Java performance

Jan 2010
Java Performance
Quest for Speed
Rajesuwer P Singaravelu

Discussed here are subtle
changes that could make
a diﬀerence.

Minor gains, of say 2ms,
could be signiﬁcant if you
are performing 100K
operations to serve a
request.

Architectural patterns are
not considered here.
Introduction

Introduction - Know Your Application
Different applications have different
performance characteristics and bottlenecks.
Performance varies across different hardware,
operating systems, compilers and virtual
machines.

Performance - Cousins
Equally important are:
Code Quality
Maintainability
Readability

Performance - Architecture
Poor performance built into your app?
Heavy use of the object-oriented paradigm (many layers
of superclasses and subclasses)
Simple tuning won t do. Re-design application or parts of
it.
Keep performance in mind when designing your
applications.

Performance - Algorithm
Use eﬃcient algorithms.
Do you use O(n2) bubblesort algorithm or a
much more eﬃcient O(n log n) quicksort ?

Avoid Creating Objects
Object creation is never free.  
This has gotten better over years, but allocating memory
is always more expensive than not allocating memory.

Reuse objects where possible.

Short Circuit
Perform a cheap test before an expensive one
if (s1 == s2 || s1.equals(s2))

Wrappers
Wrapper classes come with overhead- time and
space
Watch out for autoboxing
Consider Eﬃcient Alternatives: 
If you need to add int-s to a list or map, consider
using TIntArrayList, TIntObjectHashMap and such
from trove library.

Prefer Concrete Over Interface
Suppose you have a HashMap object. You can declare it as a
HashMap or as a generic Map:
Map myMap1 = new HashMap() vs HashMap myMap2 = new HashMap()
Which is better?
Calling through an interface reference can take longer than a call
through a concrete reference.
If you have chosen a HashMap because it ﬁts what you're doing, there is little value in calling it a Map.
Given the availability of IDEs that refactor your code for you, there's not much value in calling it a Map
even if you're not sure where the code is headed. (Again, though, public APIs are an exception: a good
API usually trumps small performance concerns.)

Prefer Static Over Virtual
Make methods static 
If you don t need access to object s ﬁelds
Can be faster 
Doesn't require a virtual method table indirection,
so can be faster.
Also good practice 
Method signature conveys that calling the method
can’t/doesn’t alter the object's state.

(Avoid) Internal Getters/Setters
Use
i = this.var
Instead of
i = getVar()
Especially inside
loops.
Which is
expensive-
method call or
instance variable
lookup?

Cache Field Lookups
Accessing object
ﬁelds is much
slower than
accessing local
variables.
Instead of:
for (int i=0; i<this.mCount; i++)
dumpItem(this.mItems[i]);
Write:
int count = this.mCount;
Item[] items = this.mItems;
for (int i = 0; i < count; i++)
dumpItems(items[i]);

Enhanced For Loop - Use Caution
For ArrayList, use:
for(int i=0; i < count; i++){ list.get(i); }
instead of:
for(String s: list){ .. }
For other collections the for-each loop will be
equivalent to explicit iterator usage.

Synchronization
Avoid synchronized methods if you can.
If you can't, synchronizing on methods rather
than on code blocks is slightly faster.

Exception
Use exceptions where you really need them.
Have a high basic cost, & their presence can
hurt compiler analysis.

Using API classes
Use Java API classes when they oﬀer native
machine performance that you can't match
using Java.

For example, arraycopy() is much faster than
using a loop to copy an array of any signiﬁcant
size.

Avoid expensive constructs
Sometimes Java constructs are so expensive that it can be
worth making your data structures or code a little more
complex to work around the problem.
For example, you can add a type id number to objects to avoid
paying the cost of an instanceof (this also allows you to use the
result in a switch).
Similarly, in a long inheritance tree you can avoid casting by
including a method that returns a value of the type you would
otherwise cast to.

Avoid expensive data structures
Expensive Java data structures can be
replaced with simpler ones at the cost of some
extra code complexity.
For example, it can be up to twice as
expensive to access a two-dimensional array
as a one-dimensional array, due to the extra
indirections.

Know your switches
When the numbers are close together, uses a fast
direct lookup.
When the numbers are further apart, uses a slower
search through a table.
This is particularly important if you're trying to
replace a sequence of if statements with a switch.

Method inlining
The Java 2 VM automatically inlines simple
methods at runtime.
Make a method look attractive to the VM to inline
(e.g.: no return value) or manually inline a method if
it doesn't break your object model.

Enums
Enums are very convenient, but unfortunately can be
painful when size and speed matter.
On first use, the class initializer invokes the <init> method
on objects representing each of the enumerated values.
Each object gets its own static field, and the full set is
stored in an array (a static field called "$VALUES").
That's a lot of code and data, just for three integers.

Use Package Scope with Inner Classes
Inner class is a totally separate
class (behind the scenes).
To make direct access to
parent class private members,
the compiler generates a
couple of synthetic methods.
The inner-class code calls
these static methods whenever
it needs to access the private
members of enclosing class.
Translation: members accessed
through accessor methods
instead of directly. (accessors are
slower than direct ﬁeld accesses)
Remedy: declare members
accessed by inner classes to
have package scope, rather than
private scope.
Flip-side: other classes in the
same package can access too;
runs counter to the standard OO
practice.

Replacing API classes
Sometimes API classes do more than you
need -with a corresponding increase in
execution time
You can write specialized versions that do less
but run faster.

Overriding API methods
Performance problems with a Java API method?
Deﬁne a subclass, override that method with your
own (hopefully more eﬃcient) version.

Strength Reduction
Use cheaper operations in place of expensive ones.
‣ += instead of ...=...+... result in fewer byte code instructions.
‣ Make a 1D array variable that points to the row, if repeatedly
accessing elements in a single row of a 2D array
‣ Shifts instead of multiplication by powers of two
‣ Multiplication instead of exponentiation, etc.
/ mathematical optimizations of this type generally have little beneﬁt
unless you are using a just-in-time compiler /

Variable allocation
For desperate optimizers only.
The ﬁrst four numeric variables or arguments in a method are accessed
using via shorter bytecode instructions, although only three are usable in
non-static methods.
If you declare your most frequently-used variables ﬁrst (e.g., loop indices),
the inner bytecode loops of your methods will be marginally shorter and
possibly faster. Note that although the number of bytecodes for the inner
loop is the same, the length of the bytecode has decreased.

Common subexpression elimination
If an expensive expression (for example, the result of a method
call) is used more than once within a block of code, calculate it
once and put it into a temporary variable for subsequent reuse.
double d = a * Math.sqrt(c); double tmp = Math.sqrt(c);
double e = b * Math.sqrt(c); double d = a * tmp;
double e = b * tmp;

Loop invariant code motion
If an expression inside a loop doesn't change after the
loop is entered (i.e. it's invariant), calculate the value of
the expression outside the loop and assign it to a
temporary variable. Then use the temporary variable
within the loop.
Note that we could also treat array.length as a loop
invariant.

Use the right algorithms & data structures
Don't use an O(n2) bubblesort algorithm to sort a
thousand elements when there's an O(n log n) quicksort
available.
Similarly, don't store a thousand items in an array that
requires an O(n) search when you could use an O(log n)
binary tree, or an O(1) Java hash table.

Wholesale is Cheaper
Perform operations in bulk instead of one at a
time.
DB access in loop: Optimize SQL to perform in
one bulk access (not same as executeBatch() )

Closing Notes
To write good, eﬃcient code: 
Understand what the code you write really does.
Make deliberate choice, not inadvertent side eﬀect: 
If you really want to allocate an iterator, by all means use
enhanced for loop syntax on a List; just make it a deliberate
choice.
Build performance into your application; then make it better.
Always think carefully about what your code is doing, and be on
the lookout for ways to speed it up.

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