无继承

有 static 修饰

static final

// 生成随机数字和字母, 
public static final String getStringRandomFinal(int length) { 
    String val = ""; 
    Random random = new Random(); 
    // 参数length,表示生成几位随机数 
    for (int i = 0; i < length; i++) { 
        String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; 
        // 输出字母还是数字 
        if ("char".equalsIgnoreCase(charOrNum)) { 
            // 输出是大写字母还是小写字母 
            // int temp = random.nextInt(2) % 2 == 0 ? 65 : 97; 
            val += (char) (random.nextInt(26) + 97); 
        } else if ("num".equalsIgnoreCase(charOrNum)) { 
            val += String.valueOf(random.nextInt(10)); 
        } 
    } 
    return val; 
}

static 非 final

// 生成随机数字和字母, 
public static String getStringRandom(int length) { 
    String val = ""; 
    Random random = new Random(); 
    // 参数length,表示生成几位随机数 
    for (int i = 0; i < length; i++) { 
        String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; 
        // 输出字母还是数字 
        if ("char".equalsIgnoreCase(charOrNum)) { 
            // 输出是大写字母还是小写字母 
            // int temp = random.nextInt(2) % 2 == 0 ? 65 : 97; 
            val += (char) (random.nextInt(26) + 97); 
        } else if ("num".equalsIgnoreCase(charOrNum)) { 
            val += String.valueOf(random.nextInt(10)); 
        } 
    } 
    return val; 
}

结果

这里使用了 OpenJDK 的 JMH 基准测试工具来测试的,结果如下:

# JMH 1.4.1 (released 903 days ago, please consider updating!) 
# VM invoker: /srv/jdk1.8.0_92/jre/bin/java 
# VM options: <none> 
# Warmup: 20 iterations, 1 s each 
# Measurement: 20 iterations, 1 s each 
# Timeout: 10 min per iteration 
# Threads: 1 thread, will synchronize iterations 
# Benchmark mode: Throughput, ops/time 
# Benchmark: org.agoncal.sample.jmh.Main.benchmark 
中间忽略了预热及测试过程,这里只显示结果 
Result: 206924.113 ±(99.9%) 7746.446 ops/s [Average] 
  Statistics: (min, avg, max) = (132107.466, 206924.113, 267265.397), stdev = 32798.937 
  Confidence interval (99.9%): [199177.667, 214670.559] 
# JMH 1.4.1 (released 903 days ago, please consider updating!) 
# VM invoker: /srv/jdk1.8.0_92/jre/bin/java 
# VM options: <none> 
# Warmup: 20 iterations, 1 s each 
# Measurement: 20 iterations, 1 s each 
# Timeout: 10 min per iteration 
# Threads: 1 thread, will synchronize iterations 
# Benchmark mode: Throughput, ops/time 
# Benchmark: org.agoncal.sample.jmh.Main.benchmarkFinal 
中间忽略了预热及测试过程,这里只显示结果 
Result: 210111.568 ±(99.9%) 8486.176 ops/s [Average] 
  Statistics: (min, avg, max) = (133813.368, 210111.568, 267525.228), stdev = 35931.001 
  Confidence interval (99.9%): [201625.392, 218597.744] 
 
# Run complete. Total time: 00:13:54 
Benchmark                       Mode  Samples       Score      Error  Units 
o.a.s.j.Main.benchmark         thrpt      200  206924.113 ± 7746.446  ops/s 
o.a.s.j.Main.benchmarkFinal    thrpt      200  210111.568 ± 8486.176  ops/s

总结:你说final的性能比非final有没有提升呢?可以说有,但几乎可以忽略不计。如果单纯地追求性能,而将所有的方法修改为 final 的话,我认为这样子是不可取的。而且这性能的差别,远远也没有网上有些人说的提升 50% 这么恐怖(有可能他们使用的是10年前的JVM来测试的吧^_^,比如 《35+ 个 Java 代码性能优化总结》这篇文章。雷总:不服?咱们来跑个分!)

分析

字节码级别的差别

StringKit.java
StringKitFinal.java

它们在字节码上的差别:

[18:52:08] emacsist:target $ diff /tmp/stringkit.log /tmp/stringkit-final.log 
1,5c1,5 
< Classfile /Users/emacsist/Documents/idea/logging/target/classes/org/agoncal/sample/jmh/StringKit.class 
<   Last modified 2017-6-15; size 1098 bytes 
<   MD5 checksum fe1ccdde26107e4037afc54c780f2c95 
<   Compiled from "StringKit.java" 
< public class org.agoncal.sample.jmh.StringKit 
--- 
> Classfile /Users/emacsist/Documents/idea/logging/target/classes/org/agoncal/sample/jmh/StringKitFinal.class 
>   Last modified 2017-6-15; size 1118 bytes 
>   MD5 checksum 410f8bf0eb723b794e4754c6eb8b9829 
>   Compiled from "StringKitFinal.java" 
> public class org.agoncal.sample.jmh.StringKitFinal 
24c24 
<   #15 = Class              #52            // org/agoncal/sample/jmh/StringKit 
--- 
>   #15 = Class              #52            // org/agoncal/sample/jmh/StringKitFinal 
32,33c32,33 
<   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKit; 
<   #24 = Utf8               getStringRandom 
--- 
>   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKitFinal; 
>   #24 = Utf8               getStringRandomFinal 
47c47 
<   #38 = Utf8               StringKit.java 
--- 
>   #38 = Utf8               StringKitFinal.java 
61c61 
<   #52 = Utf8               org/agoncal/sample/jmh/StringKit 
--- 
>   #52 = Utf8               org/agoncal/sample/jmh/StringKitFinal 
75c75 
<   public org.agoncal.sample.jmh.StringKit(); 
--- 
>   public org.agoncal.sample.jmh.StringKitFinal(); 
87c87 
<             0       5     0  this   Lorg/agoncal/sample/jmh/StringKit; 
--- 
>             0       5     0  this   Lorg/agoncal/sample/jmh/StringKitFinal; 
89c89 
<   public static java.lang.String getStringRandom(int); 
--- 
>   public static final java.lang.String getStringRandomFinal(int); 
91c91 
<     flags: ACC_PUBLIC, ACC_STATIC 
--- 
>     flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL 
187c187 
< SourceFile: "StringKit.java" 
--- 
> SourceFile: "StringKitFinal.java"

可以看到除了方法名和方法修饰符不同之外,其他的没有什么区别了。

在调用者上面的字节码差别

public void benchmark(); 
  descriptor: ()V 
  flags: ACC_PUBLIC 
  Code: 
    stack=1, locals=1, args_size=1 
       0: bipush        32 
       2: invokestatic  #2                  // Method org/agoncal/sample/jmh/StringKit.getStringRandom:(I)Ljava/lang/String; 
       5: pop 
       6: return 
    LineNumberTable: 
      line 21: 0 
      line 22: 6 
    LocalVariableTable: 
      Start  Length  Slot  Name   Signature 
          0       7     0  this   Lorg/agoncal/sample/jmh/Main; 
  RuntimeVisibleAnnotations: 
    0: #26() 
public void benchmarkFinal(); 
  descriptor: ()V 
  flags: ACC_PUBLIC 
  Code: 
    stack=1, locals=1, args_size=1 
       0: bipush        32 
       2: invokestatic  #3                  // Method org/agoncal/sample/jmh/StringKitFinal.getStringRandomFinal:(I)Ljava/lang/String; 
       5: pop 
       6: return 
    LineNumberTable: 
      line 26: 0 
      line 27: 6 
    LocalVariableTable: 
      Start  Length  Slot  Name   Signature 
          0       7     0  this   Lorg/agoncal/sample/jmh/Main; 
  RuntimeVisibleAnnotations: 
    0: #26()

可以看到,它们在调用者上面的字节码也没有什么区别,只是方法名不一样之外。

对于 JVM 来说,它是只认字节码的,既然字节码除了方法名和修饰符一样,其他都一样,那就可以大概推测它们的性能几乎可以忽略不计了。因为调用 static final 和 static 非 final 的JVM指令是一样。

无 static 修饰

方法体是一样的,只是将它们删除了 static 的修饰。

结果

# JMH version: 1.19 
# VM version: JDK 1.8.0_92, VM 25.92-b14 
# VM invoker: /srv/jdk1.8.0_92/jre/bin/java 
# VM options: <none> 
# Warmup: 20 iterations, 1 s each 
# Measurement: 20 iterations, 1 s each 
# Timeout: 10 min per iteration 
# Threads: 1 thread, will synchronize iterations 
# Benchmark mode: Throughput, ops/time 
# Benchmark: org.agoncal.sample.jmh.Main.benchmark 
中间忽略了预热及测试过程,这里只显示结果 
Result "org.agoncal.sample.jmh.Main.benchmark": 
  201306.770 ±(99.9%) 8184.423 ops/s [Average] 
  (min, avg, max) = (131889.934, 201306.770, 259928.172), stdev = 34653.361 
  CI (99.9%): [193122.347, 209491.193] (assumes normal distribution) 
# JMH version: 1.19 
# VM version: JDK 1.8.0_92, VM 25.92-b14 
# VM invoker: /srv/jdk1.8.0_92/jre/bin/java 
# VM options: <none> 
# Warmup: 20 iterations, 1 s each 
# Measurement: 20 iterations, 1 s each 
# Timeout: 10 min per iteration 
# Threads: 1 thread, will synchronize iterations 
# Benchmark mode: Throughput, ops/time 
# Benchmark: org.agoncal.sample.jmh.Main.benchmarkFinal 
中间忽略了预热及测试过程,这里只显示结果 
Result "org.agoncal.sample.jmh.Main.benchmarkFinal": 
  196871.022 ±(99.9%) 8595.719 ops/s [Average] 
  (min, avg, max) = (131182.268, 196871.022, 265522.769), stdev = 36394.814 
  CI (99.9%): [188275.302, 205466.741] (assumes normal distribution) 
 
# Run complete. Total time: 00:13:35 
Benchmark             Mode  Cnt       Score      Error  Units 
Main.benchmark       thrpt  200  201306.770 ± 8184.423  ops/s 
Main.benchmarkFinal  thrpt  200  196871.022 ± 8595.719  ops/s

分析

字节码级别的差别

[19:20:17] emacsist:target $ diff /tmp/stringkit.log /tmp/stringkit-final.log 
1,5c1,5 
< Classfile /Users/emacsist/Documents/idea/logging/target/classes/org/agoncal/sample/jmh/StringKit.class 
<   Last modified 2017-6-15; size 1110 bytes 
<   MD5 checksum f61144e86f7c17dc5d5f2b2d35fac36d 
<   Compiled from "StringKit.java" 
< public class org.agoncal.sample.jmh.StringKit 
--- 
> Classfile /Users/emacsist/Documents/idea/logging/target/classes/org/agoncal/sample/jmh/StringKitFinal.class 
>   Last modified 2017-6-15; size 1130 bytes 
>   MD5 checksum 15ce17ee17fdb5f4721f0921977b1e69 
>   Compiled from "StringKitFinal.java" 
> public class org.agoncal.sample.jmh.StringKitFinal 
24c24 
<   #15 = Class              #52            // org/agoncal/sample/jmh/StringKit 
--- 
>   #15 = Class              #52            // org/agoncal/sample/jmh/StringKitFinal 
32,33c32,33 
<   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKit; 
<   #24 = Utf8               getStringRandom 
--- 
>   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKitFinal; 
>   #24 = Utf8               getStringRandomFinal 
47c47 
<   #38 = Utf8               StringKit.java 
--- 
>   #38 = Utf8               StringKitFinal.java 
61c61 
<   #52 = Utf8               org/agoncal/sample/jmh/StringKit 
--- 
>   #52 = Utf8               org/agoncal/sample/jmh/StringKitFinal 
75c75 
<   public org.agoncal.sample.jmh.StringKit(); 
--- 
>   public org.agoncal.sample.jmh.StringKitFinal(); 
87c87 
<             0       5     0  this   Lorg/agoncal/sample/jmh/StringKit; 
--- 
>             0       5     0  this   Lorg/agoncal/sample/jmh/StringKitFinal; 
89c89 
<   public java.lang.String getStringRandom(int); 
--- 
>   public final java.lang.String getStringRandomFinal(int); 
91c91 
<     flags: ACC_PUBLIC 
--- 
>     flags: ACC_PUBLIC, ACC_FINAL 
169c169 
<             0     125     0  this   Lorg/agoncal/sample/jmh/StringKit; 
--- 
>             0     125     0  this   Lorg/agoncal/sample/jmh/StringKitFinal; 
188c188 
< SourceFile: "StringKit.java" 
--- 
> SourceFile: "StringKitFinal.java"

可以看到,字节码上除了名字和 final 修饰符差别外,其余的是一样的。

在调用者上面的字节码差别

public void benchmark(); 
  descriptor: ()V 
  flags: ACC_PUBLIC 
  Code: 
    stack=2, locals=1, args_size=1 
       0: new           #2                  // class org/agoncal/sample/jmh/StringKit 
       3: dup 
       4: invokespecial #3                  // Method org/agoncal/sample/jmh/StringKit."<init>":()V 
       7: bipush        32 
       9: invokevirtual #4                  // Method org/agoncal/sample/jmh/StringKit.getStringRandom:(I)Ljava/lang/String; 
      12: pop 
      13: return 
    LineNumberTable: 
      line 21: 0 
      line 22: 13 
    LocalVariableTable: 
      Start  Length  Slot  Name   Signature 
          0      14     0  this   Lorg/agoncal/sample/jmh/Main; 
  RuntimeVisibleAnnotations: 
    0: #30() 
public void benchmarkFinal(); 
  descriptor: ()V 
  flags: ACC_PUBLIC 
  Code: 
    stack=2, locals=1, args_size=1 
       0: new           #5                  // class org/agoncal/sample/jmh/StringKitFinal 
       3: dup 
       4: invokespecial #6                  // Method org/agoncal/sample/jmh/StringKitFinal."<init>":()V 
       7: bipush        32 
       9: invokevirtual #7                  // Method org/agoncal/sample/jmh/StringKitFinal.getStringRandomFinal:(I)Ljava/lang/String; 
      12: pop 
      13: return 
    LineNumberTable: 
      line 26: 0 
      line 27: 13 
    LocalVariableTable: 
      Start  Length  Slot  Name   Signature 
          0      14     0  this   Lorg/agoncal/sample/jmh/Main; 
  RuntimeVisibleAnnotations: 
    0: #30()

可以看到,它们除了名字不同之外,其他的JVM指令都是一样的。

总结

对于是否有 final 修饰的方法,对性能的影响可以忽略不计。因为它们生成的字节码除了 flags 标志位是否有 final 修饰不同之外,其他所有的JVM指令,都是一样的(对于方法本身,以及调用者本身的字节码都一样)。对于JVM来说,它执行的就是字节码,如果字节码都一样的话,那对于JVM来说,它就是同一样东西的了。

有继承

无 final 修饰

package org.agoncal.sample.jmh; 
import java.util.Random; 
/** 
 * Created by emacsist on 2017/6/15. 
 */ 
public abstract class StringKitAbs { 
    // 生成随机数字和字母, 
    public String getStringRandom(int length) { 
        String val = ""; 
        Random random = new Random(); 
        // 参数length,表示生成几位随机数 
        for (int i = 0; i < length; i++) { 
            String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; 
            // 输出字母还是数字 
            if ("char".equalsIgnoreCase(charOrNum)) { 
                // 输出是大写字母还是小写字母 
                // int temp = random.nextInt(2) % 2 == 0 ? 65 : 97; 
                val += (char) (random.nextInt(26) + 97); 
            } else if ("num".equalsIgnoreCase(charOrNum)) { 
                val += String.valueOf(random.nextInt(10)); 
            } 
        } 
        return val; 
    } 
}

有 final 修饰

package org.agoncal.sample.jmh; 
import java.util.Random; 
/** 
 * Created by emacsist on 2017/6/15. 
 */ 
public abstract class StringKitAbsFinal { 
    // 生成随机数字和字母, 
    public final String getStringRandomFinal(int length) { 
        String val = ""; 
        Random random = new Random(); 
        // 参数length,表示生成几位随机数 
        for (int i = 0; i < length; i++) { 
            String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; 
            // 输出字母还是数字 
            if ("char".equalsIgnoreCase(charOrNum)) { 
                // 输出是大写字母还是小写字母 
                // int temp = random.nextInt(2) % 2 == 0 ? 65 : 97; 
                val += (char) (random.nextInt(26) + 97); 
            } else if ("num".equalsIgnoreCase(charOrNum)) { 
                val += String.valueOf(random.nextInt(10)); 
            } 
        } 
        return val; 
    } 
}

测试代码

写一个类来继承上面的抽象类,以此来测试在继承中 final 有否对多态中的影响

package org.agoncal.sample.jmh; 
/** 
 * Created by emacsist on 2017/6/15. 
 */ 
public class StringKitFinal extends StringKitAbsFinal { 
}
package org.agoncal.sample.jmh; 
/** 
 * Created by emacsist on 2017/6/15. 
 */ 
public class StringKit extends StringKitAbs { 
}

然后在基准测试中:

@Benchmark 
public void benchmark() { 
    new StringKit().getStringRandom(32); 
} 
@Benchmark 
public void benchmarkFinal() { 
    new StringKitFinal().getStringRandomFinal(32); 
}

测试结果

非 final 结果

# JMH version: 1.19 
# VM version: JDK 1.8.0_92, VM 25.92-b14 
# VM invoker: /srv/jdk1.8.0_92/jre/bin/java 
# VM options: <none> 
# Warmup: 20 iterations, 1 s each 
# Measurement: 20 iterations, 1 s each 
# Timeout: 10 min per iteration 
# Threads: 1 thread, will synchronize iterations 
# Benchmark mode: Throughput, ops/time 
# Benchmark: org.agoncal.sample.jmh.Main.benchmark 
中间忽略了预热及测试过程 
Result "org.agoncal.sample.jmh.Main.benchmark": 
  213462.677 ±(99.9%) 8670.164 ops/s [Average] 
  (min, avg, max) = (135751.428, 213462.677, 264182.887), stdev = 36710.017 
  CI (99.9%): [204792.513, 222132.841] (assumes normal distribution)

有 final 结果

# JMH version: 1.19 
# VM version: JDK 1.8.0_92, VM 25.92-b14 
# VM invoker: /srv/jdk1.8.0_92/jre/bin/java 
# VM options: <none> 
# Warmup: 20 iterations, 1 s each 
# Measurement: 20 iterations, 1 s each 
# Timeout: 10 min per iteration 
# Threads: 1 thread, will synchronize iterations 
# Benchmark mode: Throughput, ops/time 
# Benchmark: org.agoncal.sample.jmh.Main.benchmarkFinal 
中间忽略了预热及测试过程 
Result "org.agoncal.sample.jmh.Main.benchmarkFinal": 
  213684.585 ±(99.9%) 8571.512 ops/s [Average] 
  (min, avg, max) = (133472.162, 213684.585, 267742.236), stdev = 36292.318 
  CI (99.9%): [205113.073, 222256.097] (assumes normal distribution)

总对比

# Run complete. Total time: 00:13:35 
Benchmark             Mode  Cnt       Score      Error  Units 
Main.benchmark       thrpt  200  213462.677 ± 8670.164  ops/s 
Main.benchmarkFinal  thrpt  200  213684.585 ± 8571.512  ops/s

它们字节码的区别

[12:12:19] emacsist:classes $ diff /tmp/StringKit.log /tmp/StringKitFinal.log 
1,5c1,5 
< Classfile /Users/emacsist/Documents/idea/logging/target/classes/org/agoncal/sample/jmh/StringKit.class 
<   Last modified 2017-6-16; size 317 bytes 
<   MD5 checksum 7f9b024adc7f39345215e3e8490cafe4 
<   Compiled from "StringKit.java" 
< public class org.agoncal.sample.jmh.StringKit extends org.agoncal.sample.jmh.StringKitAbs 
--- 
> Classfile /Users/emacsist/Documents/idea/logging/target/classes/org/agoncal/sample/jmh/StringKitFinal.class 
>   Last modified 2017-6-16; size 337 bytes 
>   MD5 checksum f54eadc79a90675d97e95f766ef88a87 
>   Compiled from "StringKitFinal.java" 
> public class org.agoncal.sample.jmh.StringKitFinal extends org.agoncal.sample.jmh.StringKitAbsFinal 
10,12c10,12 
<    #1 = Methodref          #3.#13         // org/agoncal/sample/jmh/StringKitAbs."<init>":()V 
<    #2 = Class              #14            // org/agoncal/sample/jmh/StringKit 
<    #3 = Class              #15            // org/agoncal/sample/jmh/StringKitAbs 
--- 
>    #1 = Methodref          #3.#13         // org/agoncal/sample/jmh/StringKitAbsFinal."<init>":()V 
>    #2 = Class              #14            // org/agoncal/sample/jmh/StringKitFinal 
>    #3 = Class              #15            // org/agoncal/sample/jmh/StringKitAbsFinal 
19c19 
<   #10 = Utf8               Lorg/agoncal/sample/jmh/StringKit; 
--- 
>   #10 = Utf8               Lorg/agoncal/sample/jmh/StringKitFinal; 
21c21 
<   #12 = Utf8               StringKit.java 
--- 
>   #12 = Utf8               StringKitFinal.java 
23,24c23,24 
<   #14 = Utf8               org/agoncal/sample/jmh/StringKit 
<   #15 = Utf8               org/agoncal/sample/jmh/StringKitAbs 
--- 
>   #14 = Utf8               org/agoncal/sample/jmh/StringKitFinal 
>   #15 = Utf8               org/agoncal/sample/jmh/StringKitAbsFinal 
26c26 
<   public org.agoncal.sample.jmh.StringKit(); 
--- 
>   public org.agoncal.sample.jmh.StringKitFinal(); 
32c32 
<          1: invokespecial #1                  // Method org/agoncal/sample/jmh/StringKitAbs."<init>":()V 
--- 
>          1: invokespecial #1                  // Method org/agoncal/sample/jmh/StringKitAbsFinal."<init>":()V 
38c38 
<             0       5     0  this   Lorg/agoncal/sample/jmh/StringKit; 
--- 
>             0       5     0  this   Lorg/agoncal/sample/jmh/StringKitFinal; 
40c40 
< SourceFile: "StringKit.java" 
--- 
> SourceFile: "StringKitFinal.java"

可以看到,除了它们的方法签名和方法名字不同之外其他的都是一样的,包括JVM调用指令也完全是一样的。

总结

可以看到它们几乎是一样的。

总结

基于上面的基准测试结论,我认为滥用或刻意为了所谓的提升性能,而去为每一个方法尽可能添加 final 的关键字是不可取的。使用 final ,更多的应该是根据Java对 final 的语义来定义,而不是只想着为了提升性能(而且这影响可以忽略不计)而刻意用 final.

使用 final 的情况:

final 变量: 表示只读(只初始化一次,但可多次读取)
final 方法:表示子类不可以重写。(网上认为 final 比非 final 快,就是认为它是在编译的时候已经静态绑定了,不需要在运行时再动态绑定。这个可能以前的JVM上是正确的,但在现代的JVM上,这个可以认为没什么影响,至少我在基准测试里是这样子)
final 类: 它们不能被继承,而且final类的方法,默认也是 final 的。

关于这个 final 的性能问题,我也Google了下,发现 stackoverflow 上,也有类似的问题:stackoverflow

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