String的一点特殊性质

      在常量的处理上，String和基本类型的处理有些类似的地方，那就是Constant Folding（暂且翻译成常量折叠）和常量传播。

废话不说，直接看代码：

public class Test {
public static void main(String[] args) {
System.out.println(TestFinal.str);
System.out.println(TestFinal.i);
}
}

使用的TestFinal类的代码：

public class TestFinal {
public static final int i=5;
public static final String str="Hello world";
}

直接反汇编Test.class文件：

javap -verbose Test >test.txt

结果：

Compiled from "Test.java"
public class Test extends java.lang.Object
SourceFile: "Test.java"
minor version: 0
major version: 50
Constant pool:
const #1 = Method #7.#16; // java/lang/Object."<init>":()V <br>const #2 = Field #17.#18; // java/lang/System.out:Ljava/io/PrintStream; <br>const #3 = String #19; // Hello world <br>const #4 = Method #20.#21; // java/io/PrintStream.println:(Ljava/lang/String;)V <br>const #5 = Method #20.#22; // java/io/PrintStream.println:(I)V <br>const #6 = class #23; // Test <br>const #7 = class #24; // java/lang/Object <br>const #8 = Asciz <init>; <br>const #9 = Asciz ()V; <br>const #10 = Asciz Code; <br>const #11 = Asciz LineNumberTable; <br>const #12 = Asciz main; <br>const #13 = Asciz ([Ljava/lang/String;)V; <br>const #14 = Asciz SourceFile; <br>const #15 = Asciz Test.java; <br>const #16 = NameAndType #8:#9;// "<init>":()V <br>const #17 = class #25; // java/lang/System <br>const #18 = NameAndType #26:#27;// out:Ljava/io/PrintStream; <br>const #19 = Asciz Hello world; <br>const #20 = class #28; // java/io/PrintStream <br>const #21 = NameAndType #29:#30;// println:(Ljava/lang/String;)V <br>const #22 = NameAndType #29:#31;// println:(I)V <br>const #23 = Asciz Test; <br>const #24 = Asciz java/lang/Object; <br>const #25 = Asciz java/lang/System; <br>const #26 = Asciz out; <br>const #27 = Asciz Ljava/io/PrintStream;; <br>const #28 = Asciz java/io/PrintStream; <br>const #29 = Asciz println; <br>const #30 = Asciz (Ljava/lang/String;)V; <br>const #31 = Asciz (I)V; </init></init></init>

{
public Test();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V <br> 4: return <br> LineNumberTable: <br> line 1: 0 </init>

public static void main(java.lang.String[]);
Code:
Stack=2, Locals=1, Args_size=1
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3; //String Hello world
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
11: iconst_5
12: invokevirtual #5; //Method java/io/PrintStream.println:(I)V
15: return
LineNumberTable:
line 3: 0
line 4: 8
line 5: 15

}

从里面可以看到，这个类根本没有TestFinal类的信息，使用的常量都直接替换为对应的值了。

所以，如果修改了TestFinal中的String常量和基本类型常量的值的话，一定要记得把相关的使用到该类中的变量的java文件（这里是Test.java)重新编译。

另外，众所周知，String是Immutable类，平时常见的Immutable类有很多，比如基本类型的wrapper类，Integer，Float等。但是分析问题的时候一定要注意场景，不要什么都和Immutable联系起来，比如这个例子：

public class Test {
public static void main(String[] args) {
String str="ImmutableString";
StringBuilder builder=new StringBuilder("可变的StringBuilder");
System.out.println(str);
System.out.println(builder);
change(str);
change(builder);
System.out.println(str);
System.out.println(builder);
}
public static void change(String str){
str="新的String";//
}
public static void change(StringBuilder builder){
builder=new StringBuilder("新的StringBuilder!");
}
}

只要对参数变量使用了赋值“=”处理，一切改变对外都是没有作用的，因此这个例子跟String的不变性没有任何关系。根本原因还是java的“pass by value”问题。关于这一点不想多说了，使用The Java Programming Language中的说法。