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。作用是根据class字节码文件，反解析出当前类对应的code区(汇编指令)、本地变量表、异常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_SUPER Constant pool: #1 = Methodref #5.#21 // java/lang/Object."":()V #2 = String #22 // bbbb #3 = String #23 // cccc #4 = Class #24 // com/code/baseCode/TestCode #5 = Class #25 // java/lang/Object #6 = Utf8 #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 // " ":()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." ":()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 在编译后内存情况 运行时发生情况如何查看Java bytecode指令？
https://en.wikipedia.org/wiki/List_of_Java_bytecode_instructions
Jvm系列3—字节码指令 - Gityuan博客 | 袁辉辉的技术博客
其中java byte code为
0: ldc #2 // String bbbb 2: astore_1 3: ldc #3 // String cccc 5: astore_1
ldc：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：一个引用放入到局部变量1中(也就是aaaa中)
ldc #2
astore_1
ldc #3
astore_1
图片解释：用new String()创建将代码改成如下：
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东西，并且执行指令多了。
其中执行指令如下
0: new #2 // class java/lang/String 3: dup 4: ldc #3 // String bbbb 6: invokespecial #4 // Method java/lang/String."":(Ljava/lang/String;)V 9: astore_1 10: new #2 // class java/lang/String 13: dup 14: ldc #5 // String cccc 16: invokespecial #4 // Method java/lang/String." ":(Ljava/lang/String;)V 19: astore_1
new：创建类实例
dup：复制数值，并压入栈顶
ldc：从常量池推送至栈顶
invokespecial：调用特殊实例方法（包括实例初始化方法、父类方法）
astore_1：一个引用放入到局部变量1中(也就是aaaa中)
常量池如下
Constant pool: #1 = Methodref #7.#23 // java/lang/Object."new #2 dup ldc #3 invokespecial #4 astore_1":()V #2 = Class #24 // java/lang/String #3 = String #25 // bbbb #4 = Methodref #2.#26 // java/lang/String." ":(Ljava/lang/String;)V #5 = String #27 // cccc #6 = Class #28 // com/code/baseCode/TestCode #7 = Class #29 // java/lang/Object #8 = Utf8 #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 // " ":()V #24 = Utf8 java/lang/String #25 = Utf8 bbbb #26 = NameAndType #8:#30 // " ":(Ljava/lang/String;)V #27 = Utf8 cccc #28 = Utf8 com/code/baseCode/TestCode #29 = Utf8 java/lang/Object #30 = Utf8 (Ljava/lang/String;)V
后续的 *** 作一样，因此会出现
String a = new String("bbbb");
String b = new String("bbbb");
System.out.println(a == b); // false
面试题
做下面的题是，需要知道几个知识点
1.编辑时，编译器会对确切的值进行优化：
比如String a = "a" + "b"; -> 比如String a = "ab";
final String a = "a"; String b = "a" + a; -> String b = "aa";
2.字符串变量相加会创建StringBuilder调用append方法，然后StringBuilder转换成new String
3.intern：Java查找常量池中是否有相同Unicode的字符串常量，如果有，则返回其的引用，如果没有，则在常量池中增加一个Unicode等于str的字符串并返回它的引用。
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：[java面试题]判断字符串相等，看这一篇就够了 - 知乎
结尾之前出了int类型的情况，上述是String类型的情况，后续出包装类型的情况
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