在java里，我们是怎么写for循环的代码呢。一般都是用这两种写法：
第一种：最传统的for循环写法，for(代码段a;代码段b;代码段c)，其中代码段a是初次进入for循环时执行的代码，代码段b是一个boolean的表达式，true则继续执行for循环内容，false则停止for循环，代码段c则是在for循环内部执行完后执行。
第二种：针对集合的遍历，for(类型 单个对象:集合对象)。第二种即是增强for循环。
本文的目的就是讲解增强for循环的原理。先看我的测试用例：
jdk版本:1.7.0_51
java 用例代码：
package com.onlyou.olyfinance.supply.web;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
/**
* 增强for循环编译测试
* Created by cd_huang on 2017/4/14.
*/
public class ForeachTest {
public static void superForeachTestArray(){
String[] args =new String[]{"1","2"};
for(String s:args) {
System.out.println(s);
}
}
public static void foreachTestArray(){
String[] args =new String[]{"1","2"};
String[] args2 = args;
int len = args.length;
for(int i = 0; i < len; ++i) {
String s= args2[i];
System.out.println(s);
}
}
public static void superForeachTestIterator(){
List list = Arrays.asList(new String[]{"1","2"});
for(String s:list) {
System.out.println(s);
}
}
public static void foreachTestIterator(){
List list = Arrays.asList(new String[]{"1","2"});
Iterator it = list.iterator();
while(it.hasNext()) {
String s = (String)it.next();
System.out.println(s);
}
}
}
命令行输入：C:\Users\hcd>javac c:\ForeachTest.java
生成StringTest.class文件。
命令行输入：C:\Users\hcd>javap -v c:\ForeachTest.class
生成java编译后的字节码指令。
Classfile /c:/ForeachTest.class Last modified 2017-4-14; size 1307 bytes MD5 checksum 5296942b41a468b4fa1f88e3123db02f Compiled from "ForeachTest.java" public class com.onlyou.olyfinance.supply.web.ForeachTest SourceFile: "ForeachTest.java" minor version: 0 major version: 51 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #13.#28 // java/lang/Object."":()V #2 = Class #29 // java/lang/String #3 = String #30 // 1 #4 = String #31 // 2 #5 = Fieldref #32.#33 // java/lang/System.out:Ljava/io/PrintStream; #6 = Methodref #34.#35 // java/io/PrintStream.println:(Ljava/lang/String;)V #7 = Methodref #36.#37 // java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; #8 = InterfaceMethodref #38.#39 // java/util/List.iterator:()Ljava/util/Iterator; #9 = InterfaceMethodref #40.#41 // java/util/Iterator.hasNext:()Z #10 = InterfaceMethodref #40.#42 // java/util/Iterator.next:()Ljava/lang/Object; #11 = InterfaceMethodref #38.#39 // java/util/List.iterator:()Ljava/util/Iterator; #12 = Class #43 // com/onlyou/olyfinance/supply/web/ForeachTest #13 = Class #44 // java/lang/Object #14 = Utf8 #15 = Utf8 ()V #16 = Utf8 Code #17 = Utf8 LineNumberTable #18 = Utf8 superForeachTestArray #19 = Utf8 StackMapTable #20 = Class #45 // "[Ljava/lang/String;" #21 = Utf8 foreachTestArray #22 = Utf8 superForeachTestIterator #23 = Class #46 // java/util/List #24 = Class #47 // java/util/Iterator #25 = Utf8 foreachTestIterator #26 = Utf8 SourceFile #27 = Utf8 ForeachTest.java #28 = NameAndType #14:#15 // " ":()V #29 = Utf8 java/lang/String #30 = Utf8 1 #31 = Utf8 2 #32 = Class #48 // java/lang/System #33 = NameAndType #49:#50 // out:Ljava/io/PrintStream; #34 = Class #51 // java/io/PrintStream #35 = NameAndType #52:#53 // println:(Ljava/lang/String;)V #36 = Class #54 // java/util/Arrays #37 = NameAndType #55:#56 // asList:([Ljava/lang/Object;)Ljava/util/List; #38 = Class #46 // java/util/List #39 = NameAndType #57:#58 // iterator:()Ljava/util/Iterator; #40 = Class #47 // java/util/Iterator #41 = NameAndType #59:#60 // hasNext:()Z #42 = NameAndType #61:#62 // next:()Ljava/lang/Object; #43 = Utf8 com/onlyou/olyfinance/supply/web/ForeachTest #44 = Utf8 java/lang/Object #45 = Utf8 [Ljava/lang/String; #46 = Utf8 java/util/List #47 = Utf8 java/util/Iterator #48 = Utf8 java/lang/System #49 = Utf8 out #50 = Utf8 Ljava/io/PrintStream; #51 = Utf8 java/io/PrintStream #52 = Utf8 println #53 = Utf8 (Ljava/lang/String;)V #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; { public com.onlyou.olyfinance.supply.web.ForeachTest(); flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object." ":()V 4: return LineNumberTable: line 11: 0 public static void superForeachTestArray(); flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=5, args_size=0 0: iconst_2 1: anewarray #2 // class java/lang/String 4: dup 5: iconst_0 6: ldc #3 // String 1 8: aastore 9: dup 10: iconst_1 11: ldc #4 // String 2 13: aastore 14: astore_0 15: aload_0 16: astore_1 17: aload_1 18: arraylength 19: istore_2 20: iconst_0 21: istore_3 22: iload_3 23: iload_2 24: if_icmpge 46 27: aload_1 28: iload_3 29: aaload 30: astore 4 32: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 35: aload 4 37: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 40: iinc 3, 1 43: goto 22 46: return LineNumberTable: line 14: 0 line 15: 15 line 16: 32 line 15: 40 line 18: 46 StackMapTable: number_of_entries = 2 frame_type = 255 /* full_frame */ offset_delta = 22 locals = [ class "[Ljava/lang/String;", class "[Ljava/lang/String;", int, int ] stack = [] frame_type = 248 /* chop */ offset_delta = 23 public static void foreachTestArray(); flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=5, args_size=0 0: iconst_2 1: anewarray #2 // class java/lang/String 4: dup 5: iconst_0 6: ldc #3 // String 1 8: aastore 9: dup 10: iconst_1 11: ldc #4 // String 2 13: aastore 14: astore_0 15: aload_0 16: astore_1 17: aload_0 18: arraylength 19: istore_2 20: iconst_0 21: istore_3 22: iload_3 23: iload_2 24: if_icmpge 46 27: aload_1 28: iload_3 29: aaload 30: astore 4 32: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 35: aload 4 37: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 40: iinc 3, 1 43: goto 22 46: return LineNumberTable: line 21: 0 line 22: 15 line 23: 17 line 24: 20 line 25: 27 line 26: 32 line 24: 40 line 28: 46 StackMapTable: number_of_entries = 2 frame_type = 255 /* full_frame */ offset_delta = 22 locals = [ class "[Ljava/lang/String;", class "[Ljava/lang/String;", int, int ] stack = [] frame_type = 250 /* chop */ offset_delta = 23 public static void superForeachTestIterator(); flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=3, args_size=0 0: iconst_2 1: anewarray #2 // class java/lang/String 4: dup 5: iconst_0 6: ldc #3 // String 1 8: aastore 9: dup 10: iconst_1 11: ldc #4 // String 2 13: aastore 14: invokestatic #7 // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; 17: astore_0 18: aload_0 19: invokeinterface #8, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator; 24: astore_1 25: aload_1 26: invokeinterface #9, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 31: ifeq 54 34: aload_1 35: invokeinterface #10, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 40: checkcast #2 // class java/lang/String 43: astore_2 44: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 47: aload_2 48: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 51: goto 25 54: return LineNumberTable: line 31: 0 line 32: 18 line 33: 44 line 34: 51 line 35: 54 StackMapTable: number_of_entries = 2 frame_type = 253 /* append */ offset_delta = 25 locals = [ class java/util/List, class java/util/Iterator ] frame_type = 250 /* chop */ offset_delta = 28 public static void foreachTestIterator(); flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=3, args_size=0 0: iconst_2 1: anewarray #2 // class java/lang/String 4: dup 5: iconst_0 6: ldc #3 // String 1 8: aastore 9: dup 10: iconst_1 11: ldc #4 // String 2 13: aastore 14: invokestatic #7 // Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List; 17: astore_0 18: aload_0 19: invokeinterface #11, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator; 24: astore_1 25: aload_1 26: invokeinterface #9, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 31: ifeq 54 34: aload_1 35: invokeinterface #10, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 40: checkcast #2 // class java/lang/String 43: astore_2 44: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 47: aload_2 48: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 51: goto 25 54: return LineNumberTable: line 38: 0 line 39: 18 line 40: 25 line 41: 34 line 42: 44 line 43: 51 line 44: 54 StackMapTable: number_of_entries = 2 frame_type = 253 /* append */ offset_delta = 25 locals = [ class java/util/List, class java/util/Iterator ] frame_type = 28 /* same */ }
上面的是java代码编译过后的二进制指令。如果对这些指令是什么意思有兴趣的可以看这篇博客：http://www.blogjava.net/DLevin/archive/2011/09/13/358497.html。不过单单看这个肯定是不够的。需要对java的运行时内存模型有一定的了解，明白栈帧，操作数栈，局部变量表，常量池这些东西的概念，才能理解在执行这些指令时，对应的东西里会产生什么变化。当然，要理解我说的增强for循环原理，并不需要看懂这些指令。认真观察上面的二进制指令，superForeachTestArray()方法编译后的指令，即第86行到121行，和foreachTestArray() 方法编译后的指令，即第137行到172行，两个方法编译后的指令是一样的。
superForeachTestIterator()方法编译后的指令，即第191行到222行，和foreachTestIterator() 方法编译后的指令，即第237行到268行，两个方法编译后的指令是一样的。
这说明什么问题？说明了增强for循环是一个编译前的概念，在编译后编译器会对代码进行优化。有两种对象支持增强for循环：一种是数组，编译器对数组的优化只是写法上的优化，即superForeachTestArray()方法内的增强for循环在编译后会变成foreachTestArray() 方法里for循环的代码。一种是实现了Iterable接口的对象，对于这种，其实质是拿到对象的迭代器进行遍历，即superForeachTestIterator()方法内的增强for循环在编译后会变成foreachTestIterator() 方法内的迭代器遍历。迭代器遍历的好处是安全，且它是对遍历的一种抽象，有种解耦的意味在里面。