为什么添加 try 块会使程序更快？

Question

我正在使用以下代码来测试 try 块的速度。令我惊讶的是，try 块使它更快。为什么？

public class Test {
    int value;

    public int getValue() {
        return value;
    }

    public void reset() {
        value = 0;
    }

    // Calculates without exception
    public void method1(int i) {
        value = ((value + i) / i) << 1;
        // Will never be true
        if ((i & 0xFFFFFFF) == 1000000000) {
            System.out.println("You'll never see this!");
        }
    }

    public static void main(String[] args) {
        int i;
        long l;
        Test t = new Test();

        l = System.currentTimeMillis();
        t.reset();
        for (i = 1; i < 100000000; i++) {
            t.method1(i);
        }
        l = System.currentTimeMillis() - l;
        System.out.println("method1 took " + l + " ms, result was "
                + t.getValue());

        // using a try block
        l = System.currentTimeMillis();
        t.reset();
        for (i = 1; i < 100000000; i++) {
            try {
                t.method1(i);
            } catch (Exception e) {

            }
        }

        l = System.currentTimeMillis() - l;
        System.out.println("method1 with try block took " + l + " ms, result was "
                + t.getValue());
    }
}

我的机器运行的是 64 位 Windows 7 和 64 位 JDK7。我得到了以下结果：

method1 took 914 ms, result was 2
method1 with try block took 789 ms, result was 2

我已经多次运行代码，每次都得到几乎相同的结果。

更新：

这是在 MacBook Pro、Java 6 上运行测试十次的结果。Try-catch 使该方法更快，与在 Windows 上相同。

method1 took 895 ms, result was 2
method1 with try block took 783 ms, result was 2
--------------------------------------------------
method1 took 943 ms, result was 2
method1 with try block took 803 ms, result was 2
--------------------------------------------------
method1 took 867 ms, result was 2
method1 with try block took 745 ms, result was 2
--------------------------------------------------
method1 took 856 ms, result was 2
method1 with try block took 744 ms, result was 2
--------------------------------------------------
method1 took 862 ms, result was 2
method1 with try block took 744 ms, result was 2
--------------------------------------------------
method1 took 859 ms, result was 2
method1 with try block took 765 ms, result was 2
--------------------------------------------------
method1 took 937 ms, result was 2
method1 with try block took 767 ms, result was 2
--------------------------------------------------
method1 took 861 ms, result was 2
method1 with try block took 744 ms, result was 2
--------------------------------------------------
method1 took 858 ms, result was 2
method1 with try block took 744 ms, result was 2
--------------------------------------------------
method1 took 858 ms, result was 2
method1 with try block took 749 ms, result was 2

Answer 1

当您在同一方法中有多个长时间运行的循环时，可能会触发整个方法的优化，并在第二个循环上产生不可预测的结果。避免这种情况的一种方法是；

• 为每个循环提供自己的方法
• 多次运行测试以检查结果是否可重现
• 运行测试 2 - 10 秒。

您会看到一些变化，有时结果是不确定的。即变化大于差异。

public class Test {
    int value;

    public int getValue() {
        return value;
    }

    public void reset() {
        value = 0;
    }

    // Calculates without exception
    public void method1(int i) {
        value = ((value + i) / i) << 1;
        // Will never be true
        if ((i & 0xFFFFFFF) == 1000000000) {
            System.out.println("You'll never see this!");
        }
    }

    public static void main(String[] args) {
        Test t = new Test();
        for (int i = 0; i < 5; i++) {
            testWithTryCatch(t);
            testWithoutTryCatch(t);
        }
    }

    private static void testWithoutTryCatch(Test t) {
        t.reset();
        long l = System.currentTimeMillis();
        for (int j = 0; j < 10; j++)
            for (int i = 1; i <= 100000000; i++)
                t.method1(i);

        l = System.currentTimeMillis() - l;
        System.out.println("without try/catch method1 took " + l + " ms, result was " + t.getValue());
    }

    private static void testWithTryCatch(Test t) {
        t.reset();
        long l = System.currentTimeMillis();
        for (int j = 0; j < 10; j++)
            for (int i = 1; i <= 100000000; i++)
                try {
                    t.method1(i);
                } catch (Exception ignored) {
                }

        l = System.currentTimeMillis() - l;
        System.out.println("with try/catch method1 took " + l + " ms, result was " + t.getValue());
    }
}

打印

with try/catch method1 took 9723 ms, result was 2
without try/catch method1 took 9456 ms, result was 2
with try/catch method1 took 9672 ms, result was 2
without try/catch method1 took 9476 ms, result was 2
with try/catch method1 took 8375 ms, result was 2
without try/catch method1 took 8233 ms, result was 2
with try/catch method1 took 8337 ms, result was 2
without try/catch method1 took 8227 ms, result was 2
with try/catch method1 took 8163 ms, result was 2
without try/catch method1 took 8565 ms, result was 2

从这些结果来看，使用 try/catch 可能会稍微慢一些，但并非总是如此。

在 Windows 7、Xeon E5450 和 Java 7 更新 7 上运行。

Answer 2

我用 Caliper Microbenchmark 进行了尝试，我真的看不出有什么不同。

代码如下：

public class TryCatchBenchmark extends SimpleBenchmark {

    private int value;

    public void setUp() {
        value = 0;
    }

    // Calculates without exception
    public void method1(int i) {
        value = ((value + i) / i) << 1;
        // Will never be true
        if ((i & 0xFFFFFFF) == 1000000000) {
            System.out.println("You'll never see this!");
        }
    }

    public void timeWithoutTryCatch(int reps) {
        for (int i = 1; i < reps; i++) {
            this.method1(i);
        }
    }

    public void timeWithTryCatch(int reps) {
        for (int i = 1; i < reps; i++) {
            try {
                this.method1(i);
            } catch (Exception ignore) {
            }
        }
    }

    public static void main(String[] args) {
        new Runner().run(TryCatchBenchmark.class.getName());
    }
}

结果如下：

0% 场景{vm=java, trial=0, benchmark=WithoutTryCatch} 8,23 ns; σ=0,03 ns @ 3 次试验 50% 场景{vm=java, trial=0, benchmark=WithTryCatch} 8,13 ns； σ=0,03 ns @ 3 次试验 基准 ns 线性运行时 没有TryCatch 8,23 =============================== With TryCatch 8,13 ==============================

如果我交换函数的顺序（让它们以相反的顺序运行），结果是：

0% 场景{vm=java, trial=0, benchmark=WithTryCatch} 8,21 ns; σ=0,05 ns @ 3 次试验 50% 场景{vm=java, trial=0, benchmark=WithoutTryCatch} 8,14 ns； σ=0,03 ns @ 3 次试验 基准 ns 线性运行时 With TryCatch 8,21 =============================== 没有TryCatch 8,14 ==============================

我会说它们基本上是一样的。

Answer 3

我做了一些实验。

首先，我完全确认 OP 的发现。即使删除第一个循环，或者将异常更改为一些完全不相关的异常，try catch，只要您不通过重新抛出异常来添加分支，确实会使代码更快。如果确实必须捕获异常，则代码仍然更快（例如，如果您使循环从 0 而不是 1 开始）。

我的“解释”是，JIT 是疯狂的优化机器，有时它们的性能比其他时候更好，如果没有在 JIT 级别进行非常具体的研究，您通常无法理解。有许多可能的事情可以改变（例如使用寄存器）。

This is globally what was found in a very similar case with a C# JIT.

无论如何，Java 都针对 try-catch 进行了优化。由于总是存在异常的可能性，因此通过添加 try-catch 并不会真正添加太多分支，因此发现第二个循环不比第一个循环长也就不足为奇了。

Answer 4

为了避免 JVM 和操作系统可能执行的任何隐藏优化或缓存，我首先开发了两个种子 java 程序，TryBlock 和 NoTryBlock，它们的区别在于是否使用 try 块。这两个种子程序将用于生成不同的程序以禁止 JVM 或 OS 进行隐藏优化。每次测试都会生成并编译一个新的java程序，我重复测试10次。

根据我的实验，不使用 try 块运行平均需要 9779.3 毫秒，而使用 try 块运行需要 9775.9 毫秒：它们的平均运行时间相差 3.4 毫秒（或 0.035%），这可以被视为噪音。这表明使用 void try 块（我所说的 void，我的意思是除了空指针异常之外不存在任何可能的异常）或不使用似乎对运行时间没有影响。

测试在同一台linux机器（cpu 2392MHz）和java版本“1.6.0_24”下运行。

下面是我基于种子程序生成测试程序的脚本：

for i in `seq 1 10`; do
  echo "NoTryBlock$i"
  cp NoTryBlock.java NoTryBlock$i.java
  find . -name "NoTryBlock$i.java" -print | xargs sed -i "s/NoTryBlock/NoTryBlock$i/g";
  javac NoTryBlock$i.java;
  java NoTryBlock$i
  rm NoTryBlock$i.* -f;
done

for i in `seq 1 10`; do
  echo "TryBlock$i"
  cp TryBlock.java TryBlock$i.java
  find . -name "TryBlock$i.java" -print | xargs sed -i "s/TryBlock/TryBlock$i/g";
  javac TryBlock$i.java;
  java TryBlock$i
  rm TryBlock$i.* -f;
done

这里是种子程序，首先是NoTryBlock.java

import java.util.*;
import java.lang.*;

public class NoTryBlock {
    int value;

    public int getValue() {
        return value;
    }

    public void reset() {
        value = 0;
    }

    // Calculates without exception
    public void method1(int i) {
        value = ((value + i) / i) << 1;
        // Will never be true
        if ((i & 0xFFFFFFF) == 1000000000) {
            System.out.println("You'll never see this!");
        }
    }

    public static void main(String[] args) {
        int i, j;
        long l;
        NoTryBlock t = new NoTryBlock();

        // using a try block
        l = System.currentTimeMillis();
        t.reset();
        for (j = 1; j < 10; ++j) {
          for (i = 1; i < 100000000; i++) {
              t.method1(i);
          }
        }
        l = System.currentTimeMillis() - l;
        System.out.println(
            "method1 with try block took " + l + " ms, result was "
                + t.getValue());
    }
}

第二个是TryBlock.java，在方法函数调用上使用了try-block：

import java.util.*;
import java.lang.*;

public class TryBlock {
    int value;

    public int getValue() {
        return value;
    }

    public void reset() {
        value = 0;
    }

    // Calculates without exception
    public void method1(int i) {
        value = ((value + i) / i) << 1;
        // Will never be true
        if ((i & 0xFFFFFFF) == 1000000000) {
            System.out.println("You'll never see this!");
        }
    }

    public static void main(String[] args) {
        int i, j;
        long l;
        TryBlock t = new TryBlock();

        // using a try block
        l = System.currentTimeMillis();
        t.reset();
        for (j = 1; j < 10; ++j) {
          for (i = 1; i < 100000000; i++) {
            try {
              t.method1(i);
            } catch (Exception e) {
            }
          }
        }
        l = System.currentTimeMillis() - l;
        System.out.println(
            "method1 with try block took " + l + " ms, result was "
                + t.getValue());
    }
}

下面是我的两个种子程序的差异，你可以看到除了类名，try 块是它们唯一的区别：

$ diff TryBlock.java NoTryBlock.java
4c4
<     public class TryBlock {
---
>     public class NoTryBlock {
27c27
<             TryBlock t = new TryBlock();
---
>             NoTryBlock t = new NoTryBlock();
34d33
<                 try {
36,37d34
<                 } catch (Exception e) {
<                 }
42c39
<                 "method1 with try block took " + l + " ms, result was "
---
>                 "method1 without try block took " + l + " ms, result was "

下面是输出：

method1 without try block took,9732,ms, result was 2
method1 without try block took,9756,ms, result was 2
method1 without try block took,9845,ms, result was 2
method1 without try block took,9794,ms, result was 2
method1 without try block took,9758,ms, result was 2
method1 without try block took,9733,ms, result was 2
method1 without try block took,9763,ms, result was 2
method1 without try block took,9893,ms, result was 2
method1 without try block took,9761,ms, result was 2
method1 without try block took,9758,ms, result was 2

method1 with try block took,9776,ms, result was 2
method1 with try block took,9751,ms, result was 2
method1 with try block took,9767,ms, result was 2
method1 with try block took,9726,ms, result was 2
method1 with try block took,9779,ms, result was 2
method1 with try block took,9797,ms, result was 2
method1 with try block took,9845,ms, result was 2
method1 with try block took,9784,ms, result was 2
method1 with try block took,9787,ms, result was 2
method1 with try block took,9747,ms, result was 2