Android JNI中Java與C++物件如何建立一一對應的關係

環境

系統：Mac OSX
IDE：android studio 3.0

1.JAVA物件持有C++物件的指標

• 在java類中建立long型別的變數如（long mNativeStudent ），用於儲存c++物件的指標，java程式碼如下；

public class Student {

   private long mNativeStudent = -1; // 用於存放 c++ 物件的指標
   private final String TAG = "Student";
   public
 
 Student(){
      mNativeStudent = CreateStudent();
   }

   public void setName(String name){
       SetName(name);
   }

   public String getName(){
      return GetName();
   }

   public void setGrade(int grade){
      SetGrade(grade);
   }

   public int getGrade(){
      return GetGrade();
   }

   public
 
 String print(){
      return Print();
   }

   public static String printClassName(){
      return PrintClassName();
   }

   @Override
   protected void finalize() throws Throwable {
      DestoryStudent(mNativeStudent);
      super.finalize();
   }

   private native long CreateStudent();

   private
 
 native void DestoryStudent(long nativeStuent);

   private native void SetName(String name);

   private native String GetName();

   private native void SetGrade(int grade);

   private native int GetGrade();

   private native String Print();

   private static native String PrintClassName();
}

• 在建立C++物件（如NativeStudent）的本地方法中，將C++物件的指標存入java變數中（mNativeStudent）

jlong CreateStudent(JNIEnv *env, jobject obj){
    NativeStudent *student = new(std::nothrow) NativeStudent();
    if(student){
        return (jlong)student;
    }else{
        LOGE("new NativeStudent object error.");
    }
}

• 如果要取得C++物件，則通過mNativeStudent的獲取C++物件的指標

NativeStudent * GetNativeStudentFromObj(JNIEnv *env, jobject obj){
    jclass objClazz = (jclass)env->GetObjectClass(obj);//obj為對應的Java物件
    if (objClazz == NULL){
        LOGE("env->GetObjectClass return null.");
        return nullptr;
    }
    jfieldID fid = env->GetFieldID(objClazz, "mNativeStudent", "J");
    if(fid == NULL){
        LOGE("env->GetFieldID error.");
        return nullptr;
    }
    jlong p = (jlong)env->GetLongField(obj, fid);
    return (NativeStudent *)p;
}

2.C++物件持有JAVA物件的引用

• 在C++類中建立一個long型別的變數（如long mJavaObj），用於儲存JAVA物件的引用
• 在建立C++物件時，儲存JAVA物件的引用

jlong CreateStudent(JNIEnv *env, jobject obj){
    NativeStudent *student = new(std::nothrow) NativeStudent();
    jobject gThiz = (jobject)env->NewGlobalRef(obj);//thiz為JAVA物件
    student->mJavaObj = (jint)gThiz; //c++物件儲存java引用
    if(student){
        return (jlong)student;
    }else{
        LOGE("new NativeStudent object error.");
    }
}

• 注意在銷燬c++物件是要呼叫DeleteGlobalRef方法銷燬全域性引用

void DestoryStudent(JNIEnv *env, jobject obj, jlong nativeStudent){
    NativeStudent *student = (NativeStudent*)nativeStudent;
    env->DeleteGlobalRef((jobject)student->mJavaObj); //銷燬建立的引用
    IL_SAFE_DELETE(student);
}

demo地址