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());
    }