Const.java:
1: public class Const {
2: public static final int VALUE = 600;
3: public void test() {
4: int a = VALUE;
5: System.out.println("a=" + a);
6: }
7: public static void main(String[] args) {
8: new Const().test();
9: }
10: }
ConstClient.java:
1: public class ConstClient {
2: public void test() {
3: int b = Const.VALUE;
4: System.out.println("b=" + b);
5: }
6: public static void main(String[] args) {
7: new ConstClient().test();
8: }
9: }
Compile and run both classes:
Const.java displays:
a=600
ConstClient.java displays:
b=600
So far so good, this is what we expected.
Let's try disassembling them with "javap -c Const"
1: Compiled from "Const.java"
2: public class Const extends java.lang.Object{
3: public static final int VALUE;
4: public Const();
5: Code:
6: 0: aload_0
7: 1: invokespecial #1; //Method java/lang/Object."<init>":()V
8: 4: return
9: public void test();
10: Code:
11: 0: sipush 600
12: 3: istore_1
13: 4: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
14: 7: new #3; //class java/lang/StringBuilder
15: 10: dup
16: 11: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
17: 14: ldc #5; //String a=
18: 16: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: 19: iload_1
20: 20: invokevirtual #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
21: 23: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
22: 26: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
23: 29: return
24: public static void main(java.lang.String[]);
25: Code:
26: 0: new #10; //class Const
27: 3: dup
28: 4: invokespecial #11; //Method "<init>":()V
29: 7: invokevirtual #12; //Method test:()V
30: 10: return
31: }
and "javap -c ConstClient"
1: Compiled from "ConstClient.java"
2: public class ConstClient extends java.lang.Object{
3: public ConstClient();
4: Code:
5: 0: aload_0
6: 1: invokespecial #1; //Method java/lang/Object."<init>":()V
7: 4: return
8: public void test();
9: Code:
10: 0: sipush 600
11: 3: istore_1
12: 4: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
13: 7: new #3; //class java/lang/StringBuilder
14: 10: dup
15: 11: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
16: 14: ldc #5; //String b=
17: 16: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
18: 19: iload_1
19: 20: invokevirtual #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
20: 23: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
21: 26: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
22: 29: return
23: public static void main(java.lang.String[]);
24: Code:
25: 0: new #10; //class ConstClient
26: 3: dup
27: 4: invokespecial #11; //Method "<init>":()V
28: 7: invokevirtual #12; //Method test:()V
29: 10: return
30: }
We see that from Const.class line 11 and ConstClient.class line 10, the constant value 600 is pushed onto the stack memory at slot #1.
Let's try changing Const.java constant value from 600 to 500.
1: public class Const {
2: public static final int VALUE = 500;
3: public void test() {
4: int a = VALUE;
5: System.out.println("a=" + a);
6: }
7: public static void main(String[] args) {
8: new Const().test();
9: }
10: }
Compile Const.java and run produces:
a=500
Rerun ConstClient without recompilation produces strange result:
b=600
Let's take a peek by disassembling Const.class:
1: Compiled from "Const.java"
2: public class Const extends java.lang.Object{
3: public static final int VALUE;
4: public Const();
5: Code:
6: 0: aload_0
7: 1: invokespecial #1; //Method java/lang/Object."<init>":()V
8: 4: return
9: public void test();
10: Code:
11: 0: sipush 500
12: 3: istore_1
13: 4: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
14: 7: new #3; //class java/lang/StringBuilder
15: 10: dup
16: 11: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
17: 14: ldc #5; //String a=
18: 16: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: 19: iload_1
20: 20: invokevirtual #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
21: 23: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
22: 26: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
23: 29: return
24: public static void main(java.lang.String[]);
25: Code:
26: 0: new #10; //class Const
27: 3: dup
28: 4: invokespecial #11; //Method "<init>":()V
29: 7: invokevirtual #12; //Method test:()V
30: 10: return
31: }
We see that line 11 pushes our new constant value 500 onto the stack.
Let's take another peek at ConstClient.class:
1: Compiled from "ConstClient.java"
2: public class ConstClient extends java.lang.Object{
3: public ConstClient();
4: Code:
5: 0: aload_0
6: 1: invokespecial #1; //Method java/lang/Object."<init>":()V
7: 4: return
8: public void test();
9: Code:
10: 0: sipush 600
11: 3: istore_1
12: 4: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
13: 7: new #3; //class java/lang/StringBuilder
14: 10: dup
15: 11: invokespecial #4; //Method java/lang/StringBuilder."<init>":()V
16: 14: ldc #5; //String b=
17: 16: invokevirtual #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
18: 19: iload_1
19: 20: invokevirtual #7; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
20: 23: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
21: 26: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
22: 29: return
23: public static void main(java.lang.String[]);
24: Code:
25: 0: new #10; //class ConstClient
26: 3: dup
27: 4: invokespecial #11; //Method "<init>":()V
28: 7: invokevirtual #12; //Method test:()V
29: 10: return
30: }
Line 10 tells us that for some reason our program pushes 600 onto the stack not 500 - our constant value is still 600! It seems like javac burned the constant value into ConstClient.class instead of JVM resolving it through Const.class!
- "Extra care should be taken when hot-deploying classes with public constants as they could potentially break your code during runtime."
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