JNI - Object creation with incorrect constructor

[Roger Riggs]

Hi Piotr,

Using JNI requires a quite a bit more care than the Java API, there are 
fewer checks
and guarantees.  Make sure you are calling the method/constructor you 
intend to.
What you are seeing is an artifact of JNI and the Hotspot implementation.
Double check all JNI code twice before using.

$.02, Roger


On 3/10/17 10:23 AM, Piotr Chmielewski wrote:
> When working on code with JNI Invocation API, I stumbled upon curious 
> case.
>
> Following code describes use of incorrect constructor in type when 
> creating new object.
>
>
> JNIEnv* env = ...;
>
> jclass cls_Object = env->FindClass("java/lang/Object");
> jmethodID constructor_Object = 
> env->GetMethodID(cls_Object,"<init>","()V");
> jclass cls_Date = env->FindClass("java/util/Date");
> jmethodID method_Date_toString = 
> env->GetMethodID(cls_Date,"toString","()Ljava/lang/String;");
>
> jobject obj_Date = env->NewObject(cls_Date,constructor_Object);
> jobject obj_toString = 
> env->CallObjectMethod(obj_Date,method_Date_toString);
>
> jstring str = static_cast<jstring>(obj_toString);
> jsize length = env->GetStringLength(str);
> char* dateString = new char[length];
> jsize start = 0;
> env->GetStringUTFRegion(jvmStr,start,length,data);
> std::cout << "Date: " << dateString << std::endl;
> delete[] dateString;
>
>
> This code works and prints "Thu Jan 01 01:00:00 CET 1970".
>
> Looks like "valid?" Date object is created using Object constructor - 
> partially created object.
>
> To understand the behavior, I checked OpenJDK sources for HotSpot. 
> Implementation of "NewObject" allocates space for object and executes 
> method on allocated space.
>
> Additionally, based on sources of "java.util.Date", memory for object 
> "obj_Date" is allocated and its values are set to default values. Date 
> then uses field "long fastTime" in toString method. This results as 
> object was created with "new Date(0)" instead "new Date()";
>
> Main question: Is it a bug or by design, constructor's jmethodID is 
> not checked to belong to type when creating object ?
>