为啥添加 try 块会使程序更快？

Posted 2023-03-25

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【中文标题】为啥添加 try 块会使程序更快？【英文标题】：Why adding a try block makes the program faster?为什么添加 try 块会使程序更快？ 【发布时间】：2012-10-02 22:25:02 【问题描述】：

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

【问题讨论】：

这超出了问题的范围，但是您应该使用System.nanoTime来比较数据。阅读System.currentTimeMillis vs System.nanoTime。 @RahulAgrawal 我交换了代码并得到了相同的结果。我围绕 OP 的代码做了很多测试，我证实了他的发现。是的：添加一个try catch，即使是完全不相关的错误或异常，确实可以使代码更快。如果你在 catch 中重新抛出异常，它并不会让它更快。您好，请查看***.com/questions/8423789/…，它将引导您访问 IBM 公司的论文：ibm.com/developerworks/java/library/j-benchmark1/index.html，它将向您展示如何执行正确的 java 代码基准测试。 【参考方案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。

【讨论】：

在我的机器上，testWithoutTryCatch() 总是从第三次开始变得更快。 Intel(R) Core(TM) i5-3450, PROCESSOR_IDENTIFIER=Intel64 Family 6 Model 58 Stepping 9, GenuineIntel +1，表示“变化大于差异”，这说明了一切【参考方案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。

【讨论】：

【参考方案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。

【讨论】：

【参考方案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

【讨论】：

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