jvm中的栈

向右看齐 2022-06-16 01:22 159阅读 0赞

# 1、linux下典型的进程空间 #

linux下一个典型的进程空间如下：

[![image][]][image 1]

为什么进程还要区分栈和堆，stackoverflow上的一个回答非常好：[http://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap][http_stackoverflow.com_questions_79923_what-and-where-are-the-stack-and-heap]，主要意思如下：

stack：计算架构中的[堆栈][Link 1]是以先到[先出（LIFO）][LIFO]方式添加或删除数据的[内存][Link 2]区域。

1、创建的变量出了作用域，自动释放；

2、相比较heap的分配简单高效

3、有固定大小，超出会报溢出错误

4、明确固定大小的情况下，可以在编译的时候就确定

heap

1、需要手动释放，其生命周期和手动释放时间有关

2、分配比较慢

3、可以按照程序的需求申请管理

4、动态的申请和释放

栈大小是有默认值的，如果申请的临时变量太大的话就会超过栈大小，造成栈溢出  
编译期限制栈大小，和系统限制栈深度根本是两回事。系统限制栈深是限制进程主线程的栈深，限制的是整个函数调用链的最大栈深，这个栈深是函数调用链上各个函数栈帧大小之和。编译期限制栈大小是限制单个函数栈帧的大小。  
栈的大小可以修改的。**在应用程序我们经常需要定义大的数组，数组定义成局部变量非静态变量**，那么数组就会在栈上分配，当数组超过默认栈的大小时，会引起非常内存访问

linux下的堆栈大小受环境变量设置

ulimit -a \#显示当前用户的栈大小  
ulimit -s 32768 \#将当前用户的栈大小设置为32M bytes

# 2、JVM中stack的结构 #

jvm中栈保存的是线程运行调用方法的局部变量、被调用进来的参数、它的返回值、以及运算的中间结果等等。java的栈是一帧为单位保存线程的运行状态，线程私有数据。虚拟机对java栈执行的只有二种操作，以帧为单位的入栈和出栈。

java的栈和帧在内存不必须是连续的，珍可以分布在连续的栈里，也可以分布在堆里，或者二者兼而有之，Java栈和帧的实际数据结构由虚拟机的实现者决定。

[![image][image 2]][image_image 2]

**注意进程中的栈和jvm中的栈不是一个概念，jvm中的栈是个抽象的概念，虚拟机实现程，程序员不可实现，只是一个标准。jvm中的栈是在进程的栈实现，还是在堆实现，还是二者结合，这三种情况都有。**

实际上java虚拟机就是一个栈式实现的虚拟机（主要为了摆脱对物理机的依懒，主要是寄存器）。

# 3、jvm中stack和gc的关系 #

虚拟机进行gc的时候，是否要对jvm的stack进行gc?因为stack保存的方法的参数和局部变量，调用完成后会自动释放，如果申请了堆对象，在stack也只保留了一个引用，所以，这些内容在调用完成后会自动释放，无需GC来做繁琐的工作，GC只对stack申请的堆对象进行管理。stack有自己的内存管理机制，虚拟机的实现者一般都将其于GC分开。老外有二段经典的回答不错：

观点1:

Garbage collection is a method of deallocating memory that isn't being used anymore. Sometimes the "isn't being used anymore" part is tricky. With the stack, as soon as a function returns, we can be confident (excepting programmer error) that the local variables aren't being used anymore, so they are deallocated automatically at that time in nearly every language/runtime.

观点2：

The stack is called a "stack" precisely because it is a zone of memory which is managed with a "stack policy", aka LIFO (as Last In, First Out). If allocation on the stack was not done in "the stack way" it would not be called a stack but heap.  
Garbage Collection was invented in order to cope with the problem of allocating things on a heap, i.e. such that you cannot predict which parts will be released first. GC is meant for memory allocation problems where stack management is not sufficient.

# 4、参考文档： #

[http://stackoverflow.com/questions/2447504/is-the-stack-garbage-collected-in-java][http_stackoverflow.com_questions_2447504_is-the-stack-garbage-collected-in-java]

[http://stackoverflow.com/questions/2524792/does-a-garbage-collector-collect-stack-memory-heap-memory-or-both][http_stackoverflow.com_questions_2524792_does-a-garbage-collector-collect-stack-memory-heap-memory-or-both]

[http://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap][http_stackoverflow.com_questions_79923_what-and-where-are-the-stack-and-heap]

[http://stackoverflow.com/questions/31145052/difference-between-call-stack-and-thread-stack][http_stackoverflow.com_questions_31145052_difference-between-call-stack-and-thread-stack]

[http://stackoverflow.com/questions/1762418/what-resources-are-shared-between-threads][http_stackoverflow.com_questions_1762418_what-resources-are-shared-between-threads]

[https://www.ibm.com/developerworks/cn/linux/kernel/l-thread/][https_www.ibm.com_developerworks_cn_linux_kernel_l-thread]

http://hllvm.group.iteye.com/group/forum

[image]: /images/20220616/3f777dbfc5c142d794df45adbb5ca719.png
[image 1]: https://img-blog.csdn.net/20170501225024361
[http_stackoverflow.com_questions_79923_what-and-where-are-the-stack-and-heap]: http://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap
[Link 1]: https://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29
[LIFO]: https://en.wikipedia.org/wiki/LIFO_%28computing%29
[Link 2]: https://en.wikipedia.org/wiki/Memory_%28computers%29
[image 2]: https://img-blog.csdn.net/20170501231939265
[image_image 2]: /images/20220616/0800b4f329a04937adbfea226082d608.png
[http_stackoverflow.com_questions_2447504_is-the-stack-garbage-collected-in-java]: http://stackoverflow.com/questions/2447504/is-the-stack-garbage-collected-in-java
[http_stackoverflow.com_questions_2524792_does-a-garbage-collector-collect-stack-memory-heap-memory-or-both]: http://stackoverflow.com/questions/2524792/does-a-garbage-collector-collect-stack-memory-heap-memory-or-both
[http_stackoverflow.com_questions_31145052_difference-between-call-stack-and-thread-stack]: http://stackoverflow.com/questions/31145052/difference-between-call-stack-and-thread-stack
[http_stackoverflow.com_questions_1762418_what-resources-are-shared-between-threads]: http://stackoverflow.com/questions/1762418/what-resources-are-shared-between-threads
[https_www.ibm.com_developerworks_cn_linux_kernel_l-thread]: https://www.ibm.com/developerworks/cn/linux/kernel/l-thread/