深入JAVA 的JVM核心原理解决线上各种故障【附案例】

25-03-07 20:00

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2.3.2 程序运行很长时间没有结果，如何诊断案例

3、本地方法栈（不是Java编写的代码，通过C/C++）

4.3.1 垃圾回收后，内存占用仍然很高，排查方式案例

5、元空间/方法区（-XX:MaxMetaspaceSize=8m）

5.3.2 生产环境出现元空间内存溢出问题，应该锁定这些方面

5.4 运行时常量池

5.4.1 字符串常量池JVM字节码方面原理演示

5.5 StringTable

5.5.1 StringTable常量池与串池的关系

5.6 StringTable特性

5.7 StringTable位置

5.7.1 JDK1.8 字符串常量池在堆中实例验证

5.8 StringTable垃圾回收

5.9 StringTable 性能调优（案例）

5.9.1 使用-XX:StringTableSize=大小参数增加桶的数量使StringTable性能增加案例

5.9.2 使用字符串常量池对字符串较多的场景减少内存占用案例

6、直接内存Direct Memory

6.1 直接内存定义

6.2 原理讲解

6.3 直接内存与传统方式读取大文件耗时对比案例

6.4 直接内存溢出案例

6.5 分配和使用原理

6.6 分配和回收原理及案例演示

什么是JVM?

Java Virtual Machine - java程序的运行环境（java二进制字节码的运行环境）

JVM好处？

一次编写，到处运行的基石【重点】
自动内存管理，垃圾回收功能【重点】
数据下标越界检查
多态，面向对象编程

JVM、JRE、JDK三者比较：

学习JVM有什么用？

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class="hljs-ln-code"> class="hljs-ln-line">面试

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JVM组成有哪些？

常见的JVM

JAVA 内存结构组成

程序计数器
虚拟机栈
本地方法栈
堆
方法区

1、程序计数器

1.1 程序计数器定义

Program Counter Register程序计数器（寄存器）

1.2 程序计数器作用

作用：是记住下一条jvm指令的执行地址

特点：线程私有的；不存在内存溢出，也是JVM规范中唯一没有OutOfMemoryError的区域

二进制字节码：JVM指令 —> 解释器 —> 机器码 —> CPU

程序计数器：记住下一条jvm指令的执行地址，硬件方面通过【寄存器】实现

示例：二进制字节码：jvm指令 java 源代码

2、虚拟机栈（-Xss256k）

先了解一程数据结构

栈Stack，先进后出FILO
栈-线程运行需要的内存空间
栈帧-每个方法运行时需要的内存

2.1 栈定义

Java Virtual Machine Stacks (Java虚拟机栈)

每个线程运行时所需要的内存，称为虚拟机栈
每个栈由多个栈帧（Frame）组成，对应着每次方法调用时所占用的内存
每个线程只能有一个活动栈帧，对应着当前正在执行的那个方法

2.2 栈问题

垃圾回收是否涉及栈内存？答案：栈内存不涉及垃圾回收
栈内存分配越大越好吗？答案：栈内存不是越大越好，如果设置过大，会影响可用线程的数量；比如-Xss1m、-Xss2m，在总内存不变的情况下，可用线程数量会减少
方法内的局部变量是否线程安全？答案：方法内的局部变量是线程安全，因为方法内的局部变量各自在自已独立的内存中；如果是static int 就是线程共享的，就不是线程安全；主要看变量是否是线程共享、还是线程私有

核心1：如果方法内局部变量没有逃离方法的作用范围，它是线程安全的

核心2：如果是局部变量引用了对象，并逃离方法的作用范围，需要考虑线程安全

2.3 栈内存溢出（-Xss256k）

栈帧过多导致栈内存溢出，比如：递归，我们生产环境推荐尽量不使用递归

栈帧过大导致栈内存溢出

2.3 线程运行诊断（附案例）

2.3.1 cpu占用过高，如何诊断案例

1. 用top定位哪个进程对cpu的占用过高

2. ps H -eo pid,tid,%cpu 查看linux所有进程、线程、CPU消耗情况

3. ps H -eo pid,tid,%cpu | grep 进程id 用ps命令进一步定位哪个线程引起的CPU占用过高

4. jstack 进程pid 需要将十进制的线程id转成16进制; 可以根据线程id找到有问题的线程，进一步定位问题代码的源码行号

通过上述方式找到了源代码CPU消耗过高的文件及行号

2.3.2 程序运行很长时间没有结果，如何诊断案例

nohup java -cp /root/JvmLearn-1.0-SNAPSHOT.jar com.jvm.stack.T07_StackDeadLock &

3、本地方法栈（不是Java编写的代码，通过C/C++）

4、堆（-Xmx8m）

4.1 堆的定义

通过new 关键字，创建对象都会使用堆内存
特点：
- 它是线程共享的，堆中对象需要考虑线程安全的问题
- 有垃圾回收机制

4.2 堆内存溢出问题及生产建议

代码参考：com.jvm.t02_heap.T01_HeapOutOfMemoryError

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 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 演示堆内存溢出 java.lang.OutOfMemoryError: Java heap space
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> * -Xmx8m
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生产环境建议：如果内存比较大，内存溢出不会那么快的暴露；这时，我们可以将堆内存调小，让内存溢出尽早暴露

4.3 堆内存诊断工具介绍，及实操

jps工具：查看当前系统中有哪些java进程
jmap工具：查看堆内存占用情况 jmap -heap pid
jstack 工具：线程监控
jconsole工具：图形界面的，多功能的检测工具，可以连续监测
jvisualvm工具：图形界面的，多功能的检测工具，可以连续监测；还有dump

代码参考：com.jvm.t02_heap.T02_HeapUseUpAndDown

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">/**
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 演示堆内存
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line">public class T02_HeapUseUpAndDown {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) throws InterruptedException {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println("1...");
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 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line">        byte[] array = new byte[1024 * 1024 * 10]; // 10 Mb
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Jps
Jmap -head pid 查看堆内存占用
在控制台上使用jconsole

4.3.1 垃圾回收后，内存占用仍然很高，排查方式案例

代码参考：com.jvm.t02_heap.T03_HeapAfterGcMemStillHigh

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 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 演示查看对象个数 堆转储 dump
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line">public class T03_HeapAfterGcMemStillHigh {
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 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line">//            Student student = new Student();
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 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">        Thread.sleep(1000000000L);
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 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">class Student {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">    private byte[] big = new byte[1024 * 1024];
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解决方式：jvisualvm 可以使用dump，查找最大的对象堆转储 dump（基于上述问题，使用工具进行查看）; 在测试环境下，我们可以开启dump文件记录，然后将dump文件导入到jvisualvm工具查看，占用最多的内存的对象是哪些。

5、元空间/方法区（-XX:MaxMetaspaceSize=8m）

5.1 JVM方法区定义

线程共享
在JVM启动时创建，在逻辑上属于堆的一部分（看厂商实现）
方法区也可能会内存溢出

5.2 方法区组成

5.3 方法区内存溢出

1.8 以前会导致永久代内存溢出

演示永久代内存溢出 java.lang.OutOfMemoryError: PerGen space
-XX:MaxPerSize=8m

1.8 之后会导致元空间内存溢出（系统内存）

演示元空间内存溢出 java.lang.OutOfMemoryError: Metaspace
-XX:MaxMetaspaceSize=8m

5.3.1 元空间内存溢出演示案例

Jdk1.8 参考代码：com.jvm.t03_metaspace.T01_MetaspaceOutOfMemoryError

演示元空间内存溢出 java.lang.OutOfMemoryError: Metaspace
-XX:MaxMetaspaceSize=8m

5.3.2 生产环境出现元空间内存溢出问题，应该锁定这些方面

虽然我们自己编写的程序没有大量使用动态加载类，但如果我们在使用外部一些框架时，可能大量动态加载类，就可能会导致元空间内存溢出。

场景（动态加载类），如果框架使用不合理也会导致方法区内存溢出

spring
mybatis

5.4 运行时常量池

常量池，就是一张表，虚拟机指令根据这张常量表找到要执行的类名、方法名、参数类型、字面量等信息
运行时常量池，常量池是*.class 文件中的，当该类被加载，它的常量池信息就会放入运行时常量池，并把里面的符号地址变为真实地址

5.4.1 字符串常量池JVM字节码方面原理演示

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">// 二进制字节码（类基本信息，常量池，类方法定义，包含了虚拟机指令）
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line">public class T02_StringHelloWorld {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) {
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将上述编译好的class文件进行反汇编：Javap -v HelloWord.class 反编译结果如下：

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">D:\software\Java\jdk1.8.0_211\bin\javap.exe -v com.jvm.t03_metaspace.T02_MetaspaceConstantPool
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line">Classfile /D:/lei_test_project/idea_workspace/Jvm_Learn/target/classes/com/jvm/t03_metaspace/T02_MetaspaceConstantPool.class
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line">  Last modified 2020-7-29; size 623 bytes
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line">  MD5 checksum 6b5272fbb2c0ca06c0e460818756710d
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line">  Compiled from "T02_MetaspaceConstantPool.java"
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line">public class com.jvm.t03_metaspace.T02_MetaspaceConstantPool
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line">  minor version: 0
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line">  major version: 52
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line">  flags: ACC_PUBLIC, ACC_SUPER
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">Constant pool:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">   #1 = Methodref          #6.#20         // java/lang/Object."":()V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">   #2 = Fieldref           #21.#22        // java/lang/System.out:Ljava/io/PrintStream;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line">   #3 = String             #23            // hello world!
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">   #4 = Methodref          #24.#25        // java/io/PrintStream.println:(Ljava/lang/String;)V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">   #5 = Class              #26            // com/jvm/t03_metaspace/T02_MetaspaceConstantPool
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">   #6 = Class              #27            // java/lang/Object
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">   #7 = Utf8               
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">   #8 = Utf8               ()V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">   #9 = Utf8               Code
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">  #10 = Utf8               LineNumberTable
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">  #11 = Utf8               LocalVariableTable
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">  #12 = Utf8               this
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="23"> class="hljs-ln-code"> class="hljs-ln-line">  #13 = Utf8               Lcom/jvm/t03_metaspace/T02_MetaspaceConstantPool;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="24"> class="hljs-ln-code"> class="hljs-ln-line">  #14 = Utf8               main
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="25"> class="hljs-ln-code"> class="hljs-ln-line">  #15 = Utf8               ([Ljava/lang/String;)V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="26"> class="hljs-ln-code"> class="hljs-ln-line">  #16 = Utf8               args
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="27"> class="hljs-ln-code"> class="hljs-ln-line">  #17 = Utf8               [Ljava/lang/String;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="28"> class="hljs-ln-code"> class="hljs-ln-line">  #18 = Utf8               SourceFile
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="29"> class="hljs-ln-code"> class="hljs-ln-line">  #19 = Utf8               T02_MetaspaceConstantPool.java
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="30"> class="hljs-ln-code"> class="hljs-ln-line">  #20 = NameAndType        #7:#8          // "":()V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="31"> class="hljs-ln-code"> class="hljs-ln-line">  #21 = Class              #28            // java/lang/System
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="32"> class="hljs-ln-code"> class="hljs-ln-line">  #22 = NameAndType        #29:#30        // out:Ljava/io/PrintStream;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="33"> class="hljs-ln-code"> class="hljs-ln-line">  #23 = Utf8               hello world!
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="34"> class="hljs-ln-code"> class="hljs-ln-line">  #24 = Class              #31            // java/io/PrintStream
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="35"> class="hljs-ln-code"> class="hljs-ln-line">  #25 = NameAndType        #32:#33        // println:(Ljava/lang/String;)V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="36"> class="hljs-ln-code"> class="hljs-ln-line">  #26 = Utf8               com/jvm/t03_metaspace/T02_MetaspaceConstantPool
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="37"> class="hljs-ln-code"> class="hljs-ln-line">  #27 = Utf8               java/lang/Object
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="38"> class="hljs-ln-code"> class="hljs-ln-line">  #28 = Utf8               java/lang/System
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="39"> class="hljs-ln-code"> class="hljs-ln-line">  #29 = Utf8               out
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="40"> class="hljs-ln-code"> class="hljs-ln-line">  #30 = Utf8               Ljava/io/PrintStream;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="41"> class="hljs-ln-code"> class="hljs-ln-line">  #31 = Utf8               java/io/PrintStream
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="42"> class="hljs-ln-code"> class="hljs-ln-line">  #32 = Utf8               println
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="43"> class="hljs-ln-code"> class="hljs-ln-line">  #33 = Utf8               (Ljava/lang/String;)V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="44"> class="hljs-ln-code"> class="hljs-ln-line">{
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="45"> class="hljs-ln-code"> class="hljs-ln-line">  public com.jvm.t03_metaspace.T02_MetaspaceConstantPool();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="46"> class="hljs-ln-code"> class="hljs-ln-line">    descriptor: ()V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="47"> class="hljs-ln-code"> class="hljs-ln-line">    flags: ACC_PUBLIC
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="48"> class="hljs-ln-code"> class="hljs-ln-line">    Code:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="49"> class="hljs-ln-code"> class="hljs-ln-line">      stack=1, locals=1, args_size=1
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="50"> class="hljs-ln-code"> class="hljs-ln-line">         0: aload_0
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="51"> class="hljs-ln-code"> class="hljs-ln-line">         1: invokespecial #1                  // Method java/lang/Object."":()V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="52"> class="hljs-ln-code"> class="hljs-ln-line">         4: return
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="53"> class="hljs-ln-code"> class="hljs-ln-line">      LineNumberTable:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="54"> class="hljs-ln-code"> class="hljs-ln-line">        line 11: 0
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="55"> class="hljs-ln-code"> class="hljs-ln-line">      LocalVariableTable:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="56"> class="hljs-ln-code"> class="hljs-ln-line">        Start  Length  Slot  Name   Signature
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="57"> class="hljs-ln-code"> class="hljs-ln-line">            0       5     0  this   Lcom/jvm/t03_metaspace/T02_MetaspaceConstantPool;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="58"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="59"> class="hljs-ln-code"> class="hljs-ln-line">  public static void main(java.lang.String[]);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="60"> class="hljs-ln-code"> class="hljs-ln-line">    descriptor: ([Ljava/lang/String;)V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="61"> class="hljs-ln-code"> class="hljs-ln-line">    flags: ACC_PUBLIC, ACC_STATIC
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="62"> class="hljs-ln-code"> class="hljs-ln-line">    Code:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="63"> class="hljs-ln-code"> class="hljs-ln-line">      stack=2, locals=1, args_size=1
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="64"> class="hljs-ln-code"> class="hljs-ln-line">         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="65"> class="hljs-ln-code"> class="hljs-ln-line">         3: ldc           #3                  // String hello world!
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="66"> class="hljs-ln-code"> class="hljs-ln-line">         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="67"> class="hljs-ln-code"> class="hljs-ln-line">         8: return
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="68"> class="hljs-ln-code"> class="hljs-ln-line">      LineNumberTable:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="69"> class="hljs-ln-code"> class="hljs-ln-line">        line 13: 0
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="70"> class="hljs-ln-code"> class="hljs-ln-line">        line 14: 8
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="71"> class="hljs-ln-code"> class="hljs-ln-line">      LocalVariableTable:
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="72"> class="hljs-ln-code"> class="hljs-ln-line">        Start  Length  Slot  Name   Signature
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="73"> class="hljs-ln-code"> class="hljs-ln-line">            0       9     0  args   [Ljava/lang/String;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="74"> class="hljs-ln-code"> class="hljs-ln-line">}
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="75"> class="hljs-ln-code"> class="hljs-ln-line">SourceFile: "T02_MetaspaceConstantPool.java"
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="76"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="77"> class="hljs-ln-code"> class="hljs-ln-line">Process finished with exit code 0 class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

5.5 StringTable

5.5.1 StringTable常量池与串池的关系

代码参考：com.jvm.t03_metaspace.T03_MetaspaceStringTable

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">// StringTable [ "a", "b" ,"ab" ]  hashtable 结构，不能扩容
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line">public class T03_StringTable {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line">    // 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
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line">    // ldc #2 会把 a 符号变为 "a" 字符串对象
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line">    // ldc #3 会把 b 符号变为 "b" 字符串对象
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line">    // ldc #4 会把 ab 符号变为 "ab" 字符串对象
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">        String s1 = "a"; // 懒惰的
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">        String s2 = "b";
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">        String s3 = "ab";
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line">        String s4 = s1 + s2; // new StringBuilder().append("a").append("b").toString()  new String("ab")
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">        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
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println(s3 == s4);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println(s3 == s5);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

javap -v T03_MetaspaceStringTable.class 反编译如下：

5.5.2 StringTable 字符串延迟加载

5.6 StringTable特性

常量池中的字符串仅是符号，第一次用到时才变为对象
利用串池的机制，来避免重复创建字符串对象
字符串变量拼接的原理是 StringBuilder (JDK1.8)
字符串常量拼接的原理是编译期优化
可以使用intern 方法，主动将串池中还没有的字符串对象放入串池
- 1.8 将这个字符串对象尝试放放串池，如果有则并不会放入，如果没有则放入串池，会把串池中的对象返回
- 1.6 将这个字符串对象尝试放入串池，如果有则并不会放入，如果没有会把对象复制一份，放入串池，会把串池中的对象返回

StringTable_intern_1.8

代码参考：com.jvm.t03_metaspace.T05_TestString02

5.7 StringTable位置

JDK1.6版本，字符串常量池是在永久代中；
JDK1.7 及之后版本的 JVM 已经将运行时常量池从方法区中移了出来，在 Java 堆（Heap）中开辟了一块区域存放运行时常量池。
JDK1.8开始，取消了Java方法区，取而代之的是位于直接内存的元空间（metaSpace）。

JDK1.6 与 JDK1.8字符串常量池对比

5.7.1 JDK1.8 字符串常量池在堆中实例验证

代码参考： com.jvm.t03_metaspace.T07_StringTablePosition

5.8 StringTable垃圾回收

因为在jdk1.8中，字符串常量池是放在堆中，如果堆空间不足，字符串常量池也会进行垃圾回收

代码参考：com.jvm.t04_stringtable.T08_StringTableGc

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">/**
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 演示 StringTable 垃圾回收
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> * -Xmx10m -XX:+PrintStringTableStatistics -XX:+PrintGCDetails -verbose:gc
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line">// 因为在jdk1.8中，字符串常量池是放在堆中，如果堆空间不足，字符串常量池也会进行垃圾回收
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line">public class T08_StringTableGc {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) throws InterruptedException {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">        int i = 0;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">        try {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">            for (int j = 0; j < 10000; j++) { // 前后运行100次、10000次，进行对比。j=100, j=10000
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line">                String.valueOf(j).intern();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">                i++;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">            }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">        } catch (Throwable e) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">            e.printStackTrace();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">        } finally {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">            System.out.println(i);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">        }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

5.9 StringTable 性能调优（案例）

代码参考：com.jvm.t04_stringtable.T09_StringTableSizeForPerformance

调整 -XX:StringTableSize=桶个数
考虑将字符串对象是否入池

5.9.1 使用`-XX:StringTableSize=大小`参数增加桶的数量使`StringTable`性能增加案例

class="table-box">

序号	StringTableSize大小	运行耗时（单位毫秒）
1	1009	11444
2	10009	1765
3	100009	430

将StringTable桶调小些，示例操作如下：

将StringTable桶调大些，示例操作如下：

5.9.2 使用字符串常量池对字符串较多的场景减少内存占用案例

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">/**
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 演示串池大小对性能的影响
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> * -Xms500m -Xmx500m -XX:+PrintStringTableStatistics -XX:StringTableSize=1009
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line"> * 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line"> * 字符串常量池默认桶数组大小为：60013，对字符串常量池调优主要是调节桶数据大小；如果字符串数量较多，则需要将此调大些，以减少查询复杂度（hash碰撞机率）
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line">public class T09_StringTableSizeForPerformance {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) throws IOException {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">        try (BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream("in/linux.words"), "utf-8"))) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">            String line = null;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">            long start = System.nanoTime();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line">            while (true) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">                line = reader.readLine();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">                if (line == null) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">                    break;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">                }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">                line.intern();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">            }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">            System.out.println("cost:" + (System.nanoTime() - start) / 1000000);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">        }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="23"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

说明一下：in/linux.words 大约单词量在479829个，上述代码运行结果截图，如下：

class="table-box">

读取大约48万单词	堆内存占用大小	耗时
未放入字符串池	约300兆	较短
放入字符串池	约70兆	较长

运行结果1：未放入字符串常量池中，运行情况截图

运行结果2：放入字符串常量池中，运行情况截图

6、直接内存Direct Memory

6.1 直接内存定义

常见于NIO操作时，用于数据缓冲区
分配回收成本较高，但读写性能高
不受JVM内存回收管理

6.2 原理讲解

普通内存
- 需要从用户态向内核态申请资源，即用户态会创建一个java 缓冲区byte[]，内核态会创建系统缓冲区。
直接内存
- 需要从用户态向内核态申请资源，即内核态会创建一块直接内存direct memory，这块direct memory内存可以在用户态、内核态使用。

通常使用内存（未使用直接内存） VS 直接内存，原理对比图

6.3 直接内存与传统方式读取大文件耗时对比案例

接下来，我们将对一个大约1.29G大小的视频文件进行读取并写入指定文件中，即复制。代码如下：

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">package com.jvm.t05_direct;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line">import java.io.FileInputStream;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line">import java.io.FileOutputStream;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line">import java.io.IOException;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line">import java.nio.ByteBuffer;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line">import java.nio.channels.FileChannel;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">public class T01_IoVsDirectBuffer {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">    static final String FROM = "E:\\Flink CEP.mp4";
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">    static final String TO = "E:\\a.mp4";
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line">    static final int _1Mb = 1024 * 1024;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">        io(); 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">        directBuffer(); 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">    private static void directBuffer() {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">        long start = System.nanoTime();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">        try (FileChannel from = new FileInputStream(FROM).getChannel();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="23"> class="hljs-ln-code"> class="hljs-ln-line">             FileChannel to = new FileOutputStream(TO).getChannel();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="24"> class="hljs-ln-code"> class="hljs-ln-line">        ) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="25"> class="hljs-ln-code"> class="hljs-ln-line">            ByteBuffer bb = ByteBuffer.allocateDirect(_1Mb);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="26"> class="hljs-ln-code"> class="hljs-ln-line">            while (true) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="27"> class="hljs-ln-code"> class="hljs-ln-line">                int len = from.read(bb);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="28"> class="hljs-ln-code"> class="hljs-ln-line">                if (len == -1) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="29"> class="hljs-ln-code"> class="hljs-ln-line">                    break;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="30"> class="hljs-ln-code"> class="hljs-ln-line">                }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="31"> class="hljs-ln-code"> class="hljs-ln-line">                bb.flip();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="32"> class="hljs-ln-code"> class="hljs-ln-line">                to.write(bb);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="33"> class="hljs-ln-code"> class="hljs-ln-line">                bb.clear();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="34"> class="hljs-ln-code"> class="hljs-ln-line">            }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="35"> class="hljs-ln-code"> class="hljs-ln-line">        } catch (IOException e) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="36"> class="hljs-ln-code"> class="hljs-ln-line">            e.printStackTrace();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="37"> class="hljs-ln-code"> class="hljs-ln-line">        }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="38"> class="hljs-ln-code"> class="hljs-ln-line">        long end = System.nanoTime();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="39"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println("directBuffer 用时：" + (end - start) / 1000_000.0);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="40"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="41"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="42"> class="hljs-ln-code"> class="hljs-ln-line">    private static void io() {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="43"> class="hljs-ln-code"> class="hljs-ln-line">        long start = System.nanoTime();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="44"> class="hljs-ln-code"> class="hljs-ln-line">        try (FileInputStream from = new FileInputStream(FROM);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="45"> class="hljs-ln-code"> class="hljs-ln-line">             FileOutputStream to = new FileOutputStream(TO);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="46"> class="hljs-ln-code"> class="hljs-ln-line">        ) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="47"> class="hljs-ln-code"> class="hljs-ln-line">            byte[] buf = new byte[_1Mb];
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="48"> class="hljs-ln-code"> class="hljs-ln-line">            while (true) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="49"> class="hljs-ln-code"> class="hljs-ln-line">                int len = from.read(buf);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="50"> class="hljs-ln-code"> class="hljs-ln-line">                if (len == -1) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="51"> class="hljs-ln-code"> class="hljs-ln-line">                    break;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="52"> class="hljs-ln-code"> class="hljs-ln-line">                }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="53"> class="hljs-ln-code"> class="hljs-ln-line">                to.write(buf, 0, len);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="54"> class="hljs-ln-code"> class="hljs-ln-line">            }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="55"> class="hljs-ln-code"> class="hljs-ln-line">        } catch (IOException e) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="56"> class="hljs-ln-code"> class="hljs-ln-line">            e.printStackTrace();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="57"> class="hljs-ln-code"> class="hljs-ln-line">        }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="58"> class="hljs-ln-code"> class="hljs-ln-line">        long end = System.nanoTime();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="59"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println("io 用时：" + (end - start) / 1000_000.0);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="60"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="61"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

运行耗时对比表如下：

class="table-box">

序号	传统方式 IO	直接内存directBuffer	说明
测试1	18871.591 ms	6335.745 ms	没有缓存
测试2	5710.124 ms	5497.707 ms	有缓存
测试3	7355.304 ms	5103.806 ms	有缓存

6.4 直接内存溢出案例

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">/**
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 演示直接内存溢出 java.lang.OutOfMemoryError: Direct buffer memory
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line">public class T02_DirectOutOfMemory {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line">    static int _100Mb = 1024 * 1024 * 100;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line">        List list = new ArrayList<>();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">        int i = 0;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line">        try {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">            while (true) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line">                ByteBuffer byteBuffer = ByteBuffer.allocateDirect(_100Mb);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">                list.add(byteBuffer);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">                i++;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">            }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">        } finally {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">            System.out.println(i);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">        }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">        // 方法区是jvm规范， jdk6 中对方法区的实现称为永久代
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">        //                  jdk8 对方法区的实现称为元空间
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="23"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

6.5 分配和使用原理

代码参考：com.jvm.t05_direct.T03_DirectMemoryGcBySystemGc

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">/**
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 禁用显式回收对直接内存的影响
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> * 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line"> * 因为程序调用System.gc() 会触发full gc，可能会长时间在垃圾回收
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line"> * 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line"> * 为了避免程序员显示调用System.gc(),  我们一般禁用显式调用System.gc()
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line"> * 禁用显式System.gc()，会对直接内存有影响，为此，我们需要通过unSafe类的freeMemory()方法来释放直接内存
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line">public class T03_DirectMemoryGcBySystemGc {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line">    static int _1Gb = 1024 * 1024 * 1024;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">    /*
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">     * -XX:+DisableExplicitGC 显式的
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line">     */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) throws IOException {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">        ByteBuffer byteBuffer = ByteBuffer.allocateDirect(_1Gb);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println("分配完毕...");
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">        System.in.read();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">        System.out.println("开始释放...");
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">        byteBuffer = null;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="23"> class="hljs-ln-code"> class="hljs-ln-line">        System.gc(); // 显式的垃圾回收，Full GC
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="24"> class="hljs-ln-code"> class="hljs-ln-line">        System.in.read();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="25"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="26"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

6.6 分配和回收原理及案例演示

使用了UnSafe对象完成直接内存的分配回收，并且回收需要主动调用freeMemory方法
ByteBuffer的实现类内部，使用了Cleaner（虚引用）来监测ByteBuffer对象，一旦ByteBuffer对象被垃圾回收，那么就会由ReferenceHandler线程通过Cleaner的clean方法调用freeMemory来释放直接内存

 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="1"> class="hljs-ln-code"> class="hljs-ln-line">/**
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="2"> class="hljs-ln-code"> class="hljs-ln-line"> * 直接内存分配的底层原理：Unsafe
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="3"> class="hljs-ln-code"> class="hljs-ln-line"> *
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="4"> class="hljs-ln-code"> class="hljs-ln-line"> * 虚引用关联的对象被回收了，就会触发虚引用对象的clean方法，续而调用Unsafe的freeMemory() 方法
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="5"> class="hljs-ln-code"> class="hljs-ln-line"> *
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="6"> class="hljs-ln-code"> class="hljs-ln-line"> * 6.3 分配和回收原理
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="7"> class="hljs-ln-code"> class="hljs-ln-line"> *
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="8"> class="hljs-ln-code"> class="hljs-ln-line"> * 使用了UnSafe对象完成直接内存的分配回收，并且回收需要主动调用freeMemory方法
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="9"> class="hljs-ln-code"> class="hljs-ln-line"> *
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="10"> class="hljs-ln-code"> class="hljs-ln-line"> * ByteBuffer的实现类内部，使用了Cleaner（虚引用）来监测ByteBuffer对象，一旦ByteBuffer对象被垃圾回收，
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="11"> class="hljs-ln-code"> class="hljs-ln-line"> * 那么就会由ReferenceHandler线程通过Cleaner的clean方法调用freeMemory来释放直接内存
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="12"> class="hljs-ln-code"> class="hljs-ln-line"> */
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="13"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="14"> class="hljs-ln-code"> class="hljs-ln-line">public class T04_DirectMemoryGcByUnsafe {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="15"> class="hljs-ln-code"> class="hljs-ln-line">    static int _1Gb = 1024 * 1024 * 1024;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="16"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="17"> class="hljs-ln-code"> class="hljs-ln-line">    public static void main(String[] args) throws IOException {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="18"> class="hljs-ln-code"> class="hljs-ln-line">        Unsafe unsafe = getUnsafe();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="19"> class="hljs-ln-code"> class="hljs-ln-line">        // 分配内存
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="20"> class="hljs-ln-code"> class="hljs-ln-line">        long base = unsafe.allocateMemory(_1Gb);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="21"> class="hljs-ln-code"> class="hljs-ln-line">        unsafe.setMemory(base, _1Gb, (byte) 0);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="22"> class="hljs-ln-code"> class="hljs-ln-line">        System.in.read();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="23"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="24"> class="hljs-ln-code"> class="hljs-ln-line">        // 释放内存
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="25"> class="hljs-ln-code"> class="hljs-ln-line">        unsafe.freeMemory(base);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="26"> class="hljs-ln-code"> class="hljs-ln-line">        System.in.read();
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="27"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="28"> class="hljs-ln-code"> class="hljs-ln-line"> 
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="29"> class="hljs-ln-code"> class="hljs-ln-line">    public static Unsafe getUnsafe() {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="30"> class="hljs-ln-code"> class="hljs-ln-line">        try {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="31"> class="hljs-ln-code"> class="hljs-ln-line">            Field f = Unsafe.class.getDeclaredField("theUnsafe");
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="32"> class="hljs-ln-code"> class="hljs-ln-line">            f.setAccessible(true);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="33"> class="hljs-ln-code"> class="hljs-ln-line">            Unsafe unsafe = (Unsafe) f.get(null);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="34"> class="hljs-ln-code"> class="hljs-ln-line">            return unsafe;
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="35"> class="hljs-ln-code"> class="hljs-ln-line">        } catch (NoSuchFieldException | IllegalAccessException e) {
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="36"> class="hljs-ln-code"> class="hljs-ln-line">            throw new RuntimeException(e);
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="37"> class="hljs-ln-code"> class="hljs-ln-line">        }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="38"> class="hljs-ln-code"> class="hljs-ln-line">    }
 class="hljs-ln-numbers"> class="hljs-ln-line hljs-ln-n" data-line-number="39"> class="hljs-ln-code"> class="hljs-ln-line">} class="hide-preCode-box"> class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}" onclick="hljs.signin(event)">

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