Java循环效率

第一次运行这些循环后的结果是：

for loop: 1.43100
do-while: 0.51300
while: 1.54500

但是,当我运行这三个循环至少10次时,这些循环的性能几乎相同.

for loop: 0.43200
do-while: 0.46100
while: 0.42900

JIT可以随时间优化这些循环,但是必须有一些不同之处,导致这些循环具有不同的初始性能.其实实际上有两个区别：

> do-while循环的执行次数比for循环和while循环更少

为简单起见,假设L = 1

long s1 = 0;
for (int i=0; i < L; i++) {
    for (int j = 0; j < L; j++) {
        s1 += 1;

外环：0 < 1
内循环：0 < 1
内环：1 < 1
外环：1 < 1 4次比较

int i = 0;
long s2 = 0;
do {
    i++;
    int j = 0;
    do {
        s2 += 1;
        j++;
    } while (j < L);
} while (i < L);

内环：1 < 1
外环：1 < 1 2次比较
>不同的生成字节码

为了进一步调查,我已经稍稍改变了你的班级,不会影响到班级的工作.

public class Loops {
    final static int L = 100000; // number of iterations per loop

    public static void main(String[] args) {
        int round = 10;
        while (round-- > 0) {
            forLoop();
            doWhileLoop();
            whileLoop();
        }
    }

    private static long whileLoop() {
        int i = 0;
        long s3 = 0;
        while (i++ < L) {
            int j = 0;
            while (j++ < L) {
                s3 += 1;
            }
        }
        return s3;
    }

    private static long doWhileLoop() {
        int i = 0;
        long s2 = 0;
        do {
            int j = 0;
            do {
                s2 += 1;
            } while (++j < L);
        } while (++i < L);
        return s2;
    }

    private static long forLoop() {
        long s1 = 0;
        for (int i = 0; i < L; i++) {
            for (int j = 0; j < L; j++) {
                s1 += 1;
            }
        }
        return s1;
    }
}

然后编译它并调用javap -c -s -private -l Loop来获取字节码.

首先是doWhileLoop的字节码.

0:   iconst_0        // push the int value 0 onto the stack
   1:   istore_1        // store int value into variable 1 (i)
   2:   lconst_0        // push the long 0 onto the stack
   3:   lstore_2        // store a long value in a local variable 2 (s2)
   4:   iconst_0        // push the int value 0 onto the stack
   5:   istore  4   // store int value into variable 4 (j)
   7:   lload_2     // load a long value from a local variable 2 (i)
   8:   lconst_1        // push the long 1 onto the stack
   9:   ladd        // add two longs
   10:  lstore_2        // store a long value in a local variable 2 (i)
   11:  iinc    4,1    // increment local variable 4 (j) by signed byte 1
   14:  iload   4   // load an int value from a local variable 4 (j)
   16:  iload_0     // load an int value from a local variable 0 (L)
   17:  if_icmplt   7   // if value1 is less than value2,branch to instruction at 7
   20:  iinc    1,1    // increment local variable 1 (i) by signed byte 1
   23:  iload_1     // load an int value from a local variable 1 (i)
   24:  iload_0     // load an int value from a local variable 0 (L)
   25:  if_icmplt   4   // if value1 is less than value2,branch to instruction at 4
   28:  lload_2     // load a long value from a local variable 2 (s2)
   29:  lreturn     // return a long value

现在的whileLooP的字节码：

0:   iconst_0        // push int value 0 onto the stack
   1:   istore_1        // store int value into variable 1 (i)
   2:   lconst_0        // push the long 0 onto the stack
   3:   lstore_2        // store a long value in a local variable 2 (s3)
   4:   goto        26
   7:   iconst_0        // push the int value 0 onto the stack
   8:   istore  4   // store int value into variable 4 (j)
   10:  goto        17
   13:  lload_2     // load a long value from a local variable 2 (s3)
   14:  lconst_1        // push the long 1 onto the stack
   15:  ladd        // add two longs
   16:  lstore_2        // store a long value in a local variable 2 (s3)
   17:  iload   4   // load an int value from a local variable 4 (j)
   19:  iinc    4,1    // increment local variable 4 (j) by signed byte 1
   22:  iload_0     // load an int value from a local variable 0 (L)
   23:  if_icmplt   13  // if value1 is less than value2,branch to instruction at 13
   26:  iload_1     // load an int value from a local variable 1 (i)
   27:  iinc    1,1    // increment local variable 1 by signed byte 1
   30:  iload_0     // load an int value from a local variable 0 (L)
   31:  if_icmplt   7   // if value1 is less than value2,branch to instruction at 7
   34:  lload_2     // load a long value from a local variable 2 (s3)
   35:  lreturn     // return a long value

为了使输出更加可读,我已经附加了描述每个指令基于?Java bytecode instruction listings执行的注释.

如果仔细观察,您将看到这两个字节码之间存在重大差异.
while循环(对于for循环也是如此)在字节码末尾定义了if语句(if_icmplt指令).这意味着要检查第一个循环的退出条件,必须调用到第26行的转换,并且类似地,转到第二个循环的第17行.

上述字节码是在Mac OS X上使用javac 1.6.0_45生成的.

概要

我认为不同量的比较加上在while和for循环字节码中存在goto指令是这些循环之间的性能差异的原因.