用Java实现JVM第四章《运行时数据区》

运行时数据区描述

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。

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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
2.1、配置位置:Run/Debug Configurations -> program arguments
2.2、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

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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

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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

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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

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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

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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

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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

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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

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