java.lang.String 的 + 号操作到底做了什么事情？ - 好文

前言

在之前的面试经历中，对于String的考察还是挺频繁的，大致考察以下几个知识点：

* String 常量池
* new String()
* == 和 equals 的区别
* native 方法 String.intern()
虽然面试中大体答对了，但是今天早上微信群里的一个问题我却答不上来，这个问题是这样的： String str3 = "what"; String str4
= str3 + " a nice day"; //运行时， + 相当于 new，所以堆中会有 "what a nice
day"对象，常量池中会有"what"," a nice day"两个对象，而不会有 "what a nice day"对象。
//这句话大佬们看看对不对啊，我怎么感觉不对啊 //常量池不会有"what a nice day" 对象吗？
看完这个问题，说实话我也是有点懵的，我只是知道 "what a nice day"不会在常量池，但是不知道具体的原因，后来群里的同学说 + 号是调用了
StringBuffer 的append 方法。我去证实了，发现确实调用了 append 方法，但是当时没有
调用toString()方法，我很疑惑。（最后经过证实，是StringBuilder的append 方法，不是StringBuffer）。

代码验证
public static void main(String[] args) { //#1 String str1 = "what"; //#2
String str2 = str1 + " a nice day"; //#3 System.out.println("what a nice
day".equals(str2)); //#4 System.out.println("what a nice day" == str2); }
现在有以下几个问题，小伙伴们看看是否能答出来，即使答出来了，你知道为什么吗？

* #1 str1 存放位置？
* #2 str2 存放位置？
* #3 结果是 true 还是 false？
* #4 结果是 true 还是 false？
* #5 "what a nice day" 存放在哪个位置呢？
解答分析（基于JDK1.8）

下面也不靠猜，我们直接查看生成的字节码：
localhost:test didi$ javap -verbose -p Main.class Classfile
/develop/project/string-test/out/production/classes/com/fanpan26/string/test/Main.class
Last modified 2019-11-29; size 972 bytes MD5 checksum
1d1f1a23bfe85c2f88d2f767e8aac314 Compiled from "Main.java" public class
com.fanpan26.string.test.Main minor version: 0 major version: 52 flags:
ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #13.#34 //
java/lang/Object."<init>":()V #2 = String #35 // what #3 = Class #36 //
java/lang/StringBuilder #4 = Methodref #3.#34 //
java/lang/StringBuilder."<init>":()V #5 = Methodref #3.#37 //
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; #6
= String #38 // a nice day #7 = Methodref #3.#39 //
java/lang/StringBuilder.toString:()Ljava/lang/String; #8 = Fieldref #40.#41 //
java/lang/System.out:Ljava/io/PrintStream; #9 = String #42 // what a nice day
#10 = Methodref #43.#44 // java/lang/String.equals:(Ljava/lang/Object;)Z #11 =
Methodref #45.#46 // java/io/PrintStream.println:(Z)V #12 = Class #47 //
com/fanpan26/string/test/Main #13 = Class #48 // java/lang/Object #14 = Utf8
<init> #15 = Utf8 ()V #16 = Utf8 Code #17 = Utf8 LineNumberTable #18 = Utf8
LocalVariableTable #19 = Utf8 this #20 = Utf8 Lcom/fanpan26/string/test/Main;
#21 = Utf8 main #22 = Utf8 ([Ljava/lang/String;)V #23 = Utf8 args #24 = Utf8
[Ljava/lang/String; #25 = Utf8 str1 #26 = Utf8 Ljava/lang/String; #27 = Utf8
str2 #28 = Utf8 StackMapTable #29 = Class #24 // "[Ljava/lang/String;" #30 =
Class #49 // java/lang/String #31 = Class #50 // java/io/PrintStream #32 = Utf8
SourceFile #33 = Utf8 Main.java #34 = NameAndType #14:#15 // "<init>":()V #35 =
Utf8 what #36 = Utf8 java/lang/StringBuilder #37 = NameAndType #51:#52 //
append:(Ljava/lang/String;)Ljava/lang/StringBuilder; #38 = Utf8 a nice day #39
= NameAndType #53:#54 // toString:()Ljava/lang/String; #40 = Class #55 //
java/lang/System #41 = NameAndType #56:#57 // out:Ljava/io/PrintStream; #42 =
Utf8 what a nice day #43 = Class #49 // java/lang/String #44 = NameAndType
#58:#59 // equals:(Ljava/lang/Object;)Z #45 = Class #50 // java/io/PrintStream
#46 = NameAndType #60:#61 // println:(Z)V #47 = Utf8
com/fanpan26/string/test/Main #48 = Utf8 java/lang/Object #49 = Utf8
java/lang/String #50 = Utf8 java/io/PrintStream #51 = Utf8 append #52 = Utf8
(Ljava/lang/String;)Ljava/lang/StringBuilder; #53 = Utf8 toString #54 = Utf8
()Ljava/lang/String; #55 = Utf8 java/lang/System #56 = Utf8 out #57 = Utf8
Ljava/io/PrintStream; #58 = Utf8 equals #59 = Utf8 (Ljava/lang/Object;)Z #60 =
Utf8 println #61 = Utf8 (Z)V { public com.fanpan26.string.test.Main();
descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0:
aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return
LineNumberTable: line 6: 0 LocalVariableTable: Start Length Slot Name Signature
0 5 0 this Lcom/fanpan26/string/test/Main; 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: ldc #2 // String what 2:
astore_1 3: new #3 // class java/lang/StringBuilder 6: dup 7: invokespecial #4
// Method java/lang/StringBuilder."<init>":()V 10: aload_1 11: invokevirtual #5
// Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
14: ldc #6 // String a nice day 16: invokevirtual #5 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: invokevirtual #7 // Method
java/lang/StringBuilder.toString:()Ljava/lang/String; 22: astore_2 23:
getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream; 26: ldc #9 //
String what a nice day 28: aload_2 29: invokevirtual #10 // Method
java/lang/String.equals:(Ljava/lang/Object;)Z 32: invokevirtual #11 // Method
java/io/PrintStream.println:(Z)V 35: getstatic #8 // Field
java/lang/System.out:Ljava/io/PrintStream; 38: ldc #9 // String what a nice day
40: aload_2 41: if_acmpne 48 44: iconst_1 45: goto 49 48: iconst_0 49:
invokevirtual #11 // Method java/io/PrintStream.println:(Z)V 52: return
LineNumberTable: line 9: 0 line 11: 3 line 13: 23 line 15: 35 line 16: 52
LocalVariableTable: Start Length Slot Name Signature 0 53 0 args
[Ljava/lang/String; 3 50 1 str1 Ljava/lang/String; 23 30 2 str2
Ljava/lang/String; StackMapTable: number_of_entries = 2 frame_type = 255 /*
full_frame */ offset_delta = 48 locals = [ class "[Ljava/lang/String;", class
java/lang/String, class java/lang/String ] stack = [ class java/io/PrintStream
] frame_type = 255 /* full_frame */ offset_delta = 0 locals = [ class
"[Ljava/lang/String;", class java/lang/String, class java/lang/String ] stack =
[ class java/io/PrintStream, int ] } SourceFile: "Main.java"
从 Constant pool: 中的信息可以看到，#2 #6 #9 可以解答上文中的#1,#5两个问题。

* str1 是存放在常量池的
* "what a nice day" （非str2）也是存放在常量池的.
下面我们看一下 + 操作做了什么事情，可以在Code中看到，该操作调用了 StringBuilder.append 方法
11: invokevirtual #5 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
14: ldc #6 // String a nice day 16: invokevirtual #5 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: invokevirtual #7 // Method
java/lang/StringBuilder.toString:()Ljava/lang/String;
那么到这里一切都答案都出来了

* str2 是存放在堆中。
* equals 为 true
* == 为 false
所以说其实 str1 + " a nice day" 就相当于 new StringBuilder().append(str1).append(" a
nice day");
//这两种写法生成的字节码是一样的。 //String str2 = str1 + " a nice day"; String str2 = new
StringBuilder().append(str1).append(" a nice day").toString();
而StringBuilder 的toString 方法如下：
@Override public String toString() { // 所以说 str2 其实是一个 new String，是不在常量池里面的。
return new String(value, 0, count); }
总结

通过类的字节码可以查看底层具体用什么方式实现，所以说虽然看似一个简单的String问题，其实往深处挖掘还是考察了对生成的字节码的理解。还有，遇到一个问题，不能死记答案，有些人告诉你，+
操作就是 new 对象，但是具体到底是不是或者为什么是有没有思考过呢？上文中如有错误，欢迎指出。

试一试
/** * 以下程序输出的结果是什么？ * */ public static void main(String[] args) { String str1
= "what"; String str2 = str1 + " a nice day"; System.out.println("what a nice
day".equals(str2)); System.out.println("what a nice day" == str2); } /** *
以下程序输出的结果是什么？ * */ public static void main(String[] args) { String str1 = "what
a nice day"; String str2 = new String("what a nice day");
System.out.println(str1.equals(str2)); System.out.println(str1 == str2); } /**
* 以下程序输出的结果是什么？ * */ public static void main(String[] args) { String str1 =
"what"; String str2 = str1.concat(" a nice day"); System.out.println("what a
nice day".equals(str2)); System.out.println("what a nice day" == str2);
System.out.println("what a nice day"==str2.intern()); }

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