无继承

有 static 修饰

static final

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// 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

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// 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:

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# 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:

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[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
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<   #15 = Class              #52            // org/agoncal/sample/jmh/StringKit
---
>   #15 = Class              #52            // org/agoncal/sample/jmh/StringKitFinal
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<   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKit;
<   #24 = Utf8               getStringRandom
---
>   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKitFinal;
>   #24 = Utf8               getStringRandomFinal
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<   #38 = Utf8               StringKit.java
---
>   #38 = Utf8               StringKitFinal.java
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<   #52 = Utf8               org/agoncal/sample/jmh/StringKit
---
>   #52 = Utf8               org/agoncal/sample/jmh/StringKitFinal
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<   public org.agoncal.sample.jmh.StringKit();
---
>   public org.agoncal.sample.jmh.StringKitFinal();
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<             0       5     0  this   Lorg/agoncal/sample/jmh/StringKit;
---
>             0       5     0  this   Lorg/agoncal/sample/jmh/StringKitFinal;
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<   public static java.lang.String getStringRandom(int);
---
>   public static final java.lang.String getStringRandomFinal(int);
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<     flags: ACC_PUBLIC, ACC_STATIC
---
>     flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
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< 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

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  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

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# 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

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[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
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<   #15 = Class              #52            // org/agoncal/sample/jmh/StringKit
---
>   #15 = Class              #52            // org/agoncal/sample/jmh/StringKitFinal
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<   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKit;
<   #24 = Utf8               getStringRandom
---
>   #23 = Utf8               Lorg/agoncal/sample/jmh/StringKitFinal;
>   #24 = Utf8               getStringRandomFinal
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<   #38 = Utf8               StringKit.java
---
>   #38 = Utf8               StringKitFinal.java
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<   #52 = Utf8               org/agoncal/sample/jmh/StringKit
---
>   #52 = Utf8               org/agoncal/sample/jmh/StringKitFinal
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<   public org.agoncal.sample.jmh.StringKit();
---
>   public org.agoncal.sample.jmh.StringKitFinal();
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<             0       5     0  this   Lorg/agoncal/sample/jmh/StringKit;
---
>             0       5     0  this   Lorg/agoncal/sample/jmh/StringKitFinal;
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<   public java.lang.String getStringRandom(int);
---
>   public final java.lang.String getStringRandomFinal(int);
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<     flags: ACC_PUBLIC
---
>     flags: ACC_PUBLIC, ACC_FINAL
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<             0     125     0  this   Lorg/agoncal/sample/jmh/StringKit;
---
>             0     125     0  this   Lorg/agoncal/sample/jmh/StringKitFinal;
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< 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

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  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

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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

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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

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package org.agoncal.sample.jmh;
/**
 * Created by emacsist on 2017/6/15.
 */
public class StringKitFinal extends StringKitAbsFinal {
}
 
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package org.agoncal.sample.jmh;
/**
 * Created by emacsist on 2017/6/15.
 */
public class StringKit extends StringKitAbs {
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然后在基准测试中:

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

测试结果

非 final 结果

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# 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

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# 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

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# 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

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[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。