Which part of throwing an Exception is expensive?
In Java, using throw/catch as a part of logic when there's not actually an error is generally a bad idea (in part) because throwing and catching an exception is expensive, and doing it many times in a loop is usually far slower than other control structures which don't involve throwing exceptions.
My question is, is the cost incurred in the throw/catch itself, or when creating the Exception object (since it gets a lot of runtime information including the execution stack)?
In other words, if I do
Exception e = new Exception();
but don't throw it, is that most of the cost of throwing, or is the throw + catch handling what's costly?
I'm not asking whether putting code in a try/catch block adds to the cost of executing that code, I'm asking whether catching the Exception is the expensive part, or creating (calling the constructor for) the Exception is the expensive part.
Another way of asking this is, if I made one instance of Exception and threw and caught it over and over, would that be significantly faster than creating a new Exception every time I throw?
Creating an exception object is not more expensive than creating other regular objects. The main cost is hidden in native fillInStackTrace method which walks through the call stack and collects all required information to build a stack trace: classes, method names, line numbers etc.
The myth about high exception costs comes from the fact that most of Throwable constructors implicitly call fillInStackTrace . However, there is one constructor to create a Throwable without a stack trace. It allows you to make throwables that are very fast to instantiate. Another way to create lightweight exceptions is to override fillInStackTrace .
Now what about throwing an exception?
In fact, it depends on where a thrown exception is caught .
If it is caught in the same method (or, more precisely, in the same context, since the context can include several methods due to inlining), then throw is as fast and simple as goto (of course, after JIT compilation).
However if a catch block is somewhere deeper in the stack, then JVM needs to unwind the stack frames, and this can take significantly longer. It takes even longer, if there are synchronized blocks or methods involved, because unwinding implies releasing of monitors owned by removed stack frames.
I could confirm the above statements by proper benchmarks, but fortunately I don't need to to this, since all the aspects are already perfectly covered in the post of HotSpot performance engineer Alexey Shipilëv: The Exceptional Performance of Lil' Exception.
The first operation in most Throwable constructors is to fill in the stack trace, which is where most of the expense is.
There is, however, a protected constructor with a flag to disable the stack trace. This constructor is accessible when extending Exception as well. If you create a custom exception type, you can avoid the stack trace creation and get better performance at the expense of less information.
If you create a single exception of any type by normal means, you can re-throw it many times without the overhead of filling in the stack trace. However, its stack trace will reflect where it was constructed, not where it was thrown in a particular instance.
Current versions of Java make some attempts to optimize stack trace creation. Native code is invoked to fill in the stack trace, which records the trace in a lighter-weight, native structure. Corresponding Java StackTraceElement objects are lazily created from this record only when the getStackTrace() , printStackTrace() , or other methods that require the trace are called.
If you eliminate stack trace generation, the other main cost is unwinding the stack between the throw and the catch. The fewer intervening frames encountered before the exception is caught, the faster this will be.
Design your program so that exceptions are thrown only in truly exceptional cases, and optimizations like these are hard to justify.
Theres a good write up on Exceptions here.
http://shipilev.net/blog/2014/exceptional-performance/
The conclusion being that stack trace construction and stack unwinding are the expensive parts. The code below takes advantage of a feature in 1.7 where we can turn stack traces on and off. We can then use this to see what sort of costs different scenarios have
The following are timings for Object creation alone. I've added String here so you can see that without the stack being written there's almost no difference in creating a JavaException Object and a String . With stack writing turned on the difference is dramatic ie at least one order of magnitude slower.
Time to create million String objects: 41.41 (ms)
Time to create million JavaException objects with stack: 608.89 (ms)
Time to create million JavaException objects without stack: 43.50 (ms)
The following shows how long it took to return from a throw at a particular depth a million times.
|Depth| WriteStack(ms)| !WriteStack(ms)| Diff(%)|
| 16| 1428| 243| 588 (%)|
| 15| 1763| 393| 449 (%)|
| 14| 1746| 390| 448 (%)|
| 13| 1703| 384| 443 (%)|
| 12| 1697| 391| 434 (%)|
| 11| 1707| 410| 416 (%)|
| 10| 1226| 197| 622 (%)|
| 9| 1242| 206| 603 (%)|
| 8| 1251| 207| 604 (%)|
| 7| 1213| 208| 583 (%)|
| 6| 1164| 206| 565 (%)|
| 5| 1134| 205| 553 (%)|
| 4| 1106| 203| 545 (%)|
| 3| 1043| 192| 543 (%)|
The following is almost certainly a gross over simplification...
If we take a depth of 16 with stack writing on then object creation is taking approximately ~40% of the time, the actual stack trace accounts for the vast majority of this. ~93% of instantiating the JavaException object is due to the stack trace being taken. This means that unwinding the stack in this case is taking the other 50% of the time.
When we turn off the stack trace object creation accounts for a much smaller fraction ie 20% and stack unwinding now accounts for 80% of the time.
In both cases stack unwinding takes a large portion of the overall time.
public class JavaException extends Exception {
JavaException(String reason, int mode) {
super(reason, null, false, false);
}
JavaException(String reason) {
super(reason);
}
public static void main(String[] args) {
int iterations = 1000000;
long create_time_with = 0;
long create_time_without = 0;
long create_string = 0;
for (int i = 0; i < iterations; i++) {
long start = System.nanoTime();
JavaException jex = new JavaException("testing");
long stop = System.nanoTime();
create_time_with += stop - start;
start = System.nanoTime();
JavaException jex2 = new JavaException("testing", 1);
stop = System.nanoTime();
create_time_without += stop - start;
start = System.nanoTime();
String str = new String("testing");
stop = System.nanoTime();
create_string += stop - start;
}
double interval_with = ((double)create_time_with)/1000000;
double interval_without = ((double)create_time_without)/1000000;
double interval_string = ((double)create_string)/1000000;
System.out.printf("Time to create %d String objects: %.2f (ms)n", iterations, interval_string);
System.out.printf("Time to create %d JavaException objects with stack: %.2f (ms)n", iterations, interval_with);
System.out.printf("Time to create %d JavaException objects without stack: %.2f (ms)n", iterations, interval_without);
JavaException jex = new JavaException("testing");
int depth = 14;
int i = depth;
double[] with_stack = new double[20];
double[] without_stack = new double[20];
for(; i > 0 ; --i) {
without_stack[i] = jex.timerLoop(i, iterations, 0)/1000000;
with_stack[i] = jex.timerLoop(i, iterations, 1)/1000000;
}
i = depth;
System.out.printf("|Depth| WriteStack(ms)| !WriteStack(ms)| Diff(%%)|n");
for(; i > 0 ; --i) {
double ratio = (with_stack[i] / (double) without_stack[i]) * 100;
System.out.printf("|%5d| %14.0f| %15.0f| %2.0f (%%)| n", i + 2, with_stack[i] , without_stack[i], ratio);
//System.out.printf("%dt%.2f (ms)n", i, ratio);
}
}
private int thrower(int i, int mode) throws JavaException {
ExArg.time_start[i] = System.nanoTime();
if(mode == 0) { throw new JavaException("without stack", 1); }
throw new JavaException("with stack");
}
private int catcher1(int i, int mode) throws JavaException{
return this.stack_of_calls(i, mode);
}
private long timerLoop(int depth, int iterations, int mode) {
for (int i = 0; i < iterations; i++) {
try {
this.catcher1(depth, mode);
} catch (JavaException e) {
ExArg.time_accum[depth] += (System.nanoTime() - ExArg.time_start[depth]);
}
}
//long stop = System.nanoTime();
return ExArg.time_accum[depth];
}
private int bad_method14(int i, int mode) throws JavaException {
if(i > 0) { this.thrower(i, mode); }
return i;
}
private int bad_method13(int i, int mode) throws JavaException {
if(i == 13) { this.thrower(i, mode); }
return bad_method14(i,mode);
}
private int bad_method12(int i, int mode) throws JavaException{
if(i == 12) { this.thrower(i, mode); }
return bad_method13(i,mode);
}
private int bad_method11(int i, int mode) throws JavaException{
if(i == 11) { this.thrower(i, mode); }
return bad_method12(i,mode);
}
private int bad_method10(int i, int mode) throws JavaException{
if(i == 10) { this.thrower(i, mode); }
return bad_method11(i,mode);
}
private int bad_method9(int i, int mode) throws JavaException{
if(i == 9) { this.thrower(i, mode); }
return bad_method10(i,mode);
}
private int bad_method8(int i, int mode) throws JavaException{
if(i == 8) { this.thrower(i, mode); }
return bad_method9(i,mode);
}
private int bad_method7(int i, int mode) throws JavaException{
if(i == 7) { this.thrower(i, mode); }
return bad_method8(i,mode);
}
private int bad_method6(int i, int mode) throws JavaException{
if(i == 6) { this.thrower(i, mode); }
return bad_method7(i,mode);
}
private int bad_method5(int i, int mode) throws JavaException{
if(i == 5) { this.thrower(i, mode); }
return bad_method6(i,mode);
}
private int bad_method4(int i, int mode) throws JavaException{
if(i == 4) { this.thrower(i, mode); }
return bad_method5(i,mode);
}
protected int bad_method3(int i, int mode) throws JavaException{
if(i == 3) { this.thrower(i, mode); }
return bad_method4(i,mode);
}
private int bad_method2(int i, int mode) throws JavaException{
if(i == 2) { this.thrower(i, mode); }
return bad_method3(i,mode);
}
private int bad_method1(int i, int mode) throws JavaException{
if(i == 1) { this.thrower(i, mode); }
return bad_method2(i,mode);
}
private int stack_of_calls(int i, int mode) throws JavaException{
if(i == 0) { this.thrower(i, mode); }
return bad_method1(i,mode);
}
}
class ExArg {
public static long[] time_start;
public static long[] time_accum;
static {
time_start = new long[20];
time_accum = new long[20];
};
}
The stack frames in this example are tiny compared to what you'd normally find.
You can peek at the bytecode using javap
javap -c -v -constants JavaException.class
ie this is for method 4...
protected int bad_method3(int, int) throws JavaException;
flags: ACC_PROTECTED
Code:
stack=3, locals=3, args_size=3
0: iload_1
1: iconst_3
2: if_icmpne 12
5: aload_0
6: iload_1
7: iload_2
8: invokespecial #6 // Method thrower:(II)I
11: pop
12: aload_0
13: iload_1
14: iload_2
15: invokespecial #17 // Method bad_method4:(II)I
18: ireturn
LineNumberTable:
line 63: 0
line 64: 12
StackMapTable: number_of_entries = 1
frame_type = 12 /* same */
Exceptions:
throws JavaException
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