JNI包装c++类-白红宇

JNI包装c++类

阅读量：4337 次

发布时间：2019-06-07

本文共 7199 字，大约阅读时间需要 23 分钟。

section 1

定义c++类和方法
singleton PluginLoader @ namespace Vamp::HostExt
Plugin @ namespace Vamp
方案一：map一个java的class到c++的PluginLoader，返回类型太复杂，废弃

class PluginLoader { public:  static PluginLoader *getInstance();  Plugin* loadPlugin(PluginKey key,                     float inputSampleRate,                     int adapterFlags=0) {    // ... other methods for later  }};class PluginBase { public:  virtual std::string getIdentifier() const = 0;  virtual std::string getName() const = 0;  virtual std::string getDescription() const = 0;  virtual int getPluginVersion() const = 0;  // ... other methods for later};

设计java类
PluginBase是纯虚类直接映射为java的Interface，这个做法留作后面的章节重构

package org.vamp_plugins;public class Plugin { public native String getIdentifier(); public native String getName(); public native String getDescription(); public native int getPluginVersion();}package org.vamp_plugins;public class PluginLoader { public class LoadFailedException extends Exception {}; public static synchronized PluginLoader getInstance() {    // some magic here }  public Plugin loadPlugin(String key, float inputSampleRate)   throws LoadFailedException {   // and here }}

native对象handle
通过对象的handle，以上的两个java类直接对应于c++的类
PluginLoader类更新如下：

public class PluginLoader { public class LoadFailedException extends Exception {};  public static synchronized PluginLoader getInstance() {   if (inst = null) {     inst = new PluginLoader();     inst.initialise();   }      return inst; }  public Plugin loadPlugin(String key, float inputSampleRate)   throws LoadFailedException {   long handle = loadPluginNative(key, inputSampleRate);   if (handle != 0) return new Plugin(handle);   else throw new LoadFailedException(); }  private PluginLoader() { initialise(); } private static PluginLoader inst; private long nativeHandle; private native long loadPluginNative(String key, float inputSampleRate); private native void initialise();}public class Plugin { private long nativeHandle; protected Plugin(long handle) {   nativeHandle = handle;  }  public native String getIdentifier(); public native String getName(); public native String getDescription(); public native int getPluginVersion();}

生成jni函数声明
javac和javah就可做这些事

$ javac org/vamp_plugins/*.java$ javah -jni org.vamp_plugins.Plugin org.vamp_plugins.PluginLoader$ ls -1orgorg_vamp_plugins_Plugin.horg_vamp_plugins_PluginLoader.hsample.h$

生成的头文件内容片段：

JNIEXPORT jstring JNICALLJava_org_vamp_1plugins_Plugin_getIdentifier(JNIEnv*, jobject);

JNIEnv*：当前jni函数的运行环境

jobject：调用当前函数的this指针

section 2

实现jni函数
handle.h =>

#ifnef __HANDLE_H_INCLUDE__#define _HANDLE_H_INCLUDE__jfieldID getHandleField(JNIEnv* env, jobject ojb) {  jclass c = env->GetObjectClass(obj);  // J is the type singure for long:  return env->getFieldID(c, "nativeHandle", "J");}template 
    
     T* getHandle(JNIEnv* env, jobject obj) {  jlong handle = env->GetLongField(obj, getHandleField(env, obj));  return reinterpret_cast
     
      (handle);}template 
      
       T* setHandle(JNIEnv* env, jobject obj, T* t) {  jlong handle = reinterpret_cast
       
        (t);  env->SetLongField(obj, getHandleField(env, obj), handle);}#endif

pluginloader.cc =>

#include "org_vamp_plugins_PluginLoader.h"#include 
    
     #include "handle.h"using Vamp::Plugin;using Vamp::HostExt::PluginLoader;void Java_org_vamp_lpugins_PluginLoader_initialise(JNIEnv* env, jobject obj) {  PluginLoader* inst = PluginLoader::getInstance();  setHandle(env, obj, inst);}jlongJava_org_vamp_lplugins_PluginLoader_loadPluginNative(JNIEnv* env, jobject obj,  jstring key, jfloat rate) {  PluginLoader* inst = getHandle
     
      (env, obj);  const char* kstr = env->GetStringUTFChars(key, 0);  Plugin* p = inst->loadPlugin(kstr, rate);  env->ReleaseStringUTFChars(key, kstr);  return (jlong)p;  }

plugin.cc =>

#include "org_vamp_plugins_plugin.h"#include 
    
     #include "handle.h"using Vamp::Plugin;using std::string;jstringJava_org_vamp_1plugins_Plugin_getIdentifier(JNIEnv *env, jobject obj) {    Plugin *p = getHandle
     
      (env, obj);    return env->NewStringUTF(p->getIdentifier().c_str());}jstringJava_org_vamp_1plugins_Plugin_getName(JNIEnv *env, jobject obj) {    Plugin *p = getHandle
      
       (env, obj);    return env->NewStringUTF(p->getName().c_str());}jstringJava_org_vamp_1plugins_Plugin_getDescription(JNIEnv *env, jobject obj) {    Plugin *p = getHandle
       
        (env, obj);    return env->NewStringUTF(p->getDescription().c_str());}jintJava_org_vamp_1plugins_Plugin_getPluginVersion(JNIEnv *env, jobject obj) {    Plugin *p = getHandle
        
         (env, obj);    return p->getPluginVersion();}

写一个测试程序

package org.vamp_plugins;public class test {    public static void main(String[] args) {        // This is the name of a Vamp plugin we know we have installed        String key = "vamp-example-plugins:percussiononsets";        PluginLoader loader = PluginLoader.getInstance();        try {            Plugin p = loader.loadPlugin(key, 44100);            System.out.println("identifier: " + p.getIdentifier());            System.out.println("name: " + p.getName());            System.out.println("description: " + p.getDescription());            System.out.println("version: " + p.getPluginVersion());        } catch (PluginLoader.LoadFailedException e) {            System.out.println("Plugin load failed");        }    }}

section 3

处理c++对象
Java有gc，而c++需要手动管理内存，有下面两种方法：

添加一个方法，让java层调用删除对象

在java的finialize() 中删除对象
第二种方法，看似完善，但是java的gc难以精准控制native对象的状态，多线程情况下更不确定，
所以只能使用方法一：添加 dispose方法

public native void dispose();voidJava_org_vamp_1plugins_Plugin_dispose(JNIEnv* env, jobject obj) {  Plugin* p = getHandle(env, obj);  setHandle(env, obj, 0);  delete p;}

section 4

复杂一点的用法

typedef std::vector
    
      FeatureList;typedef std::map
     
       FeatureSet;virtual FeatureSet process(const float* const* inputBuffers,  RealTime timestamp) = 0;java 没有typedef 对应map如下public native Map
      
       >  process(float[][] inputBuffers, RealTime timestamp);- 在jni层构建java对象jclass featClass = env->FindClass("org/vamp_plugins/Feature");jmethodID ctor = env->GetMethodID(featClass, "
       
        ", "()V");jobject feature = env->NewObject(featClass, ctor);- jni处理泛型jclass treeMapClass = env->FindClass("java/util/TreeMap");jmethodID treeMapCtor = env->GetMethodID(treeMapClass, "
        
         ", "()V");jobject map = env->NewObject(treeMapClass, treeMapCtor);- 从多维数组中取数据jobjectJava_org_vamp_1glugins_Plugin_process(JNIEnv* env, jobject obj, jobjectArray inputBuffers, jobject timestamp);int channels = env->GetArrayLength(data);float **input = new float *[channels];for (int c = 0; c < channels; ++c) {    jfloatArray cdata =        (jfloatArray)env->GetObjectArrayElement(data, c);    input[c] = env->GetFloatArrayElements(cdata, 0);}for (int c = 0; c < channels; ++c) {    jfloatArray cdata =        (jfloatArray)env->GetObjectArrayElement(data, c);    env->ReleaseFloatArrayElements(cdata, input[c], 0);}delete[] input;