stack 那个事物呢 也是线程私有的,有的珍爱于用户线程

1.有关概念

Java虚拟机,以下简称JVM。

1.开篇

Arvin作者又来写技术博文啦,做个细微笔记,顺便给部分人扫扫盲,近日对Java
的GC(垃圾回收机制)越发感兴趣。很已经驾驭在C/C++
那类语言中须求程序猿本人去操作对内存的管制 什么 alloc,
dealloc啦之类的。当初学OC的时候也有这么一章节。但是说到java很少有人会说到管理内存那块文化。java相比C语言在内存管理那块先进了许多,因为java的内存是活动管理的,光机关那个词就精晓很高档有木有。不过java是怎么去创造和刑释解教内存的吗。那些很有必不可少扒一扒,毕竟听大人说有个别面试上来就问java的GC是怎么work的。还有就是这一块属于jvm的基本知识。

JVM在执行java程序的长河中会把它所管理的内存划分为多少个区域。

2.准备知识

那篇作品只即使为后边的GC分析来做基础知识扫盲的,重假设讲jvm的内存分配。以前提到过jvm的内存就分2块
三个栈(stack)2个堆(Heap),其实这一个是错的,没有那样不难,依旧有点小复杂的,好了来扒一扒jvm的内存

那么些区域有各自的生命周期。有的依赖于JVM,有的倚重于用户线程。

2.JVM内存

6块区域整合。

1.结构

图片 1

那张图纸注明了,当运转java程序的时候
jvm会暴发的内存结构,而作者辈向来所说的stack 和heap 就是呼应的jvm
stack和heap(heap中的新生代和老时期那篇小说中不介绍,前边一篇GC分析的时候
会去详细介绍,近年来就把他看成jvm就好啊)

次第计数器 The pc Register

JVM可以而且支持广大推行线程。逐个JVM线程有温馨的程序计数器。任何时候,每一个JVM线程执行单个方法的代码,叫做那么些线程的脚下格局。若是不行格局不是地面的,那么程序计数器包蕴当前JVM正在推行的授命地址。借使被那几个线程当前实施的点子是当地的,那么JVM的顺序计数器的值是undefined。JVM的先后计数器充裕去持有二个回到地址或许3个地面指针在钦赐的阳台。翻译自JVM规范。

总结的话

JVM执行非本地点法,它的次序计数器存指令地址。

JVM执行本地点法,它的顺序计数器存值undefined。

tag:线程隔离的数据区

1)程序计数器(Program counter Register)

The Java Virtual Machine can support many threads of execution at
once. Each Java Virtual Machine thread has its own pc (program
counter) register. At any point, each Java Virtual Machine thread is
executing the code of a single method, namely the current method for
that thread. If that method is not native, the pc register contains
the address of the Java Virtual Machine instruction currently being
executed. If themethodcurrently being executed by the thread is native
, the value of the Java Virtual Machine’s pc register is undefined.
The Java Virtual Machine’s pc register is wide enough to hold a
returnAddress or a native pointer on the specific platform.

java官网给的解释,学过汇编的同伴应该通晓cpu中的pc register
用来储存指令的地址。 其实java中的pc
register的法则和汇编中的差异只是做的是均等件事,就是记录了目前在运转指令的地址。如若java程序运营的是native
language的下令则pc 里面存的是未定义。
其实pc的大大小小可以忽略不计因为里面存的数据量太小了。重点是要专注一下,pc
register是线程私有的,所谓的线程私有就是每1个线程有三个一见如旧的pc
register,所以唯有线程起先的时候 pc reigster才会成立,线程甘休了 pc
register自然就木有了。

Java虚拟机栈 Java Virtual Machine Stacks

逐个JVM线程有2个个体JVM栈,与线程一起被成立。二个JVM栈存frames。1个JVM栈是接近于古板语言C的栈:它有地面变量和部分变量,在点子调用和重临中起着职能。因为JVM栈从不直接被操作除了放和取frames,frames根据堆放置。对于JVM栈内存不需求用到。

首先版java虚拟机规范中,JVM栈叫Java栈。

以此标准允许JVM栈是稳定大小只怕是动态伸张以及根据计算的获取。

假定JVM栈大小是固定的,逐个JVM栈被单独的精选当栈被成立时。

种种JVM已毕可以提供程序如故用户控制基于JVM栈的早先大小,同样的动态扩充的依然contractingJVM栈,控制在最大和纤维大小。

下边是JVM栈的尤其条件

设若总结后线程要求三个更大的JVM栈比起所允许的,JVM抛出StackOverflowError.

假若JVM栈是动态增添的,当伸张中并未充分的内存还是尚未丰富的内存去开首化JVM栈的二个线程,JVM抛出OutOfMemoryError

简易的话

JVM中种种线程有一个JVM栈,与线程一起被创设。各个JVM栈用来存frames。

frame is used to store data and partial results, as well as to
perform dynamic linking, return values for methods, and dispatch
exceptions.

 Local Variables

Operand Stacks

 Dynamic Linking

Normal Method Invocation Completion

Abrupt Method Invocation Completion

frames

2)VM stack(vm 栈)

Each Java Virtual Machine thread has a private Java Virtual Machine
stack
, created at the same time as the thread. A Java Virtual Machine
stack stores frames. A Java Virtual Machine stack is analogous to the
stack of a conventional language such as C: it holds local variables
and partial results, and plays a part in method invocation and return.
Because the Java Virtual Machine stack is never manipulated directly
except to push and pop frames, frames may be heap allocated. The
memory for a Java Virtual Machine stack does not need to be
contiguous.

stack 那么些东西呢 也是线程私有的,随线程生随线程死。其实stack
那么些事物还有下级,就是stack frame。 stack frame
是指向与形式的,简单的来说,每2个method被实践的时候会创制三个stack
frame 被push到vm stack 中,方法被实施到位之后会pop出vm
stack。真正存数据的地方实际上是stack frame。vm stack类似三个成团。
stack frame中存了二种东西:

  • Local Vairable
  1. 核心项目(int,long,char,boolean…….)
  2. 目的类型的引用
  3. returnAddress的类型
  • Operand Stack
  • data frame
    如果请求vm stack 大于了
    vm可以成熟的高低,java会抛出stackoverflowerror的十二分

Native Stack 和stack的用法大概,可是是给java程序中跑native
language的时候利用的

java 堆 Heap

The Java Virtual Machine has a heap that is shared among all Java
Virtual Machine threads. The heap is the run-time data area from which
memory for all class instances and arrays is allocated.

The heap is created on virtual machine start-up. Heap storage for
objects is reclaimed by an automatic storage management system (known as
garbage collector); objects are never explicitly deallocated. The
Java Virtual Machine assumes no particular type of automatic storage
management system, and the storage management technique may be chosen
according to the implementor’s system requirements. The heap may be of a
fixed size or may be expanded as required by the computation and may be
contracted if a larger heap becomes unnecessary. The memory for the heap
does not need to be contiguous.

A Java Virtual Machine implementation may provide the programmer or the
user control over the initial size of the heap, as well as, if the heap
can be dynamically expanded or contracted, control over the maximum and
minimum heap size.

The following exceptional condition is associated with the
heap:

If a computation requires more heap than can be made available by the
automatic storage management system, the Java Virtual Machine throws
an OutOfMemoryError.

3)Heap(堆)

The Java Virtual Machine has a heap that is shared among all Java
Virtual Machine threads. The heap is the run-time data area from which
memory for all class instances and arrays is allocated.
The heap is created on virtual machine start-up. Heap storage for
objects is reclaimed by an automatic storage management system (known
as a garbage collector); objects are never explicitly deallocated.
The Java Virtual Machine assumes no particular type of automatic
storage management system, and the storage management technique may be
chosen according to the implementor’s system requirements. The heap
may be of a fixed size or may be expanded as required by the
computation and may be contracted if a larger heap becomes
unnecessary. The memory for the heap does not need to be contiguous.

堆是占内存最大一块的地方,他是线程共享的也等于说在java程序运维的时候创造的截止java程序为止。主假如存放实例对象和数组数据。也随即GC爆发最多的地点。别的说一点
堆中存放的多少的大体地址不是一连的
学过汇编的同伴应该可以知晓。即使那么些地点须要的大大小小大于了vm
所接受的分寸会保outofmemoryerror 相当于风传中的OOM

方法区 Method Area

The Java Virtual Machine has a method area that is shared among all
Java Virtual Machine threads. The method area is analogous to the
storage area for compiled code of a conventional language or analogous
to the “text” segment in an operating system process. It stores
per-class structures such as the run-time constant pool, field and
method data, and the code for methods and constructors, including the
special methods
(§2.9)
used in class and instance initialization and interface initialization.

The method area is created on virtual machine start-up. Although the
method area is logically part of the heap, simple implementations may
choose not to either garbage collect or compact it. This specification
does not mandate the location of the method area or the policies used to
manage compiled code. The method area may be of a fixed size or may be
expanded as required by the computation and may be contracted if a
larger method area becomes unnecessary. The memory for the method area
does not need to be contiguous.

A Java Virtual Machine implementation may provide the programmer or the
user control over the initial size of the method area, as well as, in
the case of a varying-size method area, control over the maximum and
minimum method area size.

The following exceptional condition is associated with the method
area:

If memory in the method area cannot be made available to satisfy an
allocation request, the Java Virtual Machine throws an OutOfMemoryError.

4)Method Area(方法区)

The Java Virtual Machine has a method area that is shared among all
Java Virtual Machine threads. The method area is analogous to the
storage area for compiled code of a conventional language or analogous
to the “text” segment in an operating system process. It stores
per-class structures such as the run-time constant pool, field and
method data, and the code for methods and constructors, including the
special methods used in class and instance initialization and
interface initialization.

方法区也是线程共享的显假诺用来存储 vm
已经加载号的类消息,静态变量,等等。同时在方块区里面有一块是
常量池,约等于我们平昔用final创设出来的变量都会被平放这几个里面。这几个地点发生GC相比少,不过倘使跨越大小也会抛出OOM的那多少个

运营时常量池 Run-Time Constant Pool

run-time constant pool is a per-class or per-interface run-time
representation of the constant_pool table in a class file
(§4.4).
It contains several kinds of constants, ranging from numeric literals
known at compile-time to method and field references that must be
resolved at run-time. The run-time constant pool serves a function
similar to that of a symbol table for a conventional programming
language, although it contains a wider range of data than a typical
symbol table.

运作时常量池是每一个类如故各个接口在类公事的常量池表的运维时表示。他饱含了三种常量,从编译时的数字文字到格局和引用字段,必须在运转时化解。运营时常量池服务方程式类似高满堂规编程语言的符号表,固然它包罗了二个大范围的多少而不是卓绝群伦的符号表。

Each run-time constant pool is allocated from the Java Virtual
Machine’s method area
(
§2.5.4).
The run-time constant pool for a class or interface is constructed when
the class or interface is created
(§5.3)
by the Java Virtual Machine.

The following exceptional condition is associated with the construction
of the run-time constant pool for a class or
interface:

When creating a class or interface, if the construction of the run-time
constant pool requires more memory than can be made available in the
method area of the Java Virtual Machine, the Java Virtual Machine throws
an OutOfMemoryError.

See §5 (Loading, Linking, and
Initializing
)
 for
information about the construction of the run-time constant pool.

2.栗子

上边介绍了一多种的内存分布每一块都有和好的效率和性格,大家接下去拿二个板栗来作为实例分析一下:

Object obj = new Object();

大约吗,可是深扒仍可以够扒出很多事物的。拆分成2块来开展辨析。
Object obj 在对应的stack frame中的local
variable表中以reference类型出现。
new Object()呢
在heap中开辟了一块以存储object类型全数实例数据的内存。heap中还非得蕴涵相应的对象型数据类型(interface,super
class,class….)
reference
里面就是2个针对性对象的引用所以以后的题材就是哪些把她们二个链接起来(=)。有2种艺术可以链接,不一样的vm采纳不一样的主意:
办法1)指向heap中的句炳,然后由句炳指向实际的实例,什么看头呢,就是直接指向。即使是应用那种措施那么在heap中毫无疑问会开出一块存放句炳的内存

heap中2块地方 句炳池和实例池,句炳用来找到实例和对象类型数据
办法2)间接访问,就是打消了句炳了。直接存储实例的地方,直接访问到实例数据

二者的利害:句炳的话,借使数据发现改变移动
reference里面的值是不会变的。直接访问的话效能更好速度更快,sum hotspot
vm就是用的平素访问.

率先篇小说到那边甘休啦。主要介绍了vm的内存分配,如若大家要测试oom可以修改vm对应的参数。

本地点法栈 Native Method Stacks

An implementation of the Java Virtual Machine may use conventional
stacks, colloquially called “C stacks,” to support native methods
(methods written in a language other than the Java programming
language). Native method stacks may also be used by the implementation
of an interpreter for the Java Virtual Machine’s instruction set in a
language such as C. Java Virtual Machine implementations that cannot
load nativemethods and that do not themselves rely on conventional
stacks need not supply native method stacks. If supplied, native method
stacks are typically allocated per thread when each thread is created.

This specification permits native method stacks either to be of a fixed
size or to dynamically expand and contract as required by the
computation. If the native method stacks are of a fixed size, the size
of each native method stack may be chosen independently when that stack
is created.

A Java Virtual Machine implementation may provide the programmer or the
user control over the initial size of the native method stacks, as well
as, in the case of varying-size native method stacks, control over the
maximum and minimum method stack sizes.

The following exceptional conditions are associated with native method
stacks:

If the computation in a thread requires a larger native method stack
than is permitted, the Java Virtual Machine throws
a StackOverflowError.

If native method stacks can be dynamically expanded and native method
stack expansion is attempted but insufficient memory can be made
available, or if insufficient memory can be made available to create the
initial native method stack for a new thread, the Java Virtual Machine
throws an OutOfMemoryError.

参考

深切了解Java虚拟机 周志明 著

JVM规范链接