Java语法糖之foreach

谁践踏了优雅 2022-05-09 01:14 276阅读 0赞

### JAVA集合foreach ###

for-each其实只是java提供的语法糖。语法糖是编程语言提供的一些便于程序员书写代码的语法，是编译器提供给程序员的糖衣，编译时会对这些语法特殊处理。语法糖虽然不会带来实质性的改进，但是在提高代码可读性，提高语法严谨性，减少编码错误机会上确实做出了很大贡献；  
Java要求集合必须实现Iterable接口，才能使用for-each语法糖遍历该集合的实例；  
先看个简单的例子：

public static void main(String[] args) {
            List<Integer> list = Arrays.asList(12, 30, 88);
            for (Integer num : list) {
                System.err.println(num);
            }
        }

使用javap -v 反编译

Classfile /Users/Robert/gitlab/learn/target/classes/com/quancheng/SockerServer.class
      Last modified 2018-9-29; size 1126 bytes
      MD5 checksum d6c68f72bdc08b156eda0315c1bdb0c9
      Compiled from "SockerServer.java"
    public class com.quancheng.SockerServer
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #11.#34        // java/lang/Object."<init>":()V
       #2 = Class              #35            // java/lang/Integer
       #3 = Methodref          #2.#36         // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       #4 = Methodref          #37.#38        // java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
       #5 = InterfaceMethodref #39.#40        // java/util/List.iterator:()Ljava/util/Iterator;
       #6 = InterfaceMethodref #41.#42        // java/util/Iterator.hasNext:()Z
       #7 = InterfaceMethodref #41.#43        // java/util/Iterator.next:()Ljava/lang/Object;
       #8 = Fieldref           #44.#45        // java/lang/System.err:Ljava/io/PrintStream;
       #9 = Methodref          #46.#47        // java/io/PrintStream.println:(Ljava/lang/Object;)V
      #10 = Class              #48            // com/quancheng/SockerServer
      #11 = Class              #49            // java/lang/Object
      #12 = Utf8               <init>
      #13 = Utf8               ()V
      #14 = Utf8               Code
      #15 = Utf8               LineNumberTable
      #16 = Utf8               LocalVariableTable
      #17 = Utf8               this
      #18 = Utf8               Lcom/quancheng/SockerServer;
      #19 = Utf8               main
      #20 = Utf8               ([Ljava/lang/String;)V
      #21 = Utf8               num
      #22 = Utf8               Ljava/lang/Integer;
      #23 = Utf8               args
      #24 = Utf8               [Ljava/lang/String;
      #25 = Utf8               list
      #26 = Utf8               Ljava/util/List;
      #27 = Utf8               LocalVariableTypeTable
      #28 = Utf8               Ljava/util/List<Ljava/lang/Integer;>;
      #29 = Utf8               StackMapTable
      #30 = Class              #50            // java/util/List
      #31 = Class              #51            // java/util/Iterator
      #32 = Utf8               SourceFile
      #33 = Utf8               SockerServer.java
      #34 = NameAndType        #12:#13        // "<init>":()V
      #35 = Utf8               java/lang/Integer
      #36 = NameAndType        #52:#53        // valueOf:(I)Ljava/lang/Integer;
      #37 = Class              #54            // java/util/Arrays
      #38 = NameAndType        #55:#56        // asList:([Ljava/lang/Object;)Ljava/util/List;
      #39 = Class              #50            // java/util/List
      #40 = NameAndType        #57:#58        // iterator:()Ljava/util/Iterator;
      #41 = Class              #51            // java/util/Iterator
      #42 = NameAndType        #59:#60        // hasNext:()Z
      #43 = NameAndType        #61:#62        // next:()Ljava/lang/Object;
      #44 = Class              #63            // java/lang/System
      #45 = NameAndType        #64:#65        // err:Ljava/io/PrintStream;
      #46 = Class              #66            // java/io/PrintStream
      #47 = NameAndType        #67:#68        // println:(Ljava/lang/Object;)V
      #48 = Utf8               com/quancheng/SockerServer
      #49 = Utf8               java/lang/Object
      #50 = Utf8               java/util/List
      #51 = Utf8               java/util/Iterator
      #52 = Utf8               valueOf
      #53 = Utf8               (I)Ljava/lang/Integer;
      #54 = Utf8               java/util/Arrays
      #55 = Utf8               asList
      #56 = Utf8               ([Ljava/lang/Object;)Ljava/util/List;
      #57 = Utf8               iterator
      #58 = Utf8               ()Ljava/util/Iterator;
      #59 = Utf8               hasNext
      #60 = Utf8               ()Z
      #61 = Utf8               next
      #62 = Utf8               ()Ljava/lang/Object;
      #63 = Utf8               java/lang/System
      #64 = Utf8               err
      #65 = Utf8               Ljava/io/PrintStream;
      #66 = Utf8               java/io/PrintStream
      #67 = Utf8               println
      #68 = Utf8               (Ljava/lang/Object;)V
    {
      public com.quancheng.SockerServer();
        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 11: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lcom/quancheng/SockerServer;
    
      public static void main(java.lang.String[]);
        descriptor: ([Ljava/lang/String;)V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=4, locals=4, args_size=1
             0: iconst_3
             1: anewarray     #2                  // class java/lang/Integer
             4: dup
             5: iconst_0
             6: bipush        12
             8: invokestatic  #3                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
            11: aastore
            12: dup
            13: iconst_1
            14: bipush        30
            16: invokestatic  #3                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
            19: aastore
            20: dup
            21: iconst_2
            22: bipush        88
            24: invokestatic  #3                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
            27: aastore
            28: invokestatic  #4                  // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
            31: astore_1
            32: aload_1
            33: invokeinterface #5,  1            // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
            38: astore_2
            39: aload_2
            40: invokeinterface #6,  1            // InterfaceMethod java/util/Iterator.hasNext:()Z
            45: ifeq          68
            48: aload_2
            49: invokeinterface #7,  1            // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
            54: checkcast     #2                  // class java/lang/Integer
            57: astore_3
            58: getstatic     #8                  // Field java/lang/System.err:Ljava/io/PrintStream;
            61: aload_3
            62: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
            65: goto          39
            68: return
          LineNumberTable:
            line 13: 0
            line 14: 32
            line 15: 58
            line 16: 65
            line 17: 68
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
               58       7     3   num   Ljava/lang/Integer;
                0      69     0  args   [Ljava/lang/String;
               32      37     1  list   Ljava/util/List;
          LocalVariableTypeTable:
            Start  Length  Slot  Name   Signature
               32      37     1  list   Ljava/util/List<Ljava/lang/Integer;>;
          StackMapTable: number_of_entries = 2
            frame_type = 253 /* append */
              offset_delta = 39
              locals = [ class java/util/List, class java/util/Iterator ]
            frame_type = 250 /* chop */
              offset_delta = 28
    }

注意看这行JVM指令：

33: invokeinterface #5,  1            // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;

从上面的字节码指令可以看出foreach语法实际调用的还是List.iterator()方法然后使用迭代器进行迭代，这就是集合foreach的语法糖；

for (Integer num : list) {
        System.err.println(num);
    }

由Jvm编译期处理后等效于：

for (I #i = Expression.iterator(); #i.hasNext(); ) {
        {VariableModifier} TargetType Identifier =
            (TargetType) #i.next();
        Statement
    }

### java数组foreach ###

数组的实现机制跟List完全不一样，因为数组并没有实现Iterator接口，并没有采用Iterable实现转换。真正的解析结果如下所示：

T[] #a = Expression;
    L1: L2: ... Lm:
    for (int #i = 0; #i < #a.length; #i++) {
        {VariableModifier} TargetType Identifier = #a[#i];
        Statement
    }

*说明：javap是JDK自带的反汇编器，可以查看java编译器为我们生成的字节码*

### 【知识点】 ###

*  for-each遍历的集合对象不能为null  
    既然对集合的for-each遍历实际上是使用迭代器，会调用集合对象的iterator()方法获得迭代器，那么，对null集合的for-each遍历，就会在null集合对象上调用方法，势必会抛出空指针异常；
 *  for-each遍历时不能改变正在遍历的集合  
    因为在使用迭代器遍历集合时，不能够改变集合，所以for-each遍历时改变集合，同样会引发ConcurrentModificationException异常；