是否指定异常？

Question

我有一个小的性能问题，当使用try-catch子句时，最好指定可以获得的确切异常，或者只使用exception它更好？例子：

try {
    whatever
} catch (NullPointerException ex) {
    whatever
}

或者如果你不介意什么样的例外：

try {
    whatever
} catch (Exception ex) {
    whatever
}

因为我知道你可以使用不同的异常来触发不同的效果，但我只是要求性能。

Answer 1

在这种情况下，两者都不是。try/catch比较昂贵，应该节约使用。手动检查null比捕获NullPointerException更好。

Answer 2

我建议这里的正确答案是根据编程原因使用适当的异常处理，而不是性能。如果（忽略性能）捕获一个NullPointerExcsion会更合适，那么就这样做。

例外情况应该是例外情况。它们应该很少发生，因此异常处理中的性能应该不如正确性重要。

如果您的应用程序定期预测这种情况，那么它应该通过某种机制来处理，而不是异常。如果您担心性能，这一点尤其正确，因为抛出异常总是代价高昂的。

Answer 3

根据我的测试，性能没有显著差异。

每次运行都会尝试每个场景中的1000万次，然后以纳秒和舍入秒为单位比较运行时间。这实际上与我最初的假设相反，因为我认为捕捉一个可丢弃的会显示出显著的改善。

我也开始意识到这部分可能是由于优化器的影响，因此我创建了一个更复杂的示例，其中包括下面的伪随机数，认为这将减轻优化器对代码的任何潜在影响。

（我不会教你如何正确使用cat块，因为这个问题是关于性能的，而不是最佳实践。）

下面有很多数据！

运行1结果：

Exception: 7196141955 (7.196s)
NumberFormatException: 7736401837 (7.736s)
Throwable: 6818656505 (6.819s)

运行2个结果：

Exception: 7262897545 (7.263s)
NumberFormatException: 7056116050 (7.056s)
Throwable: 7108232206 (7.108s)

运行3个结果：

Exception: 7088967045 (7.089s)
NumberFormatException: 7020495455 (7.020s)
Throwable: 7192925684 (7.193s)

运行4个结果：

Exception: 6916917328 (6.917s)
NumberFormatException: 7690084994 (7.690s)
Throwable: 6906011513 (6.906s)

运行5个结果：

Exception: 7247571874 (7.248s)
NumberFormatException: 6818511040 (6.819s)
Throwable: 6813286603 (6.813s)

代码

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Test {

    private static final int TRIALS = 10000000;
    private static final int NANOS_IN_SECOND = 1000000000;
    private static final int DECIMAL_PRECISION = 3;
    private static final RoundingMode ROUNDING_MODE = RoundingMode.HALF_UP;

    public static void main(String[] args) {

        long firstStart = System.nanoTime();

        for(int i = 0; i < TRIALS; i++) {
            try {
                throw new NumberFormatException();
            }
            catch(Exception e) {

            }
        }

        long firstEnd = System.nanoTime();

        long secondStart = System.nanoTime();

        for(int i = 0; i < TRIALS; i++) {
            try {
                throw new NumberFormatException();
            }
            catch(NumberFormatException e) {

            }
        }

        long secondEnd = System.nanoTime();

        long thirdStart = System.nanoTime();

        for(int i = 0; i < TRIALS; i++) {
            try {
                throw new NumberFormatException();
            }
            catch(Throwable e) {

            }
        }

        long thirdEnd = System.nanoTime();

        long exception = firstEnd - firstStart;
        long numberFormatException = secondEnd - secondStart;
        long throwable = thirdEnd - thirdStart;

        BigDecimal exceptionSeconds = new BigDecimal((double)exception / (double)NANOS_IN_SECOND);
        BigDecimal numberFormatExceptionSeconds = new BigDecimal((double)numberFormatException / (double)NANOS_IN_SECOND);
        BigDecimal throwableSeconds = new BigDecimal((double)throwable / (double)NANOS_IN_SECOND);

        exceptionSeconds = exceptionSeconds.setScale(DECIMAL_PRECISION, ROUNDING_MODE);
        numberFormatExceptionSeconds = numberFormatExceptionSeconds.setScale(DECIMAL_PRECISION, ROUNDING_MODE);
        throwableSeconds = throwableSeconds.setScale(DECIMAL_PRECISION, ROUNDING_MODE);

        System.out.println("Exception: " + exception + " (" + exceptionSeconds + "s)");
        System.out.println("NumberFormatException: " + numberFormatException + " (" + numberFormatExceptionSeconds + "s)");
        System.out.println("Throwable: " + throwable + " (" + throwableSeconds + "s)");

    }

}

更复杂的伪随机码

我创建它是为了确保优化器不会简单地“忽略”整个抛出/捕获过程，因为我意识到代码块将始终流向catch。通过尝试一个整数。parseInt（）在随机选择的字符串上（但总是无效的），这意味着编译器直到运行时才能知道通过for（）循环的给定运行是否有效。

正如第一次实验所预期的，三种情景之间没有显著差异。

运行1结果：

Exception: 10988431371 (10.988s)
NumberFormatException: 11360698958 (11.361s)
Throwable: 10539041505 (10.539s)

运行2个结果：

Exception: 12468860076 (12.469s)
NumberFormatException: 11852429194 (11.852s)
Throwable: 11859547560 (11.860s)

运行3个结果：

Exception: 10618082779 (10.618s)
NumberFormatException: 10718252324 (10.718s)
Throwable: 10327709072 (10.328s)

运行4个结果：

Exception: 11031135405 (11.031s)
NumberFormatException: 10689877480 (10.690s)
Throwable: 10668345685 (10.668s)

运行5个结果：

Exception: 11513727192 (11.514s)
NumberFormatException: 11581826079 (11.582s)
Throwable: 12488301109 (12.488s)

代码

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Random;

public class Test {

    private static final int TRIALS = 10000000;
    private static final int NANOS_IN_SECOND = 1000000000;
    private static final int DECIMAL_PRECISION = 3;
    private static final RoundingMode ROUNDING_MODE = RoundingMode.HALF_UP;

    private static final String[] TEST_STRINGS = {
        "lawl",
        "rofl",
        "trololo",
        "foo",
        "bar"
    };

    private static final Random RANDOM = new Random();



    public static void main(String[] args) {

        long firstStart = System.nanoTime();

        for(int i = 0; i < TRIALS; i++) {
            try {
                Integer.parseInt(TEST_STRINGS[RANDOM.nextInt(TEST_STRINGS.length)]);
            }
            catch(Exception e) {

            }
        }

        long firstEnd = System.nanoTime();

        long secondStart = System.nanoTime();

        for(int i = 0; i < TRIALS; i++) {
            try {
                Integer.parseInt(TEST_STRINGS[RANDOM.nextInt(TEST_STRINGS.length)]);
            }
            catch(NumberFormatException e) {

            }
        }

        long secondEnd = System.nanoTime();

        long thirdStart = System.nanoTime();

        for(int i = 0; i < TRIALS; i++) {
            try {
                Integer.parseInt(TEST_STRINGS[RANDOM.nextInt(TEST_STRINGS.length)]);
            }
            catch(Throwable e) {

            }
        }

        long thirdEnd = System.nanoTime();

        long exception = firstEnd - firstStart;
        long numberFormatException = secondEnd - secondStart;
        long throwable = thirdEnd - thirdStart;

        BigDecimal exceptionSeconds = new BigDecimal((double)exception / (double)NANOS_IN_SECOND);
        BigDecimal numberFormatExceptionSeconds = new BigDecimal((double)numberFormatException / (double)NANOS_IN_SECOND);
        BigDecimal throwableSeconds = new BigDecimal((double)throwable / (double)NANOS_IN_SECOND);

        exceptionSeconds = exceptionSeconds.setScale(DECIMAL_PRECISION, ROUNDING_MODE);
        numberFormatExceptionSeconds = numberFormatExceptionSeconds.setScale(DECIMAL_PRECISION, ROUNDING_MODE);
        throwableSeconds = throwableSeconds.setScale(DECIMAL_PRECISION, ROUNDING_MODE);

        System.out.println("Exception: " + exception + " (" + exceptionSeconds + "s)");
        System.out.println("NumberFormatException: " + numberFormatException + " (" + numberFormatExceptionSeconds + "s)");
        System.out.println("Throwable: " + throwable + " (" + throwableSeconds + "s)");

    }

}