String拼接字符串效率低，你知道原因吗？

面试官Q1：请问为什么String用"+"拼接字符串效率低下，最好能从JVM角度谈谈吗？

对于这个问题，我们先来看看如下代码：

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);}
}

打印结果：

abcdef
abcdef

从上面代码示例中，我们看到两种方式拼接的字符串打印的结果是一样的。但这只是表面上的，实际内部运行不一样。

两者究竟有什么不一样？

为了看到两者的不同，对代码做如下调整：

public class StringTest {public static void main(String[] args) {String a = "abc";String b = "def";String c = a + b;System.out.Println(c);}
}

我们看看编译完成后它是什么样子：

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"

首先看到使用了一个指针指向一个常量池中的对象内容为“abc”，而另一个指针指向“def”，此时通过new申请了一个StringBuilder，然后调用这个StringBuilder的初始化方法；然后分别做了两次append操作，然后最后做一个toString()操作；可见String的+在编译后会被编译为StringBuilder来运行，我们知道这里做了一个new StringBuilder的操作，并且做了一个toString的操作，如果你对JVM有所了解，凡是new出来的对象绝对不会放在常量池中，toString会发生一次内容拷贝，但是也不会在常量池中，所以在这里常量池String+常量池String放在了堆中。

 

我们再来看看另外一种情况，用同样的方式来看看结果是什么：

代码如下：

public class StringTest {public static void main(String[] args) {String c = "abc" + "def";System.out.println(c);}
}

我们也来看看它编译完成后是什么样子：

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"

这一次编译完后的代码比前面少了很多，而且，仔细看，你会发现14行处，编译的过程中直接变成了"abcdef"，这是为什么呢？因为当发生“abc” + “def”在同一行发生时，JVM在编译时就认为这个加号是没有用处的，编译的时候就直接变成

String d = "abcdef";

同理如果出现：String a =“a” + 1，编译时候就会变成：String a = “a1″;

再补充一个例子：

final String a = "a";
final String b = "ab";
String c = a + b;

在编译时候，c部分会被编译为：String c = “aab”;但是如果a或b有任意一个不是final的，都会new一个新的对象出来；其次再补充下，如果a和b，是某个方法返回回来的，不论方法中是final类型的还是常量什么的，都不会被在编译时将数据编译到常量池，因为编译器并不会跟踪到方法体里面去看你做了什么，其次只要是变量就是可变的，即使你认为你看到的代码是不可变的，但是运行时是可以被切入的。

那么效率问题从何说起？

那说了这么多，也没看到有说效率方面的问题呀？

其实上面两个例子，连接字符串行表达式很简单，那么"+"和StringBuilder基本是一样的，但如果结构比较复杂，如使用循环来连接字符串，那么产生的Java Byte Code就会有很大的区别。我们再来看看下面一段代码：

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);}
}

上面代码反编译后的结果如下：

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
}

我们可以看到，虽然编译器将"+"转换成了StringBuilder，但创建StringBuilder对象的位置却在for语句内部。这就意味着每执行一次循环，就会创建一个StringBuilder对象（对于本例来说，是创建了10个StringBuilder对象），虽然Java有垃圾回收器，但这个回收器的工作时间是不定的。如果不断产生这样的垃圾，那么仍然会占用大量的资源。解决这个问题的方法就是在程序中直接使用StringBuilder来连接字符串，代码如下：

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());}
}

上面代码反编译后的结果如下：

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
}

从上面的反编译结果可以看出，创建StringBuilder的代码被放在了for语句外。虽然这样处理在源程序中看起来复杂，但却换来了更高的效率，同时消耗的资源也更少了。

 

所以，从上述几个例子中我们得出的结论是：String采用连接运算符(+)效率低下，都是上述循环、大批量数据情况造成的，每做一次"+"就产生个StringBuilder对象，然后append后就扔掉。下次循环再到达时重新产生个StringBuilder对象，然后append字符串，如此循环直至结束。如果我们直接采用StringBuilder对象进行append的话，我们可以节省创建和销毁对象的时间。如果只是简单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"