如何创建静态JNI环境指针? [英] How to create static JNI Environment Pointer?
问题描述
这里我在JAVA中创建一个类,其中我有函数(callback),我必须从C文件中调用它。
Here I create a class in JAVA in which I have function (callback) which I must call from C file.
class DSMInitializeClassParameter {
/**
* Callback function for DSM Initialize.
*/
public void DSMInitializeCallback( ) {
// Write Message To Logs.
System.out.println( "Dsm Initialize Callback called." );
}
}
因为我已经创建了native方法, 。
For that I have created native method which must be called.
public class DsmLibraryTest extends Activity {
// Some code ....
// Create a DSMInitializeClassParameter class object.
DSMInitializeClassParameter object = new DSMInitializeClassParameter();
// Call native method with given object.
nativeMethod( object );
// Some code ....
// Implementation of native method.
public native int nativeMethod(DSMInitializeClassParameter classObject);
}
在 C p>
In C file I have following:
dsmResult_t dsmInitializeCall( dsmResult_t status, void * pUserData, dsmEvent_t * hEvent ) {
(*env)->CallVoidMethod(env, classObject, mid);
}
JNIEXPORT jint JNICALL Java_com_Dsm_Test_DsmLibraryTest_nativeMethod(JNIEnv* env, jobject obj, jobject classObject) {
// This function loads a locally-defined class. It searches the directories and zip
// files specified by the CLASSPATH environment variable for the class with the specified name.
jclass cls = (*env)->FindClass( env, "com/Dsm/Test/DSMInitializeClassParameter" );
// Get java Method.
jmethodID mid = (*env)->GetMethodID(env, cls, "DSMInitializeCallback", "()V");
// If no method was found return -1;
if( mid == NULL ) {
return -1;
}
// Call DSM Initialize Callback Function and return value.
return dsmInitialize( dsmInitializeCall, NULL );
}
如何查看我想调用 env) - > CallVoidMethod(env,classObject,mid); 从
, dsmInitializeCall
函数,但是如果我没有< c $ c> env classObject
和 mid
我尝试使用静态,但它不工作。
how you can see I want to call (*env)->CallVoidMethod(env, classObject, mid);
from dsmInitializeCall
function, but how I can call it if I have no env
, classObject
and mid
I try with statics but it not work.
推荐答案
通常 unsafe 缓存 JNIEnv *
实例并继续使用,因为它根据当前活动的线程而有所不同。您可以保存 JavaVM *
实例,该实例将永远不会更改。在本地初始化函数中,调用 GetJavaVM
并传递 JavaVM
指针的地址:
It's generally unsafe to cache a JNIEnv*
instance and keep using it, as it varies depending on the currently active thread. You can save a JavaVM*
instance, which will never change. In a native initializer function, call GetJavaVM
and pass it the address of a JavaVM
pointer:
static JavaVM *jvm;
JNIEXPORT void JNICALL Java_SomeClass_init(JNIEnv *env, jclass) {
int status = (*env)->GetJavaVM(env, &jvm);
if(status != 0) {
// Fail!
}
}
现在你可以使用 JavaVM *
以获取当前 JNIEnv *
与 AttachCurrentThread
:
Now you can use that JavaVM*
to get the current JNIEnv*
with AttachCurrentThread
:
dsmResult_t dsmInitializeCall( dsmResult_t status, void * pUserData, dsmEvent_t * hEvent ) {
JNIEnv *env;
(*jvm)->AttachCurrentThread(jvm, (void **)&env, NULL);
(*env)->CallVoidMethod(env, classObject, mid);
}
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