jvm字节码解析i++和++

喜欢ヅ旅行 2022-04-16 06:55 227阅读 0赞

i++和++i是编码中比较常用的代码，并且也是初学者容易混淆的。我们知道i++是先赋值再+自己，而++i是先+自己再赋值。为什么是这样呢？他们之间效率对比又是怎么样呢？本文我们从字节码层面来分析。  
前面的文章介绍过了jvm字节码的基本知识还有如何分析字节码，我们现在还是写个小demo

public class TestSelfAdd {
        public static void f1() {
            int i=0;
            int j = i++;
            System.out.println(j);
        }
        public static void f2() {
            int i=0;
            int j = ++i;
            System.out.println(j);
        }
    }

编译成字节码之后：（如何编译成字节码看上一篇文章）

Classfile /G:/WorkingProjects/learn/project/test/out/production/test/TestSelfAdd.class
      Last modified 2018-11-19; size 580 bytes
      MD5 checksum fc262482f3054fd5f4c2bb18edf2355a
      Compiled from "TestSelfAdd.java"
    public class TestSelfAdd
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #5.#20         // java/lang/Object."<init>":()V
       #2 = Fieldref           #21.#22        // java/lang/System.out:Ljava/io/PrintStream;
       #3 = Methodref          #23.#24        // java/io/PrintStream.println:(I)V
       #4 = Class              #25            // TestSelfAdd
       #5 = Class              #26            // java/lang/Object
       #6 = Utf8               <init>
       #7 = Utf8               ()V
       #8 = Utf8               Code
       #9 = Utf8               LineNumberTable
      #10 = Utf8               LocalVariableTable
      #11 = Utf8               this
      #12 = Utf8               LTestSelfAdd;
      #13 = Utf8               f1
      #14 = Utf8               i
      #15 = Utf8               I
      #16 = Utf8               j
      #17 = Utf8               f2
      #18 = Utf8               SourceFile
      #19 = Utf8               TestSelfAdd.java
      #20 = NameAndType        #6:#7          // "<init>":()V
      #21 = Class              #27            // java/lang/System
      #22 = NameAndType        #28:#29        // out:Ljava/io/PrintStream;
      #23 = Class              #30            // java/io/PrintStream
      #24 = NameAndType        #31:#32        // println:(I)V
      #25 = Utf8               TestSelfAdd
      #26 = Utf8               java/lang/Object
      #27 = Utf8               java/lang/System
      #28 = Utf8               out
      #29 = Utf8               Ljava/io/PrintStream;
      #30 = Utf8               java/io/PrintStream
      #31 = Utf8               println
      #32 = Utf8               (I)V
    {
      public TestSelfAdd();
        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 1: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   LTestSelfAdd;
    
      public static void f1();
        descriptor: ()V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=2, args_size=0
             0: iconst_0
             1: istore_0
             2: iload_0
             3: iinc          0, 1
             6: istore_1
             7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
            10: iload_1
            11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
            14: return
          LineNumberTable:
            line 3: 0
            line 4: 2
            line 5: 7
            line 6: 14
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                2      13     0     i   I
                7       8     1     j   I
    
      public static void f2();
        descriptor: ()V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=2, args_size=0
             0: iconst_0
             1: istore_0
             2: iinc          0, 1
             5: iload_0
             6: istore_1
             7: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
            10: iload_1
            11: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
            14: return
          LineNumberTable:
            line 8: 0
            line 9: 2
            line 10: 7
            line 11: 14
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                2      13     0     i   I
                7       8     1     j   I
    }
    SourceFile: "TestSelfAdd.java"

我们抽取了他们关键的部分：

i++                                                   ++i
             0: iconst_0                                         0: iconst_0 
             1: istore_0                                         1: istore_0
             2: iload_0                                          2: iinc          0, 1
             3: iinc          0, 1                               5: iload_0
             6: istore_1                                         6: istore_1
            10: iload_1                                          10: iload_1

我们仔细分析，最后iload\_1赋值输出的是本地变量表的索引为1的元素，本地变量表那个元素的来源是栈，栈的元素来源又是本地变量表，iinc 0,1的意思是本地变量表索引为0的元素增加1。在栈的元素更新之前，也就是之心iload\_0之前，++i的代码已经iinc过了，而i++的没有，所以最终iload\_1之后，i++结果为0，而++i为1。  
分析完i++和++i的原理，我们思考一个问题，他们谁的效率高？  
同样我们写个demo：

public class TestSelfAddEfficiency {
        public static void f1() {
            for(int i=0;i<10;i++) {
                System.out.println(i);
            }
        }
        public static void f2() {
            for(int i=0;i<10;++i) {
                System.out.println(i);
            }
        }
    }

编译之后看字节码，可以看到两个函数他们的字节码是一样的，所以效率上没有差别

Classfile /G:/WorkingProjects/learn/project/test/out/production/test/TestSelfAddEfficiency.class
      Last modified 2018-11-19; size 646 bytes
      MD5 checksum a78a263c0750a489f5999bc1a6905ad2
      Compiled from "TestSelfAddEfficiency.java"
    public class TestSelfAddEfficiency
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #5.#20         // java/lang/Object."<init>":()V
       #2 = Fieldref           #21.#22        // java/lang/System.out:Ljava/io/PrintStream;
       #3 = Methodref          #23.#24        // java/io/PrintStream.println:(I)V
       #4 = Class              #25            // TestSelfAddEfficiency
       #5 = Class              #26            // java/lang/Object
       #6 = Utf8               <init>
       #7 = Utf8               ()V
       #8 = Utf8               Code
       #9 = Utf8               LineNumberTable
      #10 = Utf8               LocalVariableTable
      #11 = Utf8               this
      #12 = Utf8               LTestSelfAddEfficiency;
      #13 = Utf8               f1
      #14 = Utf8               i
      #15 = Utf8               I
      #16 = Utf8               StackMapTable
      #17 = Utf8               f2
      #18 = Utf8               SourceFile
      #19 = Utf8               TestSelfAddEfficiency.java
      #20 = NameAndType        #6:#7          // "<init>":()V
      #21 = Class              #27            // java/lang/System
      #22 = NameAndType        #28:#29        // out:Ljava/io/PrintStream;
      #23 = Class              #30            // java/io/PrintStream
      #24 = NameAndType        #31:#32        // println:(I)V
      #25 = Utf8               TestSelfAddEfficiency
      #26 = Utf8               java/lang/Object
      #27 = Utf8               java/lang/System
      #28 = Utf8               out
      #29 = Utf8               Ljava/io/PrintStream;
      #30 = Utf8               java/io/PrintStream
      #31 = Utf8               println
      #32 = Utf8               (I)V
    {
      public TestSelfAddEfficiency();
        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 1: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   LTestSelfAddEfficiency;
    
      public static void f1();
        descriptor: ()V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=1, args_size=0
             0: iconst_0
             1: istore_0
             2: iload_0
             3: bipush        10
             5: if_icmpge     21
             8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
            11: iload_0
            12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
            15: iinc          0, 1
            18: goto          2
            21: return
          LineNumberTable:
            line 3: 0
            line 4: 8
            line 3: 15
            line 6: 21
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                2      19     0     i   I
          StackMapTable: number_of_entries = 2
            frame_type = 252 /* append */
              offset_delta = 2
              locals = [ int ]
            frame_type = 250 /* chop */
              offset_delta = 18
    
      public static void f2();
        descriptor: ()V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=2, locals=1, args_size=0
             0: iconst_0
             1: istore_0
             2: iload_0
             3: bipush        10
             5: if_icmpge     21
             8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
            11: iload_0
            12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
            15: iinc          0, 1
            18: goto          2
            21: return
          LineNumberTable:
            line 8: 0
            line 9: 8
            line 8: 15
            line 11: 21
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                2      19     0     i   I
          StackMapTable: number_of_entries = 2
            frame_type = 252 /* append */
              offset_delta = 2
              locals = [ int ]
            frame_type = 250 /* chop */
              offset_delta = 18
    }
    SourceFile: "TestSelfAddEfficiency.java"