String拼接字符串效率低，你知道原因吗？
面試官Q1：請問為什么String用"+"拼接字符串效率低下，最好能從JVM角度談談嗎？
對于這個問題，我們先來看看如下代碼：
public class StringTest {public static void main(String[] args) {String a = "abc";String b = "def";String c = a + b;String d = "abc" + "def";System.out.Println(c);System.out.Println(d);} }
打印結果：
abcdef
abcdef 從上面代碼示例中，我們看到兩種方式拼接的字符串打印的結果是一樣的。但這只是表面上的，實際內部運行不一樣。
兩者究竟有什么不一樣？
為了看到兩者的不同，對代碼做如下調整：
public class StringTest {public static void main(String[] args) {String a = "abc";String b = "def";String c = a + b;System.out.Println(c);} }
我們看看編譯完成后它是什么樣子：
C:\Users\GRACE\Documents>javac StringTest.java2C:\Users\GRACE\Documents>javap -verbose StringTest3Classfile /C:/Users/GRACE/Documents/StringTest.class4 Last modified 2018-7-21; size 607 bytes5 MD5 checksum a2729f11e22d7e1153a209e5ac968b986 Compiled from "StringTest.java"7public class StringTest8 minor version: 09 major version: 52 10 flags: ACC_PUBLIC, ACC_SUPER 11Constant pool: 12 #1 = Methodref #11.#20 // java/lang/Object."<init>":()V 13 #2 = String #21 // abc 14 #3 = String #22 // def 15 #4 = Class #23 // java/lang/StringBuilder 16 #5 = Methodref #4.#20 // java/lang/StringBuilder."<init>":()V 17 #6 = Methodref #4.#24 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 18 #7 = Methodref #4.#25 // java/lang/StringBuilder.toString:()Ljava/lang/String; 19 #8 = Fieldref #26.#27 // java/lang/System.out:Ljava/io/PrintStream; 20 #9 = Methodref #28.#29 // java/io/PrintStream.println:(Ljava/lang/String;)V 21 #10 = Class #30 // StringTest 22 #11 = Class #31 // java/lang/Object 23 #12 = Utf8 <init> 24 #13 = Utf8 ()V 25 #14 = Utf8 Code 26 #15 = Utf8 LineNumberTable 27 #16 = Utf8 main 28 #17 = Utf8 ([Ljava/lang/String;)V 29 #18 = Utf8 SourceFile 30 #19 = Utf8 StringTest.java 31 #20 = NameAndType #12:#13 // "<init>":()V 32 #21 = Utf8 abc 33 #22 = Utf8 def 34 #23 = Utf8 java/lang/StringBuilder 35 #24 = NameAndType #32:#33 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 36 #25 = NameAndType #34:#35 // toString:()Ljava/lang/String; 37 #26 = Class #36 // java/lang/System 38 #27 = NameAndType #37:#38 // out:Ljava/io/PrintStream; 39 #28 = Class #39 // java/io/PrintStream 40 #29 = NameAndType #40:#41 // println:(Ljava/lang/String;)V 41 #30 = Utf8 StringTest 42 #31 = Utf8 java/lang/Object 43 #32 = Utf8 append 44 #33 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder; 45 #34 = Utf8 toString 46 #35 = Utf8 ()Ljava/lang/String; 47 #36 = Utf8 java/lang/System 48 #37 = Utf8 out 49 #38 = Utf8 Ljava/io/PrintStream; 50 #39 = Utf8 java/io/PrintStream 51 #40 = Utf8 println 52 #41 = Utf8 (Ljava/lang/String;)V 53{ 54 public StringTest(); 55 descriptor: ()V 56 flags: ACC_PUBLIC 57 Code: 58 stack=1, locals=1, args_size=1 59 0: aload_0 60 1: invokespecial #1 // Method java/lang/Object."<init>":()V 61 4: return 62 LineNumberTable: 63 line 1: 0 64 65 public static void main(java.lang.String[]); 66 descriptor: ([Ljava/lang/String;)V 67 flags: ACC_PUBLIC, ACC_STATIC 68 Code: 69 stack=2, locals=4, args_size=1 70 0: ldc #2 // String abc 71 2: astore_1 72 3: ldc #3 // String def 73 5: astore_2 74 6: new #4 // class java/lang/StringBuilder 75 9: dup 76 10: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V 77 13: aload_1 78 14: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 79 17: aload_2 80 18: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 81 21: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 82 24: astore_3 83 25: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream; 84 28: aload_3 85 29: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 86 32: return 87 LineNumberTable: 88 line 3: 0 89 line 4: 3 90 line 5: 6 91 line 6: 25 92 line 7: 32 93} 94SourceFile: "StringTest.java"
首先看到使用了一個指針指向一個常量池中的對象內容為“abc”，而另一個指針指向“def”，此時通過new申請了一個StringBuilder，然后調用這個StringBuilder的初始化方法；然后分別做了兩次append操作，然后最后做一個toString()操作；可見String的+在編譯后會被編譯為StringBuilder來運行，我們知道這里做了一個new StringBuilder的操作，并且做了一個toString的操作，如果你對JVM有所了解，凡是new出來的對象絕對不會放在常量池中，toString會發生一次內容拷貝，但是也不會在常量池中，所以在這里常量池String+常量池String放在了堆中。
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我們再來看看另外一種情況，用同樣的方式來看看結果是什么：
代碼如下：
public class StringTest {public static void main(String[] args) {String c = "abc" + "def";System.out.println(c);} }
我們也來看看它編譯完成后是什么樣子：
C:\Users\GRACE\Documents>javac StringTest.java23C:\Users\GRACE\Documents>javap -verbose StringTest4Classfile /C:/Users/GRACE/Documents/StringTest.class5 Last modified 2018-7-21; size 426 bytes6 MD5 checksum c659d48ff8aeb45a3338dea5d129f5937 Compiled from "StringTest.java"8public class StringTest9 minor version: 0 10 major version: 52 11 flags: ACC_PUBLIC, ACC_SUPER 12Constant pool: 13 #1 = Methodref #6.#15 // java/lang/Object."<init>":()V 14 #2 = String #16 // abcdef 15 #3 = Fieldref #17.#18 // java/lang/System.out:Ljava/io/PrintStream; 16 #4 = Methodref #19.#20 // java/io/PrintStream.println:(Ljava/lang/String;)V 17 #5 = Class #21 // StringTest 18 #6 = Class #22 // java/lang/Object 19 #7 = Utf8 <init> 20 #8 = Utf8 ()V 21 #9 = Utf8 Code 22 #10 = Utf8 LineNumberTable 23 #11 = Utf8 main 24 #12 = Utf8 ([Ljava/lang/String;)V 25 #13 = Utf8 SourceFile 26 #14 = Utf8 StringTest.java 27 #15 = NameAndType #7:#8 // "<init>":()V 28 #16 = Utf8 abcdef 29 #17 = Class #23 // java/lang/System 30 #18 = NameAndType #24:#25 // out:Ljava/io/PrintStream; 31 #19 = Class #26 // java/io/PrintStream 32 #20 = NameAndType #27:#28 // println:(Ljava/lang/String;)V 33 #21 = Utf8 StringTest 34 #22 = Utf8 java/lang/Object 35 #23 = Utf8 java/lang/System 36 #24 = Utf8 out 37 #25 = Utf8 Ljava/io/PrintStream; 38 #26 = Utf8 java/io/PrintStream 39 #27 = Utf8 println 40 #28 = Utf8 (Ljava/lang/String;)V 41{ 42 public StringTest(); 43 descriptor: ()V 44 flags: ACC_PUBLIC 45 Code: 46 stack=1, locals=1, args_size=1 47 0: aload_0 48 1: invokespecial #1 // Method java/lang/Object."<init>":()V 49 4: return 50 LineNumberTable: 51 line 1: 0 52 53 public static void main(java.lang.String[]); 54 descriptor: ([Ljava/lang/String;)V 55 flags: ACC_PUBLIC, ACC_STATIC 56 Code: 57 stack=2, locals=2, args_size=1 58 0: ldc #2 // String abcdef 59 2: astore_1 60 3: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream; 61 6: aload_1 62 7: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 63 10: return 64 LineNumberTable: 65 line 3: 0 66 line 4: 3 67 line 5: 10 68} 69SourceFile: "StringTest.java"
這一次編譯完后的代碼比前面少了很多，而且，仔細看，你會發現14行處，編譯的過程中直接變成了"abcdef"，這是為什么呢？因為當發生“abc” + “def”在同一行發生時，JVM在編譯時就認為這個加號是沒有用處的，編譯的時候就直接變成
String d = "abcdef";
同理如果出現：String a =“a” + 1，編譯時候就會變成：String a = “a1″;
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再補充一個例子：
final String a = "a"; final String b = "ab"; String c = a + b;
在編譯時候，c部分會被編譯為：String c = “aab”;但是如果a或b有任意一個不是final的，都會new一個新的對象出來；其次再補充下，如果a和b，是某個方法返回回來的，不論方法中是final類型的還是常量什么的，都不會被在編譯時將數據編譯到常量池，因為編譯器并不會跟蹤到方法體里面去看你做了什么，其次只要是變量就是可變的，即使你認為你看到的代碼是不可變的，但是運行時是可以被切入的。
那么效率問題從何說起？
那說了這么多，也沒看到有說效率方面的問題呀？
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其實上面兩個例子，連接字符串行表達式很簡單，那么"+"和StringBuilder基本是一樣的，但如果結構比較復雜，如使用循環來連接字符串，那么產生的Java Byte Code就會有很大的區別。我們再來看看下面一段代碼：
import java.util.*; public class StringTest {public static void main(String[] args){String s = "";Random rand = new Random();for (int i = 0; i < 10; i++){s = s + rand.nextInt(1000) + " ";}System.out.println(s);} }
上面代碼反編譯后的結果如下：
C:\Java\jdk1.8.0_171\bin>javap -c E:\StringTest.class Picked up _JAVA_OPTIONS: -Xmx512M Compiled from "StringTest.java" public class StringTest {public StringTest();Code:0: aload_01: invokespecial #8 // Method java/lang/Object."<init>":()V4: returnpublic static void main(java.lang.String[]);Code://String s = "";0: ldc #16 // String2: astore_1//Random rand = new Random();3: new #18 // class java/util/Random6: dup7: invokespecial #20 // Method java/util/Random."<init>":()V10: astore_2//StringBuilder result = new StringBuilder();11: iconst_012: istore_313: goto 49//s = (new StringBuilder(String.valueOf(s))).append(rand.nextInt(1000)).append(" ").toString();16: new #21 // class java/lang/StringBuilder19: dup20: aload_121: invokestatic #23 // Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;24: invokespecial #29 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V27: aload_228: sipush 100031: invokevirtual #32 // Method java/util/Random.nextInt:(I)I34: invokevirtual #36 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;37: ldc #40 // String39: invokevirtual #42 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;42: invokevirtual #45 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;45: astore_146: iinc 3, 149: iload_350: bipush 1052: if_icmplt 16//System.out.println(s);55: getstatic #49 // Field java/lang/System.out:Ljava/io/PrintStream;58: aload_159: invokevirtual #55 // Method java/io/PrintStream.println:(Ljava/lang/String;)V62: return }
我們可以看到，雖然編譯器將"+"轉換成了StringBuilder，但創建StringBuilder對象的位置卻在for語句內部。這就意味著每執行一次循環，就會創建一個StringBuilder對象（對于本例來說，是創建了10個StringBuilder對象），雖然Java有垃圾回收器，但這個回收器的工作時間是不定的。如果不斷產生這樣的垃圾，那么仍然會占用大量的資源。解決這個問題的方法就是在程序中直接使用StringBuilder來連接字符串，代碼如下：
import java.util.Random; public class StringTest {public static void main(String[] args) {Random rand = new Random();StringBuilder result = new StringBuilder();for (int i = 0; i < 10; i++) {result.append(rand.nextInt(1000));result.append(" ");}System.out.println(result.toString());} }
上面代碼反編譯后的結果如下：
C:\Java\jdk1.8.0_171\bin>javap -c E:\Dubbo\Demo\bin\StringTest.class Picked up _JAVA_OPTIONS: -Xmx512M Compiled from "StringTest.java" public class StringTest {public StringTest();Code:0: aload_01: invokespecial #8 // Method java/lang/Object."<init>":()V4: returnpublic static void main(java.lang.String[]);Code://Random rand = new Random();0: new #16 // class java/util/Random3: dup4: invokespecial #18 // Method java/util/Random."<init>":()V7: astore_1//StringBuilder result = new StringBuilder();8: new #19 // class java/lang/StringBuilder11: dup12: invokespecial #21 // Method java/lang/StringBuilder."<init>":()V15: astore_2//for(int i = 0; i < 10; i++)16: iconst_017: istore_318: goto 43//result.append(rand.nextInt(1000));21: aload_222: aload_123: sipush 100026: invokevirtual #22 // Method java/util/Random.nextInt:(I)I29: invokevirtual #26 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;32: pop//result.append(" ");33: aload_234: ldc #30 // String36: invokevirtual #32 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;39: pop40: iinc 3, 143: iload_344: bipush 1046: if_icmplt 21//System.out.println(result.toString());49: getstatic #35 // Field java/lang/System.out:Ljava/io/PrintStream;52: aload_253: invokevirtual #41 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;56: invokevirtual #45 // Method java/io/PrintStream.println:(Ljava/lang/String;)V59: return }
從上面的反編譯結果可以看出，創建StringBuilder的代碼被放在了for語句外。雖然這樣處理在源程序中看起來復雜，但卻換來了更高的效率，同時消耗的資源也更少了。
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所以，從上述幾個例子中我們得出的結論是：String采用連接運算符(+)效率低下，都是上述循環、大批量數據情況造成的，每做一次"+"就產生個StringBuilder對象，然后append后就扔掉。下次循環再到達時重新產生個StringBuilder對象，然后append字符串，如此循環直至結束。如果我們直接采用StringBuilder對象進行append的話，我們可以節省創建和銷毀對象的時間。如果只是簡單的字面量拼接或者很少的字符串拼接，性能都是差不多的。
C:\Users\GRACE\Documents>javac?StringTest.java
2C:\Users\GRACE\Documents>javap?-verbose?StringTest
3Classfile?/C:/Users/GRACE/Documents/StringTest.class
4??Last?modified?2018-7-21;?size?607?bytes
5??MD5?checksum?a2729f11e22d7e1153a209e5ac968b98
6??Compiled?from?"StringTest.java"
7public?class?StringTest
8??minor?version:?0
9??major?version:?52
10??flags:?ACC_PUBLIC,?ACC_SUPER
11Constant?pool:
12???#1?=?Methodref??????????#11.#20????????//?java/lang/Object."<init>":()V
13???#2?=?String?????????????#21????????????//?abc
14???#3?=?String?????????????#22????????????//?def
15???#4?=?Class??????????????#23????????????//?java/lang/StringBuilder
16???#5?=?Methodref??????????#4.#20?????????//?java/lang/StringBuilder."<init>":()V
17???#6?=?Methodref??????????#4.#24?????????//?java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
18???#7?=?Methodref??????????#4.#25?????????//?java/lang/StringBuilder.toString:()Ljava/lang/String;
19???#8?=?Fieldref???????????#26.#27????????//?java/lang/System.out:Ljava/io/PrintStream;
20???#9?=?Methodref??????????#28.#29????????//?java/io/PrintStream.println:(Ljava/lang/String;)V
21??#10?=?Class??????????????#30????????????//?StringTest
22??#11?=?Class??????????????#31????????????//?java/lang/Object
23??#12?=?Utf8???????????????<init>
24??#13?=?Utf8???????????????()V
25??#14?=?Utf8???????????????Code
26??#15?=?Utf8???????????????LineNumberTable
27??#16?=?Utf8???????????????main
28??#17?=?Utf8???????????????([Ljava/lang/String;)V
29??#18?=?Utf8???????????????SourceFile
30??#19?=?Utf8???????????????StringTest.java
31??#20?=?NameAndType????????#12:#13????????//?"<init>":()V
32??#21?=?Utf8???????????????abc
33??#22?=?Utf8???????????????def
34??#23?=?Utf8???????????????java/lang/StringBuilder
35??#24?=?NameAndType????????#32:#33????????//?append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
36??#25?=?NameAndType????????#34:#35????????//?toString:()Ljava/lang/String;
37??#26?=?Class??????????????#36????????????//?java/lang/System
38??#27?=?NameAndType????????#37:#38????????//?out:Ljava/io/PrintStream;
39??#28?=?Class??????????????#39????????????//?java/io/PrintStream
40??#29?=?NameAndType????????#40:#41????????//?println:(Ljava/lang/String;)V
41??#30?=?Utf8???????????????StringTest
42??#31?=?Utf8???????????????java/lang/Object
43??#32?=?Utf8???????????????append
44??#33?=?Utf8???????????????(Ljava/lang/String;)Ljava/lang/StringBuilder;
45??#34?=?Utf8???????????????toString
46??#35?=?Utf8???????????????()Ljava/lang/String;
47??#36?=?Utf8???????????????java/lang/System
48??#37?=?Utf8???????????????out
49??#38?=?Utf8???????????????Ljava/io/PrintStream;
50??#39?=?Utf8???????????????java/io/PrintStream
51??#40?=?Utf8???????????????println
52??#41?=?Utf8???????????????(Ljava/lang/String;)V
53{
54??public?StringTest();
55????descriptor:?()V
56????flags:?ACC_PUBLIC
57????Code:
58??????stack=1,?locals=1,?args_size=1
59?????????0:?aload_0
60?????????1:?invokespecial?#1??????????????????//?Method?java/lang/Object."<init>":()V
61?????????4:?return
62??????LineNumberTable:
63????????line?1:?0
64
65??public?static?void?main(java.lang.String[]);
66????descriptor:?([Ljava/lang/String;)V
67????flags:?ACC_PUBLIC,?ACC_STATIC
68????Code:
69??????stack=2,?locals=4,?args_size=1
70?????????0:?ldc???????????#2??????????????????//?String?abc
71?????????2:?astore_1
72?????????3:?ldc???????????#3??????????????????//?String?def
73?????????5:?astore_2
74?????????6:?new???????????#4??????????????????//?class?java/lang/StringBuilder
75?????????9:?dup
76????????10:?invokespecial?#5??????????????????//?Method?java/lang/StringBuilder."<init>":()V
77????????13:?aload_1
78????????14:?invokevirtual?#6??????????????????//?Method?java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
79????????17:?aload_2
80????????18:?invokevirtual?#6??????????????????//?Method?java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
81????????21:?invokevirtual?#7??????????????????//?Method?java/lang/StringBuilder.toString:()Ljava/lang/String;
82????????24:?astore_3
83????????25:?getstatic?????#8??????????????????//?Field?java/lang/System.out:Ljava/io/PrintStream;
84????????28:?aload_3
85????????29:?invokevirtual?#9??????????????????//?Method?java/io/PrintStream.println:(Ljava/lang/String;)V
86????????32:?return
87??????LineNumberTable:
88????????line?3:?0
89????????line?4:?3
90????????line?5:?6
91????????line?6:?25
92????????line?7:?32
93}
94SourceFile:?"StringTest.java"
轉載于:https://www.cnblogs.com/shanheyongmu/p/9583381.html
總結
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