面试官问他Java中“a”+“b”究竟会产生几个对象？

最近在群里聊起这个话题。

有人在群里提问说“刚才面试官问他：‘Java中“a”+“b”究竟会产生几个对象？’”。

群里的各路豪杰就开始了一波波的解答。其实这也是一个老生常谈的问题。

今天我们就试图对这个问题进行一次剖析。

首先我们新建一个类AB1.java，如下：

public class AB1 {
    String str="a"+"b";
}

然后编译此类javac AB1.java，得到AB1.class文件。

通过idea把class文件打开，发现“a”+“b”被合并到一块了。可以看出反编译器是直接从class文件中取到了“ab”，而不是“a”+“b”。

也就是说，我们在javac编译此类的时候，编译器已经智能的把“a”+“b”合并为了“ab”，这算是编译器优化。

"a"+“b”

上面只是表象，现在我们通过javap -v AB1.class看看具体的字节码指令：

hezhuofandeMacBook-Pro:datastructure hezhuofan$ javap -v AB1.class Classfile /Users/hezhuofan/Documents/importsource_dev/concurrency-samples/src/main/java/com/importsource/datastructure/AB1.class Last modified Apr 25, 2018; size 274 bytes MD5 checksum 76e8f9d06c49826518eed49957bcf526 Compiled from "AB1.java" public class com.importsource.datastructure.AB1 minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #5.#14 // java/lang/Object."<init>":()V #2 = String #15 // ab #3 = Fieldref #4.#16 // com/importsource/datastructure/AB1.str:Ljava/lang/String; #4 = Class #17 // com/importsource/datastructure/AB1 #5 = Class #18 // java/lang/Object #6 = Utf8 str #7 = Utf8 Ljava/lang/String; #8 = Utf8 <init> #9 = Utf8 ()V #10 = Utf8 Code #11 = Utf8 LineNumberTable #12 = Utf8 SourceFile #13 = Utf8 AB1.java #14 = NameAndType #8:#9 // "<init>":()V #15 = Utf8 ab #16 = NameAndType #6:#7 // str:Ljava/lang/String; #17 = Utf8 com/importsource/datastructure/AB1 #18 = Utf8 java/lang/Object { java.lang.String str; descriptor: Ljava/lang/String; flags: public com.importsource.datastructure.AB1(); descriptor: ()V flags: ACC_PUBLIC Code: stack=2, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: aload_0 5: ldc #2 // String ab 7: putfield #3 // Field str:Ljava/lang/String; 10: return LineNumberTable: line 6: 0 line 7: 4 } SourceFile: "AB1.java"

但是如果我们换成如下，情况就会改变。

a+b

现在我们新建一个AB.java:

public class AB {
    String a = "a";
    String b = "b";
    String result = a + b ;
}

然后我们javac AB.java生成class，然后使用idea打开AB.class:

发现result的a+b被移动到了构造函数里。

然后用javap -v AB.class来来细节：

hezhuofandeMacBook-Pro:datastructure hezhuofan$ javap -v AB.class Classfile /Users/hezhuofan/Documents/importsource_dev/concurrency-samples/src/main/java/com/importsource/datastructure/AB.class Last modified Apr 25, 2018; size 504 bytes MD5 checksum fa64cc63e24478ad1026aec48ed45243 Compiled from "AB.java" public class com.importsource.datastructure.AB minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #12.#23 // java/lang/Object."<init>":()V #2 = String #13 // a #3 = Fieldref #11.#24 // com/importsource/datastructure/AB.a:Ljava/lang/String; #4 = String #15 // b #5 = Fieldref #11.#25 // com/importsource/datastructure/AB.b:Ljava/lang/String; #6 = Class #26 // java/lang/StringBuilder #7 = Methodref #6.#23 // java/lang/StringBuilder."<init>":()V #8 = Methodref #6.#27 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; #9 = Methodref #6.#28 // java/lang/StringBuilder.toString:()Ljava/lang/String; #10 = Fieldref #11.#29 // com/importsource/datastructure/AB.result:Ljava/lang/String; #11 = Class #30 // com/importsource/datastructure/AB #12 = Class #31 // java/lang/Object #13 = Utf8 a #14 = Utf8 Ljava/lang/String; #15 = Utf8 b #16 = Utf8 result #17 = Utf8 <init> #18 = Utf8 ()V #19 = Utf8 Code #20 = Utf8 LineNumberTable #21 = Utf8 SourceFile #22 = Utf8 AB.java #23 = NameAndType #17:#18 // "<init>":()V #24 = NameAndType #13:#14 // a:Ljava/lang/String; #25 = NameAndType #15:#14 // b:Ljava/lang/String; #26 = Utf8 java/lang/StringBuilder #27 = NameAndType #32:#33 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder; #28 = NameAndType #34:#35 // toString:()Ljava/lang/String; #29 = NameAndType #16:#14 // result:Ljava/lang/String; #30 = Utf8 com/importsource/datastructure/AB #31 = Utf8 java/lang/Object #32 = Utf8 append #33 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder; #34 = Utf8 toString #35 = Utf8 ()Ljava/lang/String; { java.lang.String a; descriptor: Ljava/lang/String; flags: java.lang.String b; descriptor: Ljava/lang/String; flags: java.lang.String result; descriptor: Ljava/lang/String; flags: public com.importsource.datastructure.AB(); descriptor: ()V flags: ACC_PUBLIC Code: stack=3, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: aload_0 5: ldc #2 // String a 7: putfield #3 // Field a:Ljava/lang/String; 10: aload_0 11: ldc #4 // String b 13: putfield #5 // Field b:Ljava/lang/String; 16: aload_0 17: new #6 // class java/lang/StringBuilder 20: dup 21: invokespecial #7 // Method java/lang/StringBuilder."<init>":()V 24: aload_0 25: getfield #3 // Field a:Ljava/lang/String; 28: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 31: aload_0 32: getfield #5 // Field b:Ljava/lang/String; 35: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 38: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 41: putfield #10 // Field result:Ljava/lang/String; 44: return LineNumberTable: line 6: 0 line 7: 4 line 8: 10 line 9: 16 } SourceFile: "AB.java"

你会发现在这种情况下，StringBuilder介入了，它负责把a+b给拼接起来。

也就是等价于

StringBuilder sb=new StringBuilder(); sb.append(a); sb.append(b); result=sb.toString();

上面画的黑体的指令就是这段代码。

new String("ab")

那么new String("ab")呢？

代码：

public class ABNew {
    public static void main(String[] args) {
         String str=new String("ab");
    }
}

依然是通过javac编译后，通过javap -v ABNEW.class来查看：

hezhuofandeMacBook-Pro:datastructure hezhuofan$ javap -v ABNew.class Classfile /Users/hezhuofan/Documents/importsource_dev/concurrency-samples/src/main/java/com/importsource/datastructure/ABNew.class Last modified Apr 26, 2018; size 364 bytes MD5 checksum 224e8d6e5f5b8bd353f6ff043b198d67 Compiled from "ABNew.java" public class com.importsource.datastructure.ABNew minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #6.#15 // java/lang/Object."<init>":()V #2 = Class #16 // java/lang/String #3 = String #17 // ab #4 = Methodref #2.#18 // java/lang/String."<init>":(Ljava/lang/String;)V #5 = Class #19 // com/importsource/datastructure/ABNew #6 = Class #20 // java/lang/Object #7 = Utf8 <init> #8 = Utf8 ()V #9 = Utf8 Code #10 = Utf8 LineNumberTable #11 = Utf8 main #12 = Utf8 ([Ljava/lang/String;)V #13 = Utf8 SourceFile #14 = Utf8 ABNew.java #15 = NameAndType #7:#8 // "<init>":()V #16 = Utf8 java/lang/String #17 = Utf8 ab #18 = NameAndType #7:#21 // "<init>":(Ljava/lang/String;)V #19 = Utf8 com/importsource/datastructure/ABNew #20 = Utf8 java/lang/Object #21 = Utf8 (Ljava/lang/String;)V { public com.importsource.datastructure.ABNew(); 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 public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=3, locals=2, args_size=1 0: new #2 // class java/lang/String 3: dup 4: ldc #3 // String ab 6: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V 9: astore_1 10: return LineNumberTable: line 8: 0 line 9: 10 } SourceFile: "ABNew.java"

可以发现，“ab”是在常量池中，new String()这个动作触发new指令强制新建了一个对象。所以是创建了两个对象。这一点也可以从下面的代码中得到验证：

System.out.println("ab"==str);//false

总结

当“a”+“b”时，在javac编译阶段就会被优化成“ab”存储常量池。当是a+b时，会使用StringBuilder进行拼接。new String时，则会额外强制新建一个对象。

所以第一种情况下只会有一个对象，就是str；第二种情况下是三个对象 a、b、result，同时创建了StringBuilder一个对象，所以一共是四个对象。第三种情况new String时，则是两个对象。

ps：本文所在环境Java8。另有误的地方望指正。