Android NDK 开发

Android NDK 开发

JNI 基础

1. Java 与 C/C++ 基本类型对应

Java	jni	占用大小
byte	jbyte	1
short	jshort	2
int	jint	4
float	jfloat	4
long	jlong	8
double	jdouble	8
char	jchar	2
boolean	jboolean	1
void	void

定义在

JDK_HOME/include/jni.h

JDK_HOME/include/platform/jni_md.h

1. 引用类型对应

Java引用类型	Java jni
Class	jclass
Object	jobject
String	jstring
Array	jarray,jbooleanArray,jbyteArray,jcharArray….
throwable	jthrowable

1. 域和方法IDs
   jfiledID,jmethodID

   struct _jfieldID;              /* opaque structure */ 
   typedef struct _jfieldID *jfieldID;   /* field IDs */ 
   
   struct _jmethodID;              /* opaque structure */ 
   typedef struct _jmethodID *jmethodID; /* method IDs */ 

2. jvalue Type

   typedef union jvalue { 
   jboolean z; 
   jbyte    b; 
   jchar    c; 
   jshort   s; 
   jint     i; 
   jlong    j; 
   jfloat   f; 
   jdouble  d; 
   jobject  l; 
   } jvalue; 

3. 类型签名(Type Signatures)

Type Signature	Java Type
Z	boolean
B	byte
C	char
S	short
I	int
J	long
F	float
D	Double
L fully-qualified-class;	full-qualified-class
[ type	type[]
(arg-types)ret-type	method type

对于Java方法:
long f (int n,String s,int[] arr);

有如下签名:
(ILjava/lang/String;[I)J
需要注意的是引用类型String,必须像这样Ljava/lang/String; 分号不能少。
参考

JNI 几个重要函数及变量

1. System.loadLibrary()
   导入so库函数。比如你的库是libhelljni.so 那么导入是这样的System.loadLibrary(“helljni”)
   该函数调用一般是在静态代码块中

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

2. JNIEnv
   JNIEnv JNI接口指针 用于调用方法等

3. JNI_OnLoad
   在加载so之后需要做的工作，主动注入native函数需要实现

JNI 主动注册和被动注册 native 函数

1. 调用 RegisterNatives 主动注册JNI函数

   Android 系统模块一般都用这个方法。具体可查看源码
   frameworks\base\core\jni
   frameworks\base\services\jni
   fremeworks\base\media\jni

   我们来拿源码Media部分代码看看这个这过程这么做的。
   首先，需要在Java代码中声明native函数
   framework/base/media/java/android/media/MediaScanner

   package android.media;
   .......
   public class MediaScanner
   {
      static {
          System.loadLibrary("media_jni");
          native_init();
      }
      .........
          private static native final void native_init();
          private native final void native_setup();
          private native final void native_finalize();
   }

   C++实现/framework/base/media/jni/android_media_MediaScanner.cpp

   // This function gets a field ID, which in turn causes class initialization.
   // It is called from a static block in MediaScanner, which won't run until the
   // first time an instance of this class is used.
   static void android_media_MediaScanner_native_init(JNIEnv *env)
   {
     ALOGV("native_init");
     jclass clazz = env->FindClass(kClassMediaScanner);
     if (clazz == NULL) {
         return;
     }
   
     fields.context = env->GetFieldID(clazz, "mNativeContext", "I");
     if (fields.context == NULL) {
         return;
     }
   }
   .......
   static JNINativeMethod gMethods[] = {
   ......
   
   {
       "native_init",
       "()V",
       (void *)android_media_MediaScanner_native_init
   },
   
   {
       "native_setup",
       "()V",
       (void *)android_media_MediaScanner_native_setup
   },
   
   {
       "native_finalize",
       "()V",
       (void *)android_media_MediaScanner_native_finalize
   },
   };
   ........
   // This function only registers the native methods, and is called from
   // JNI_OnLoad in android_media_MediaPlayer.cpp
   int register_android_media_MediaScanner(JNIEnv *env)
   {
     return AndroidRuntime::registerNativeMethods(env,
               kClassMediaScanner, gMethods, NELEM(gMethods));
   }

   真正导入so然后调用JNI_OnLoad 然后注册registerNativeMethods否则找不到native函数
   我们看看framework/base/media/jni/android_media_MediaPlayer.cpp

   ......
   extern int register_android_media_MediaScanner(JNIEnv *env);
   ......
   jint JNI_OnLoad(JavaVM* vm, void* reserved)
   {
   JNIEnv* env = NULL;
   jint result = -1;
   
   if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
       ALOGE("ERROR: GetEnv failed\n");
       goto bail;
   }
   assert(env != NULL);
   
   ........
   
   if (register_android_media_MediaScanner(env) < 0) {
       ALOGE("ERROR: MediaScanner native registration failed\n");
       goto bail;
   }
   .......
   
   /* success -- return valid version number */
   result = JNI_VERSION_1_4;
   
   bail:
     return result;
   }
   

2. 被动注册Native函数

   这个很多jni编写教程都是这种方法,大致流程是这样的

   • 编写Java native 代码，javac 生产class(Android Studio 可以自动构建)
   • 使用javah命令，生产包含JNI本地函数原型的头文件
     一般生产的函数原型头文件是这样的

      /* DO NOT EDIT THIS FILE - it is machine generated */
   
       #include 
      /* Header for class com_XXX_XXXX_utils_NativeSpeedTest    */
   
     #ifndef _Included_com_XXX_XXXX_utils_NativeSpeedTest
     #define _Included_com_XXX_XXXX_utils_NativeSpeedTest
     #ifdef __cplusplus
     extern "C" {
     #endif
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    startDownloadWireless
        * Signature: ([Ljava/lang/String;II)I
        */
       JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_startDownloadWireless
         (JNIEnv *env, jclass cclass, jobjectArray jurls, jint jconns , jint jduration);
   
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    signatureVerify
        * Signature: (Ljava/lang/String;)I
        */
       JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_signatureVerify
         (JNIEnv *env, jclass cclass, jstring jstr);
   
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    init
        * Signature: (Ljava/lang/String;)I
        */
       JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_init
         (JNIEnv *env, jclass cclass, jstring jstr);
   
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    uninit
        * Signature: ()V
        */
       JNIEXPORT void JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_uninit
         (JNIEnv *env, jclass cclass);
   
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    startDownload
        * Signature: ([Ljava/lang/String;II)I
        */
       JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_startDownload
         (JNIEnv *env, jclass cclass, jobjectArray jurls, jint jconns, jint jduration);
   
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    statUpload
        * Signature: ([Ljava/lang/String;II)I
        */
       JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_statUpload
         (JNIEnv *env, jclass cclass, jobjectArray jurls, jint jconns, jint jduration);
   
       /*
        * Class:     com_XXX_XXXX_utils_NativeSpeedTest
        * Method:    callBack
        * Signature: (Ljava/lang/String;Ljava/lang/String;)I
        */
       JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_callBack
         (JNIEnv *env, jclass thiz, jstring jclassname, jstring jmethodname);
   
       #ifdef __cplusplus
       }
       #endif
   
   #endif
   

   自动生产的话是没有定义变量像这样

   
/*
* Class: com_XXX_XXXX_utils_NativeSpeedTest
* Method: callBack
* Signature: (Ljava/lang/String;Ljava/lang/String;)I
*/
JNIEXPORT jint JNICALL Java_com_XXX_XXXX_utils_NativeSpeedTest_callBack
(JNIEnv , jclass, jstring, jstring);

   自己加上变量名

   • 实现JNI本地函数
     然后新建.cpp文件实现上面函数

   • 生产C 共享库
     ndk-build 生产so文件，Android studio 需要做ndk配置。
     build.gradle

     android{
      defaultConfig{
             ndk{
                 moduleName "XXX"
              ldLibs "log"
              abiFilters "armeabi", "armeabi-v7a", "x86"
             }
      }
     }

     在main/下建立 jni和jniLibs

     gradle.properties加入

     android.useDeprecatedNdk=true

   • 通过JNI，调用JNI本地函数

这两种方式建议用主动注册方式(或者动态注册),编写起来函数名简便，且不需要VM去查找函数省时间。
参考