和上次int a = 1 后续
测试代码
javac TestCode.java
public class TestCode { public static void main(String[] args) { // 此处写aaaa是便于看字节码 String aaaa = "bbbb"; aaaa = "cccc"; }}反编译
javap -v -p -l TestCode(l是小写的L)
注解：javap是jdk自带的反解析工具。作用是依据class字节码文件，反解析出以后类对应的code区(汇编指令)、本地变量表、异样表和代码行偏移量映射表、常量池等等信息。
Classfile xxxxx/com/code/baseCode/TestCode.class Last modified 2021-11-22; size 475 bytes MD5 checksum 0a108c035194f620555926cf7d113c8b Compiled from "TestCode.java"public class com.code.baseCode.TestCode minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPERConstant pool: #1 = Methodref #5.#21 // java/lang/Object."<init>":()V #2 = String #22 // bbbb #3 = String #23 // cccc #4 = Class #24 // com/code/baseCode/TestCode #5 = Class #25 // java/lang/Object #6 = Utf8 <init> #7 = Utf8 ()V #8 = Utf8 Code #9 = Utf8 LineNumberTable #10 = Utf8 LocalVariableTable #11 = Utf8 this #12 = Utf8 Lcom/code/baseCode/TestCode; #13 = Utf8 main #14 = Utf8 ([Ljava/lang/String;)V #15 = Utf8 args #16 = Utf8 [Ljava/lang/String; #17 = Utf8 aaaa #18 = Utf8 Ljava/lang/String; #19 = Utf8 SourceFile #20 = Utf8 TestCode.java #21 = NameAndType #6:#7 // "<init>":()V #22 = Utf8 bbbb #23 = Utf8 cccc #24 = Utf8 com/code/baseCode/TestCode #25 = Utf8 java/lang/Object{ public com.code.baseCode.TestCode(); 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 8: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lcom/code/baseCode/TestCode; public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=1, locals=2, args_size=1 0: ldc #2 // String bbbb 2: astore_1 3: ldc #3 // String cccc 5: astore_1 6: return LineNumberTable: line 11: 0 line 12: 3 line 55: 6 LocalVariableTable: Start Length Slot Name Signature 0 7 0 args [Ljava/lang/String; 3 4 1 aaaa Ljava/lang/String;}SourceFile: "TestCode.java"反编译后，会发现aaaa存在于Constant pool（常量池）和LocalVariableTable（本地变量表）中
同时发现和int a = 1的区别在于字符串“bbbb”和“cccc”也会在常量池中
JVM中常量池的划分，请看上篇文章
图片解释
java1.8的内存布局
在编译后内存状况
运行时产生状况
如何查看Java bytecode指令？
https://en.wikipedia.org/wiki...
http://gityuan.com/2015/10/24...
其中java byte code为
0: ldc #2 // String bbbb2: astore_13: ldc #3 // String cccc5: astore_1ldc：push a constant #index from a constant pool (String, int, float, Class, java.lang.invoke.MethodType, java.lang.invoke.MethodHandle, or a dynamically-computed constant) onto the stack
意思就是：int、float或String型等常量从常量池推送至栈顶
astore_1：store a reference into local variable 1
意思就是将：一个援用放入到局部变量1中(也就是aaaa中)
ldc #2
astore_1
ldc #3
astore_1
图片解释：用new String()创立
将代码改成如下：
public class TestCode { public static void main(String[] args) { String aaaa = new String("bbbb"); aaaa = new String("cccc"); }}则会发现常量池多了一些货色，并且执行指令多了。
其中执行指令如下
0: new #2 // class java/lang/String3: dup4: ldc #3 // String bbbb6: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V9: astore_110: new #2 // class java/lang/String13: dup14: ldc #5 // String cccc16: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V19: astore_1new：创立类实例
dup：复制数值，并压入栈顶
ldc：从常量池推送至栈顶
invokespecial：调用非凡实例办法（包含实例初始化办法、父类办法）
astore_1：一个援用放入到局部变量1中(也就是aaaa中)
常量池如下
Constant pool: #1 = Methodref #7.#23 // java/lang/Object."<init>":()V #2 = Class #24 // java/lang/String #3 = String #25 // bbbb #4 = Methodref #2.#26 // java/lang/String."<init>":(Ljava/lang/String;)V #5 = String #27 // cccc #6 = Class #28 // com/code/baseCode/TestCode #7 = Class #29 // java/lang/Object #8 = Utf8 <init> #9 = Utf8 ()V #10 = Utf8 Code #11 = Utf8 LineNumberTable #12 = Utf8 LocalVariableTable #13 = Utf8 this #14 = Utf8 Lcom/code/baseCode/TestCode; #15 = Utf8 main #16 = Utf8 ([Ljava/lang/String;)V #17 = Utf8 args #18 = Utf8 [Ljava/lang/String; #19 = Utf8 aaaa #20 = Utf8 Ljava/lang/String; #21 = Utf8 SourceFile #22 = Utf8 TestCode.java #23 = NameAndType #8:#9 // "<init>":()V #24 = Utf8 java/lang/String #25 = Utf8 bbbb #26 = NameAndType #8:#30 // "<init>":(Ljava/lang/String;)V #27 = Utf8 cccc #28 = Utf8 com/code/baseCode/TestCode #29 = Utf8 java/lang/Object #30 = Utf8 (Ljava/lang/String;)Vnew #2
dup
ldc #3
invokespecial #4
astore_1
后续的操作一样，因而会呈现
String a = new String("bbbb");String b = new String("bbbb");System.out.println(a == b); // false面试题
做上面的题是，须要晓得几个知识点
1.编辑时，编译器会对确切的值进行优化：
比方String a = "a" + "b"; -> 比方String a = "ab";
final String a = "a"; String b = "a" + a; -> String b = "aa";
2.字符串变量相加会创立StringBuilder调用append办法，而后StringBuilder转换成new String
3.intern：Java查找常量池中是否有雷同Unicode的字符串常量，如果有，则返回其的援用，如果没有，则在常量池中减少一个Unicode等于str的字符串并返回它的援用。
String s11 = "a";String s22 = "bc";String s1 = "abc";String s2 = "a" + "bc";String s3 = new String("a") + "bc";String s4 = new String("a") + new String("bc");String s5 = new String("abc");String s6 = s11 + s22;System.out.println(s1 == s2);System.out.println(s1 == s3);System.out.println(s1 == s4);System.out.println(s1 == s5);System.out.println(s1 == s6);System.out.println(s3 == s4);System.out.println(s4 == s5);System.out.println(s1 == s5.intern());System.out.println(s1 == s6.intern());System.out.println(s1.equals(s5));System.out.println(s1.equals(s6));答案解释：https://zhuanlan.zhihu.com/p/...
结尾
之前出了int类型的状况，上述是String类型的状况，后续出包装类型的状况