静态块与静态方法 - 初始化静态字段 [英] Static block vs static method - initializing static fields

问题描述

出于好奇，我测量了静态块和静态方法初始化器之间的性能。首先，我在两个单独的java类中实现了上述方法，如下所示：

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:

首先：

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);    
    }
}

第二名：

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);    
    }
}

然后我执行这个小批量脚本测量它100次并将值放在一个文件中。 batchFile.bat First Second dum.res.txt

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

之后，我写了这段代码用来计算Dummy和Muddy测量值的平均值和标准差。

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

这是我得到的结果：

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

在我的其他机器上它也是类似的...每次我测试它。

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

现在我想知道，为什么会这样？我检查了字节码，Second.class在调用System.currentTimeMillis（）之间有一条指令（调用静态initList（））。
他们都做同样的事情，但为什么第一个更慢？我只能通过查看字节码来解释它，因为这是我第一次触摸 javap ;我还不懂字节码。

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.

推荐答案

我认为静态块版本比静态方法版本慢的原因可能是由于不同的JIT优化他们得到...

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 ...

有关更多有趣的信息，请参阅这篇有趣的文章： Java秘密：是否解释了静态块？

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

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