java final 關鍵字 -- 常量部分
靜態常量的定義又可以分兩種情況：一種是定義時賦值，一種是靜態方法塊中賦值
定義時賦值，如下代碼：
/**
* Created by Jokul on 2018/1/17.
*/
public class FinalTest {
private static final String A = "av";
public static void main(String[] args) {
System.out.println(A);
}
}
此種情況，由編譯器在編譯時就將 “av” 常量值放入了常量區，不存在A這個變量。
靜態方法塊中賦值，如下代碼：
/**
* Created by Jokul on 2018/1/17.
*/
public class FinalTest {
private static final String A;
static {
A = "av";
}
public static void main(String[] args) {
System.out.println(A);
}
}
看一下class文件的指令，如下所示：
Classfile /D:/ideaProjects/final-test/target/classes/com/test/java/FinalTest.class
Last modified 2018-1-17; size 652 bytes
MD5 checksum b2a05ed23493ecfc5d34dd7a7a64f41a
Compiled from "FinalTest.java"
public class com.test.java.FinalTest
SourceFile: "FinalTest.java"
minor version: 0
major version: 49
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #7.#24 // java/lang/Object."<init>":()V
#2 = Fieldref #25.#26 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Fieldref #6.#27 // com/test/java/FinalTest.A:Ljava/lang/String;
#4 = Methodref #28.#29 // java/io/PrintStream.println:(Ljava/lang/String;)V
#5 = String #30 // av
#6 = Class #31 // com/test/java/FinalTest
#7 = Class #32 // java/lang/Object
#8 = Utf8 A
#9 = Utf8 Ljava/lang/String;
#10 = Utf8 <init>
#11 = Utf8 ()V
#12 = Utf8 Code
#13 = Utf8 LineNumberTable
#14 = Utf8 LocalVariableTable
#15 = Utf8 this
#16 = Utf8 Lcom/test/java/FinalTest;
#17 = Utf8 main
#18 = Utf8 ([Ljava/lang/String;)V
#19 = Utf8 args
#20 = Utf8 [Ljava/lang/String;
#21 = Utf8 <clinit>
#22 = Utf8 SourceFile
#23 = Utf8 FinalTest.java
#24 = NameAndType #10:#11 // "<init>":()V
#25 = Class #33 // java/lang/System
#26 = NameAndType #34:#35 // out:Ljava/io/PrintStream;
#27 = NameAndType #8:#9 // A:Ljava/lang/String;
#28 = Class #36 // java/io/PrintStream
#29 = NameAndType #37:#38 // println:(Ljava/lang/String;)V
#30 = Utf8 av
#31 = Utf8 com/test/java/FinalTest
#32 = Utf8 java/lang/Object
#33 = Utf8 java/lang/System
#34 = Utf8 out
#35 = Utf8 Ljava/io/PrintStream;
#36 = Utf8 java/io/PrintStream
#37 = Utf8 println
#38 = Utf8 (Ljava/lang/String;)V
{
public com.test.java.FinalTest();
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/test/java/FinalTest;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: getstatic #3 // Field A:Ljava/lang/String;
6: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
9: return
LineNumberTable:
line 23: 0
line 24: 9
LocalVariableTable:
Start Length Slot Name Signature
0 10 0 args [Ljava/lang/String;
static {};
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: ldc #5 // String av
2: putstatic #3 // Field A:Ljava/lang/String;
5: return
LineNumberTable:
line 12: 0
line 13: 5
}
從class文件的指令中可以看出（看黃底標示的指令），在類被加載時 執行static{} 代碼塊時會將 "av" 字面值賦值給 靜態常量A，然後在main() 方法中獲取了常量A的值，所以靜態方法塊對靜態常量賦值是在類加載階段完成的
靜態常量相關內容介紹完了，下面介紹實例常量，實例常量跟靜態常量一樣也有兩種賦值方式，一種是定義時賦值，一種是構造函數中賦值，但最終編譯後都是構造函數中賦值，我們下面一起看一下
先看定義時賦值，代碼如下：
/**
* Created by Jokul on 2018/1/17.
*/
public class FinalTest {
private final String b = "bv";
public static void main(String[] args) {
FinalTest test = new FinalTest();
System.out.println(test.b);
}
}
class指令如下：
Classfile /D:/ideaProjects/final-test/target/classes/com/test/java/FinalTest.class
Last modified 2018-1-17; size 708 bytes
MD5 checksum a28964e208e3fd7b3e8368a8688be37d
Compiled from "FinalTest.java"
public class com.test.java.FinalTest
SourceFile: "FinalTest.java"
minor version: 0
major version: 49
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #9.#27 // java/lang/Object."<init>":()V
#2 = String #28 // bv
#3 = Fieldref #4.#29 // com/test/java/FinalTest.b:Ljava/lang/String;
#4 = Class #30 // com/test/java/FinalTest
#5 = Methodref #4.#27 // com/test/java/FinalTest."<init>":()V
#6 = Fieldref #31.#32 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref #9.#33 // java/lang/Object.getClass:()Ljava/lang/Class;
#8 = Methodref #34.#35 // java/io/PrintStream.println:(Ljava/lang/String;)V
#9 = Class #36 // java/lang/Object
#10 = Utf8 b
#11 = Utf8 Ljava/lang/String;
#12 = Utf8 ConstantValue
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 LocalVariableTable
#18 = Utf8 this
#19 = Utf8 Lcom/test/java/FinalTest;
#20 = Utf8 main
#21 = Utf8 ([Ljava/lang/String;)V
#22 = Utf8 args
#23 = Utf8 [Ljava/lang/String;
#24 = Utf8 test
#25 = Utf8 SourceFile
#26 = Utf8 FinalTest.java
#27 = NameAndType #13:#14 // "<init>":()V
#28 = Utf8 bv
#29 = NameAndType #10:#11 // b:Ljava/lang/String;
#30 = Utf8 com/test/java/FinalTest
#31 = Class #37 // java/lang/System
#32 = NameAndType #38:#39 // out:Ljava/io/PrintStream;
#33 = NameAndType #40:#41 // getClass:()Ljava/lang/Class;
#34 = Class #42 // java/io/PrintStream
#35 = NameAndType #43:#44 // println:(Ljava/lang/String;)V
#36 = Utf8 java/lang/Object
#37 = Utf8 java/lang/System
#38 = Utf8 out
#39 = Utf8 Ljava/io/PrintStream;
#40 = Utf8 getClass
#41 = Utf8 ()Ljava/lang/Class;
#42 = Utf8 java/io/PrintStream
#43 = Utf8 println
#44 = Utf8 (Ljava/lang/String;)V
{
public com.test.java.FinalTest();
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 bv
7: putfield #3 // Field b:Ljava/lang/String;
10: return
LineNumberTable:
line 6: 0
line 9: 4
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/test/java/FinalTest;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #4 // class com/test/java/FinalTest
3: dup
4: invokespecial #5 // Method "<init>":()V
7: astore_1
8: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
11: aload_1
12: invokevirtual #7 // Method java/lang/Object.getClass:()Ljava/lang/Class;
15: pop
16: ldc #2 // String bv
18: invokevirtual #8 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
21: return
LineNumberTable:
line 21: 0
line 23: 8
line 24: 21
LocalVariableTable:
Start Length Slot Name Signature
0 22 0 args [Ljava/lang/String;
8 14 1 test Lcom/test/java/FinalTest;
}
請看標黃的指令行，我們在代碼中並未編寫構造函數，更沒有在構造函數中給實例常量b 賦值，編譯器自動都給加上了，同時編譯器在使用常量時做優化，直接使用了常量區的常量值，請看main方法中標黃的指令，接下來再來看我們主動在構造函數中給實例常量b賦值，代碼如下：
/**
* Created by Jokul on 2018/1/17.
*/
public class FinalTest {
private final String b;
public FinalTest(){
b = "bv";
}
public static void main(String[] args) {
FinalTest test = new FinalTest();
System.out.println(test.b);
}
}
class指令如下：
Classfile /D:/ideaProjects/final-test/target/classes/com/test/java/FinalTest.class
Last modified 2018-1-17; size 642 bytes
MD5 checksum 3254d93a7430c59105e2a9532bd3d95b
Compiled from "FinalTest.java"
public class com.test.java.FinalTest
SourceFile: "FinalTest.java"
minor version: 0
major version: 49
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #8.#25 // java/lang/Object."<init>":()V
#2 = String #26 // bv
#3 = Fieldref #4.#27 // com/test/java/FinalTest.b:Ljava/lang/String;
#4 = Class #28 // com/test/java/FinalTest
#5 = Methodref #4.#25 // com/test/java/FinalTest."<init>":()V
#6 = Fieldref #29.#30 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref #31.#32 // java/io/PrintStream.println:(Ljava/lang/String;)V
#8 = Class #33 // java/lang/Object
#9 = Utf8 b
#10 = Utf8 Ljava/lang/String;
#11 = Utf8 <init>
#12 = Utf8 ()V
#13 = Utf8 Code
#14 = Utf8 LineNumberTable
#15 = Utf8 LocalVariableTable
#16 = Utf8 this
#17 = Utf8 Lcom/test/java/FinalTest;
#18 = Utf8 main
#19 = Utf8 ([Ljava/lang/String;)V
#20 = Utf8 args
#21 = Utf8 [Ljava/lang/String;
#22 = Utf8 test
#23 = Utf8 SourceFile
#24 = Utf8 FinalTest.java
#25 = NameAndType #11:#12 // "<init>":()V
#26 = Utf8 bv
#27 = NameAndType #9:#10 // b:Ljava/lang/String;
#28 = Utf8 com/test/java/FinalTest
#29 = Class #34 // java/lang/System
#30 = NameAndType #35:#36 // out:Ljava/io/PrintStream;
#31 = Class #37 // java/io/PrintStream
#32 = NameAndType #38:#39 // println:(Ljava/lang/String;)V
#33 = Utf8 java/lang/Object
#34 = Utf8 java/lang/System
#35 = Utf8 out
#36 = Utf8 Ljava/io/PrintStream;
#37 = Utf8 java/io/PrintStream
#38 = Utf8 println
#39 = Utf8 (Ljava/lang/String;)V
{
public com.test.java.FinalTest();
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 bv
7: putfield #3 // Field b:Ljava/lang/String;
10: return
LineNumberTable:
line 15: 0
line 16: 4
line 17: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/test/java/FinalTest;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #4 // class com/test/java/FinalTest
3: dup
4: invokespecial #5 // Method "<init>":()V
7: astore_1
8: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
11: aload_1
12: getfield #3 // Field b:Ljava/lang/String;
15: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
18: return
LineNumberTable:
line 21: 0
line 23: 8
line 24: 18
LocalVariableTable:
Start Length Slot Name Signature
0 19 0 args [Ljava/lang/String;
8 11 1 test Lcom/test/java/FinalTest;
}
從以上標黃指令可以看出，給實例常量b賦值仍然是在構造函數中進行的，這次編譯器並未對使用實例常量的地方進行編譯優化，而是采用 getfield 來獲取實例常量的值。
綜上所述，可以得出如下表所示的結論：
發生的階段如下表所示：
自己原創，希望跟大家交流討論，有不當的地方請指正
註：本文中使用到的查看class文件的工具，使用的是 javap –verbose 命令
java final 關鍵字 -- 常量部分