分类： Java
2009-03-17 18:41:27
There's no way around it: Android-powered devices are embedded devices. Modern handsets may be more like small handheld computers than mere phones these days, but even the fastest, highest-end handset doesn't even come close to the capabilities of even a modest desktop system.
That's why it's very important to consider performance when you write Android applications. These systems are not that fast to begin with and they are also constrained by their battery life. This means that there's not a lot of horsepower to spare, so when you write Android code it's important to write it as efficiently as possible.
This page describes a number of things that developers can do to make their Android code run more efficiently. By following the tips on this page, you can help make sure your code runs as efficiently as possible.
Contents
There are two basic rules for resource-constrained systems:
All the tips below follow from these two basic tenets.
Some would argue that much of the advice on this page amounts to "premature optimization." While it's true that micro-optimizations sometimes make it harder to develop efficient data structures and algorithms, on embedded devices like handsets you often simply have no choice. For instance, if you bring your assumptions about VM performance on desktop machines to Android, you're quite likely to write code that exhausts system memory. This will bring your application to a crawl — let alone what it will do to other programs running on the system!
That's why these guidelines are important. Android's success depends on the user experience that your applications provide, and that user experience depends in part on whether your code is responsive and snappy, or slow and aggravating. Since all our applications will run on the same devices, we're all in this together, in a way. Think of this document as like the rules of the road you had to learn when you got your driver's license: things run smoothly when everybody follows them, but when you don't, you get your car smashed up.
Before we get down to brass tacks, a brief observation: nearly all issues described below are valid whether or not the VM features a JIT compiler. If I have two methods that accomplish the same thing, and the interpreted execution of foo() is faster than bar(), then the compiled version of foo() will probably be as fast or faster than compiled bar(). It is unwise to rely on a compiler to "save" you and make your code fast enough.
Object creation is never free. A generational GC with per-thread allocation pools for temporary objects can make allocation cheaper, but allocating memory is always more expensive than not allocating memory.
If you allocate objects in a user interface loop, you will force a periodic garbage collection, creating little "hiccups" in the user experience.
Thus, you should avoid creating object instances you don't need to. Some examples of things that can help:
A somewhat more radical idea is to slice up multidimensional arrays into parallel single one-dimension arrays:
Generally speaking, avoid creating short-term temporary objects if you can. Fewer objects created mean less-frequent garbage collection, which has a direct impact on user experience.
When processing strings, don't hesitate to use specialty methods like String.indexOf(), String.lastIndexOf(), and their cousins. These are typically implemented in C/C++ code that easily runs 10-100x faster than doing the same thing in a Java loop.
The flip side of that advice is that punching through to a native method is more expensive than calling an interpreted method. Don't use native methods for trivial computation, if you can avoid it.
Suppose you have a HashMap object. You can declare it as a HashMap or as a generic Map:
Map myMap1 = new HashMap();
HashMap myMap2 = new HashMap();
Which is better?
Conventional wisdom says that you should prefer Map, because it allows you to change the underlying implementation to anything that implements the Map interface. Conventional wisdom is correct for conventional programming, but isn't so great for embedded systems. Calling through an interface reference can take 2x longer than a virtual method 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.)
If you don't need to access an object's fields, make your method static. It can be called faster, because it doesn't require a virtual method table indirection. It's also good practice, because you can tell from the method signature that calling the method can't alter the object's state.
In native languages like C++ it's common practice to use getters (e.g.
i = getCount()) instead of accessing the
field directly (i = mCount). This is an
excellent habit for C++, because the compiler can usually inline the access, and
if you need to restrict or debug field access you can add the code at any
time.
On Android, this is a bad idea. Virtual method calls are expensive, much more so than instance field lookups. It's reasonable to follow common object-oriented programming practices and have getters and setters in the public interface, but within a class you should always access fields directly.
Accessing object fields is much slower than accessing local variables. Instead of writing:
for (int i = 0; i < this.mCount; i++)
dumpItem(this.mItems[i]);
You should write:
int count = this.mCount;
Item[] items = this.mItems;
for (int i = 0; i < count; i++)
dumpItems(items[i]);
(We're using an explicit "this" to make it clear that these are member variables.)
A similar guideline is never call a method in the second clause of a "for" statement. For example, the following code will execute the getCount() method once per iteration, which is a huge waste when you could have simply cached the value as an int:
for (int i = 0; i < this.getCount(); i++)
dumpItems(this.getItem(i));
It's also usually a good idea to create a local variable if you're going to be accessing an instance field more than once. For example:
protected void drawHorizontalScrollBar(Canvas canvas, int width, int height) {
if (isHorizontalScrollBarEnabled()) {
int size = mScrollBar.getSize(false);
if (size <= 0) {
size = mScrollBarSize;
}
mScrollBar.setBounds(0, height - size, width, height);
mScrollBar.setParams(
computeHorizontalScrollRange(),
computeHorizontalScrollOffset(),
computeHorizontalScrollExtent(), false);
mScrollBar.draw(canvas);
}
}
That's four separate lookups of the member field mScrollBar. By caching mScrollBar in a local stack
variable, the four member field lookups become four stack variable references,
which are much more efficient.
Incidentally, method arguments have the same performance characteristics as local variables.
Consider the following declaration at the top of a class:
static int intVal = 42;
static String strVal = "Hello, world!";
The compiler generates a class initializer method, called intVal, and extracts a reference from the classfile
string constant table for strVal. When these
values are referenced later on, they are accessed with field lookups.
We can improve matters with the "final" keyword:
static final int intVal = 42;
static final String strVal = "Hello, world!";
The class no longer requires a intVal will use the integer value
42 directly, and accesses to strVal will use
a relatively inexpensive "string constant" instruction instead of a field
lookup.
Declaring a method or class "final" does not confer any immediate performance benefits, but it does allow certain optimizations. For example, if the compiler knows that a "getter" method can't be overridden by a sub-class, it can inline the method call.
You can also declare local variables final. However, this has no definitive performance benefits. For local variables, only use "final" if it makes the code clearer (or you have to, e.g. for use in an anonymous inner class).
The enhanced for loop (also sometimes known as "for-each" loop) can be used for collections that implement the Iterable interface. With these objects, an iterator is allocated to make interface calls to hasNext() and next(). With an ArrayList, you're better off walking through it directly, but for other collections the enhanced for loop syntax will be equivalent to explicit iterator usage.
Nevertheless, the following code shows an acceptable use of the enhanced for loop:
public class Foo {
int mSplat;
static Foo mArray[] = new Foo[27];
public static void zero() {
int sum = 0;
for (int i = 0; i < mArray.length; i++) {
sum += mArray[i].mSplat;
}
}
public static void one() {
int sum = 0;
Foo[] localArray = mArray;
int len = localArray.length;
for (int i = 0; i < len; i++) {
sum += localArray[i].mSplat;
}
}
public static void two() {
int sum = 0;
for (Foo a: mArray) {
sum += a.mSplat;
}
}
}
zero() retrieves the static field twice and gets the array length once for every iteration through the loop.
one() pulls everything out into local variables, avoiding the lookups.
two() uses the enhanced for loop syntax introduced in version 1.5 of the Java programming language. The code generated by the compiler takes care of copying the array reference and the array length to local variables, making it a good choice for walking through all elements of an array. It does generate an extra local load/store in the main loop (apparently preserving "a"), making it a teensy bit slower and 4 bytes longer than one().
To summarize all that a bit more clearly: enhanced for loop syntax performs well with arrays, but be cautious when using it with Iterable objects since there is additional object creation.
Enums are very convenient, but unfortunately can be painful when size and speed matter. For example, this:
public class Foo {
public enum Shrubbery { GROUND, CRAWLING, HANGING }
}
turns into a 900 byte .class file (Foo$Shrubbery.class). On first use, the
class initializer invokes the
This:
Shrubbery shrub = Shrubbery.GROUND;
causes a static field lookup. If "GROUND" were a static final int, the compiler would treat it as a known constant and inline it.
The flip side, of course, is that with enums you get nicer APIs and some compile-time value checking. So, the usual trade-off applies: you should by all means use enums for public APIs, but try to avoid them when performance matters.
In some circumstances it can be helpful to get enum integer values through
the ordinal() method. For example,
replace:
for (int n = 0; n < list.size(); n++) {
if (list.items[n].e == MyEnum.VAL_X)
// do stuff 1
else if (list.items[n].e == MyEnum.VAL_Y)
// do stuff 2
}
with:
int valX = MyEnum.VAL_X.ordinal();
int valY = MyEnum.VAL_Y.ordinal();
int count = list.size();
MyItem items = list.items();
for (int n = 0; n < count; n++)
{
int valItem = items[n].e.ordinal();
if (valItem == valX)
// do stuff 1
else if (valItem == valY)
// do stuff 2
}
In some cases, this will be faster, though this is not guaranteed.
Consider the following class definition:
public class Foo {
private int mValue;
public void run() {
Inner in = new Inner();
mValue = 27;
in.stuff();
}
private void doStuff(int value) {
System.out.println("Value is " + value);
}
private class Inner {
void stuff() {
Foo.this.doStuff(Foo.this.mValue);
}
}
}
The key things to note here are that we define an inner class (Foo$Inner) that directly accesses a private method and a private instance field in the outer class. This is legal, and the code prints "Value is 27" as expected.
The problem is that Foo$Inner is technically (behind the scenes) a totally separate class, which makes direct access to Foo's private members illegal. To bridge that gap, the compiler generates a couple of synthetic methods:
/*package*/ static int Foo.access$100(Foo foo) {
return foo.mValue;
}
/*package*/ static void Foo.access$200(Foo foo, int value) {
foo.doStuff(value);
}
The inner-class code calls these static methods whenever it needs to access the "mValue" field or invoke the "doStuff" method in the outer class. What this means is that the code above really boils down to a case where you're accessing member fields through accessor methods instead of directly. Earlier we talked about how accessors are slower than direct field accesses, so this is an example of a certain language idiom resulting in an "invisible" performance hit.
We can avoid this problem by declaring fields and methods accessed by inner classes to have package scope, rather than private scope. This runs faster and removes the overhead of the generated methods. (Unfortunately it also means the fields could be accessed directly by other classes in the same package, which runs counter to the standard OO practice of making all fields private. Once again, if you're designing a public API you might want to carefully consider using this optimization.)
Before the release of the Pentium CPU, it was common for game authors to do as much as possible with integer math. With the Pentium, the floating point math co-processor became a built-in feature, and by interleaving integer and floating-point operations your game would actually go faster than it would with purely integer math. The common practice on desktop systems is to use floating point freely.
Unfortunately, embedded processors frequently do not have hardware floating point support, so all operations on "float" and "double" are performed in software. Some basic floating point operations can take on the order of a millisecond to complete.
Also, even for integers, some chips have hardware multiply but lack hardware divide. In such cases, integer division and modulus operations are performed in software — something to think about if you're designing a hash table or doing lots of math.
To illustrate some of our ideas, here is a table listing the approximate run times for a few basic actions. Note that these values should NOT be taken as absolute numbers: they are a combination of CPU and wall clock time, and will change as improvements are made to the system. However, it is worth noting how these values apply relative to each other — for example, adding a member variable currently takes about four times as long as adding a local variable.
| Action | Time |
|---|---|
| Add a local variable | 1 |
| Add a member variable | 4 |
| Call String.length() | 5 |
| Call empty static native method | 5 |
| Call empty static method | 12 |
| Call empty virtual method | 12.5 |
| Call empty interface method | 15 |
| Call Iterator:next() on a HashMap | 165 |
| Call put() on a HashMap | 600 |
| Inflate 1 View from XML | 22,000 |
| Inflate 1 LinearLayout containing 1 TextView | 25,000 |
| Inflate 1 LinearLayout containing 6 View objects | 100,000 |
| Inflate 1 LinearLayout containing 6 TextView objects | 135,000 |
| Launch an empty activity | 3,000,000 |
The best way to write good, efficient code for embedded systems is to understand what the code you write really does. 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, not an inadvertent side effect.
Forewarned is forearmed! Know what you're getting into! Insert your favorite maxim here, but always think carefully about what your code is doing, and be on the lookout for ways to speed it up.