java.lang.String 的 + 号操作到底做了什么事情？

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());

    }