今天小伙伴和我分享一个有意思的一段代码
public static void main(string [] args){
String str1 = "ABC";
String str2 = "ABC";
final String Str3 = "AB";
String str4 = Str3 + "C";
System.out.println(str1==str2);//true
System.out.println(str2==str4);//true
}
当final修饰Str3 是常亮时，str2 == str4
public static void main(string [] args){
String str1 = "ABC";
String str2 = "ABC";
String Str3 = "AB";
String str4 = Str3 + "C";
System.out.println(str1==str2);//true
System.out.println(str2==str4);//false
}
当Str3为普通变量时，str2 != str4。So Why？
再看一下编译后的class文件
//Str3 为 fianl 可以看到常量池中只有 ABC，AB因为没有用到所以编译器优化了
Constant pool:
#1 = Methodref #6.#19 // java/lang/Object."<init>":()V
#2 = String #20 // ABC 常量池中只有ABC
#3 = Fieldref #21.#22 // java/lang/System.out:Ljava/io/PrintStream;
#4 = Methodref #23.#24 // java/io/PrintStream.println:(Z)V
#5 = Class #25 // A
#6 = Class #26 // 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 StackMapTable
#14 = Class #27 // "[Ljava/lang/String;"
#15 = Class #28 // java/lang/String
#16 = Class #29 // java/io/PrintStream
#17 = Utf8 SourceFile
#18 = Utf8 A.java
#19 = NameAndType #7:#8 // "<init>":()V
#20 = Utf8 ABC
#21 = Class #30 // java/lang/System
#22 = NameAndType #31:#32 // out:Ljava/io/PrintStream;
#23 = Class #29 // java/io/PrintStream
#24 = NameAndType #33:#34 // println:(Z)V
#25 = Utf8 A
#26 = Utf8 java/lang/Object
#27 = Utf8 [Ljava/lang/String;
#28 = Utf8 java/lang/String
#29 = Utf8 java/io/PrintStream
#30 = Utf8 java/lang/System
#31 = Utf8 out
#32 = Utf8 Ljava/io/PrintStream;
#33 = Utf8 println
#34 = Utf8 (Z)V
//Str3 为 普通变量 ，可以看到常量池中有 ABC AB 以及 C
Constant pool:
#1 = Methodref #12.#25 // java/lang/Object."<init>":()V
#2 = String #26 // ABC
#3 = String #27 // AB
#4 = Class #28 // java/lang/StringBuilder
#5 = Methodref #4.#25 // java/lang/StringBuilder."<init>":()V
#6 = Methodref #4.#29 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#7 = String #30 // C
#8 = Methodref #4.#31 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#9 = Fieldref #32.#33 // java/lang/System.out:Ljava/io/PrintStream;
#10 = Methodref #34.#35 // java/io/PrintStream.println:(Z)V
#11 = Class #36 // A
#12 = Class #37 // java/lang/Object
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 main
#18 = Utf8 ([Ljava/lang/String;)V
#19 = Utf8 StackMapTable
#20 = Class #38 // "[Ljava/lang/String;"
#21 = Class #39 // java/lang/String
#22 = Class #40 // java/io/PrintStream
#23 = Utf8 SourceFile
#24 = Utf8 A.java
#25 = NameAndType #13:#14 // "<init>":()V
#26 = Utf8 ABC
#27 = Utf8 AB
#28 = Utf8 java/lang/StringBuilder
#29 = NameAndType #41:#42 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#30 = Utf8 C
#31 = NameAndType #43:#44 // toString:()Ljava/lang/String;
#32 = Class #45 // java/lang/System
#33 = NameAndType #46:#47 // out:Ljava/io/PrintStream;
#34 = Class #40 // java/io/PrintStream
#35 = NameAndType #48:#49 // println:(Z)V
#36 = Utf8 A
#37 = Utf8 java/lang/Object
#38 = Utf8 [Ljava/lang/String;
#39 = Utf8 java/lang/String
#40 = Utf8 java/io/PrintStream
#41 = Utf8 append
#42 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#43 = Utf8 toString
#44 = Utf8 ()Ljava/lang/String;
#45 = Utf8 java/lang/System
#46 = Utf8 out
#47 = Utf8 Ljava/io/PrintStream;
#48 = Utf8 println
#49 = Utf8 (Z)V
String str4 = Str3 + "天";
首先 编译器理解C 是一个常亮，当Str3是变量时，编译器理解str4就是一个 变量 + 常量=变量，只能在运行时赋值。当Str3是常量时，编译器理解str4就是一个 常量 + 常量=常量，所以str4在处理化过程中就会预处理，在常量池中查找，并且赋值。
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