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2020-11-19 17:34:16 +08:00
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docs/Java/JVM/JVM系列-第9章-StringTable(字符串常量池).md
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@@ -10,6 +10,7 @@ keywords: JVM，虚拟机。
description: JVM系列-第9章-StringTable(字符串常量池)。
cover: 'https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/logo.png'
top_img: 'https://cdn.jsdelivr.net/gh/youthlql/lql_img/blog/top_img.jpg'
abbrlink: ee2ba71e
date: 2020-11-16 12:38:02
---
@@ -166,11 +167,11 @@ str 的内容并没有变：“test ok” 位于字符串常量池中的另一
4. 在JDK7中，StringTable的长度默认值是60013，StringTablesize设置没有要求
5. 在JDK8中，StringTable的长度默认值是60013，StringTable可以设置的最小值为1009
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0001.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0001.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0002.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0002.png">
@@ -276,11 +277,11 @@ String 的内存分配
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0003.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0003.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0004.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0004.png">
@@ -382,23 +383,23 @@ public class StringTest4 {
1、程序启动时已经加载了 2293 个字符串常量
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0005.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0005.png">
2、加载了一个换行符（println），所以多了一个
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0006.jpg">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0006.jpg">
3、加载了字符串常量 “1”~“9”
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0007.jpg">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0007.jpg">
4、加载字符串常量 “10”
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0008.jpg">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0008.jpg">
5、之后的字符串"1" 到 "10"不会再次加载
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0009.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0009.png">
@@ -425,7 +426,7 @@ class Memory {
分析运行时内存（foo() 方法是实例方法，其实图中少了一个 this 局部变量）
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0010.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0010.png">
@@ -493,7 +494,7 @@ class Memory {
IDEA 反编译 class 文件后，来看这个问题
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0011.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0011.png">
@@ -929,7 +930,7 @@ public class StringNewTest {
5. `23 ldc #8 <b>` ：在字符串常量池中放入 “b”（如果之前字符串常量池中没有 “b” 的话）
6. `31 invokevirtual #9 <java/lang/StringBuilder.toString>` ：调用 StringBuilder 的 toString() 方法，会生成一个 String 对象
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0012.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0012.png">
@@ -988,13 +989,13 @@ JDK6 ：正常眼光判断即可
* new String() 即在堆中
* str.intern() 则把字符串放入常量池中
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0013.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0013.png">
JDK7及后续版本，**注意大坑**
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0014.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0014.png">
@@ -1053,11 +1054,11 @@ public class StringExer1 {
**JDK6**
![image-20201116113423492](https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0015.png)
![image-20201116113423492](https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0015.png)
**JDK7/8**
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0016.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0016.png">
@@ -1080,7 +1081,7 @@ public class StringExer1 {
}
```
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0017.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0017.png">
**练习3**
@@ -1169,11 +1170,11 @@ public class StringIntern2 {
arr[i] = new String(String.valueOf(data[i % data.length]));
```
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0018.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0018.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0019.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0019.png">
2、使用 intern() 方法：由于数组中字符串的引用都指向字符串常量池中的字符串，所以程序需要维护的 String 对象更少，内存占用也更低
@@ -1182,11 +1183,11 @@ arr[i] = new String(String.valueOf(data[i % data.length]));
arr[i] = new String(String.valueOf(data[i % data.length])).intern();
```
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0020.png" >
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0020.png" >
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0021.png">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0021.png">
@@ -1218,11 +1219,11 @@ public class StringGCTest {
* Number of entries 和 Number of literals 明显没有 100000
* 以上两点均说明 StringTable 区发生了垃圾回收
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0022.jpg">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0022.jpg">
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img/JVM/chapter_009/0023.jpg"/>
<img src="https://cdn.jsdelivr.net/gh/youthlql/lql_img_002/JVM/chapter_009/0023.jpg"/>