Why defining class as final improves JVM performance?

Question

Quoting from http://sites.google.com/site/gson/gson-design-document:

Why are most classes in Gson marked as final?

While Gson provides a fairly extensible architecture by providing pluggable serializers and deserializers, Gson classes were not specifically designed to be extensible. Providing non-final classes would have allowed a user to legitimately extend Gson classes, and then expect that behavior to work in all subsequent revisions. We chose to limit such use-cases by marking classes as final, and waiting until a good use-case emerges to allow extensibility. Marking a class final also has a minor benefit of providing additional optimization opportunities to Java compiler and virtual machine.

Why is this the case? [If I would guess: of JVM knows class is final it does not maintain method override tables? Are there any other reasons?]

What is the benefit in performance?

Does this applies to classes that are frequency instantiated (POJO?) or perhaps to class that are holders static methods (Utility classes) ?

Are methods defined as final also can theoretically improve performance?

Are there any implications?

Thank you, Maxim.

Exosphere

I'm sure that if you searched SO, you'll find an answer to this; I remember one just a few days ago. But regardless, I recommend that you ignore that last sentence; the rest of the paragraph is what's important. – Electroencephalogram

I'm asking for learning only purposes. Would love a link because I haven't found such (Did ran a quick search before asking). – Exosphere

Agreed, the benefit of marking classes final has more to do with documenting their design (or lack thereof) for extensibility than performance. (On the other hand, that's not what @Maxim Veksler asked...) – Indies

I know no relatively sane JVM that skips CHA (class hierarchy analysis), so final classes improve nothing. – Cupronickel

From what I understand, it's the JIT that does any optimization on final not the JVM. See this article for more details. – Hersey

Answer 1

Virtual (overridden) methods generally are implemented via some sort of table (vtable) that is ultimately a function pointer. Each method call has the overhead of having to go through that pointer. When classes are marked final then all of the methods cannot be overridden and the use of a table is not needed anymore - this it is faster.

Some VMs (like HotSpot) may do things more intelligently and know when methods are/are not overridden and generate faster code as appropriate.

Here is some more specific info on HotSpot. And some general info too.

Breakwater

Answer 2

An old, apparently no longer but still largely relevant, article on this from IBM developerWorks, which states:

The common perception is that declaring classes or methods final makes it easier for the compiler to inline method calls, but this perception is incorrect (or at the very least, greatly overstated).

final classes and methods can be a significant inconvenience when programming -- they limit your options for reusing existing code and extending the functionality of existing classes. While sometimes a class is made final for a good reason, such as to enforce immutability, the benefits of using final should outweigh the inconvenience. Performance enhancement is almost always a bad reason to compromise good object-oriented design principles, and when the performance enhancement is small or nonexistent, this is a bad trade-off indeed.

Also see this related answer on another question. There's also the equivalent question for .Net, discussed here. SO discussion, "Are final methods inlined?" On a question titled "What optimizations are going to be useless tomorrow," this one appears on the list.

Note also that there is an entangling of the effects of final classes vs. final methods. You may get some performance benefit (again, I don't have a good reference) for final methods for sure, as it could cue the JIT to do inlining it couldn't otherwise do (or not so simply). You get the same effect when you mark the class final, which means that all the methods are suddenly final as well. Note that the Sun/Oracle folks claim that HotSpot can usually do this with or without the final keyword. Are there any additional effects from having the class itself final?

For reference, links to the JLS on final methods and final classes.

Indies

It was true in the VERY old days. When we banged rocks together to get ones and zeros and nulls. – Mirk

@Thorbjørn: Banging rocks together sounds lots more productive than most of the software development I see... ;-) Presumably HotSpot does all the right things -- can we assume that all the Java compilers and JVMs have finally caught up? – Indies

@andersoj, high performant JVMs must do this to get enough speed (trading for memory). Small JVM's like JamVM cannot do as aggressive optimizations to stay smalle, so they cannot get quite as fast. – Mirk

@Thorbjørn: To clarify -- the time/space tradeoff you mention is the space to keep around inlined pieces of code (vs. efficiency in executing this code)? – Indies

@andersoj. No, it is the space the JVM needs for its own internal datastructures describing the program being run. Basically the more calculations you can remember for later, the more advanced program transformations you can do. – Mirk

@Thorbjørn: Ah, good, that helps. I am not a JVM geek, but seem to have glancing blows with folks who are from time to time. Is there a canonical "VM nuances 101" to point folks to? – Indies

@andersoj, I am not familiar with one. Also the various JVM code bases are vastly different so it is hard to generalize. IBM Developerworks has some nice articles. – Mirk

@Thorbjørn: Opened a new question on this topic: #3964143 – Indies

Answer 3

Not knowing the implementation of every particular JVM, I would theoretically say that if a JVM knows that a pointer to an object is a pointer to a type that is final, it can do non-virtual function calls (i.e., direct vs. indirect) to a member functions (i.e., no indirection through a function pointer), which may result in faster execution. This may also in turn lead to inlinining possibilities.

Ecto

The Sun JVM is smart enough that if no subclasses have been loaded, it treats the base class's methods as non-virtual. – Ist

Yes, but if it does that it also needs to recognize when a new subclass is loaded and force recompilation of any call to an – Brunt

(continued - BTW how retarded is it that I can't fix my comment?) ... base class method that was inferred to be final. The JVM is not likely to keep track of every such method so it will probably need to recompile all methods that invoke any of that base class's methods. – Brunt

Answer 4

Marking classes as final allows further optimizations to be applied during the JIT stage.

If you are calling a virtual method on a non-final class, you don't know whether the proper implementation is the one defined in that class, or some sub-class that you don't know about.

However, if you have a reference to a final class, you know the specific implementation that is required.

Consider:

A extends B
B extends C

B myInstance = null;
if(someCondition)
   myInstance = new B();
else
   myInstance = new C();
myInstance.toString();

In this case, the JIT can't know whether C's implementation of toString() or B's implementation of toString() will be called. However, if B is marked as final, it is impossible for any implementation other than B's to be the proper implementation

Mordred

Is there documentation that HotSpot or any of the other major JVM vendors exploit this? Has anyone measured the effect recently in production code? – Indies

The JVM does know if there's a subclass because it keeps track of which classes have been loaded. If B and C are never loaded then it knows all of A's methods can be called directly. – Ist

Answer 5

No difference, that's just speculation. The only situation where it has sense are classes like String, etc where jvm treat them differently.

Insurmountable