如何查看.java文件的字節碼（原碼）
出自於:https://www.cnblogs.com/tomasman/p/6751751.html
直接了解foreach底層有些困難,我們需要從更簡單的例子著手.下面上一個簡單例子:
1 public class Simple {
2
3 public static void main(String[] args) {
4 int i = 5;
5 System.out.println(i);
6 }
7 }
找到其字節碼文件所在目錄並在目錄下打開終端(Windows系統是在目錄下shift+鼠標右鍵選擇在此打開powershell窗口)
輸入指令:javac -Simple.class >SimpleRunc
目錄中得到SimpleRunc文件,打開它,會看到如下代碼(裏面有我的註釋):
1 Compiled from "Simple.java"
2 public class cn._012thDay._foreach.Simple {
3 public cn._012thDay._foreach.Simple();
4 Code:
5 0: aload_0 將第一個引用型本地變量推送至棧頂;
6 1: invokespecial #8 // Method java/lang/Object."<init>":()V
7 調用超類構造方法;
8 4: return
9
10 public static void main(java.lang.String[]);
11 Code:
12 0: iconst_5 將int型5推送至棧頂;
13 1: istore_1 將棧頂int型數據存入第二個本地變量;
14 此處推測:第一個本地變量是超類;
15
16
17 2: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
18 獲取本地靜態域並將其壓入棧頂;即獲取靜態屬性壓入棧頂
19 5: iload_1 將第二個int型本地變量推送至棧頂;
20 6: invokevirtual #22 // Method java/io/PrintStream.println:(I)V
21 調用實例方法;
22 獲取類屬性,加載入棧,打印棧頂,即5
23
24 9: return
25 }
如果不懂指令意思,可查詢JVM指令表.這裏我說明一下步驟:
第一步:加載超類Object類
第二步:將int類型的5壓入棧頂,然後將5存入本地變量1
第三部:獲取靜態屬性
第四步:加載本地變量1,即將5推送至棧頂
第五步:打印
for循環
1 public class ForSimple {
2
3 public static void main(String[] args) {
4 for (int i = 5; i > 0; i-=2) {
5 System.out.println(i);
6 }
7 }
8 }
其實這個例子foreach就做不了,因為foreach遍歷必須要有一個數組.
javap -c:
Compiled from "ForSimple.java"
public class cn._012thDay._foreach.ForSimple {
public cn._012thDay._foreach.ForSimple();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_5
1: istore_1
2: goto 15
5: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
8: iload_1
9: invokevirtual #22 // Method java/io/PrintStream.println:(I)V
12: iinc 1, -2
15: iload_1
16: ifgt 5
19: return
}
main方法第2行goto 15代表無條件跳轉至第15行加載變量1,值是5
16行:ifgt 5 如果int型大於零則回到第5行,5>0
5~9行:打印,5
12行:變量1自增-2,即5變為3
15行:加載變量1,值是3
16行:ifgt 5 如果int型大於零則回到第5行,3>0
5~9行:打印,3
12行:變量1自增-2,即3變為1
15行:加載變量1,值是1
16行:ifgt 5 如果int型大於零則回到第5行,1>0
5~9行:打印,1
12行:變量1自增-2,即1變為-1
15行:加載變量1,值是-1
16行:ifgt 5 如果int型大於零則回到第5行,-1 <0
19行:return main方法結束
for循環打印結果為5,3,1
foreach遍歷
先new一個int[]數組,看看數據是如何存儲的:
1 public class SimpleDemo {
2
3 public static void main(String[] args) {
4 // TODO Auto-generated method stub
5 int[]arr = new int[3];
6 arr[0] = 10;
7 arr[1] = 20;
8 arr[2] = 30;
9 }
10 }
Compiled from "SimpleDemo.java"
public class cn._012thDay._foreach.SimpleDemo {
public cn._012thDay._foreach.SimpleDemo();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_3
1: newarray int
3: astore_1
4: aload_1
5: iconst_0
6: bipush 10
8: iastore
9: aload_1
10: iconst_1
11: bipush 20
13: iastore
14: aload_1
15: iconst_2
16: bipush 30
18: iastore
19: return
}
前3行創建基本類型(int)數組,長度為3,存入本地引用型變量1
將索引為0的位置壓入10並存儲
將索引為1的位置壓入20並存儲
將索引為2的位置壓入30並存儲
接下來開始遍歷,加入for循環:
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
Compiled from "SimpleDemo.java"
public class cn._012thDay._foreach.SimpleDemo {
public cn._012thDay._foreach.SimpleDemo();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_3
1: newarray int
3: astore_1
4: aload_1
5: iconst_0
6: bipush 10
8: iastore
9: aload_1
10: iconst_1
11: bipush 20
13: iastore
14: aload_1
15: iconst_2
16: bipush 30
18: iastore
19: iconst_0
20: istore_2
21: goto 36
24: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
27: aload_1
28: iload_2
29: iaload
30: invokevirtual #22 // Method java/io/PrintStream.println:(I)V
33: iinc 2, 1
36: iload_2
37: aload_1
38: arraylength
39: if_icmplt 24
42: return
}
上面代碼大家應該不那麽陌生了,前面18行存入數組,第19行開始創建了一個新的變量int型值為0,存入變量2.然後用變量2和數組長度作比較,小於數組長度就回到第24行打印,這是一個典型的for循環.
整個遍歷中不考慮超類的加載總共創建了兩個本地變量,即arr[3]和int i,用arr[3]的長度3和i進行比較,符合條件輸出arr[i].輸出結果為10,20,30
下面終於輪到我們的主角foreach登場了,刪除for循環,新增foreach叠代:
for (int item : arr) {
System.out.println(item);
}
Compiled from "SimpleDemo.java"
public class cn._012thDay._foreach.SimpleDemo {
public cn._012thDay._foreach.SimpleDemo();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_3
1: newarray int
3: astore_1
4: aload_1
5: iconst_0
6: bipush 10
8: iastore
9: aload_1
10: iconst_1
11: bipush 20
13: iastore
14: aload_1
15: iconst_2
16: bipush 30
18: iastore
19: aload_1 load local1 :0
20: dup copy
21: astore 5 int[] local5 = local1
23: arraylength 3
24: istore 4 int local4 = 3
26: iconst_0 0
27: istore_3 int local3 = 0
28: goto 46
31: aload 5 load local5 : int[3]
33: iload_3 load local3 : 0..
34: iaload arr[0..]進棧
35: istore_2 int local2 = arr[0..]
36: getstatic #16 // Field java/lang/System.out:Ljava/io/PrintStream;
39: iload_2 load local2 :arr[0..]
40: invokevirtual #22 // Method java/io/PrintStream.println:(I)V
43: iinc 3, 1 local3 +=1
46: iload_3 load local3 :0..
47: iload 4 load local4 :3
49: if_icmplt 31 local3 < local4 ? go line31:next line
52: return
}
以上代碼我加入了註釋,這裏大家應該可以看懂了,不考慮超類加載,foreach總共創建了5個本地變量:
local1是原始數組,引用類型
local5是原始數組副本,引用類型
local4是副本數組長度,int類型
local3是0,int類型
local2是arr[i]的副本,int類型
總結:
1.for循環和foreach循環底層創建變量數不同,對於遍歷int[]類型數組,for循環底層創建2個本地變量,而foreach底層創建5個本地變量;
2.for循環直接對數組進行操作,foreach對數組副本進行操作;
由於foreach是對數組副本操作,開發中可能導致的問題:
附上java代碼和javap -c代碼
1 public class ForeachDemo {
2
3 public static void main(String[] args) {
4 // TODO Auto-generated method stub
5
6 String[] s1 = new String[3];
7 for (String item : s1) {//這裏的s1實際是s1副本
8 item = new String("b");
9 System.out.println(item);//這裏可以把副本中的每項打印出來
10 }
11 print(s1);//打印s1是null,因為s1在內存地址中沒有任何變化
12
13 }
14
15 private static void print(String[] s) {
16 // TODO Auto-generated method stub
17 for (int i = 0; i < s.length; i++) {
18 System.out.print(s[i]+" ");
19 }
20 }
21
22 }
Compiled from "ForeachDemo.java"
public class cn._012thDay._foreach.ForeachDemo {
public cn._012thDay._foreach.ForeachDemo();
Code:
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_3
1: anewarray #16 // class java/lang/String
4: astore_1
5: aload_1
6: dup
7: astore 5
9: arraylength
10: istore 4
12: iconst_0
13: istore_3
14: goto 42
17: aload 5
19: iload_3
20: aaload
21: astore_2
22: new #16 // class java/lang/String
25: dup
26: ldc #18 // String b
28: invokespecial #20 // Method java/lang/String."<init>":(Ljava/lang/String;)V
31: astore_2
32: getstatic #23 // Field java/lang/System.out:Ljava/io/PrintStream;
35: aload_2
36: invokevirtual #29 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
39: iinc 3, 1
42: iload_3
43: iload 4
45: if_icmplt 17
48: aload_1
49: invokestatic #34 // Method print:([Ljava/lang/String;)V
52: return
}
javap -c代碼第7行新建了String[]數組副本變量5,之後一直在對副本進行操作,直到48行aload_1,然後打印,此時不難看出foreach中進行的所有操作都沒有對本地變量1(即原數組)的值產生任何影響.
所以main方法最後一行打印數組s1,其結果一定是3個null
如何查看.java文件的字節碼（原碼）