section 1

定義c++類和方法
singleton PluginLoader @ namespace Vamp::HostExt
Plugin @ namespace Vamp
方案一：map一個java的class到c++的PluginLoader，返回類型太復雜，廢棄

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 };

設計java類
PluginBase是純虛類直接映射為java的Interface，這個做法留作后面的章節重構

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對象handle
  通過對象的handle，以上的兩個java類直接對應于c++的類
  PluginLoader類更新如下：

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函數聲明
  javac和javah就可做這些事

$ javac org/vamp_plugins/*.java $ javah -jni org.vamp_plugins.Plugin org.vamp_plugins.PluginLoader $ ls -1 org org_vamp_plugins_Plugin.h org_vamp_plugins_PluginLoader.h sample.h $

生成的頭文件內容片段：

JNIEXPORT jstring JNICALL Java_org_vamp_1plugins_Plugin_getIdentifier(JNIEnv*, jobject);

JNIEnv*：當前jni函數的運行環境
jobject：調用當前函數的this指針

section 2

實現jni函數
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 <typename T> T* getHandle(JNIEnv* env, jobject obj) {jlong handle = env->GetLongField(obj, getHandleField(env, obj));return reinterpret_cast<T*>(handle); }template <typename T> T* setHandle(JNIEnv* env, jobject obj, T* t) {jlong handle = reinterpret_cast<jlong>(t);env->SetLongField(obj, getHandleField(env, obj), handle); }#endif

pluginloader.cc =>

#include "org_vamp_plugins_PluginLoader.h" #include <vamp-hostsdk/PluginLoader.h> #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); }jlong Java_org_vamp_lplugins_PluginLoader_loadPluginNative(JNIEnv* env, jobject obj,jstring key, jfloat rate) {PluginLoader* inst = getHandle<PluginLoader>(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 <vamp-hostsdk/plugin.h> #include "handle.h"using Vamp::Plugin; using std::string;jstring Java_org_vamp_1plugins_Plugin_getIdentifier(JNIEnv *env, jobject obj) {Plugin *p = getHandle<Plugin>(env, obj);return env->NewStringUTF(p->getIdentifier().c_str()); }jstring Java_org_vamp_1plugins_Plugin_getName(JNIEnv *env, jobject obj) {Plugin *p = getHandle<Plugin>(env, obj);return env->NewStringUTF(p->getName().c_str()); }jstring Java_org_vamp_1plugins_Plugin_getDescription(JNIEnv *env, jobject obj) {Plugin *p = getHandle<Plugin>(env, obj);return env->NewStringUTF(p->getDescription().c_str()); }jint Java_org_vamp_1plugins_Plugin_getPluginVersion(JNIEnv *env, jobject obj) {Plugin *p = getHandle<Plugin>(env, obj);return p->getPluginVersion(); }

寫一個測試程序

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 installedString 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

• 處理c++對象
  Java有gc，而c++需要手動管理內存，有下面兩種方法：

添加一個方法，讓java層調用刪除對象

在java的finialize() 中刪除對象
第二種方法，看似完善，但是java的gc難以精準控制native對象的狀態，多線程情況下更不確定，
所以只能使用方法一：添加 dispose方法

public native void dispose(); void Java_org_vamp_1plugins_Plugin_dispose(JNIEnv* env, jobject obj) {Plugin* p = getHandle(env, obj);setHandle(env, obj, 0);delete p; }

section 4

• 復雜一點的用法

typedef std::vector<Feature> FeatureList; typedef std::map<int, FeatureList> FeatureSet; virtual FeatureSet process(const float* const* inputBuffers,RealTime timestamp) = 0;java 沒有typedef 對應map如下 public native Map<Integer, ArrayList<Feature>>process(float[][] inputBuffers, RealTime timestamp); - 在jni層構建java對象 jclass featClass = env->FindClass("org/vamp_plugins/Feature"); jmethodID ctor = env->GetMethodID(featClass, "<init>", "()V"); jobject feature = env->NewObject(featClass, ctor); - jni處理泛型 jclass treeMapClass = env->FindClass("java/util/TreeMap"); jmethodID treeMapCtor = env->GetMethodID(treeMapClass, "<init>", "()V"); jobject map = env->NewObject(treeMapClass, treeMapCtor); - 從多維數組中取數據 jobject Java_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;

That's all, thx for reading, hope it's been useful
譯自：http://thebreakfastpost.com/2012/03/06/wrapping-a-c-library-with-jni-part-1/

轉載于:https://www.cnblogs.com/octave/p/5073958.html

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