JVM-JVM内存结构(一)

程序计数器

Program Counter Register程序计数器(寄存器)

程序计数器在物理层上是通过寄存器实现的

• 作用：记住下一条jvm指令的执行地址
• 特点
  1. 是线程私有的(每个线程都有属于自己的程序计数器)
  2. 不会存在内存溢出

虚拟机栈

• 每个线程运行时所需要的内存称为虚拟机栈
• 每个栈由多个栈帧组成，对应着每次方法调用时所占的内存
• 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

栈内存溢出(StackOverflowError)

• 栈帧过多导致内存溢出
• 栈帧过大导致内存溢出

java编译工具：

jstack 线程id：可以根据线程id找到有问题的线程进一步定位到有问题的代码行数

本地方法栈(Native Method stack)

java代码在完成一些需求时，需要调用一些底层的，如c/c++代码，那么就需要本地方法栈

例如hashCode()方法，就是一个本地方法

public native int hashCode();/*** Indicates whether some other object is "equal to" this one.* <p>* The {@code equals} method implements an equivalence relation* on non-null object references:* <ul>* <li>It is <i>reflexive</i>: for any non-null reference value*     {@code x}, {@code x.equals(x)} should return*     {@code true}.* <li>It is <i>symmetric</i>: for any non-null reference values*     {@code x} and {@code y}, {@code x.equals(y)}*     should return {@code true} if and only if*     {@code y.equals(x)} returns {@code true}.* <li>It is <i>transitive</i>: for any non-null reference values*     {@code x}, {@code y}, and {@code z}, if*     {@code x.equals(y)} returns {@code true} and*     {@code y.equals(z)} returns {@code true}, then*     {@code x.equals(z)} should return {@code true}.* <li>It is <i>consistent</i>: for any non-null reference values*     {@code x} and {@code y}, multiple invocations of*     {@code x.equals(y)} consistently return {@code true}*     or consistently return {@code false}, provided no*     information used in {@code equals} comparisons on the*     objects is modified.* <li>For any non-null reference value {@code x},*     {@code x.equals(null)} should return {@code false}.* </ul>** <p>* An equivalence relation partitions the elements it operates on* into <i>equivalence classes</i>; all the members of an* equivalence class are equal to each other. Members of an* equivalence class are substitutable for each other, at least* for some purposes.** @implSpec* The {@code equals} method for class {@code Object} implements* the most discriminating possible equivalence relation on objects;* that is, for any non-null reference values {@code x} and* {@code y}, this method returns {@code true} if and only* if {@code x} and {@code y} refer to the same object* ({@code x == y} has the value {@code true}).** In other words, under the reference equality equivalence* relation, each equivalence class only has a single element.** @apiNote* It is generally necessary to override the {@link #hashCode() hashCode}* method whenever this method is overridden, so as to maintain the* general contract for the {@code hashCode} method, which states* that equal objects must have equal hash codes.** @param   obj   the reference object with which to compare.* @return  {@code true} if this object is the same as the obj*          argument; {@code false} otherwise.* @see     #hashCode()* @see     java.util.HashMap*/