用Java实现JVM第四章《运行时数据区》

悠悠 2023-01-07 02:29 105阅读 0赞

\*\*![f82ce6a445f5cd8d94c746592ef24da2.png][]

## 案例介绍 ##

本案例初步实现运行时数据区里；线程、Java虚拟机栈、帧、操作数栈、局部变量表。

> 在运行Java程序时，Java虚拟机需要使用内存来存放各种各样的数据。Java虚拟机规范把这些内存区域叫作运行时数据区。运行时数据区可以分为两类：一类是多线程共享的，另一类则是线程私有的。多线程共享的运行时数据区需要在Java虚拟机启动时创建好在Java虚拟机推出时销毁。线程私有的运行时数据区则在创建线程时才创建，线程退出时销毁。

> 线程私有的运行时数据区用于辅助执行Java字节码。每个线程都有自己的pc寄存器（Program Counter）和Java虚拟机栈（JVM Stack）。Java虚拟机栈又由栈帧（Stack Frame，后面简称帧）构成，帧中保存方法执行的状态，包括局部变量表（Local Variable）和操作数栈（Operand Stack）等。在任一时刻，某一线程肯定是在执行某个方法。这个方法叫作该线程的当前方法；执行该方法的帧叫作线程的当前帧；声明该方法的类叫作当前类。如果当前方法是Java方法，则pc寄存器中存放当前正在执行的Java虚拟机指令的地址，否则，当前方法是本地方法，pc寄存器中的值没有明确定义。

Run-Time Data Area
    ├── Thread
    │   └── pc
    │  	└── Jvm Stack  
    │       └── Frame
    │  	        ├── Local Variable
    │  	        └── Operand Stack
    └── Heap
        ├── Method Area
    	│   └── Class
    	│       ├── Run-Time
    	│ 	    └── Constant Pool
    	└── Object

## 环境准备 ##

1.  jdk 1.8.0
2.  IntelliJ IDEA Community Edition 2018.3.1 x64

## 配置信息 ##

1.  调试配置
    
    1.  配置位置：Run/Debug Configurations -> program arguments
    2.  配置内容：-Xjre “C:\\Program Files\\Java\\jdk1.8.0\_161\\jre” E:\\itstack\\git\\istack-demo\\itstack-demo-jvm\\itstack-demo-jvm-04\\target\\test-classes\\org\\itstack\\demo\\test\\HelloWorld

## 代码示例 ##

itstack-demo-jvm-04
    ├── pom.xml
    └── src
        └── main
        │    └── java
        │        └── org.itstack.demo.jvm
    	│             ├── classfile
        │             │   ├── attributes   {BootstrapMethods/Code/ConstantValue...}
        │             │   ├── constantpool {CONSTANT_TAG_CLASS/CONSTANT_TAG_FIELDREF/CONSTANT_TAG_METHODREF...}
        │             │   ├── ClassFile.java
        │             │   ├── ClassReader.java
        │             │   └── MemberInfo.java	
        │             ├── classpath
        │             │   ├── impl
        │             │   │   ├── CompositeEntry.java
        │             │   │   ├── DirEntry.java 
        │             │   │   ├── WildcardEntry.java 
        │             │   │   └── ZipEntry.java    
        │             │   ├── Classpath.java
        │             │   └── Entry.java   
        │             ├── rtda
        │             │   ├── Frame.java
        │             │   ├── JvmStack.java
        │             │   ├── LocalVars.java
        │             │   ├── OperandStack.java
        │             │   ├── Slot.java	
        │             │   └── Thread.java
        │             ├── Cmd.java
        │             └── Main.java
        └── test
             └── java
                 └── org.itstack.demo.test
                     └── HelloWorld.java

> Frame.java

package org.itstack.demo.jvm.rtda;
    
    /**
     * http://www.itstack.org
     * create by fuzhengwei on 2019/4/26
     * 栈帧
     */
    public class Frame {
    
        //stack is implemented as linked list
        Frame lower;
    
        //局部变量表
        private LocalVars localVars;
    
        //操作数栈
        private OperandStack operandStack;
    
        public Frame(int maxLocals, int maxStack) {
            this.localVars = new LocalVars(maxLocals);
            this.operandStack = new OperandStack(maxStack);
        }
    
        public LocalVars localVars(){
            return localVars;
        }
    
        public OperandStack operandStack(){
            return operandStack;
        }
    
    }

> JvmStack.java

package org.itstack.demo.jvm.rtda;
    
    /**
     * http://www.itstack.org
     * create by fuzhengwei on 2019/4/26
     * 虚拟机栈
     */
    public class JvmStack {
    
        private int maxSize;
        private int size;
        private Frame _top;
    
        public JvmStack(int maxSize) {
            this.maxSize = maxSize;
        }
    
        public void push(Frame frame) {
            if (this.size > this.maxSize) {
                throw new StackOverflowError();
            }
    
            if (this._top != null) {
                frame.lower = this._top;
            }
    
            this._top = frame;
            this.size++;
        }
    
        public Frame pop() {
            if (this._top == null) {
                throw new RuntimeException("jvm stack is empty!");
            }
    
            Frame top = this._top;
            this._top = top.lower;
            top.lower = null;
            this.size--;
    
            return top;
        }
    
        public Frame top(){
            if (this._top == null){
                throw new RuntimeException("jvm stack is empty!");
            }
            return this._top;
        }
    
    }

> LocalVars.java

package org.itstack.demo.jvm.rtda;
    
    /**
     * http://www.itstack.org
     * create by fuzhengwei on 2019/4/26
     * 局部变量表
     */
    public class LocalVars {
    
        private Slot[] slots;
    
        public LocalVars(int maxLocals) {
            if (maxLocals > 0) {
                slots = new Slot[maxLocals];
                for (int i = 0; i < maxLocals; i++) {
                    slots[i] = new Slot();
                }
            }
        }
    
        public void setInt(int idx, int val) {
            this.slots[idx].num = val;
        }
    
        public int getInt(int idx) {
            return slots[idx].num;
        }
    
        public void setFloat(int idx, float val) {
            this.slots[idx].num = (Float.valueOf(val)).intValue();
        }
    
        public Float getFloat(int idx) {
            int num = this.slots[idx].num;
            return (float) num;
        }
    
        public void setLong(int idx, long val) {
            this.slots[idx].num = (int) val;
            this.slots[idx + 1].num = (int) (val >> 32);
        }
    
        public Long getLong(int idx) {
            int low = this.slots[idx].num;
            int high = this.slots[idx + 1].num;
            return ((long) high << 32) | (long) low;
        }
    
        public void setDouble(int idx, double val) {
            setLong(idx, (long) val);
        }
    
        public Double getDouble(int idx) {
            return Double.valueOf(getLong(idx));
        }
    
        public void setRef(int idx, Object ref) {
            slots[idx].ref = ref;
        }
    
        public Object getRef(int idx) {
            return slots[idx].ref;
        }
    
    }

> OperandStack.java

package org.itstack.demo.jvm.rtda;
    
    /**
     * http://www.itstack.org
     * create by fuzhengwei on 2019/4/26
     * 操作数栈
     */
    public class OperandStack {
    
        private int size = 0;
        private Slot[] slots;
    
        public OperandStack(int maxStack) {
            if (maxStack > 0) {
                slots = new Slot[maxStack];
                for (int i = 0; i < maxStack; i++) {
                    slots[i] = new Slot();
                }
            }
        }
    
        public void pushInt(int val) {
            slots[size].num = val;
            size++;
        }
    
        public int popInt(){
            size --;
            return slots[size].num;
        }
    
        public void pushRef(Object ref){
            slots[size].ref = ref;
            size++;
        }
    
        public Object popRef(){
            size --;
            Object ref = slots[size].ref;
            slots[size].ref = null;
            return ref;
        }
    
    }

> Slot.java

package org.itstack.demo.jvm.rtda;
    
    /**
     * http://www.itstack.org
     * create by fuzhengwei on 2019/4/26
     * 数据槽
     */
    public class Slot {
    
        int num;
        Object ref;
    
    }

> Thread.java

package org.itstack.demo.jvm.rtda;
    
    /**
     * http://www.itstack.org
     * create by fuzhengwei on 2019/4/26
     * 线程
     */
    public class Thread {
    
        //Program Counter 寄存器
        private int pc;
    
        //虚拟机栈
        private JvmStack stack;
    
        public Thread(){
            this.stack = new JvmStack(1024);
        }
    
        public int pc(){
            return this.pc;
        }
    
        public void setPC(int pc){
            this.pc = pc;
        }
    
        public void pushFrame(Frame frame){
            this.stack.push(frame);
        }
    
        public Frame popFrame(){
            return this.stack.pop();
        }
    
        public Frame currentFrame(){
            return this.stack.top();
        }
    
    }

## 测试结果 ##

100
    -100
    null
    -100

> 本文在开源项目：[https://github.com/Android-Alvin/Android-LearningNotes][https_github.com_Android-Alvin_Android-LearningNotes] 中已收录，里面包含不同方向的自学编程路线、面试题集合/面经、及系列技术文章等，资源持续更新中…  
> \*\*

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