String str ="a" + "b" 与 String str = new String("a") + new String("b") 的区别（jdk1.8环境利用javap命令探究）
今天，又在思考一个常见的问题，那就是字符串创建对象系列。发现了一个很多人都模糊不清的问题，那就是
String str =“a” + “b” 与 Strin str = new String(“c”) + new String(“d”) 把对象创建在了哪里？创建了几个字符串对象？
话不多出，show my code：
String str =“a” + “b”
package com.bestqiang.set;
/**
* @author BestQiang
*/
public class Test2 {
public static void main(String[] args) {
String str = "a" + "b";
}
}
使用 javap进行解析：
Classfile /D:/workspace_idea/ data_structure/out/production/Container/com/bestqiang/set/Test2.class
Last modified 2019-6-18; size 446 bytes
MD5 checksum a78553412dae26fae704645a9bf0b82b
Compiled from "Test2.java"
public class com.bestqiang.set.Test2
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #4.#20 // java/lang/Object."":()V
#2 = String #21 // ab
#3 = Class #22 // com/bestqiang/set/Test2
#4 = Class #23 // java/lang/Object
#5 = Utf8
#6 = Utf8 ()V
#7 = Utf8 Code
#8 = Utf8 LineNumberTable
#9 = Utf8 LocalVariableTable
#10 = Utf8 this
#11 = Utf8 Lcom/bestqiang/set/Test2;
#12 = Utf8 main
#13 = Utf8 ([Ljava/lang/String;)V
#14 = Utf8 args
#15 = Utf8 [Ljava/lang/String;
#16 = Utf8 str
#17 = Utf8 Ljava/lang/String;
#18 = Utf8 SourceFile
#19 = Utf8 Test2.java
#20 = NameAndType #5:#6 // "":()V
#21 = Utf8 ab
#22 = Utf8 com/bestqiang/set/Test2
#23 = Utf8 java/lang/Object
{
public com.bestqiang.set.Test2();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/bestqiang/set/Test2;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=2, args_size=1
0: ldc #2 // String ab
2: astore_1
3: return
LineNumberTable:
line 8: 0
line 11: 3
LocalVariableTable:
Start Length Slot Name Signature
0 4 0 args [Ljava/lang/String;
3 1 1 str Ljava/lang/String;
}
SourceFile: "Test2.java"
Process finished with exit code 0
你认为 String str = “a” + “b”;这是创建了几个对象呢？没错，你没看错，就是只创建了一个对象
- String常量的累加操作中，编译器会进行优化，如果是 String str = “a” + “b”,那相当于String str = “ab”; ，常量池中会创建一个ab的对象。 ldc #2 // String ab 然后 将int,float或String型常量值从常量池中推送至栈顶即可。等于只在常量池中创建了一个对象，jdk1.8的常量池在java堆中，等于在java堆的常量池创建了一个对象。
String str = new String(“a”) + new String(“b”)
package com.bestqiang.set;
/**
* @author BestQiang
*/
public class Test2 {
public static void main(String[] args) {
String str1 = new String("a") + new String("b");
}
}
javap：
Classfile /D:/workspace_idea/ data_structure/out/production/Container/com/bestqiang/set/Test2.class
Last modified 2019-6-18; size 686 bytes
MD5 checksum 14d133b0ba6751a29914b05da34af598
Compiled from "Test2.java"
public class com.bestqiang.set.Test2
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #11.#27 // java/lang/Object."":()V
#2 = Class #28 // java/lang/StringBuilder
#3 = Methodref #2.#27 // java/lang/StringBuilder."":()V
#4 = Class #29 // java/lang/String
#5 = String #30 // a
#6 = Methodref #4.#31 // java/lang/String."":(Ljava/lang/String;)V
#7 = Methodref #2.#32 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#8 = String #33 // b
#9 = Methodref #2.#34 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#10 = Class #35 // com/bestqiang/set/Test2
#11 = Class #36 // java/lang/Object
#12 = Utf8
#13 = Utf8 ()V
#14 = Utf8 Code
#15 = Utf8 LineNumberTable
#16 = Utf8 LocalVariableTable
#17 = Utf8 this
#18 = Utf8 Lcom/bestqiang/set/Test2;
#19 = Utf8 main
#20 = Utf8 ([Ljava/lang/String;)V
#21 = Utf8 args
#22 = Utf8 [Ljava/lang/String;
#23 = Utf8 str1
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 SourceFile
#26 = Utf8 Test2.java
#27 = NameAndType #12:#13 // "":()V
#28 = Utf8 java/lang/StringBuilder
#29 = Utf8 java/lang/String
#30 = Utf8 a
#31 = NameAndType #12:#37 // "":(Ljava/lang/String;)V
#32 = NameAndType #38:#39 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#33 = Utf8 b
#34 = NameAndType #40:#41 // toString:()Ljava/lang/String;
#35 = Utf8 com/bestqiang/set/Test2
#36 = Utf8 java/lang/Object
#37 = Utf8 (Ljava/lang/String;)V
#38 = Utf8 append
#39 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#40 = Utf8 toString
#41 = Utf8 ()Ljava/lang/String;
{
public com.bestqiang.set.Test2();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/bestqiang/set/Test2;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=4, locals=2, args_size=1
0: new #2 // class java/lang/StringBuilder
3: dup
4: invokespecial #3 // Method java/lang/StringBuilder."":()V
7: new #4 // class java/lang/String
10: dup
11: ldc #5 // String a
13: invokespecial #6 // Method java/lang/String."":(Ljava/lang/String;)V
16: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: new #4 // class java/lang/String
22: dup
23: ldc #8 // String b
25: invokespecial #6 // Method java/lang/String."":(Ljava/lang/String;)V
28: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
31: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: astore_1
35: return
LineNumberTable:
line 9: 0
line 11: 35
LocalVariableTable:
Start Length Slot Name Signature
0 36 0 args [Ljava/lang/String;
35 1 1 str1 Ljava/lang/String;
}
SourceFile: "Test2.java"
Process finished with exit code 0
从javap的解析结果可以看出，在常量池中生成了 a 和 b两个字符串对象，并没有ab这个字符串对象。在堆中用两个new创建了两个String对象，然后通过StringBuilder对象调用append方法进行两次字符串的添加操作，最后用StringBuilder的toString方法进行返回了一个堆中的字符串对象。
所以，Strin str = new String(“a”) + new String(“b”) 在堆中创建了三个字符串对象（a字符串对象，b字符串对象，ab字符串对象），在堆中的常量池中创建了2个字符串对象（a字符串对象，b字符串对象），一共创建了5个对象。
总结：
- String str =“a” + “b” 创建了1个字符串对象，在java堆中的常量池中
- String str = new String(“a”) + new String(“b”) 创建了5个字符串对象，3个在java堆中，2个在java堆的常量池中。
希望大家能够从上面的分析有所收获，上面的分析如有谬论，请大家指出，共同探究。