.class.getClassLoader()频繁调用是否有较大性能开销？

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前言

今天工作中与同事就classLoader获取方式的性能差异展开了争论。

问题概括为：在一个频繁被调用的方法中，通过本地static变量获取classLoader和通过.class.getClassLoader到底哪种方式更加性能优越，开销更少？

测试环境

环境项 环境信息
操作系统 macOS Catalina 10.15.4
处理器 2.4 GHz 八核Intel Core i9
内存 64 GB 2667 MHz DDR4
Java Version openjdk version “1.8.0_282”
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_282-b08)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.282-b08, mixed mode)

验证

测试类准备

下面我写了一个测试类来验证两种获取classloader方式的开销差异。为了避免一些cache影响以及运行时的差异，两个testcase分成2个独立的进程分别运行，统计时间开销。

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package com.kaimingwan.questions;

import com.google.common.base.Stopwatch;
import com.kaimingwan.questions.model.Student;
import java.util.concurrent.TimeUnit;
import lombok.extern.slf4j.Slf4j;
import org.junit.Test;

/**
 * @author wanshao create time is  2022/2/23
 **/
@Slf4j
public class StaticFieldPerfTest {

  private static ClassLoader staticStuClzLoader = Student.class.getClassLoader();

  @Test
  public void useStaticClassLoad() {
    int count = 1000000;
    Stopwatch stopwatch = Stopwatch.createStarted();
    // 1. Get class loader from static field
    for (int i = 0; i < count; i++) {
      getClzLoaderByStatic();
    }
    stopwatch.stop();

    log.info("Get class loader from static cost "+stopwatch.elapsed(TimeUnit.NANOSECONDS)+" ns.");

  }

  @Test
  public void useNativeClassLoad() {
    int count = 1000000;
    Stopwatch stopwatch = Stopwatch.createStarted();
    // 1. Get class loader from static field
    for (int i = 0; i < count; i++) {
      getClzLoaderByStatic();
    }
    stopwatch.stop();

    log.info("Get class loader from native cost "+stopwatch.elapsed(TimeUnit.NANOSECONDS)+" ns.");

  }

  private ClassLoader getClzLoaderByStatic() {
    return staticStuClzLoader;
  }

  private ClassLoader getClzLoaderByNativeKeyword() {
    return Student.class.getClassLoader();
  }
}

测试结果

Static访问方式

测试循环次数 耗时
1000万 平均300万~400万纳秒
1亿 平均300万~400万纳秒
10亿 平均300万~400万纳秒

Native访问方式

测试循环次数 耗时
1000万 平均300万~400万纳秒
1亿 平均300万~400万纳秒
10亿 平均300万~400万纳秒

单纯从测试结果来看两种方式差异很小，而且循环次数对其实际的开销影响非常小。

汇编指令查看

通过反编译代码我们看下具体的汇编指令，了解两种方式的底层差异。

代码上做了一些调整，使得汇编内容更加精简。下面展示对应的源码和字节码。使用IDEA的jclasslib查看字节码汇编信息。
NativeLoad

源码

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package com.kaimingwan.questions;


import com.kaimingwan.questions.model.Student;

/**
 * @author wanshao create time is  2022/2/23
 **/
public class NativeLoad {

  public static void main(String[] args) {
    NativeLoad nativeLoad = new NativeLoad();
    for (int i = 0; i < 100; i++) {
      nativeLoad.useNativeClassLoad();
    }
  }

  public void useNativeClassLoad() {
    ClassLoader clzLoader = Student.class.getClassLoader();
  }
}

字节码

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Classfile /Users/wanshao/projects/java-questions/target/classes/com/kaimingwan/questions/NativeLoad.class
  Last modified 2022-2-25; size 814 bytes
  MD5 checksum 15dd15338ee0440807c7742e76473e7f
  Compiled from "NativeLoad.java"
public class com.kaimingwan.questions.NativeLoad
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #7.#29         // java/lang/Object."<init>":()V
   #2 = Class              #30            // com/kaimingwan/questions/NativeLoad
   #3 = Methodref          #2.#29         // com/kaimingwan/questions/NativeLoad."<init>":()V
   #4 = Methodref          #2.#31         // com/kaimingwan/questions/NativeLoad.useNativeClassLoad:()V
   #5 = Class              #32            // com/kaimingwan/questions/model/Student
   #6 = Methodref          #33.#34        // java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;
   #7 = Class              #35            // java/lang/Object
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               LineNumberTable
  #12 = Utf8               LocalVariableTable
  #13 = Utf8               this
  #14 = Utf8               Lcom/kaimingwan/questions/NativeLoad;
  #15 = Utf8               main
  #16 = Utf8               ([Ljava/lang/String;)V
  #17 = Utf8               i
  #18 = Utf8               I
  #19 = Utf8               args
  #20 = Utf8               [Ljava/lang/String;
  #21 = Utf8               nativeLoad
  #22 = Utf8               StackMapTable
  #23 = Class              #30            // com/kaimingwan/questions/NativeLoad
  #24 = Utf8               useNativeClassLoad
  #25 = Utf8               clzLoader
  #26 = Utf8               Ljava/lang/ClassLoader;
  #27 = Utf8               SourceFile
  #28 = Utf8               NativeLoad.java
  #29 = NameAndType        #8:#9          // "<init>":()V
  #30 = Utf8               com/kaimingwan/questions/NativeLoad
  #31 = NameAndType        #24:#9         // useNativeClassLoad:()V
  #32 = Utf8               com/kaimingwan/questions/model/Student
  #33 = Class              #36            // java/lang/Class
  #34 = NameAndType        #37:#38        // getClassLoader:()Ljava/lang/ClassLoader;
  #35 = Utf8               java/lang/Object
  #36 = Utf8               java/lang/Class
  #37 = Utf8               getClassLoader
  #38 = Utf8               ()Ljava/lang/ClassLoader;
{
  public com.kaimingwan.questions.NativeLoad();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 9: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/kaimingwan/questions/NativeLoad;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: new           #2                  // class com/kaimingwan/questions/NativeLoad
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: astore_1
         8: iconst_0
         9: istore_2
        10: iload_2
        11: bipush        100
        13: if_icmpge     26
        16: aload_1
        17: invokevirtual #4                  // Method useNativeClassLoad:()V
        20: iinc          2, 1
        23: goto          10
        26: return
      LineNumberTable:
        line 12: 0
        line 13: 8
        line 14: 16
        line 13: 20
        line 16: 26
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           10      16     2     i   I
            0      27     0  args   [Ljava/lang/String;
            8      19     1 nativeLoad   Lcom/kaimingwan/questions/NativeLoad;
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 10
          locals = [ class com/kaimingwan/questions/NativeLoad, int ]
        frame_type = 250 /* chop */
          offset_delta = 15

  public void useNativeClassLoad();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=1
         0: ldc           #5                  // class com/kaimingwan/questions/model/Student
         2: invokevirtual #6                  // Method java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;
         5: astore_1
         6: return
      LineNumberTable:
        line 19: 0
        line 20: 6
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lcom/kaimingwan/questions/NativeLoad;
            6       1     1 clzLoader   Ljava/lang/ClassLoader;
}
SourceFile: "NativeLoad.java"

StaticLoad
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package com.kaimingwan.questions;

import com.kaimingwan.questions.model.Student;

/**
 * @author wanshao create time is  2022/2/23
 **/
public class StaticLoad {

  private static ClassLoader staticStuClzLoader = Student.class.getClassLoader();

  public static void main(String[] args) {
    StaticLoad staticLoad = new StaticLoad();
    for (int i = 0; i < 100; i++) {
      staticLoad.useStaticClassLoad();
    }

  }


  public void useStaticClassLoad() {
    ClassLoader clzLoader = staticStuClzLoader;
  }
}

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Classfile /Users/wanshao/projects/java-questions/target/classes/com/kaimingwan/questions/StaticLoad.class
  Last modified 2022-2-25; size 909 bytes
  MD5 checksum f7da301c6a526fb272a2339ece5ffcd4
  Compiled from "StaticLoad.java"
public class com.kaimingwan.questions.StaticLoad
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#32         // java/lang/Object."<init>":()V
   #2 = Class              #33            // com/kaimingwan/questions/StaticLoad
   #3 = Methodref          #2.#32         // com/kaimingwan/questions/StaticLoad."<init>":()V
   #4 = Methodref          #2.#34         // com/kaimingwan/questions/StaticLoad.useStaticClassLoad:()V
   #5 = Fieldref           #2.#35         // com/kaimingwan/questions/StaticLoad.staticStuClzLoader:Ljava/lang/ClassLoader;
   #6 = Class              #36            // com/kaimingwan/questions/model/Student
   #7 = Methodref          #37.#38        // java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;
   #8 = Class              #39            // java/lang/Object
   #9 = Utf8               staticStuClzLoader
  #10 = Utf8               Ljava/lang/ClassLoader;
  #11 = Utf8               <init>
  #12 = Utf8               ()V
  #13 = Utf8               Code
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               Lcom/kaimingwan/questions/StaticLoad;
  #18 = Utf8               main
  #19 = Utf8               ([Ljava/lang/String;)V
  #20 = Utf8               i
  #21 = Utf8               I
  #22 = Utf8               args
  #23 = Utf8               [Ljava/lang/String;
  #24 = Utf8               staticLoad
  #25 = Utf8               StackMapTable
  #26 = Class              #33            // com/kaimingwan/questions/StaticLoad
  #27 = Utf8               useStaticClassLoad
  #28 = Utf8               clzLoader
  #29 = Utf8               <clinit>
  #30 = Utf8               SourceFile
  #31 = Utf8               StaticLoad.java
  #32 = NameAndType        #11:#12        // "<init>":()V
  #33 = Utf8               com/kaimingwan/questions/StaticLoad
  #34 = NameAndType        #27:#12        // useStaticClassLoad:()V
  #35 = NameAndType        #9:#10         // staticStuClzLoader:Ljava/lang/ClassLoader;
  #36 = Utf8               com/kaimingwan/questions/model/Student
  #37 = Class              #40            // java/lang/Class
  #38 = NameAndType        #41:#42        // getClassLoader:()Ljava/lang/ClassLoader;
  #39 = Utf8               java/lang/Object
  #40 = Utf8               java/lang/Class
  #41 = Utf8               getClassLoader
  #42 = Utf8               ()Ljava/lang/ClassLoader;
{
  public com.kaimingwan.questions.StaticLoad();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 8: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/kaimingwan/questions/StaticLoad;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: new           #2                  // class com/kaimingwan/questions/StaticLoad
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: astore_1
         8: iconst_0
         9: istore_2
        10: iload_2
        11: bipush        100
        13: if_icmpge     26
        16: aload_1
        17: invokevirtual #4                  // Method useStaticClassLoad:()V
        20: iinc          2, 1
        23: goto          10
        26: return
      LineNumberTable:
        line 13: 0
        line 14: 8
        line 15: 16
        line 14: 20
        line 18: 26
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           10      16     2     i   I
            0      27     0  args   [Ljava/lang/String;
            8      19     1 staticLoad   Lcom/kaimingwan/questions/StaticLoad;
      StackMapTable: number_of_entries = 2
        frame_type = 253 /* append */
          offset_delta = 10
          locals = [ class com/kaimingwan/questions/StaticLoad, int ]
        frame_type = 250 /* chop */
          offset_delta = 15

  public void useStaticClassLoad();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=2, args_size=1
         0: getstatic     #5                  // Field staticStuClzLoader:Ljava/lang/ClassLoader;
         3: astore_1
         4: return
      LineNumberTable:
        line 22: 0
        line 23: 4
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/kaimingwan/questions/StaticLoad;
            4       1     1 clzLoader   Ljava/lang/ClassLoader;

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #6                  // class com/kaimingwan/questions/model/Student
         2: invokevirtual #7                  // Method java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;
         5: putstatic     #5                  // Field staticStuClzLoader:Ljava/lang/ClassLoader;
         8: return
      LineNumberTable:
        line 10: 0
}
SourceFile: "StaticLoad.java"

从汇编结果来看，两者差异主要是指令层面使用的差异。staic load使用了GETSTATIC，而native load则使用了invoke special。但是他们本质上都是同一个方法引用的调用，所以两者本质上使用不会有太大的差别。

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java/lang/Class.getClassLoader:()Ljava/lang/ClassLoader;