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Does use of final keyword in Java improve the performance?
(14 answers)
Closed 5 years ago.
Does declaring variables as final inside a method brings any benefit from a performance/memory point of view in the latest java versions?
I am not talking here about any other benefit.
This question Does use of final keyword in Java improve the performance? was addresed almost 7 years ago, some progress has been done since then.
No, final keyword on a local variable has no performance impact, and it cannot have even in theory, since .class files do not retain this information.
See this answer for details.
final keyword has no direct performance impact if we will talk about variables, but can improve performance in some use cases.
Example:
When iterating in a loop, if the variable is final then JIT will not check for any cause which can change the value of the list (reference).
final allows a variable to be inlined into the calling code, so that could cause some memory and performance improvement. This is used when you have a number of public static final Strings in a class (reference).
final keyword improves performance. Not just JVM can cache final variable but also application can cache frequently use final variables (check here for 3 and 4).
final keyword allows JVM to optimized method, variable or class.
Related
This question already has answers here:
Does use of final keyword in Java improve the performance?
(14 answers)
Closed 7 years ago.
Now, I recently ran into a recommendation that you should use the keyword final as wide as possible. This is good in order to prevent a programmer from shooting his own leg - that is, reassign the variable that should not be reassigned.
But, does it serve any other goal? That is, can JVM use information about the final variables in order to optimize the bytecode somehow, so it would ran faster (build a better pipelining or use it in a multithreaded environment)? Or is just a syntactic sugar that minimizes the possibility of errors during code development?
IBM states:
Like many myths about Java performance, the erroneous belief that declaring classes or methods as final results in better performance is widely held but rarely examined. The argument goes that declaring a method or class as final means that the compiler can inline method calls more aggressively, because it knows that at run time this is definitely the version of the method that's going to be called. But this is simply not true. Just because class X is compiled against final class Y doesn't mean that the same version of class Y will be loaded at run time. So the compiler cannot inline such cross-class method calls safely, final or not. Only if a method is private can the compiler inline it freely, and in that case, the final keyword would be redundant.
As far as variables go, Java is smart enough to figure that a variable is not being changed anywhere in the method, and use this knowledge for optimization purposes. It does not need you to mark a variable final in order to know that.
Methods are a slightly different story: when you mark a method final, Java can invoke such method faster, because it no longer needs to check for its overrides. Still, hotspot is smart enough to figure out that there are no overrides, with or without the use of final.
Generally, though, this recommendation is intended to make your code easier to read and understand by others: making a variable final tells you readers that you are making a constant; making a class final tells your readers that the class is not designed for inheritance; making a method final tells your readers that the logic of that method should stay invariant across the inheritance hierarchy. In the long run, this information is more valuable than the potential to optimize your running code.
I think this recommendation is a bit too unspecific.
For example; I would recommend to avoid using final on classes and methods by default (because final classes break unit tests, respectively force you to use specific unit test frameworks to overcome final methods/classes).
I also find that using final for method parameters is just a waste of "screen space" (and thus: wasting "energy" on the side of the reader).
On the other hand, having only final attributes for classes can turn objects of such classes into immutable thingies; and that is a good thing.
As you see; there are many pros and cons on the usage of final; and I think that "readability" most often wins over "potential" performance improvements.
final methods may or may not be inlined by the JVM until they after they are loaded by the JVM. So, if you're sure the method is not going to be redefined, mark it as final.
Constants should always be static final since they will be immutable and jvm does not need to keep track of these variables since they will never change.
This question already has answers here:
Multithreading and booleans
(2 answers)
Closed 9 years ago.
If I have a class holding a private boolean made public by a setter and getter method
, would I have to set those methods as synchronized if I want to read and write to
that boolean from different threads?
Synchronizing access of a simple value is often unnecessary. Generally all you need is to mark it volatile which is less restrictive and more informative.
It all really depends a lot on how you access the value.
In some cases using an AtomicBoolean can be the best approach. This provides slightly different guarantees to volatile.
See question Java: volatile boolean vs AtomicBoolean question for more details.
Taking a look at the classes in java/util/concurrent/atomic might be useful to you. Such as AtomicBoolean.
Yes, making setters and getters synchronized is a good idea. To read or write a boolean is NOT atomic command , so in rare cases it may cause you some problems. ( you cannot be sure if you reading form RAM or cashe). Unless it is volatile.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Why would one mark local variables and method parameters as “final” in Java?
I used PMD in my code and it always tells me to make method local variables final. Can someone tell me how this effects the general performance apart from making the code more readable.
There's no effect on performance and it's debatable whether it's more readable. Java should have made variables final by default.
The biggest value of final is that it prevents programming errors. Regarding performance, I'd think the compiler can figure out the last write to a variable in most cases and do the necessary optimizations.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Is there any performance reason to declare method parameters final in Java?
Why would one mark local variables and method parameters as “final” in Java?
I am using PMD to see the code violations.
Inside a webService Method, I have this below code
public ServiceRequest getData()
{
Status status = new Status();
// code
}
What PMD is suggesting me is that, this local variable status could be declared as final.
My question is, making it final would result in any performance improvements or if not what benefits the code could get?
Taken from the following article: http://www.javapractices.com/topic/TopicAction.do?Id=23
clearly communicates your intent
allows the compiler and virtual machine to perform minor optimizations
clearly flags items which are simpler in behaviour - final says, "If you are looking for complexity, you won't find it here."
This is also discussed in this question: Can excessive use of final hurt more than do good?
final indicates the local variable won't be changed. My feeling is that methods should be so short you should be able to easily understand them and so making the variable final may be a bit redundant.
I prefer to make fields final because making the whole class so short, is a serious limitation. Also fields can have thread safety issues which local variables do not.
I dont know about performance-benefits by making status final, but PMD is suggesting you this, because probably you are never writing on status after its first initialization.
So what you gain by making it final is just that your code is less error-prone - if you declare it final, you cant overwrite it by mistake...
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Is there any performance reason to declare method parameters final in Java?
Does use of final keyword in Java improve the performance?
So there's a class-level object variable that's allocated upon object creation and stays put for the lifetime of the object:
class MyClass
{
private Rect rc = new Rect();
//...
}
What do I gain be declaring it final? Same question for method-level variables.
it is all completely implementation specific.
main reason to use final is to ensure that variable value is not allowed to be change over time. it's a matter of you code logic and not optimization.
Declaring variables final does have benefits that come from immutability. If used properly your code can be more thread safe, provided the variable you're making final does not have internal state that could be changed unexpectedly and effect other threads.
See the other answers here too about JVM optimizations.
Final has one obvious use, where it makes the object/variable immutable, now can this feature help in performance gains?
Quoting: The Final Word On the final Keyword, the performance gain you get using final is:
A field doesn't need to be ever reloaded, since its value is
guaranteed never to change.
But I see a flaw in the above statement, the value won't be reloaded if it's set only once and final guarantees that it will never be reloaded again, but this does not mean using final directly helps you in performance gain, the performance gain we get here is due to the property that if we make sure we do not set the to something else, it will anyway, won't be reloaded.
So, you answer:
Although, it is implementation specific to the JVM and atleast in the HotSpot, you will get no performance gains from using final. Read more: Does use of final keyword in Java improve the performance?
IBM also says you won't get ant performance gain out of final.
This might not give you 100% answer. But i have a point which i know.
There is lot of optimization gained by the jvm when you declare variables as final.
I remember a good example where i have if-else conditions in my program and in the condition since i used final variable, complier has evaluted which part of if-else is valid and it stripped out the other lines and i have seen it with javap. I will get you that example soon.
It allows for some optimization via memoization. For example, if you have a Rectangle class with final height and final width, then your rectangle's area function only needs to compute it once, since it knows that height and width cannot change.
I doubt that final fields have a hughe impact on performance, unless you have shared object in multiple threads.
However declaring methods final or even classes will help the jit to determine wether a method can be overridden, or can't. If the jit is certain the method is no where overwritten he might remove the method lookup and use a direct jump (jump subroutine). Also Jits like to inline small final methods.
Besides the obvious compile-time protection from modifications (*) you do gain some form of optimized access to its value. The compiler may for example replace the references of primitive variables with literal values. The runtime can for example place copies of the its content in each thread's private memory space to avoid accessing main memory. It does depend on each JVM implementation.
For local variables you do again have gains, both at compile-time and at run-time, mostly in the form of thread-access optimizations. Bear in mind though, that you will only ever notice these gains in hot spots of your code, code that executes hundreds or thousands of times per second.
(*) final only protects from compile-time modifications after the introduction of Accessible Objects.