从JNI卸载JAR文件

Question

I have a problem with the Java Native Interface.

I use the JNI in my application, everything is working fine except the fact that I'm unable to unload / release the lock from the .JAR file im accessing with the JNI.

Here is the destructor I'm using:

destructor Schnittstelle.Destroy;
begin
try
if (assigned(FJNIEnv)) then
begin
  ShowMessage('Destruktor');

  FJNIEnv.Free;
  FJNIEnv := nil;

  FJavaVM.JavaVM^.DetachCurrentThread(FJavaVM.JavaVM);
  FJavaVM.JavaVM^.DestroyJavaVM(FJavaVM.JavaVM);

  FJavaVM.Free;
  FJavaVM := nil;

  inherited Destroy;
end;
except
  ShowMessage('Java VM kann nicht freigegeben werden.');
  Exit;
end;
end;

After the desctructor is called, I try to change the JARs file name to test if the JNI stopped accessing the file, but thats not the case.

If I terminate my delphi application the JAR file is usable.

Thank you!

Answer 1

You not specify in your question how you load the classes in the JAR file, directly via JNI o indirectly thru some Java call.

Anyway I make the assumption that class loading inside the JVM, either with Class.forName(String) or ClassLoader.loadClass(String) uses the same mechanisms as the JNI FindClass :

FindClass

In JDK release 1.1, 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.

Since Java 2 SDK release 1.2, the Java security model allows non-system classes to load and call native methods. FindClass locates the class loader associated with the current native method; that is, the class loader of the class that declared the native method. If the native method belongs to a system class, no class loader will be involved. Otherwise, the proper class loader will be invoked to load and link the named class.

Since Java 2 SDK release 1.2, when FindClass is called through the Invocation Interface , there is no current native method or its associated class loader. In that case, the result of ClassLoader.getSystemClassLoader is used. This is the class loader the virtual machine creates for applications , and is able to locate classes listed in the java.class.path property.

Also see this other considerations over unloading of the JVM :

It seems that the JVM is not completely unloaded until the program exits.

I guess that the system classloader that is referencing the jar file and the classes in it is not closed until the JVM is unloaded, so I guess that it is not possible to unload the jar until the program exits.