Static block vs static method - initializing stati

问题:

Out of curiosity, I measured the performance between static block and static method initializer. First, I implemented the above mentioned methods in two separate java classes, like so:

First:

class Dummy {
    static java.util.List<Integer> lista = new java.util.ArrayList<Integer>();
    static {
        for(int i=0; i < 1000000; ++i) {
            lista.add(new Integer(i));
        }
    }
}

public class First {
    public static void main(String[] args) { 
        long st = System.currentTimeMillis();
            Dummy d = new Dummy();
        long end = System.currentTimeMillis() - st;
        System.out.println(end);    
    }
}

Second:

class Muddy {
    static java.util.List<Integer> lista = new java.util.ArrayList<Integer>();
    public static void initList() {
        for(int i=0; i < 1000000; ++i) {
            lista.add(new Integer(i));
        }
    }
}

public class Second {
    public static void main(String[] args) { 
        long st = System.currentTimeMillis();
            Muddy.initList();
            Muddy m = new Muddy();
        long end = System.currentTimeMillis() - st;
        System.out.println(end);    
    }
}

Then I executed this little batch script to measure it 100 times and put the values in a file. batchFile.bat First Second dum.res.txt

After that, I wrote this piece of code to calculate mean value and standard deviation of Dummy's and Muddy's measured values.

This is the result that I've got:

First size: 100 Second size: 100
First       Sum: 132    Std. deviation: 13
Second      Sum: 112    Std. deviation: 9

And it is similar on my other machines...every time I test it.

Now I'm wondering, why is it so? I checked the bytecode and Second.class has one instruction more (call to static initList()) between calls to System.currentTimeMillis(). They both do the same thing, but why is the First one slower? I can't really reason it out just by looking at the bytecode, since this was my first time touching javap; I don't understand bytecode yet.

回答1:

I think that the reason why the static block version is slower than the static method version could be due to the different JIT optimization that they get ...

See this interesting article for more interesting information : Java Secret: Are static blocks interpreted?

回答2:

Here's my guess as to the reason for this:

The initialization you are doing is creating enough objects that it is causing one or more garbage collections.

When the initialization is called from the static block, it is done during the class initialization rather than during simple method execution. During class initialization, the garbage detector may have a little more work to do (because the execution stack is longer, for example) than during simple method execution, even though the contents of the heap are almost the same.

To test this, you could try adding -Xms200m or something to your java commands; this should eliminate the need to garbage collect during the initialization you are doing.