener;//
JNIEnv* _env;
jmethodID _changeTextMethodID;
jclass _class;
extern "C"
JNIEXPORT jstring JNICALL
Java_com_toy_key_cjni_JNIHelper_stringFromJNI(JNIEnv *env, jobject thiz) {
//當調用stringFromJNI時調用registerListener中NativeListener的changeText回調函數
_env->CallVoidMethod(_listener, _changeTextMethodID, _env->NewStringUTF("你猜我說啥了"));
//創建變量std並賦值爲Hello from C++
std::string hello = "Hello from C++";
//返回數據
return env->NewStringUTF(hello.c_str());
}
extern "C"
JNIEXPORT void JNICALL
Java_com_toy_key_cjni_JNIHelper_registerListener(JNIEnv *env, jclass clazz, jobject listener) {
// TODO: implement registerListener()
//將對應的類和方法存放臨時變量
_listener = env->NewGlobalRef(listener);//獲取對象類
_env = env;
_class = _env->GetObjectClass(_listener);//獲取實體類
_changeTextMethodID = _env->GetMethodID(_class, "changeText", "(Ljava/lang/String;)V");//獲取方法
}
4、編輯MainActivity
//實現接口NativeListener
public class MainActivity extends AppCompatActivity implements NativeListener {
TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tv = findViewById(R.id.sample_text);
//綁定監聽
registerListener(this);
}
public void onViewClick(View view) {
//調用c庫的stringFromJNI方法獲取返回值並從Toast中顯示
String str = stringFromJNI();
Toast.makeText(this,str,Toast.LENGTH_SHORT).show();
}
@Override
public void changeText(String msg) {//實現接口NativeListener 的chaneText方法。修改文字
tv.setText(msg);
}
}
5、activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="@+id/tv_content"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="10dp"
android:text="Hello World!"
android:onClick="onViewClick"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
六、運行結果
點擊後的效果
注意:若是之前編譯運行過的,可能會出現庫引用錯誤的衝突,所以在新編譯運行時記得清理一下
先刪除這框起來的兩個文件,然後在clean 項目,再運行就可以。
七、C/C++內容延展
在寫邏輯是有個方獲取,其中涉及到了參數類型
_changeTextMethodID = _env->GetMethodID(_class, "changeText", "(Ljava/lang/String;)V");//獲取方法
這裏的慘(Ljava/lang/String;)表示String參數,V表示的void方法
對應表NDK中的規則如下:
字符簽名 | jni中類型 | java中類型 |
---|---|---|
V | void | void |
Z | jboolean | boolean |
I | jint | int |
J | jlong | long |
D | jdouble | double |
F | jfloat | float |
B | jbyte | byte |
C | jchar | char |
S | jshort | short |
對於數組而言,需要以"["開始,組合以上規則即可,具體對應關係表如下:
字符簽名 | jni中類型 | java中類型 |
---|---|---|
[Z | jbooleanArray | boolean[] |
[I | jintArray | int[] |
[J | jlongArray | long[] |
[D | jdoubleArray | double[] |
[F | jfloatArray | float[] |
[B | jbyteArray | byte[] |
[C | jcharArray | char[] |
[S | jshortArray | short[] |
其中要注意的對象類型數據後面要跟“;”,比如上面的“String;”
其寫法爲 包名+對象如 String 就是 java/long/String;
暫時就寫這麼多吧,作爲一個簡單記錄,以後有機會再深入寫。