Android NDK中C++调用Java

Posted 2022-12-10 startFromWeb

调用JAVA中的静态方法

//调用静态方法
jclass cls = env->FindClass("test/Demo");
jmethodID mid = env->GetStaticMethodID(cls, "getHelloWorld","()Ljava/lang/String;");
jstring msg = (jstring)env->CallStaticObjectMethod(cls, mid);	
cout<<JStringToCString(env, msg);

调用JAVA中的静态属性

//调用静态方法
jclass cls = env->FindClass("test/Demo");
jfieldID fid = env->GetStaticFieldID(cls, "COUNT","I");
int count = (int)env->GetStaticIntField(cls, fid);	
cout<<count<<endl;

调用JAVA中的带参数构造函数

//调用构造函数
jclass cls = env->FindClass("test/Demo");
jmethodID mid = env->GetMethodID(cls,"<init>","(Ljava/lang/String;)V");
const char szTest[] = "电信";
jstring arg = NewJString(env, szTest);
jobject demo = env->NewObject(cls,mid,arg);
//验证是否构造成功
mid = env->GetMethodID(cls, "getMessage","()Ljava/lang/String;");
jstring msg = (jstring)env->CallObjectMethod(demo, mid);	
cout<<JStringToCString(env, msg);

传入传出数组

//传入传出数组
//构造数组
long		arrayCpp[] = 1,3,5,7,9;
jintArray array = env->NewIntArray(5);
env->SetIntArrayRegion(array, 0, 5, arrayCpp);
//传入数组
jclass cls = env->FindClass("test/Demo");
jobject obj = env->AllocObject(cls);
jmethodID mid = env->GetMethodID(cls,"setCounts","([I)V");
env->CallVoidMethod(obj, mid, array);
//获取数组
mid = env->GetMethodID(cls,"getCounts","()[I");
jintArray msg = (jintArray)env->CallObjectMethod(obj, mid, array);
int len =env->GetArrayLength(msg);
jint* elems =env-> GetIntArrayElements(msg, 0);
for(int i=0; i< len; i++)

    cout<<"ELEMENT "<<i<<" IS "<<elems[i]<<endl;

env->ReleaseIntArrayElements(msg, elems, 0);

异常处理  

由于调用了Java的方法，因此难免产生操作的异常信息，如JAVA函数返回的异常，或者调用JNI方法（如GetMethodID）时抛出的异常。这些异常没有办法通过C++本身的异常处理机制来捕捉到，但JNI可以通过一些函数来获取Java中抛出的异常信息。

//异常处理
jclass cls = env->FindClass("test/Demo");
jobject obj = env->AllocObject(cls);
jmethodID mid = env->GetMethodID(cls,"throwExcp","()V");
env->CallVoidMethod(obj, mid);
//获取异常信息
string exceptionInfo = "";
jthrowable excp = 0;
excp = env->ExceptionOccurred();	
if(excp)

    jclass cls = env->GetObjectClass(excp);
    env->ExceptionClear();
    jmethodID mid = env->GetMethodID(cls, "toString","()Ljava/lang/String;");
    jstring msg = (jstring) env->CallObjectMethod(excp, mid);
    out<<JStringToCString(env, msg)<<endl;	
    env->ExceptionClear();

 多线程中注意事项

JNIEnv和jobject对象都不能跨线程使用

 

    对于jobject，解决办法是

    a、m_obj = m_env->NewGlobalRef(obj);//创建一个全局变量  

    b、jobject obj = m_env->AllocObject(m_cls);//在每个线程中都生成一个对象

 

    对于JNIEnv，解决办法是在每个线程中都重新生成一个env

 

    JNIEnv *env;  

    m_jvm->AttachCurrentThread((void **)&env, NULL);