在清理未使用对象的引用之前，如何强制垃圾收集器首先调用 finalize 方法

Question

I have a scenario which uses a C library using JNA:
I have override the finalize method where I send the call to delete the object that is created through C.
My code looks somewhat like this:

protected void finalize() throws Throwable
    {
        if (LicensingAPIHelper.instance == null)
            throw new NullPointerException("LicensingAPIHelper is not initialized");
                   LicensingAPIHelper.instance.sntl_licensing_app_context_delete(nativeAppContext.getValue());
    }

sntl_licensing_app_context_delete is the API, which deletes the object created in the C library.
nativeAppContext is PointerByReference and nativeAppContext.getValue() sends the pointer of the object to be deleted.
What is happening right now is, when GC is called a crash occurs because the nativeAppContext reference is cleaned first by GC and since it does not find any reference so when it tries to get the value for sending to C library it crashes.

Is there any way to enforce GC to call finalize method first before cleaning up the objects?

In this case, I assume GC cleans up the object first and then calls the finalize method.

Answer 1

It is and it is only guaranteed that the finalize() method is called before the object is cleaned by the GC.

What you are trying to do is just not what finalize() is made for. I never known when the GC will clean up the instance you used. It's not like a descructor in C++. It will not immediatelly be called after a variable gets out of scope. Therefore using finalize() for your cleanup code will hold open your C library for much more time then needed.

What you are looking for is probably the close() method of the interfaces java.io.Closeable or java.lang.AutoCloseable .

Instead of putting your code in the finalize() method but it in there and mark your class implements AutoCloseable . Then when you construct your instance of your class use a try-with-resouces block. When this block gets exited, the close() -method will be called automatically.

try (MyClass myInstance = new MyClass()) {
    // use myInstance here
}
// myInstance.close() will have been called here automatically