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.
     * 

     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * 

     * 
It is reflexive: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * 
It is symmetric: 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}.
     * 
It is transitive: 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}.
     * 
It is consistent: 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.
     * 
For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.
     * 
     *
     * 

     * An equivalence relation partitions the elements it operates on
     * into equivalence classes; 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
     */