JVM 调优实战--垃圾回收的常见算法

淡淡的烟草味﹌ 2023-10-07 08:16 24阅读 0赞

**目录**

--------------------

引用计数法

原理

优缺点

标记清除法

原理

优缺点

标记压缩算法

原理

优缺点

复制算法

原理

JVM年轻代内存空间

优缺点

分代算法

总结

--------------------

> 自动化的管理内存资源，垃圾回收机制必须要有一套算法来进行计算，哪些是有效的对象，哪些是无效的对象，对于无效的对象就要进行回收处理。
> 
> 常见的垃圾回收算法有：**引用计数法、标记清除法、标记压缩法、复制算法、分代算法**等。

## 引用计数法 ##

> ![20200116131523746.png][]
> 
> ### 原理 ###
> 
> ![20200116131711419.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 1][]
> 
> ### 优缺点 ###
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 2][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 3][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 4][]

## 标记清除法 ##

> 标记清除算法也叫根可达算法。在垃圾回收器执行过程中会暂停所有的工作线程。
> 
> ![20200116134224325.png][]
> 
> ### 原理 ###
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 5][]
> 
> ![20200116134504923.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 6][]
> 
> ![20200116134658398.png][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 7][]
> 
> ![20200116134836659.png][]
> 
> ### 优缺点 ###
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 8][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 9][]

## 标记压缩算法 ##

> ![20200116135643730.png][]
> 
> ### 原理 ###
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 10][]
> 
> ### 优缺点 ###
> 
> ![20200116135946914.png][]

## 复制算法 ##

> ### 原理 ###
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 11][]
> 
> 将存活对象从from区复制到to区：
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 12][]
> 
> 清空from区，并且将to区的存活对象移动到from区，相当于颠倒一下位置：
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 13][]
> 
> ### JVM年轻代内存空间 ###
> 
> 使用的就是复制算法：
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 14][]
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 15][]
> 
> ### 优缺点 ###
> 
> ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly96aGFuZ3h1ZWxpYW5nLmJsb2cuY3Nkbi5uZXQ_size_16_color_FFFFFF_t_70 16][]

## 分代算法 ##

> ![20200116141448320.png][]

## 总结 ##

> 目前还没有一种完美的垃圾回收算法，每一种算法既有优点又有缺点，因此根据垃圾回收对象的特点进行选择，才是最明智的选择。
> 
> 分代算法就是如此（因地制宜），根据回收对象的特点进行选择：
> 
> 在JVM中，年轻代适合使用复制算法，因为垃圾对象较多；
> 
> 老年代适合使用标记清除或者标记压缩算法，因为垃圾较少，存活对象较多。

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