for+foreach 详解

我会带着你远行 2022-05-20 00:43 98阅读 0赞

*  简介
 *  for
    
     *  源码
     *  idea反编译
     *  javap反编译
 *  foreach
    
     *  源码
     *  反编译
        
         *  idea反编译
         *  javap反编译
 *  最佳实践
    
     *  for和foreach 遍历的坑
        
         *  foreach循环
         *  for循环
         *  for循环修复
     *  Date
        
         *  开始日期小于等于结束日期
         *  开始日期小于结束日期

# 简介 #

1.  foreach 的语法糖实现
2.  for和foreach 遍历的坑

为什么要结合java汇编分析?

因为 java 可以有语法糖,但是汇编绝对不会有语法糖,拨开语法糖的面纱,去看看他们的真面目

# for #

## 源码 ##

public class For { 
    
        public static void main(String[] args) {
            List<String> strings = new ArrayList<>();
            strings.add("Brown");
    
            for (int index = 0;index<strings.size();index++){
                System.out.println(strings.get(index));
            }
        }
    }

## idea反编译 ##

public class For { 
        public For() {
        }
    
        public static void main(String[] args) {
            List<String> strings = new ArrayList();
            strings.add("Brown");
    
            for(int index = 0; index < strings.size(); ++index) {
                System.out.println((String)strings.get(index));
            }
    
        }
    }

未能激起丝毫的变化

## javap反编译 ##

public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=3, locals=3, args_size=1
             0: new           #2                  // class java/util/ArrayList
             3: dup
             4: invokespecial #3                  // Method java/util/ArrayList."<init>":()V
             7: astore_1
             8: aload_1
             9: ldc           #4                  // String Brown
            11: invokeinterface #5,  2            // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z
            16: pop
            17: iconst_0
            18: istore_2
            19: iload_2
            20: aload_1
            21: invokeinterface #6,  1            // InterfaceMethod java/util/List.size:()I
            26: if_icmpge     51
            29: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
            32: aload_1
            33: iload_2
            34: invokeinterface #8,  2            // InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
            39: checkcast     #9                  // class java/lang/String
            42: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
            45: iinc          2, 1
            48: goto          19
            51: return
    
    }

逐字逐句翻译为java 代码

List<String> strings = new ArrayList<>();//0-4
        String str = "Brown";//8-9 
        strings.add(str);//7,11-16
        int i = 0; //17-18
        if(i>= strings.size()){
       //19-26
            goto return // 跳到方法结束
        }
    
        PrintStream pw = System.out;//29-32
        Object obj = strings.get(i);//33-34
        String str = (String)obj;//39 checkcast 由强转产生
        pw.println(str);//42
        i++;//45
        goto  if(i>= strings.size())//48 跳转到 i和size 判断的地方
        return // 可以理解为结尾的一个大括号,跳出方法体

由于压根没有goto的语法,所以得翻译成符合java语言规范的代码:

List<String> strings = new ArrayList<>();
    strings.add("Brown");
    int i = 0;
    for(;i<string.size();){
        System.out.println((String)strings.get(i));
        i++;
    }

这里可以发现,每次调用 i< strings.size() 的时候. List的size 方法都会被调用一次

# foreach #

## 源码 ##

例子:

public class Foreach { 
    
        public static void main(String[] args) {
            List<String> strings = new ArrayList<>();
            strings.add("Brown");
    
            for (String name:strings){
                System.out.println(name);
            }
        }
    }

此时,控制台输出: Brown

## 反编译 ##

### idea反编译 ###

用 idea 自带的反编译

public class Foreach { 
        public Foreach() {
        }
    
        public static void main(String[] args) {
            List<String> strings = new ArrayList();
            strings.add("Brown");
            Iterator var2 = strings.iterator();
    
            while(var2.hasNext()) {
                String name = (String)var2.next();
                System.out.println(name);
            }
    
        }
    }

非常智能的分析出来了,foreach 编译为字节码之后,实际就是调用Iterator进行遍历.

### javap反编译 ###

javap -v com/aya/Foreach.class

public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=4, args_size=1 0: new #2 // class java/util/ArrayList 3: dup 4: invokespecial #3 // Method java/util/ArrayList."<init>":()V 7: astore_1 8: aload_1 9: ldc #4 // String Brown 11: invokeinterface #5, 2 // InterfaceMethod java/util/List.add:(Ljava/lang/Object;)Z 16: pop 17: aload_1 18: invokeinterface #6, 1 // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator; 23: astore_2 24: aload_2 25: invokeinterface #7, 1 // InterfaceMethod java/util/Iterator.hasNext:()Z 30: ifeq 53 33: aload_2 34: invokeinterface #8, 1 // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object; 39: checkcast #9 // class java/lang/String 42: astore_3 43: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream; 46: aload_3 47: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 50: goto 24 53: return }

从字节码的层面来看,foreach 就是一个while

逐字逐句翻译伪代码如下:

List<String> strings = new ArrayList<>();//0-4, 通过LocalVariableTable 拿到类型和变量名,类型通过LocalVariableTypeTable 获得
    strings.add("Brown");//8-16
    Iterator iterator = strings.iterator();//17-18
    Boolean has = iterator.hasNext() //23-25
    if(has){
       //30 ifeq 栈顶为0 就跳到结束
        String next = (String)iterator.next(); // 33-39 , checkcase在强转时产生这个字节码
        System.out.println(next);//42-47
        goto has = iterator.hasNext() //50 由于java 没有goto 语法,所以if判断应该改为while
    }
    return;

整理伪代码逻辑:

List<String> strings = new ArrayList<>();
    strings.add("Brown");
    Iterator iterator = strings.iterator();
    while(iterator.hasNext()){
        System.out.println((String)iterator.next());
    }

idea 分析还是更加智能一些,在不是必要的情况下,用idea 反编译就可以.

这里可以发现,每次遍历的时候,都是调用Iterator 的hasNext和next 方法, iterator只调用了一次

# 最佳实践 #

## for和foreach 遍历的坑 ##

### foreach循环 ###

public List<String> getList(){
            ArrayList<String> strings = new ArrayList<String>(){
                @Override
                public Iterator<String> iterator() {
                    System.out.println("iterator 被调用");
                    return super.iterator();
                }
            };
    
            strings.add("Brown");
            strings.add("Carl");
            return strings;
        }
    
        @Test
        public void testForEach (){
            for (String name:getList()){
                System.out.println(name);
            }
        }

控制台依次输出:  
1. iterator 被调用  
2. Brown  
3. Carl

### for循环 ###

public List<String> getList(){
            System.out.println("getList 被调用");
            ArrayList<String> strings = new ArrayList<String>();
            strings.add("Brown");
            strings.add("Carl");
            return strings;
        }
    
        @Test
        public void testForEach (){
            for (int i = 0;i<getList().size();i++){
                System.out.println(getList().get(i));
            }
        }

控制台输出:  
1. getList 被调用  
2. getList 被调用  
3. Brown  
4. getList 被调用  
5. getList 被调用  
6. Carl  
7. getList 被调用

### for循环修复 ###

所以千万不要在for 循环时做方法调用哦.可以写成如下的形式:

@Test
        public void testForEachFix(){
            List<String> list = getList();
            for (String name: list){
                System.out.println(name);
            }
        }

此时控制台输出:  
1. getList 被调用  
2. Brown  
3. Carl

如果你已经理解`字节码汇编指令`分析for和foreach 的特性后. 这个问题是不是已经不再疑惑了呢?

## Date ##

### 开始日期小于等于结束日期 ###

@Test
        public void testLessEqualDate (){
    
            Date now = new Date();
            Date start = new Date(now.getTime());
            Date end = new Date(now.getTime());
            end.setDate(end.getDate()+2);
    
            for (;start.compareTo(end)<=0;start.setDate(start.getDate()+1)){
                System.out.println(start);
            }
        }

### 开始日期小于结束日期 ###

@Test
        public void testLessDate (){
    
            Date now = new Date();
            Date start = new Date(now.getTime());
            Date end = new Date(now.getTime());
            end.setDate(end.getDate()+2);;
    
            for (;start.before(end);start.setDate(start.getDate()+1)){
                System.out.println(start);
            }
        }