[jvm-packages] Minor improvements to the CMake build (dmlc#2379)

* [jvm-packages] Fixed JNI_OnLoad overload

It does not compile on Windows without proper export flags.

* [jvm-packages] Use JNI types directly where appropriate

* Removed lib hack from CMake build

Prior to this commit the CMake build use hardcoded lib prefix for
libxgboost and libxgboost4j. Unfortunatelly this did not play well with
Windows, which does not use the lib- prefix.

CMakeLists.txt

@@ -94,8 +94,6 @@ if(MSVC)
 else()
     set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${PROJECT_SOURCE_DIR}/lib)
     set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${PROJECT_SOURCE_DIR})
-    #Prevent shared library being called liblibxgboost.so on Linux
-    set(CMAKE_SHARED_LIBRARY_PREFIX "")
 endif()
 
 set(LINK_LIBRARIES dmlccore rabit)
@@ -124,11 +122,12 @@ endif()
 add_library(objxgboost OBJECT ${SOURCES})
 set_target_properties(${objxgboost} PROPERTIES POSITION_INDEPENDENT_CODE 1)
 
-add_library(libxgboost SHARED $<TARGET_OBJECTS:objxgboost> ${CUDA_OBJS})
-add_executable(xgboost $<TARGET_OBJECTS:objxgboost> ${CUDA_OBJS})
+add_executable(runxgboost $<TARGET_OBJECTS:objxgboost> ${CUDA_OBJS})
+set_target_properties(runxgboost PROPERTIES OUTPUT_NAME xgboost)
+target_link_libraries(runxgboost ${LINK_LIBRARIES})
 
+add_library(xgboost SHARED $<TARGET_OBJECTS:objxgboost> ${CUDA_OBJS})
 target_link_libraries(xgboost ${LINK_LIBRARIES})
-target_link_libraries(libxgboost ${LINK_LIBRARIES})
 
 option(JVM_BINDINGS "Build JVM bindings" OFF)
 
@@ -137,11 +136,11 @@ if(JVM_BINDINGS)
 
     include_directories(${JNI_INCLUDE_DIRS} jvm-packages/xgboost4j/src/native)
 
-    add_library(libxgboost4j SHARED
+    add_library(xgboost4j SHARED
         $<TARGET_OBJECTS:objxgboost>
         ${CUDA_OBJS}
         jvm-packages/xgboost4j/src/native/xgboost4j.cpp)
-    target_link_libraries(libxgboost4j
+    target_link_libraries(xgboost4j
         ${LINK_LIBRARIES}
         ${JNI_LIBRARIES})
 endif()

jvm-packages/xgboost4j/src/native/xgboost4j.cpp

@@ -24,19 +24,15 @@
 // helper functions
 // set handle
 void setHandle(JNIEnv *jenv, jlongArray jhandle, void* handle) {
-#ifdef __APPLE__
-  jlong out = (long) handle;
-#else
-  int64_t out = (int64_t) handle;
-#endif
+  jlong out = (jlong) handle;
   jenv->SetLongArrayRegion(jhandle, 0, 1, &out);
 }
 
 // global JVM
 static JavaVM* global_jvm = nullptr;
 
 // overrides JNI on load
-jint JNI_OnLoad(JavaVM *vm, void *reserved) {
+JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
   global_jvm = vm;
   return JNI_VERSION_1_6;
 }
@@ -76,7 +72,7 @@ XGB_EXTERN_C int XGBoost4jCallbackDataIterNext(
         batch, jenv->GetFieldID(batchClass, "featureValue", "[F"));
       XGBoostBatchCSR cbatch;
       cbatch.size = jenv->GetArrayLength(joffset) - 1;
-      cbatch.offset = reinterpret_cast<long *>(
+      cbatch.offset = reinterpret_cast<jlong *>(
           jenv->GetLongArrayElements(joffset, 0));
       if (jlabel != nullptr) {
         cbatch.label = jenv->GetFloatArrayElements(jlabel, 0);