首先看這樣一個面試題
// StringTable [ "a", "b" ,"ab" ] hashtable 結構，不能擴容
public class Demo1_22 {
// 常量池中的信息，都會被加載到運行時常量池中， 這時 a b ab 都是常量池中的符號，還沒有變為 java 字符串對象
// ldc #2 會把 a 符號變為 "a" 字符串對象
// ldc #3 會把 b 符號變為 "b" 字符串對象
// ldc #4 會把 ab 符號變為 "ab" 字符串對象
public static void main(String[] args) {
String s1 = "a"; // 懶惰的
String s2 = "b";
String s3 = "ab";
String s4 = s1 + s2; // new StringBuilder().append("a").append("b").toString() new String("ab")
String s5 = "a" + "b"; // javac 在編譯期間的優化，結果已經在編譯期確定為ab
System.out.println(s3 == s4); // false
System.out.println(s3 == s5); // true
}
}
我們從字節碼的角度來分析結果
首先反編譯該類代字節碼,輸入命令：
javap -v Demo1_22.class // -v顯示詳細信息
得到結果：
Classfile /D:/work_folder/java_studying/idea_workspace/jvm/out/production/jvm/cn/itcast/jvm/t1/stringtable/Demo1_22.class
Last modified 2019-12-8; size 1039 bytes
MD5 checksum 9efed67948c7dacf9ddb854ca18f8c2b
Compiled from "Demo1_22.java"
public class cn.itcast.jvm.t1.stringtable.Demo1_22
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #12.#36 // java/lang/Object."<init>":()V
#2 = String #37 // a
#3 = String #38 // b
#4 = String #39 // ab
#5 = Class #40 // java/lang/StringBuilder
#6 = Methodref #5.#36 // java/lang/StringBuilder."<init>":()V
#7 = Methodref #5.#41 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#8 = Methodref #5.#42 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#9 = Fieldref #43.#44 // java/lang/System.out:Ljava/io/PrintStream;
#10 = Methodref #45.#46 // java/io/PrintStream.println:(Z)V
#11 = Class #47 // cn/itcast/jvm/t1/stringtable/Demo1_22
#12 = Class #48 // java/lang/Object
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 LocalVariableTable
#18 = Utf8 this
#19 = Utf8 Lcn/itcast/jvm/t1/stringtable/Demo1_22;
#20 = Utf8 main
#21 = Utf8 ([Ljava/lang/String;)V
#22 = Utf8 args
#23 = Utf8 [Ljava/lang/String;
#24 = Utf8 s1
#25 = Utf8 Ljava/lang/String;
#26 = Utf8 s2
#27 = Utf8 s3
#28 = Utf8 s4
#29 = Utf8 s5
#30 = Utf8 StackMapTable
#31 = Class #23 // "[Ljava/lang/String;"
#32 = Class #49 // java/lang/String
#33 = Class #50 // java/io/PrintStream
#34 = Utf8 SourceFile
#35 = Utf8 Demo1_22.java
#36 = NameAndType #13:#14 // "<init>":()V
#37 = Utf8 a
#38 = Utf8 b
#39 = Utf8 ab
#40 = Utf8 java/lang/StringBuilder
#41 = NameAndType #51:#52 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#42 = NameAndType #53:#54 // toString:()Ljava/lang/String;
#43 = Class #55 // java/lang/System
#44 = NameAndType #56:#57 // out:Ljava/io/PrintStream;
#45 = Class #50 // java/io/PrintStream
#46 = NameAndType #58:#59 // println:(Z)V
#47 = Utf8 cn/itcast/jvm/t1/stringtable/Demo1_22
#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 println
#59 = Utf8 (Z)V
{
public cn.itcast.jvm.t1.stringtable.Demo1_22();
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 4: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcn/itcast/jvm/t1/stringtable/Demo1_22;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=6, args_size=1
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: astore 4
29: ldc #4 // String ab
31: astore 5
33: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
36: aload_3
37: aload 4
39: if_acmpne 46
42: iconst_1
43: goto 47
46: iconst_0
47: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
50: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
53: aload_3
54: aload 5
56: if_acmpne 63
59: iconst_1
60: goto 64
63: iconst_0
64: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
67: return
LineNumberTable:
line 11: 0
line 12: 3
line 13: 6
line 14: 9
line 15: 29
line 16: 33
line 17: 50
line 18: 67
LocalVariableTable:
Start Length Slot Name Signature
0 68 0 args [Ljava/lang/String;
3 65 1 s1 Ljava/lang/String;
6 62 2 s2 Ljava/lang/String;
9 59 3 s3 Ljava/lang/String;
29 39 4 s4 Ljava/lang/String;
33 35 5 s5 Ljava/lang/String;
StackMapTable: number_of_entries = 4
frame_type = 255 /* full_frame */
offset_delta = 46
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass 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, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
stack = [ class java/io/PrintStream, int ]
frame_type = 79 /* same_locals_1_stack_item */
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, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
stack = [ class java/io/PrintStream, int ]
}
SourceFile: "Demo1_22.java"
我們從中摘取最重要的一段：
Code:
stack=3, locals=6, args_size=1
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: astore 4
29: ldc #4 // String ab
31: astore 5
上面每句的意思可參照jvm指令手冊去進行閱讀
0: ldc #2 // String a
ldc 把常量池中的項壓入棧,即在Constant pool中找到#2,然后將其壓入棧。也就是String a
astore_1 // astore_1 將引用類型或returnAddress類型值存入局部變量1
也就是LocalVariableTable中的這一行
3 65 1 s1 Ljava/lang/String;
其他類似，
在索引9的位置：
9: new #5 // class java/lang/StringBuilder
這 就是創建了一個StringBuilder對象，
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
"
即
invokespecial 根據編譯時類型來調用實例方法
16: aload_1 // aload_1 從局部變量1中裝載引用類型值,就是之前的String a
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
調用了StringBuilder對象的append方法，
后面幾句類似。
接着：
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
調用了StringBuilder對象的toString方法。
綜合下來：
這一句虛擬機底層就干了這個事，
String s4 = s1 + s2; // new StringBuilder().append("a").append("b").toString()
我們來看看StringBuilder對象的toString方法。
@Override
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
可以發現是創建了一個字符串對象：new String("ab")
而這一行：
String s5 = "a" + "b"; // javac 在編譯期間的優化，結果已經在編譯期確定為ab
對應：
29: ldc #4 // String ab
31: astore 5
即直接拿到了#4，可在常量池中對應找下。也就是"ab"v這個字符串。
所以：
new StringBuilder().append("a").append("b").toString() new String("ab")自然s3和s5都是從常量池中拿的"ab"
而s4是通過new StringBuilder().append("a").append("b").toString() // new String("ab")
即在堆中的對象
可參照jvm指令手冊進行理解，隨手百度即可。
總結：
-
常量池中的字符串僅是符號，第一次用到時才變為對象
-
利用串池的機制，來避免重復創建字符串對象
-
字符串變量拼接的原理是 StringBuilder （1.8）
-
字符串常量拼接的原理是編譯期優化