当我们使用异常来执行边界检查时会发生什么？

使用异常处理空检查，边界检查，文件存在检查等操作会在抛出异常时引入大量开销。

What happens when we use exceptions to perform boundary checks?

Using exceptions for handling operations like null checking, bounds checking, file existance checking introduces a lot of overhead whenever the exception is thrown.

如果您只是检查边界，您会做什么：

What you would have done if you simply checked the bounds:

• 检查数组的大小是否为0而不是


• 返回结果

使用基于异常的检查时您实际在做什么：

What you actually are doing when using exception-based checking:

• 检查数组的边界


• 启动java异常机制（及其所有开销）


• 创建一个新的异常对象


• 转储整个堆栈跟踪


• 用新堆栈数据填充新创建的对象


• 捕获异常


• 返回结果

• check the bounds of the array
• initiate java exceptions mechanism (with all its overhead)
• create a new Exception object
• dump entire stack trace
• fill the newly created object with all stack data
• catch the exception
• return the result

通过这个简单的测试程序，我测量了两种类型的数组边界检查的速度。

With this simple test program, I have measured the speed of both types of array boundary checks.

public class BoundsCheckTest {

    final static int[] array = new int[1];
    final static Random gen = new Random();

    public static void main(String[] args){

        boolean ret = false;
        int tries = 100000000;
        long timestart = System.nanoTime();

        for (int a=0; a< tries; a++) {
            ret = method1();
        }
        long timeend1 = System.nanoTime();
        System.out.println();

        for (int a=0; a< tries; a++) {
            ret = metod2();
        }
        long timeend2 = System.nanoTime();
        System.out.println();


        long t1 = timeend1-timestart;
        long t2 = timeend2-timeend1;
        System.out.println("\ntime 1=["+t1+"]\n     2=["+t2+"]"+
                 "\ndiff=["+Math.abs(t1-t2)+"] percent diff=["+(100d*t2/t1-100)+"]");

    }

    private static boolean metod2() {
        try {
            int val = array[gen.nextInt(2)];
            return true;
        } catch (Exception e) {
            return false;
        }
    }

    private static boolean method1() {
        return array.length < gen.nextInt(2);
    }

}

结果：

JDK 7，eclipse 以模式运行：

JDK 7, eclipse Run as mode:

time check=[911620628]
       exc=[1192569638]
diff=[280949010] percent diff=[30.818632375220886]

JDK 7，eclipse 调试模式：

JDK 7, eclipse Debug mode:

time check=[931243924]
       exc=[651480777121]
diff=[650549533197] percent diff=[69858.12378809143]

禁用调试时的速度损失不是很显着，尽管它是可见的：没有例外的代码速度提高了约30％（大约50％的错误回报）。调试模式下的速度损失是惊人的。基于异常的代码比正常的直接数组大小检查运行速度慢约700倍。

The speed loss with debugging disabled is not very significant, though it is visible: code without exceptions is about 30% faster (for roughly 50% of false returns). The speed loss in debug mode is astonishing. The exception-based code runs about 700 times slower than the normal straight-up array size check.

例外背后的一般思想是允许一种处理 exceptiona 条件的方法。在这种情况下，根本没有异常条件 - 范围检查只是代码的正常部分。仅仅因为这个原因，在这种情况下不应该使用例外。

The general idea behind exceptions is to allow a way to handle exceptiona conditions. In this case, there is no exceptional condition at all - the range check is just a normal part of code. For that reason alone the exception should not be used in this situation.