String

在 java 里，对 string 对象进行赋值，发生什么情况呢？

package test;

public class TestStr {
	
	private static void test02()
	{
		String str01 = "aaa";
		System.out.println(str01);
		str01 = "bbb";
		System.out.println(str01);
		str01 = "aaa1";
		System.out.println(str01);
	}
	
	public static void main(String[] args) {
		test02();
	}
}

$jdk\bin\javap -verbose -private TestStr
Compiled from "TestStr.java"

public class test.TestStr extends java.lang.Object
  SourceFile: "TestStr.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class        #2;     //  test/TestStr
const #2 = Asciz        test/TestStr;
const #3 = class        #4;     //  java/lang/Object
const #4 = Asciz        java/lang/Object;
const #5 = Asciz        <init>;
const #6 = Asciz        ()V;
const #7 = Asciz        Code;
const #8 = Method       #3.#9;  //  java/lang/Object."<init>":()V
const #9 = NameAndType  #5:#6;//  "<init>":()V
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       LocalVariableTable;
const #12 = Asciz       this;
const #13 = Asciz       Ltest/TestStr;;
const #14 = Asciz       test02;
const #15 = String      #16;    //  aaa
const #16 = Asciz       aaa;
const #17 = Field       #18.#20;        //  java/lang/System.out:Ljava/io/PrintS
tream;
const #18 = class       #19;    //  java/lang/System
const #19 = Asciz       java/lang/System;
const #20 = NameAndType #21:#22;//  out:Ljava/io/PrintStream;
const #21 = Asciz       out;
const #22 = Asciz       Ljava/io/PrintStream;;
const #23 = Method      #24.#26;        //  java/io/PrintStream.println:(Ljava/l
ang/String;)V
const #24 = class       #25;    //  java/io/PrintStream
const #25 = Asciz       java/io/PrintStream;
const #26 = NameAndType #27:#28;//  println:(Ljava/lang/String;)V
const #27 = Asciz       println;
const #28 = Asciz       (Ljava/lang/String;)V;
const #29 = String      #30;    //  bbb
const #30 = Asciz       bbb;
const #31 = String      #32;    //  aaa1
const #32 = Asciz       aaa1;
const #33 = Asciz       str01;
const #34 = Asciz       Ljava/lang/String;;
const #35 = Asciz       main;
const #36 = Asciz       ([Ljava/lang/String;)V;
const #37 = Method      #1.#38; //  test/TestStr.test02:()V
const #38 = NameAndType #14:#6;//  test02:()V
const #39 = Asciz       args;
const #40 = Asciz       [Ljava/lang/String;;
const #41 = Asciz       SourceFile;
const #42 = Asciz       TestStr.java;

{
public test.TestStr();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 3: 0
  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      5      0    this       Ltest/TestStr;

private static void test02();
  Code:
   Stack=2, Locals=1, Args_size=0
   0:   ldc     #15; //String aaa
   2:   astore_0
   3:   getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   6:   aload_0
   7:   invokevirtual   #23; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   10:  ldc     #29; //String bbb
   12:  astore_0
   13:  getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   16:  aload_0
   17:  invokevirtual   #23; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   20:  ldc     #31; //String aaa1
   22:  astore_0
   23:  getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   26:  aload_0
   27:  invokevirtual   #23; //Method java/io/PrintStream.println:(Ljava/lang/St
ring;)V
   30:  return
  LineNumberTable:
   line 7: 0
   line 8: 3
   line 9: 10
   line 10: 13
   line 11: 20
   line 12: 23
   line 13: 30
  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   3      28      0    str01       Ljava/lang/String;

public static void main(java.lang.String[]);
  Code:
   Stack=0, Locals=1, Args_size=1
   0:   invokestatic    #37; //Method test02:()V
   3:   return
  LineNumberTable:
   line 16: 0
   line 17: 3
  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      4      0    args       [Ljava/lang/String;

}

可以看到，在常量表里生成三个常量 aaa, bbb, aaa1。