StringTable
首先看这样一个面试题
// StringTable [ "a", "b" ,"ab" ] hashtable 结构，不能扩容
public class Demo1_22 {
// 常量池中的信息，都会被加载到运行时常量池中， 这时 a b ab 都是常量池中的符号，还没有变为 java 字符串对象
// ldc #2 会把 a 符号变为 "a" 字符串对象
// ldc #3 会把 b 符号变为 "b" 字符串对象
// ldc #4 会把 ab 符号变为 "ab" 字符串对象
public static void main(String[] args) {
String s1 = "a"; // 懒惰的
String s2 = "b";
String s3 = "ab";
String s4 = s1 + s2; // new StringBuilder().append("a").append("b").toString() new String("ab")
String s5 = "a" + "b"; // javac 在编译期间的优化，结果已经在编译期确定为ab
System.out.println(s3 == s4); // false
System.out.println(s3 == s5); // true
}
}
我们从字节码的角度来分析结果
首先反编译该类代字节码,输入命令：
javap -v Demo1_22.class // -v显示详细信息
得到结果：
Classfile /D:/work_folder/java_studying/idea_workspace/jvm/out/production/jvm/cn/itcast/jvm/t1/stringtable/Demo1_22.class
Last modified 2019-12-8; size 1039 bytes
MD5 checksum 9efed67948c7dacf9ddb854ca18f8c2b
Compiled from "Demo1_22.java"
public class cn.itcast.jvm.t1.stringtable.Demo1_22
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #12.#36 // java/lang/Object."<init>":()V
#2 = String #37 // a
#3 = String #38 // b
#4 = String #39 // ab
#5 = Class #40 // java/lang/StringBuilder
#6 = Methodref #5.#36 // java/lang/StringBuilder."<init>":()V
#7 = Methodref #5.#41 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#8 = Methodref #5.#42 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#9 = Fieldref #43.#44 // java/lang/System.out:Ljava/io/PrintStream;
#10 = Methodref #45.#46 // java/io/PrintStream.println:(Z)V
#11 = Class #47 // cn/itcast/jvm/t1/stringtable/Demo1_22
#12 = Class #48 // java/lang/Object
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 LocalVariableTable
#18 = Utf8 this
#19 = Utf8 Lcn/itcast/jvm/t1/stringtable/Demo1_22;
#20 = Utf8 main
#21 = Utf8 ([Ljava/lang/String;)V
#22 = Utf8 args
#23 = Utf8 [Ljava/lang/String;
#24 = Utf8 s1
#25 = Utf8 Ljava/lang/String;
#26 = Utf8 s2
#27 = Utf8 s3
#28 = Utf8 s4
#29 = Utf8 s5
#30 = Utf8 StackMapTable
#31 = Class #23 // "[Ljava/lang/String;"
#32 = Class #49 // java/lang/String
#33 = Class #50 // java/io/PrintStream
#34 = Utf8 SourceFile
#35 = Utf8 Demo1_22.java
#36 = NameAndType #13:#14 // "<init>":()V
#37 = Utf8 a
#38 = Utf8 b
#39 = Utf8 ab
#40 = Utf8 java/lang/StringBuilder
#41 = NameAndType #51:#52 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#42 = NameAndType #53:#54 // toString:()Ljava/lang/String;
#43 = Class #55 // java/lang/System
#44 = NameAndType #56:#57 // out:Ljava/io/PrintStream;
#45 = Class #50 // java/io/PrintStream
#46 = NameAndType #58:#59 // println:(Z)V
#47 = Utf8 cn/itcast/jvm/t1/stringtable/Demo1_22
#48 = Utf8 java/lang/Object
#49 = Utf8 java/lang/String
#50 = Utf8 java/io/PrintStream
#51 = Utf8 append
#52 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#53 = Utf8 toString
#54 = Utf8 ()Ljava/lang/String;
#55 = Utf8 java/lang/System
#56 = Utf8 out
#57 = Utf8 Ljava/io/PrintStream;
#58 = Utf8 println
#59 = Utf8 (Z)V
{
public cn.itcast.jvm.t1.stringtable.Demo1_22();
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 4: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcn/itcast/jvm/t1/stringtable/Demo1_22;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=6, args_size=1
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: astore 4
29: ldc #4 // String ab
31: astore 5
33: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
36: aload_3
37: aload 4
39: if_acmpne 46
42: iconst_1
43: goto 47
46: iconst_0
47: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
50: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
53: aload_3
54: aload 5
56: if_acmpne 63
59: iconst_1
60: goto 64
63: iconst_0
64: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
67: return
LineNumberTable:
line 11: 0
line 12: 3
line 13: 6
line 14: 9
line 15: 29
line 16: 33
line 17: 50
line 18: 67
LocalVariableTable:
Start Length Slot Name Signature
0 68 0 args [Ljava/lang/String;
3 65 1 s1 Ljava/lang/String;
6 62 2 s2 Ljava/lang/String;
9 59 3 s3 Ljava/lang/String;
29 39 4 s4 Ljava/lang/String;
33 35 5 s5 Ljava/lang/String;
StackMapTable: number_of_entries = 4
frame_type = 255 /* full_frame */
offset_delta = 46
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
stack = [ class java/io/PrintStream ]
frame_type = 255 /* full_frame */
offset_delta = 0
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
stack = [ class java/io/PrintStream, int ]
frame_type = 79 /* same_locals_1_stack_item */
stack = [ class java/io/PrintStream ]
frame_type = 255 /* full_frame */
offset_delta = 0
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String, class java/lang/String, c
lass java/lang/String ]
stack = [ class java/io/PrintStream, int ]
}
SourceFile: "Demo1_22.java"
我们从中摘取最重要的一段：
Code:
stack=3, locals=6, args_size=1
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: astore 4
29: ldc #4 // String ab
31: astore 5
上面每句的意思可参照jvm指令手册去进行阅读
0: ldc #2 // String a
ldc 把常量池中的项压入栈,即在Constant pool中找到#2,然后将其压入栈。也就是String a
astore_1 // astore_1 将引用类型或returnAddress类型值存入局部变量1
也就是LocalVariableTable中的这一行
3 65 1 s1 Ljava/lang/String;
其他类似，
在索引9的位置：
9: new #5 // class java/lang/StringBuilder
这 就是创建了一个StringBuilder对象，
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
""
StringTable的更多相关文章
- ue4 StringTable
StringTable 用法很简单可以参考 https://blog.csdn.net/u012801153/article/details/80393531 这里只说说上面文章中没提到的小技巧 T ...
- JVM 专题十六：StringTable
1. String的基本特性 String:字符串,使用一对""引起来表示. String声明为final的,不可被继承. String实现了Serializable接口:表示字符 ...
- StringTable结构以及基本调优
StringTable底层实现类似于HashTable,由数组和链表组成,数组又称为桶数组.比如有这样一段代码: public class Demo4 { public static void mai ...
- 【JVM之内存与垃圾回收篇】StringTable
StringTable String的基本特性 String:字符串,使用一对 "" 引起来表示 String s1 = "Nemo"; // 字面量的定义方式 ...
- JVM系列之:String.intern和stringTable
目录 简介 intern简介 intern和字符串字面量常量 分析intern返回的String对象 分析实际的问题 G1中的去重功能 总结 简介 StringTable是什么?它和String.in ...
- 看了这篇文章，我搞懂了StringTable
好好学习,天天向上 本文已收录至我的Github仓库DayDayUP:github.com/RobodLee/DayDayUP,欢迎Star,更多文章请前往:目录导航 前言 String应该是Java ...
- JVM字符串常量池StringTable
String的基本特性 String:字符串,使用一对""引起来表示. String sl = "hello"://字面量的定义方式: String s2 = ...
- 第 13 章 StringTable详解
目录 第 13 章 StringTable 1.String 的基本特性 1.1.String 概述 1.2.String 的基本特征 1.3.String 的底层结构 2.String 的内存分配 ...
- JVM 中的StringTable
是什么 字符串常量池是 JVM 中的一个重要结构,用于存储JVM运行时产生的字符串.在JDK7之前在方法区中,存储的是字符串常量.而字符串常量池在 JDK7 开始移入堆中,随之而来的是除了存储字符串常 ...
随机推荐
- Kubernetes CNI网络插件
CNI 容器网络接口,就是在网络解决方案由网络插件提供,这些插件配置容器网络则通过CNI定义的接口来完成,也就是CNI定义的是容器运行环境与网络插件之间的接口规范.这个接口只关心容器的网络连接,在创建 ...
- https网站部署成功 CA证书申请 IIS配置
https://www.cnblogs.com/lichunting/p/9274422.html#4276645 参考文档解决方案
- javascript字符串加密解密函数
代码如下: /** * 加密函数 * @param str 待加密字符串 * @returns {string} */ function str_encrypt(str) { var c = Stri ...
- django迁移脚本
执行migrate报错的解决办法: 想知道migrate为什么报错,需要先了解migrate到底做了什么事情 migrate做了什么事情? 1.将相关的迁移脚本翻译成sql语句,然后在数据库中执行 2 ...
- 201871010133-赵永军《面向对象程序设计（java）》第二周学习总结
201871010133-赵永军<面向对象程序设计(java)>第二周学习总结 项目 内容 这个作业属于哪个课程 https://www.cnblogs.com/nwnu-daizh/ 这 ...
- pointnet
无序性:虽然输入的点云是有顺序的,但是显然这个顺序不应当影响结果.点之间的交互:每个点不是独立的,而是与其周围的一些点共同蕴含了一些信息,因而模型应当能够抓住局部的结构和局部之间的交互.变换不变性:比 ...
- [Algorithm] 118. Pascal's Triangle
Given a non-negative integer numRows, generate the first numRows of Pascal's triangle. In Pascal's t ...
- javascript中 encodeURIComponent() 与 encodeURI() 的区别
前言:js 中仅有的几个全局函数中,有两个全局函数可以用来编码url 字符串. 一.encodeURIComponent() 将转义用于分隔 URI 各个部分的标点符号 ,也就是可以编码 " ...
- 关于Visual Studio源代码文件的行尾
我们都知道,UNIX只使用换行符(linefeed)来结束每一行,而DOS传统上使用CR+LF来结束每一行,Visual Studio应该完全在DOS世界中,但不管出于什么原因,当我们从代码服务器上获 ...
- Json、Pickle
目录 Json pickle Json JSON是一个序列化模块,一种用于存储和交换数据的语法. JSON是用JavaScript对象表示法(JavaScript object notation)格式 ...