JAVA GC(垃圾回收机制)-1Java内存区域及外存溢出特别。

1.连锁概念

Java虚拟机,以下简称JVM。

1.开篇

Arvin小编又来形容技术博文啦,做个不大笔记,顺便让一部分丁扫扫盲,最近对Java
的GC(垃圾回收机制)特别感谢兴趣。很已经懂得在C/C++
这仿佛语言中需要程序猿自己失去操作对内存的管住 什么 alloc,
dealloc啦之类的。当初学OC的下啊有诸如此类一章节节。但是说交java很少有人会说到管理内存这块文化。java相比C语言在内存管理这块先进了无数,因为java的内存是活动管理之,光机关这个词即明白死高档有木有。但是java是怎么去创造和释放内存的吗。这个可怜有必不可少扒一熬,毕竟听说有些面试上来便问java的GC是怎么work的。还有即使是当时无异片属于jvm的基本知识。

JVM在履java程序的历程中会拿它们所管理的内存划分为多单区域。

2.预备知识

眼看首文章要是也后面的GC分析来开基础知识扫盲的,主要是讲jvm的内存分配。以前提到过jvm的内存就分割2片
一个仓房(stack)一个积聚(Heap),其实是是拂的,没有这样简单,还是来接触小复杂的,好了来扒一扒jvm的内存

这些区域发生各自的生命周期。有的依赖让JVM,有的拄让用户线程。

2.JVM内存

6片区域组成。

1.结构

图片 1

顿时张图片表明了,当运行java程序的时光
jvm会产生的内存结构,而我们平常所说的stack 和heap 就是对应之jvm
stack和heap(heap中的新生代与老年代就篇稿子中无介绍,后面同样首GC分析的时候
会去详细介绍,目前即拿他当做jvm就哼啊)

程序计数器 The pc Register

JVM可以同时支持多尽线程。每个JVM线程有投机之次序计数器。任何时刻,每个JVM线程执行单个方法的代码,叫做那个线程的时法。如果不行方式不是当地的,那么程序计数器包含当前JVM正在行之下令地址。如果被这个线程当前推行之计是本地的,那么JVM的次序计数器的值是undefined。JVM的次第计数器足够去有一个归地址或者一个地方指针在指定的平台。翻译自JVM规范。

简易的话

JVM执行非本地方法,它的主次计数器存指令地址。

JVM执行本地方法,它的次第计数器存值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官网给的说明,学了汇编的伴侣应该懂得cpu中的pc register
用来囤指令的地点。 其实java中的pc
register的法则与汇编中之不均等可做的凡平等件事,就是记录了手上于运行指令的地方。如果java程序运行的凡native
language的通令则pc 里面存的是勿定义。
其实pc的高低可以忽略不计因为里面存的数据量太小了。重点是如注意一下,pc
register是线程私有的,所谓的线程私有就是各一个线程有一个相应的pc
register,所以只有线程开始之上 pc reigster才会创建,线程结束了 pc
register自然就木有了。

Java虚拟机栈 Java Virtual Machine Stacks

每个JVM线程有一个个体JVM栈,与线程一起吃创造。一个JVM栈存frames。一个JVM栈是接近于人情语言C的库:它有本地变量和有变量,在措施调用和归中自在作用。因为JVM栈从不直接让操作除了加大与取frames,frames按照堆放置。对于JVM栈内存不欲用到。

先是版java虚拟机规范着,JVM栈叫Java栈。

斯标准允许JVM栈是稳大小还是是动态扩展以及因计算的获取。

假若JVM栈大小是定点的,每个JVM栈被单独的挑当栈被创造时。

每个JVM实现好提供程序还是用户控制因JVM栈的始发大小,同样的动态扩展的要contractingJVM栈,控制以最充分以及极其小尺寸。

下是JVM栈的不得了条件

比方算后线程要求一个还怪之JVM栈比从所兴的,JVM抛来StackOverflowError.

要JVM栈是动态扩展的,当扩展中没有足够的内存还是没足够的内存去初始化JVM栈的一个线程,JVM抛出OutOfMemoryError

简言之的话

JVM中每个线程有一个JVM栈,与线程一起让创造。每个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 这个事物呢 也是线程私有的,随线程生随线程死。其实stack
这个事物还有下级,就是stack frame。 stack frame
是对准跟方法的,简单的吧,每一个method被实践之时光会创一个stack
frame 被push到vm stack 中,方法让实施得后会pop出vm
stack。真正存数据的地方实在是stack frame。vm stack类似一个成团。
stack frame中满怀了三栽东西:

  • Local Vairable

  • 主导类型(int,long,char,boolean…….)

  • 目标类型的援
  • returnAddress的类型

  • Operand Stack

  • data frame
    若要vm stack 大于了
    vm可以成熟之轻重,java会废弃来stackoverflowerror的不得了

Native Stack 和stack的用法差不多,但是是吃java程序中跑native
language的时段下的

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.

堆是霸占内存最特别一片的地方,他是线程共享的也就是说在java程序运行的时创建的直到java程序结束。主要是存实例对象与数组数据。也马上GC发生最多之地方。另外说一样触及
堆中存放的数目的物理地址不是接二连三的
学过汇编的同伙应该可以清楚。如果这地方要求的大小大于了vm
所领的轻重缓急会保outofmemoryerror 也即是风传着之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要是因此来储存 vm
已经加载号的接近信息,静态变量,等等。同时于方块区里面有同片是
常量池,也就是咱们平素因故final创建出来的变量都见面被停放这个里面。这个地方发生GC比较少,但是倘若跨越大小也会扔来OOM的不得了

运行时量池 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.

运行时量池是每个接近或每个接口在类似公事之常量池表的运作时表示。他带有了几栽常量,从编译时之数字文字及方法和援字段,必须于运行时解决。运行时量池服务方程式类似于正规编程语言的号表,尽管它包含了一个良范围之数要非是第一流的符号表。

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

面介绍了一样文山会海的内存分布每一样块都有温馨的作用及特性,我们接下拿一个板栗来当实例分析一下:

Object obj = new Object();

简短吧,但是深扒还是得扒出很多物的。拆分成2块来进展解析。
Object obj 在相应的stack frame中之local
variable表中因reference类型出现。
new Object()呢
在heap中开拓了一样片以存储object类型所有实例数据的内存。heap中尚须含有相应的对象型数据类型(interface,super
class,class….)
reference
里面就是是一个对准对象的援所以现在之题目便是哪些拿她们2个链接起来(=)。有2种植方法可以链接,不同的vm采用不同之计:
措施1)指向heap中的句炳,然后由句炳指向真正的实例,什么意思吧,就是间接指向。如果是利用这种办法那么当heap中自然会起出一致片存放句炳的内存

heap中2片地方 句炳池和实例池,句炳用来找到实例和目标类型数据
计2)直接看,就是取消了句炳了。直接存储实例的地点,直接看到实例数据

两头的得失:句炳的话,如果数量发现改变移动
reference里面的价是免见面转换的。直接看的话语效率还好快更快,sum hotspot
vm就是用之直访问.

首先首文章到此了啦。主要介绍了vm的内存分配,如果大家而测试oom可以改vm对应之参数。

当地方法栈 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

深刻明Java虚拟机 周志明 著

JVM规范链接

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